{"paper_id":"493995a3-c919-4d99-a896-e4bbd2ef148b","body_text":"Survival improvements in zoo-housed giraffes lead to surplus males: A long-term analysis of longevity and mortality in Japanese zoos | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Survival improvements in zoo-housed giraffes lead to surplus males: A long-term analysis of longevity and mortality in Japanese zoos Megu Gunji, Yuuki Kawata, Masato Yayota, Satoshi Kusuda, Isao Shimizu This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7858970/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Surplus animals exceeding housing capacity pose a major challenge for zoo population management. In polygynous species such as giraffes ( Giraffa camelopardalis ), where fewer males than females can be accommodated per facility, male surpluses are particularly problematic. Although the proportion of males among zoo-housed giraffes has increased globally in recent decades, the demographic drivers remain unclear. Here, we analysed Japanese zoo studbook data (1980–2023) to examine population trends, birth sex ratios, and sex-specific survival to elucidate factors influencing the population sex balance. Our analysis showed that the number of males rose sharply after 2012 and surpassed females for the first time in 2021, indicating the emergence of a pronounced male surplus. Survival analyses showed that both sexes experienced higher survival following husbandry reforms around 2010, but the improvement was disproportionately greater in males. Although females still exhibited higher survivorship, the survival gap between sexes narrowed and disappeared in the most recent cohorts, shifting the adult sex ratio toward parity and accelerating the male surplus. This study demonstrates that male surplus can emerge as an unintended consequence of successful husbandry improvements rather than biased birth ratios or reduced culling. These findings provide key implications for proactive sex ratio management in zoos. Biological sciences/Ecology Earth and environmental sciences/Ecology Biological sciences/Evolution Biological sciences/Zoology giraffe longevity studbook surplus zoo Figures Figure 1 Figure 2 Figure 3 Figure 4 Introduction In zoos, appropriate population management and improved breeding techniques are essential [ 1 ]. However, these efforts can sometimes lead to excessive population growth. Individuals that exceed the facility’s management capacity are termed “surplus animals,” which burden facilities and resources while raising concerns about animal welfare and genetic diversity. Therefore, preventing the occurrence of surplus individuals has become a major challenge in modern zoo population management [ 2 , 3 , 4 ]. Giraffes ( Giraffa camelopardalis ) are currently a representative species facing a serious surplus animal problem. The European Association of Zoos and Aquaria (EAZA) recommends housing in herds consisting of multiple females with their offspring and a dominant male [ 5 ]. While this housing structure mimics the wild herd structure, the large disparity in the number of males and females that can be accommodated in a single facility increases the likelihood of male surplus. Furthermore, over the past 20 years, the proportion of adult male giraffes in zoo populations has increased globally. The adult male-to-female ratio steadily rose from 0.47:1 in 2000 to 0.68:1 in 2022 [ 6 ]. In 2014, Copenhagen Zoo euthanised a healthy young male giraffe under such surplus conditions, attracting worldwide media attention and sparking ethical debate [ 7 , 8 ]. Although castration is sometimes used as a countermeasure for surplus individuals, surgical procedures on large mammals carry high risks [ 9 ] and do not provide a complete solution. Therefore, in zoo-housed giraffes, managing the population numbers and sex ratio balance is an urgent issue. A recent study using a global database of zoo-housed giraffes [ 6 ] proposed three possibilities for the increase in the proportion of adult males: (1) reduced male mortality due to improved husbandry conditions in recent years, (2) decreased culling since 2014, and (3) a combination of both factors. However, this analysis compared mortality risk between birth cohorts and between sexes, and did not directly examine whether recent reductions in mortality were particularly pronounced in males. Furthermore, the global database aggregates data from multiple regions with vastly different husbandry practices, making it difficult to assess the effects of husbandry condition developments and the extent of culling clearly. Therefore, the demographic mechanisms generating the male surplus remain unresolved. Japan provides an ideal case study to fill this knowledge gap. Currently, approximately 200 giraffes are housed in Japanese zoos, and breeding conditions have improved since 2010 following the introduction of international guidelines. However, unlike in Europe, surplus individuals have not been culled because of cultural and institutional reasons. Therefore, the Japanese population provides a natural experimental setting to examine whether improvements in husbandry conditions and associated changes in survival rates cause a male surplus. Here, we analyse long-term studbook data (1980–2023) from Japanese zoos to (1) quantify recent trends in population size by sex, (2) evaluate whether survivorship improve following enhanced husbandry conditions, and if confirmed, assess whether this improvement is particularly pronounced in males, and (3) examine other demographic factors such as birth sex ratio and import/export patterns. Together, these analyses aim to clarify the demographic processes driving the emergence of surplus males. Results Population changes of captive individuals In Japan, the population of zoo-housed giraffes subsequently increased until 1989, and then declined for approximately 15 years before rising again in 2014 (Fig. 1 ). Since 1957, females have consistently outnumbered males. However, the male population has increased rapidly since 2012, exceeding the female population in 2021. Specifically, the population grew from 60 males and 87 females (sex ratio of 0.69:1) in 2012 to 101 males and 96 females (sex ratio of 1.05:1) in 2023. Over this period, the female population increased by 1.1 times while the male population increased by 1.7 times. The number of zoos housing giraffes remained relatively stable (54 in 2012, 55 in 2023), but the number of males per zoo rose from 1.11 in 2012 to 1.84 in 2023. In 2023, 28 zoos housed at least 2 males, with 9 zoos housing 3 or more males. Survivorship analysis A Cox proportional hazard analysis revealed that males had consistently lower survivorship than females in both juvenile and adult stages (Table 3 ). The hazard ratio for males relative to females was 1.303 in juveniles and 1.417 in adults, indicating a 30–40% higher mortality risk in males. Survivorship also improved markedly across birth cohorts compared with the 1980s (reference group). For individuals born in the 2000s, hazard ratios were 0.856 in juveniles and 0.802 in adults; for those born in the 2010s, these ratios declined to 0.420 and 0.294, respectively. Analysis by sex revealed that the improvements in survivorship were especially pronounced in males (Figs. 2 , 3 ; Table 4 ). The hazard ratios for juvenile males declined to 0.358 in the 2010s compared with the 1980s, whereas those for juvenile females declined to 0.514. A similar trend was observed in adults, with hazard ratios decreased to 0.263 in males and 0.343 in females in the 2010s. For adults, the hazard ratio in the most recent cohort may have been overestimated due to the large proportion of censored data. Table 3 Survivorship analyses (Cox proportional hazards) of giraffes ( Giraffa camelopardalis ) kept in Japanese zoos from 1960 to 2023; models include distinct periods (birth cohorts 1980–1989, 1990–1999, 2000–2009, 2010–2019) and sex as factors. Model Hazard ratio (95%CI) z p up to 4 years of age reference: females (n = 462) males (n = 473) 1.303 (1.131–1.501) 3.672 < 0.001 *** 1980–1989 (n = 296) 1990–1999 (n = 278) 0.907 (0.769–1.070) -1.157 0.247 2000–2009 (n = 184) 0.856 (0.705–1.039) -1.171 0.116 2010–2019 (n = 177) 0.420 (0.321–0.549) -6.339 < 0.001 *** ≥ 4 years of age reference: females (n = 261) males (n = 232) 1.417 (1.145–1.754) 3.201 0.00137 ** 1980–1989 (n = 132) 1990–1999 (n = 144) 0.922 (0.726–1.172) -0.662 0.508 2000–2009 (n = 93) 0.802 (0.600–1.079) -1.455 0.146 2010–2019 (n = 124) 0.294 (0.168–0.515) -4.272 < 0.001 *** Table 4 Survivorship analyses (Cox proportional hazards) by sex of giraffes ( Giraffa camelopardalis ) kept in Japanese zoos from 1960 to 2023; models include distinct periods (birth cohorts 1980–1989, 1990–1999, 2000–2009, 2010–2019) as factors. Model Hazard ratio (95%CI) z p Male: up to 4 years of age reference: 1980–1989 (n = 151) 1990–1999 (n = 131) 0.885 (0.699–1.120) -1.018 0.309 2000–2009 (n = 96) 0.883 (0.677–1.153) -0.913 0.361 2010–2019 (n = 95) 0.358 (0.249–0.517) -5.494 < 0.001 *** Male: ≥ 4 years of age reference: 1980–1989 (n = 59) 1990–1999 (n = 65) 0.954 (0.667–1.363) -0.261 0.794 2000–2009 (n = 41) 0.753 (0.485–1.170) -1.263 0.206 2010–2019 (n = 67) 0.263 (0.124–0.555) -3.500 < 0.001 *** Female: up to 4 years of age reference: 1980–1989 (n = 145) 1990–1999 (n = 147) 0.928 (0.735–1.172) -0.625 0.532 2000–2009 (n = 88) 0.831 (0.626–1.103) -1.280 0.200 2010–2019 (n = 82) 0.514 (0.346–0.762) -3.312 < 0.001 *** Female: ≥ 4 years of age reference: 1980–1989 (n = 73) 1990–1999 (n = 79) 0.904 (0.651–1.255) -0.603 0.546 2000–2009 (n = 52) 0.850 (0.568–1.273) -0.789 0.430 2010–2019 (n = 57) 0.343 (0.146–0.803) -2.466 0.0137 * Kaplan–Meier survival curves similarly showed different trends between males and females (Fig. 4 ). Significant sex differences in survival probability likely occurred in the 1980s, 1990s, and 2000s (Log-rank test, p < 0.05) but disappeared in the 2010s (p = 0.99). Thus, although males generally had lower survival probability than females, the magnitude of improvement over time was greater for males, and sex difference was effectively eliminated in the most recent cohort. Similar to a Cox proportional hazard analysis, note that the results are inaccurate due to insufficient time elapsed since birth for individuals born in the 2010s. Instead, we compared survival probability at age four, when all individuals in this cohort had reached that age by the end of 2023. In the 2000s cohort, survival probability at age four was 0.427 for males and 0.591 for females, whereas in the 2010s cohort it was 0.705 for males and 0.695 for females. This indicates a marked improvement in juvenile survival, particularly among males, in the most recent cohort. Number and sex ratio of births The number of giraffe births in Japanese zoos has increased from the 1960s to the 1980s, peaking in the late 1980s, and has declined thereafter (Table 1 ). Since 2000, the average annual number of births has been 18.25 ± 3.67. Table 1 Number of births per decade. 1960s 1970s 1980s 1990s 2000s 2010s 2020s Total Male 15 75 170 142 98 95 38 633 Female 18 82 159 155 90 82 33 619 Unknown 3 5 10 5 0 2 0 25 Across the entire period since 1960, 633 males, 619 females, and 25 sex unknowns were recorded, indicating that the male proportion was 50.6% for known sex individuals. Among known-sex individuals born after 2000, the proportion of males was 53.0% (231 males and 205 females), although no significant difference was observed in the sex ratio of newborn individuals between before and after 2000 (chi-square test, p = 0.2325). Import and export situation The number of individuals imported from wild and foreign zoos or exported to zoos abroad is shown in Table 2 . A total of 26 males and 70 females were imported directly from the wild, but no wild individuals have been introduced since 1981. Imports from foreign zoos peaked in the 1970s and declined sharply after 1990; of the 138 imported individuals, 68.8% were female, resulting in a significantly female-biased sex ratio. In contrast, exports to foreign zoos were rare and unbiased (20 males and 22 females). Table 2 Number of imported and exported giraffes per decade. 1950s 1960s 1970s 1980s 1990s 2000s 2010s 2020s Total Imported from the wild or foreign zoo Male 7 12 15 7 1 0 1 0 43 Female 9 14 47 23 0 0 2 0 95 Exported to foreign zoo Male 0 2 3 7 6 2 0 0 20 Female 0 1 4 10 5 2 0 0 22 Discussion This study revealed that the population of zoo-housed male giraffes in Japan has increased rapidly since 2012, suggesting that a significant surplus of males is developing, even without culling. Moreover, our survival analyses provide the first clear evidence that survivorship has improved for both sexes in Japanese zoos since the 2010s, with disproportionately greater improvements in males. Although females continued to outlive males overall, the survival gap steadily narrowed and effectively disappeared in the most recent cohort. Thus, male-biased improvements in survival appear to have shifted the adult sex ratio toward parity, ultimately accelerating the emergence of surplus males. The timing of the marked survivorship improvements coincides with nationwide changes in husbandry practices, following the translation of the EAZA Giraffe Husbandry and Management Guidelines into Japanese in 2010 [ 10 ]. During this period, Japanese zoos substantially revised their nutritional protocols, reducing reliance on grass hay and low-fibre pellets and shifting toward alfalfa hay and browse [ 11 , 12 ], thereby improving nutritional quality. Previous studies have shown that such nutritional improvements contribute to longer lifespans in giraffes [ 13 , 14 ]. At the same time, the widespread adoption of long-term hormone monitoring allowed better estimation of conception and parturition in giraffids and other ungulates [ 11 , 15 , 16 , 17 ]. This facilitated more effective pregnancy and neonatal management, which likely improved juvenile survival. Although the precise mechanisms remain unclear, these reforms appear to have conferred relatively greater benefits to males, which previously suffered higher baseline mortality. Since 1960, the sex ratio of newborns in Japan has remained at approximately 1:1, indicating no significant bias (see Table 1 ). After 2000, the proportion of males increased slightly to 52.9%, although the value remained within a similar range compared to that of European zoos (52%) [ 5 ]. Although higher male birth ratios have been reported in wild populations (around 60%) [ 18 ], no indication existed of a significantly higher male birth rate relative to female births in the zoo-housed population. Furthermore, international introductions were clearly biased toward females, with the frequency of female imports exceeding that of males by more than twofold. This is considered one factor contributing to the formation of a population with an excess of females prior to 1990. In recent years, due to advances in domestic breeding, the number of individuals introduced from overseas has been extremely low, and most individuals introduced in the past died by around 2000. This is considered another reason for the disappearance of female dominance in the domestic population after 2000. In summary, the primary cause of the male surplus of giraffes in Japanese zoos was the improvement in male mortality rates that became pronounced after the 2010s, while the effects of sex ratio at birth and international introductions remained secondary factors. Thus, male surplus has emerged as a side effect of the “success” of improved husbandry. Improvements in zoo environments have been reported to reduce mortality rates among various species of animals other than giraffes [ 19 , 20 , 21 , 22 , 23 ]. Therefore, similar phenomena involving surplus males may occur in other species as well. The findings of this study provide valuable information for developing sex ratio management strategies for zoo-housed populations, not only for giraffes, but also for other species, particularly polygynous ones. Interestingly, improvements in husbandry conditions may not only lead to increased survival rates but may also potentially influence the sex ratio of newborns. The hypothesis that improved maternal condition increases the probability of male births [ 24 ] has been supported across multiple animal species, including ruminants [ 25 , 26 , 27 ]. Our data show the sex ratio at birth in Japanese zoos remained close to parity, not supporting this prediction at present; but if husbandry practices continue to improve, such effects could emerge in the future. Therefore, this hypothesis highlights the importance of continuing to monitor changes in population dynamics. Materials and Methods To clarify sex-specific demographic changes in Japanese zoo-housed giraffes, we referenced the ‘2023 Giraffe Studbook Japanese Regional’, which lists almost all individuals kept in Japan. For each sex, we examined the (1) annual population size as of 31 December, (2) number of births, and (3) number of giraffes imported from or exported to foreign zoos. For the survivorship analyses, we calculated the number of days each individual survived based on birth and death dates, or until 31 December 2023 for living individuals. For individuals still alive at the end of the study period (31 December 2023), survival time was right censored at that date. Since few individuals were born before 1980, we focused on those born between 1980 and 2023. Individuals lacking birth or death dates or who became untraceable (i.e., data lacking after transfer overseas) were excluded from the analysis. The final dataset consisted of 1006 individuals (males: 511, females: 495). We conducted a Cox proportional hazard analysis in R [28] using the survival package [29]. In these analyses, coefficients < 1 (i.e., the 95% confidence interval excludes 1) indicated reduced mortality risk compared with the reference group. Birth cohorts were defined as four discrete categories: 1980–1989, 1990–1999, 2000–2009, 2010–2019. The significance level was set at p < 0.05. As in Scherer et al. 2024, survivorship for juveniles was calculated using individuals under the age of four, and survivorship for adults was calculated using all individuals that had reached the age of four. For the adult survivorship analyses (≥ 4 years), time-at-risk was measured from the 4th birthday (left-truncated at age 4) until death or censoring on 31 December 2023. Kaplan–Meier survival analysis was also conducted, and log-rank tests were applied to compare sexes and birth cohorts. The studbook categorises the giraffes into six subspecies-based lineage groups: reticulated giraffe ( Giraffa camelopardalis reticulata ), Masai giraffe ( G. c. tippelskirchi ), Cape giraffe ( G. c. giraffa ), Angola giraffe ( G. c. angolensis ), Nigerian giraffe ( G. c. peralta ), and Rothschild’s giraffe ( G. c. rothschildi ). The reticulated giraffe lineage group includes pure reticulated giraffes as well as hybrids between reticulated giraffes and other subspecies (mostly Rothschild’s giraffes). The remaining hybrid giraffes were grouped together in a seventh group. Our analysis did not consider differences between lineages. Declarations Acknowledgements We express our gratitude to JAZA for allowing us to use the 2023 Giraffe Studbook of the Japanese Region. There is no funding to report. Author contributions M.G. and Y. K. conceived and designed the study, analyzed the data, and wrote the manuscript. S.K. and M.Y. contributed to the study design and part of the discussion. I.S. organized the data and contributed to part of the discussion. All authors reviewed and approved the final version of the manuscript. Competing interests The authors declare no competing interests. Data availability All data supporting the findings of this study are provided in the Supplementary Information. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {\"props\":{\"pageProps\":{\"initialData\":{\"identity\":\"rs-7858970\",\"acceptedTermsAndConditions\":true,\"allowDirectSubmit\":true,\"archivedVersions\":[],\"articleType\":\"Article\",\"associatedPublications\":[],\"authors\":[{\"id\":529483591,\"identity\":\"235330a1-7d54-4285-a046-6e515a909933\",\"order_by\":0,\"name\":\"Megu 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2023.\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"Figure12023resized.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-7858970/v1/0a4a44c77c66c3108fd904d2.png\"},{\"id\":93551887,\"identity\":\"804b1aab-bc6b-442a-a2e0-03d2d211fa42\",\"added_by\":\"auto\",\"created_at\":\"2025-10-15 05:28:02\",\"extension\":\"png\",\"order_by\":2,\"title\":\"Figure 2\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":416267,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003eJuvenile survivorship (from birth to 4 years of age) in the zoo-housed giraffe (\\u003cem\\u003eGiraffa camelopardalis\\u003c/em\\u003e) population in Japan, separated by sex and birth cohort.\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"Figure2.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-7858970/v1/2ff139ba2d393de6b63b14f5.png\"},{\"id\":93551995,\"identity\":\"51c73e14-ef73-40b4-8509-af70888c61ae\",\"added_by\":\"auto\",\"created_at\":\"2025-10-15 05:36:02\",\"extension\":\"png\",\"order_by\":3,\"title\":\"Figure 3\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":546595,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003eAdult survivorship (≥4 years of age) in the zoo-housed giraffe (\\u003cem\\u003eGiraffa camelopardalis\\u003c/em\\u003e) population in Japan, separated by sex and birth cohort.\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"Figure3.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-7858970/v1/3a31761ce0f416d37bf9a215.png\"},{\"id\":93551998,\"identity\":\"de271c04-d8ef-49e4-9826-7e9d4387a449\",\"added_by\":\"auto\",\"created_at\":\"2025-10-15 05:36:02\",\"extension\":\"png\",\"order_by\":4,\"title\":\"Figure 4\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":315366,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003eKaplan–Meier survival curves for zoo-housed giraffes (\\u003cem\\u003eGiraffa camelopardalis\\u003c/em\\u003e) in Japan in each birth cohort. Males and females are shown in blue and red, respectively, with shaded areas representing the 95% Confidence Interval (CI). The table below the graph indicates the number of individuals at risk at each time point.\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"Figure4resized.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-7858970/v1/fc74a4ee47f975fd6f796698.png\"},{\"id\":98441336,\"identity\":\"1423a804-608f-412d-ae94-41b2d8050c31\",\"added_by\":\"auto\",\"created_at\":\"2025-12-17 17:05:13\",\"extension\":\"pdf\",\"order_by\":0,\"title\":\"\",\"display\":\"\",\"copyAsset\":false,\"role\":\"manuscript-pdf\",\"size\":2106190,\"visible\":true,\"origin\":\"\",\"legend\":\"\",\"description\":\"\",\"filename\":\"manuscript.pdf\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-7858970/v1/ebb4b145-45b4-4328-8c9c-7c6835dfea2c.pdf\"},{\"id\":93551886,\"identity\":\"e007fff9-6ec5-49af-b275-e76153e32ee7\",\"added_by\":\"auto\",\"created_at\":\"2025-10-15 05:28:02\",\"extension\":\"xlsx\",\"order_by\":0,\"title\":\"\",\"display\":\"\",\"copyAsset\":false,\"role\":\"supplement\",\"size\":42932,\"visible\":true,\"origin\":\"\",\"legend\":\"\",\"description\":\"\",\"filename\":\"Coxanalysisdatasubmit2.xlsx\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-7858970/v1/af7cfd870ef26e0fc411cf2d.xlsx\"}],\"financialInterests\":\"No competing interests reported.\",\"formattedTitle\":\"\\u003cp\\u003e\\u003cstrong\\u003eSurvival improvements in zoo-housed giraffes lead to surplus males: A long-term analysis of longevity and mortality in Japanese zoos\\u003c/strong\\u003e\\u003c/p\\u003e\",\"fulltext\":[{\"header\":\"Introduction\",\"content\":\"\\u003cp\\u003eIn zoos, appropriate population management and improved breeding techniques are essential [\\u003cspan citationid=\\\"CR1\\\" class=\\\"CitationRef\\\"\\u003e1\\u003c/span\\u003e]. However, these efforts can sometimes lead to excessive population growth. Individuals that exceed the facility\\u0026rsquo;s management capacity are termed \\u0026ldquo;surplus animals,\\u0026rdquo; which burden facilities and resources while raising concerns about animal welfare and genetic diversity. Therefore, preventing the occurrence of surplus individuals has become a major challenge in modern zoo population management [\\u003cspan citationid=\\\"CR2\\\" class=\\\"CitationRef\\\"\\u003e2\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR3\\\" class=\\\"CitationRef\\\"\\u003e3\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR4\\\" class=\\\"CitationRef\\\"\\u003e4\\u003c/span\\u003e].\\u003c/p\\u003e\\u003cp\\u003eGiraffes (\\u003cem\\u003eGiraffa camelopardalis\\u003c/em\\u003e) are currently a representative species facing a serious surplus animal problem. The European Association of Zoos and Aquaria (EAZA) recommends housing in herds consisting of multiple females with their offspring and a dominant male [\\u003cspan citationid=\\\"CR5\\\" class=\\\"CitationRef\\\"\\u003e5\\u003c/span\\u003e]. While this housing structure mimics the wild herd structure, the large disparity in the number of males and females that can be accommodated in a single facility increases the likelihood of male surplus. Furthermore, over the past 20 years, the proportion of adult male giraffes in zoo populations has increased globally. The adult male-to-female ratio steadily rose from 0.47:1 in 2000 to 0.68:1 in 2022 [\\u003cspan citationid=\\\"CR6\\\" class=\\\"CitationRef\\\"\\u003e6\\u003c/span\\u003e]. In 2014, Copenhagen Zoo euthanised a healthy young male giraffe under such surplus conditions, attracting worldwide media attention and sparking ethical debate [\\u003cspan citationid=\\\"CR7\\\" class=\\\"CitationRef\\\"\\u003e7\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR8\\\" class=\\\"CitationRef\\\"\\u003e8\\u003c/span\\u003e]. Although castration is sometimes used as a countermeasure for surplus individuals, surgical procedures on large mammals carry high risks [\\u003cspan citationid=\\\"CR9\\\" class=\\\"CitationRef\\\"\\u003e9\\u003c/span\\u003e] and do not provide a complete solution. Therefore, in zoo-housed giraffes, managing the population numbers and sex ratio balance is an urgent issue.\\u003c/p\\u003e\\u003cp\\u003eA recent study using a global database of zoo-housed giraffes [\\u003cspan citationid=\\\"CR6\\\" class=\\\"CitationRef\\\"\\u003e6\\u003c/span\\u003e] proposed three possibilities for the increase in the proportion of adult males: (1) reduced male mortality due to improved husbandry conditions in recent years, (2) decreased culling since 2014, and (3) a combination of both factors. However, this analysis compared mortality risk between birth cohorts and between sexes, and did not directly examine whether recent reductions in mortality were particularly pronounced in males. Furthermore, the global database aggregates data from multiple regions with vastly different husbandry practices, making it difficult to assess the effects of husbandry condition developments and the extent of culling clearly. Therefore, the demographic mechanisms generating the male surplus remain unresolved.\\u003c/p\\u003e\\u003cp\\u003eJapan provides an ideal case study to fill this knowledge gap. Currently, approximately 200 giraffes are housed in Japanese zoos, and breeding conditions have improved since 2010 following the introduction of international guidelines. However, unlike in Europe, surplus individuals have not been culled because of cultural and institutional reasons. Therefore, the Japanese population provides a natural experimental setting to examine whether improvements in husbandry conditions and associated changes in survival rates cause a male surplus. Here, we analyse long-term studbook data (1980\\u0026ndash;2023) from Japanese zoos to (1) quantify recent trends in population size by sex, (2) evaluate whether survivorship improve following enhanced husbandry conditions, and if confirmed, assess whether this improvement is particularly pronounced in males, and (3) examine other demographic factors such as birth sex ratio and import/export patterns. Together, these analyses aim to clarify the demographic processes driving the emergence of surplus males.\\u003c/p\\u003e\"},{\"header\":\"Results\",\"content\":\"\\u003cdiv id=\\\"Sec3\\\" class=\\\"Section2\\\"\\u003e\\u003ch2\\u003ePopulation changes of captive individuals\\u003c/h2\\u003e\\u003cp\\u003eIn Japan, the population of zoo-housed giraffes subsequently increased until 1989, and then declined for approximately 15 years before rising again in 2014 (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig1\\\" class=\\\"InternalRef\\\"\\u003e1\\u003c/span\\u003e). Since 1957, females have consistently outnumbered males. However, the male population has increased rapidly since 2012, exceeding the female population in 2021. Specifically, the population grew from 60 males and 87 females (sex ratio of 0.69:1) in 2012 to 101 males and 96 females (sex ratio of 1.05:1) in 2023. Over this period, the female population increased by 1.1 times while the male population increased by 1.7 times.\\u003c/p\\u003e\\u003cp\\u003e\\u003c/p\\u003e\\u003cp\\u003eThe number of zoos housing giraffes remained relatively stable (54 in 2012, 55 in 2023), but the number of males per zoo rose from 1.11 in 2012 to 1.84 in 2023. In 2023, 28 zoos housed at least 2 males, with 9 zoos housing 3 or more males.\\u003c/p\\u003e\\u003c/div\\u003e\\n\\u003ch3\\u003eSurvivorship analysis\\u003c/h3\\u003e\\n\\u003cp\\u003eA Cox proportional hazard analysis revealed that males had consistently lower survivorship than females in both juvenile and adult stages (Table\\u0026nbsp;\\u003cspan refid=\\\"Tab1\\\" class=\\\"InternalRef\\\"\\u003e3\\u003c/span\\u003e). The hazard ratio for males relative to females was 1.303 in juveniles and 1.417 in adults, indicating a 30\\u0026ndash;40% higher mortality risk in males. Survivorship also improved markedly across birth cohorts compared with the 1980s (reference group). For individuals born in the 2000s, hazard ratios were 0.856 in juveniles and 0.802 in adults; for those born in the 2010s, these ratios declined to 0.420 and 0.294, respectively. Analysis by sex revealed that the improvements in survivorship were especially pronounced in males (Figs.\\u0026nbsp;\\u003cspan refid=\\\"Fig2\\\" class=\\\"InternalRef\\\"\\u003e2\\u003c/span\\u003e, \\u003cspan refid=\\\"Fig3\\\" class=\\\"InternalRef\\\"\\u003e3\\u003c/span\\u003e; Table\\u0026nbsp;\\u003cspan refid=\\\"Tab2\\\" class=\\\"InternalRef\\\"\\u003e4\\u003c/span\\u003e). The hazard ratios for juvenile males declined to 0.358 in the 2010s compared with the 1980s, whereas those for juvenile females declined to 0.514. A similar trend was observed in adults, with hazard ratios decreased to 0.263 in males and 0.343 in females in the 2010s. For adults, the hazard ratio in the most recent cohort may have been overestimated due to the large proportion of censored data.\\u003c/p\\u003e\\u003cp\\u003e\\u003cdiv class=\\\"gridtable\\\"\\u003e\\u003ctable float=\\\"Yes\\\" id=\\\"Tab1\\\" border=\\\"1\\\"\\u003e\\u003ccaption language=\\\"En\\\"\\u003e\\u003cdiv class=\\\"CaptionNumber\\\"\\u003eTable 3\\u003c/div\\u003e\\u003cdiv class=\\\"CaptionContent\\\"\\u003e\\u003cp\\u003eSurvivorship analyses (Cox proportional hazards) of giraffes (\\u003cem\\u003eGiraffa camelopardalis\\u003c/em\\u003e) kept in Japanese zoos from 1960 to 2023; models include distinct periods (birth cohorts 1980\\u0026ndash;1989, 1990\\u0026ndash;1999, 2000\\u0026ndash;2009, 2010\\u0026ndash;2019) and sex as factors.\\u003c/p\\u003e\\u003c/div\\u003e\\u003c/caption\\u003e\\u003ccolgroup cols=\\\"5\\\"\\u003e\\u003cdiv align=\\\"left\\\" class=\\\"colspec\\\" colname=\\\"c1\\\" colnum=\\\"1\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"left\\\" class=\\\"colspec\\\" colname=\\\"c2\\\" colnum=\\\"2\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"left\\\" class=\\\"colspec\\\" colname=\\\"c3\\\" colnum=\\\"3\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"left\\\" class=\\\"colspec\\\" colname=\\\"c4\\\" colnum=\\\"4\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"left\\\" class=\\\"colspec\\\" colname=\\\"c5\\\" colnum=\\\"5\\\"\\u003e\\u003c/div\\u003e\\u003cthead\\u003e\\u003ctr\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003eModel\\u003c/p\\u003e\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u0026nbsp;\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003eHazard ratio (95%CI)\\u003c/p\\u003e\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003ez\\u003c/p\\u003e\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003ep\\u003c/p\\u003e\\u003c/th\\u003e\\u003c/tr\\u003e\\u003c/thead\\u003e\\u003ctbody\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colspan=\\\"5\\\" nameend=\\\"c5\\\" namest=\\\"c1\\\"\\u003e\\u003cp\\u003eup to 4 years of age\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003ereference:\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u0026nbsp;\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u0026nbsp;\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c4\\\"\\u003e\\u0026nbsp;\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u0026nbsp;\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003efemales (n\\u0026thinsp;=\\u0026thinsp;462)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003emales (n\\u0026thinsp;=\\u0026thinsp;473)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e1.303 (1.131\\u0026ndash;1.501)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e3.672\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e\\u0026lt;\\u0026thinsp;0.001 ***\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e1980\\u0026ndash;1989 (n\\u0026thinsp;=\\u0026thinsp;296)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e1990\\u0026ndash;1999 (n\\u0026thinsp;=\\u0026thinsp;278)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e0.907 (0.769\\u0026ndash;1.070)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e-1.157\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e0.247\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u0026nbsp;\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e2000\\u0026ndash;2009 (n\\u0026thinsp;=\\u0026thinsp;184)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e0.856 (0.705\\u0026ndash;1.039)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e-1.171\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e0.116\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u0026nbsp;\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e2010\\u0026ndash;2019 (n\\u0026thinsp;=\\u0026thinsp;177)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e0.420 (0.321\\u0026ndash;0.549)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e-6.339\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e\\u0026lt;\\u0026thinsp;0.001 ***\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colspan=\\\"5\\\" nameend=\\\"c5\\\" namest=\\\"c1\\\"\\u003e\\u003cp\\u003e\\u0026ge;\\u0026thinsp;4 years of age\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003ereference:\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u0026nbsp;\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u0026nbsp;\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c4\\\"\\u003e\\u0026nbsp;\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u0026nbsp;\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003efemales (n\\u0026thinsp;=\\u0026thinsp;261)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003emales (n\\u0026thinsp;=\\u0026thinsp;232)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e1.417 (1.145\\u0026ndash;1.754)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e3.201\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e0.00137 **\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e1980\\u0026ndash;1989 (n\\u0026thinsp;=\\u0026thinsp;132)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e1990\\u0026ndash;1999 (n\\u0026thinsp;=\\u0026thinsp;144)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e0.922 (0.726\\u0026ndash;1.172)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e-0.662\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e0.508\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u0026nbsp;\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e2000\\u0026ndash;2009 (n\\u0026thinsp;=\\u0026thinsp;93)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e0.802 (0.600\\u0026ndash;1.079)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e-1.455\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e0.146\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u0026nbsp;\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e2010\\u0026ndash;2019 (n\\u0026thinsp;=\\u0026thinsp;124)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e0.294 (0.168\\u0026ndash;0.515)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e-4.272\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e\\u0026lt;\\u0026thinsp;0.001 ***\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003c/tbody\\u003e\\u003c/colgroup\\u003e\\u003c/table\\u003e\\u003c/div\\u003e\\u003c/p\\u003e\\u003cp\\u003e\\u003c/p\\u003e\\u003cp\\u003e\\u003c/p\\u003e\\u003cp\\u003e\\u003cdiv class=\\\"gridtable\\\"\\u003e\\u003ctable float=\\\"Yes\\\" id=\\\"Tab2\\\" border=\\\"1\\\"\\u003e\\u003ccaption language=\\\"En\\\"\\u003e\\u003cdiv class=\\\"CaptionNumber\\\"\\u003eTable 4\\u003c/div\\u003e\\u003cdiv class=\\\"CaptionContent\\\"\\u003e\\u003cp\\u003eSurvivorship analyses (Cox proportional hazards) by sex of giraffes (\\u003cem\\u003eGiraffa camelopardalis\\u003c/em\\u003e) kept in Japanese zoos from 1960 to 2023; models include distinct periods (birth cohorts 1980\\u0026ndash;1989, 1990\\u0026ndash;1999, 2000\\u0026ndash;2009, 2010\\u0026ndash;2019) as factors.\\u003c/p\\u003e\\u003c/div\\u003e\\u003c/caption\\u003e\\u003ccolgroup cols=\\\"5\\\"\\u003e\\u003cdiv align=\\\"left\\\" class=\\\"colspec\\\" colname=\\\"c1\\\" colnum=\\\"1\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"left\\\" class=\\\"colspec\\\" colname=\\\"c2\\\" colnum=\\\"2\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"left\\\" class=\\\"colspec\\\" colname=\\\"c3\\\" colnum=\\\"3\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"left\\\" class=\\\"colspec\\\" colname=\\\"c4\\\" colnum=\\\"4\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"left\\\" class=\\\"colspec\\\" colname=\\\"c5\\\" colnum=\\\"5\\\"\\u003e\\u003c/div\\u003e\\u003cthead\\u003e\\u003ctr\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003eModel\\u003c/p\\u003e\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u0026nbsp;\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003eHazard ratio (95%CI)\\u003c/p\\u003e\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003ez\\u003c/p\\u003e\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003ep\\u003c/p\\u003e\\u003c/th\\u003e\\u003c/tr\\u003e\\u003c/thead\\u003e\\u003ctbody\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colspan=\\\"5\\\" nameend=\\\"c5\\\" namest=\\\"c1\\\"\\u003e\\u003cp\\u003eMale: up to 4 years of age\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003ereference:\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u0026nbsp;\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u0026nbsp;\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c4\\\"\\u003e\\u0026nbsp;\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u0026nbsp;\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e1980\\u0026ndash;1989 (n\\u0026thinsp;=\\u0026thinsp;151)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e1990\\u0026ndash;1999 (n\\u0026thinsp;=\\u0026thinsp;131)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e0.885 (0.699\\u0026ndash;1.120)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e-1.018\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e0.309\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u0026nbsp;\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e2000\\u0026ndash;2009 (n\\u0026thinsp;=\\u0026thinsp;96)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e0.883 (0.677\\u0026ndash;1.153)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e-0.913\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e0.361\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u0026nbsp;\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e2010\\u0026ndash;2019 (n\\u0026thinsp;=\\u0026thinsp;95)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e0.358 (0.249\\u0026ndash;0.517)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e-5.494\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e\\u0026lt;\\u0026thinsp;0.001 ***\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colspan=\\\"5\\\" nameend=\\\"c5\\\" namest=\\\"c1\\\"\\u003e\\u003cp\\u003eMale: \\u0026ge; 4 years of age\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003ereference:\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u0026nbsp;\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u0026nbsp;\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c4\\\"\\u003e\\u0026nbsp;\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u0026nbsp;\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e1980\\u0026ndash;1989 (n\\u0026thinsp;=\\u0026thinsp;59)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e1990\\u0026ndash;1999 (n\\u0026thinsp;=\\u0026thinsp;65)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e0.954 (0.667\\u0026ndash;1.363)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e-0.261\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e0.794\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u0026nbsp;\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e2000\\u0026ndash;2009 (n\\u0026thinsp;=\\u0026thinsp;41)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e0.753 (0.485\\u0026ndash;1.170)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e-1.263\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e0.206\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u0026nbsp;\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e2010\\u0026ndash;2019 (n\\u0026thinsp;=\\u0026thinsp;67)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e0.263 (0.124\\u0026ndash;0.555)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e-3.500\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e\\u0026lt;\\u0026thinsp;0.001 ***\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colspan=\\\"5\\\" nameend=\\\"c5\\\" namest=\\\"c1\\\"\\u003e\\u003cp\\u003eFemale: up to 4 years of age\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003ereference:\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u0026nbsp;\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u0026nbsp;\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c4\\\"\\u003e\\u0026nbsp;\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u0026nbsp;\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e1980\\u0026ndash;1989 (n\\u0026thinsp;=\\u0026thinsp;145)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e1990\\u0026ndash;1999 (n\\u0026thinsp;=\\u0026thinsp;147)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e0.928 (0.735\\u0026ndash;1.172)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e-0.625\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e0.532\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u0026nbsp;\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e2000\\u0026ndash;2009 (n\\u0026thinsp;=\\u0026thinsp;88)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e0.831 (0.626\\u0026ndash;1.103)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e-1.280\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e0.200\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u0026nbsp;\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e2010\\u0026ndash;2019 (n\\u0026thinsp;=\\u0026thinsp;82)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e0.514 (0.346\\u0026ndash;0.762)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e-3.312\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e\\u0026lt;\\u0026thinsp;0.001 ***\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colspan=\\\"5\\\" nameend=\\\"c5\\\" namest=\\\"c1\\\"\\u003e\\u003cp\\u003eFemale: \\u0026ge; 4 years of age\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003ereference:\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u0026nbsp;\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u0026nbsp;\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c4\\\"\\u003e\\u0026nbsp;\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u0026nbsp;\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e1980\\u0026ndash;1989 (n\\u0026thinsp;=\\u0026thinsp;73)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e1990\\u0026ndash;1999 (n\\u0026thinsp;=\\u0026thinsp;79)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e0.904 (0.651\\u0026ndash;1.255)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e-0.603\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e0.546\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u0026nbsp;\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e2000\\u0026ndash;2009 (n\\u0026thinsp;=\\u0026thinsp;52)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e0.850 (0.568\\u0026ndash;1.273)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e-0.789\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e0.430\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u0026nbsp;\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e2010\\u0026ndash;2019 (n\\u0026thinsp;=\\u0026thinsp;57)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e0.343 (0.146\\u0026ndash;0.803)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e-2.466\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e0.0137 *\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003c/tbody\\u003e\\u003c/colgroup\\u003e\\u003c/table\\u003e\\u003c/div\\u003e\\u003c/p\\u003e\\u003cp\\u003eKaplan\\u0026ndash;Meier survival curves similarly showed different trends between males and females (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig4\\\" class=\\\"InternalRef\\\"\\u003e4\\u003c/span\\u003e). Significant sex differences in survival probability likely occurred in the 1980s, 1990s, and 2000s (Log-rank test, p\\u0026thinsp;\\u0026lt;\\u0026thinsp;0.05) but disappeared in the 2010s (p\\u0026thinsp;=\\u0026thinsp;0.99). Thus, although males generally had lower survival probability than females, the magnitude of improvement over time was greater for males, and sex difference was effectively eliminated in the most recent cohort. Similar to a Cox proportional hazard analysis, note that the results are inaccurate due to insufficient time elapsed since birth for individuals born in the 2010s. Instead, we compared survival probability at age four, when all individuals in this cohort had reached that age by the end of 2023. In the 2000s cohort, survival probability at age four was 0.427 for males and 0.591 for females, whereas in the 2010s cohort it was 0.705 for males and 0.695 for females. This indicates a marked improvement in juvenile survival, particularly among males, in the most recent cohort.\\u003c/p\\u003e\\u003cp\\u003e\\u003c/p\\u003e\\n\\u003ch3\\u003eNumber and sex ratio of births\\u003c/h3\\u003e\\n\\u003cp\\u003eThe number of giraffe births in Japanese zoos has increased from the 1960s to the 1980s, peaking in the late 1980s, and has declined thereafter (Table\\u0026nbsp;\\u003cspan refid=\\\"Tab3\\\" class=\\\"InternalRef\\\"\\u003e1\\u003c/span\\u003e). Since 2000, the average annual number of births has been 18.25\\u0026thinsp;\\u0026plusmn;\\u0026thinsp;3.67.\\u003c/p\\u003e\\u003cp\\u003e\\u003cdiv class=\\\"gridtable\\\"\\u003e\\u003ctable float=\\\"Yes\\\" id=\\\"Tab3\\\" border=\\\"1\\\"\\u003e\\u003ccaption language=\\\"En\\\"\\u003e\\u003cdiv class=\\\"CaptionNumber\\\"\\u003eTable 1\\u003c/div\\u003e\\u003cdiv class=\\\"CaptionContent\\\"\\u003e\\u003cp\\u003eNumber of births per decade.\\u003c/p\\u003e\\u003c/div\\u003e\\u003c/caption\\u003e\\u003ccolgroup cols=\\\"9\\\"\\u003e\\u003cdiv align=\\\"left\\\" class=\\\"colspec\\\" colname=\\\"c1\\\" colnum=\\\"1\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"char\\\" char=\\\".\\\" class=\\\"colspec\\\" colname=\\\"c2\\\" colnum=\\\"2\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"char\\\" char=\\\".\\\" class=\\\"colspec\\\" colname=\\\"c3\\\" colnum=\\\"3\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"char\\\" char=\\\".\\\" class=\\\"colspec\\\" colname=\\\"c4\\\" colnum=\\\"4\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"char\\\" char=\\\".\\\" class=\\\"colspec\\\" colname=\\\"c5\\\" colnum=\\\"5\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"char\\\" char=\\\".\\\" class=\\\"colspec\\\" colname=\\\"c6\\\" colnum=\\\"6\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"char\\\" char=\\\".\\\" class=\\\"colspec\\\" colname=\\\"c7\\\" colnum=\\\"7\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"char\\\" char=\\\".\\\" class=\\\"colspec\\\" colname=\\\"c8\\\" colnum=\\\"8\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"char\\\" char=\\\".\\\" class=\\\"colspec\\\" colname=\\\"c9\\\" colnum=\\\"9\\\"\\u003e\\u003c/div\\u003e\\u003cthead\\u003e\\u003ctr\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u0026nbsp;\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e1960s\\u003c/p\\u003e\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e1970s\\u003c/p\\u003e\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e1980s\\u003c/p\\u003e\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e1990s\\u003c/p\\u003e\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c6\\\"\\u003e\\u003cp\\u003e2000s\\u003c/p\\u003e\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c7\\\"\\u003e\\u003cp\\u003e2010s\\u003c/p\\u003e\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c8\\\"\\u003e\\u003cp\\u003e2020s\\u003c/p\\u003e\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c9\\\"\\u003e\\u003cp\\u003eTotal\\u003c/p\\u003e\\u003c/th\\u003e\\u003c/tr\\u003e\\u003c/thead\\u003e\\u003ctbody\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003eMale\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e15\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e75\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e170\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e142\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c6\\\"\\u003e\\u003cp\\u003e98\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c7\\\"\\u003e\\u003cp\\u003e95\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c8\\\"\\u003e\\u003cp\\u003e38\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c9\\\"\\u003e\\u003cp\\u003e633\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003eFemale\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e18\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e82\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e159\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e155\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c6\\\"\\u003e\\u003cp\\u003e90\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c7\\\"\\u003e\\u003cp\\u003e82\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c8\\\"\\u003e\\u003cp\\u003e33\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c9\\\"\\u003e\\u003cp\\u003e619\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003eUnknown\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e3\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e5\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e10\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e5\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c6\\\"\\u003e\\u003cp\\u003e0\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c7\\\"\\u003e\\u003cp\\u003e2\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c8\\\"\\u003e\\u003cp\\u003e0\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c9\\\"\\u003e\\u003cp\\u003e25\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003c/tbody\\u003e\\u003c/colgroup\\u003e\\u003c/table\\u003e\\u003c/div\\u003e\\u003c/p\\u003e\\u003cp\\u003eAcross the entire period since 1960, 633 males, 619 females, and 25 sex unknowns were recorded, indicating that the male proportion was 50.6% for known sex individuals. Among known-sex individuals born after 2000, the proportion of males was 53.0% (231 males and 205 females), although no significant difference was observed in the sex ratio of newborn individuals between before and after 2000 (chi-square test, p\\u0026thinsp;=\\u0026thinsp;0.2325).\\u003c/p\\u003e\\n\\u003ch3\\u003eImport and export situation\\u003c/h3\\u003e\\n\\u003cp\\u003eThe number of individuals imported from wild and foreign zoos or exported to zoos abroad is shown in Table\\u0026nbsp;\\u003cspan refid=\\\"Tab4\\\" class=\\\"InternalRef\\\"\\u003e2\\u003c/span\\u003e. A total of 26 males and 70 females were imported directly from the wild, but no wild individuals have been introduced since 1981. Imports from foreign zoos peaked in the 1970s and declined sharply after 1990; of the 138 imported individuals, 68.8% were female, resulting in a significantly female-biased sex ratio. In contrast, exports to foreign zoos were rare and unbiased (20 males and 22 females).\\u003c/p\\u003e\\u003cp\\u003e\\u003cdiv class=\\\"gridtable\\\"\\u003e\\u003ctable float=\\\"Yes\\\" id=\\\"Tab4\\\" border=\\\"1\\\"\\u003e\\u003ccaption language=\\\"En\\\"\\u003e\\u003cdiv class=\\\"CaptionNumber\\\"\\u003eTable 2\\u003c/div\\u003e\\u003cdiv class=\\\"CaptionContent\\\"\\u003e\\u003cp\\u003eNumber of imported and exported giraffes per decade.\\u003c/p\\u003e\\u003c/div\\u003e\\u003c/caption\\u003e\\u003ccolgroup cols=\\\"11\\\"\\u003e\\u003cdiv align=\\\"left\\\" class=\\\"colspec\\\" colname=\\\"c1\\\" colnum=\\\"1\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"left\\\" class=\\\"colspec\\\" colname=\\\"c2\\\" colnum=\\\"2\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"char\\\" char=\\\".\\\" class=\\\"colspec\\\" colname=\\\"c3\\\" colnum=\\\"3\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"char\\\" char=\\\".\\\" class=\\\"colspec\\\" colname=\\\"c4\\\" colnum=\\\"4\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"char\\\" char=\\\".\\\" class=\\\"colspec\\\" colname=\\\"c5\\\" colnum=\\\"5\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"char\\\" char=\\\".\\\" class=\\\"colspec\\\" colname=\\\"c6\\\" colnum=\\\"6\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"char\\\" char=\\\".\\\" class=\\\"colspec\\\" colname=\\\"c7\\\" colnum=\\\"7\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"char\\\" char=\\\".\\\" class=\\\"colspec\\\" colname=\\\"c8\\\" colnum=\\\"8\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"char\\\" char=\\\".\\\" class=\\\"colspec\\\" colname=\\\"c9\\\" colnum=\\\"9\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"char\\\" char=\\\".\\\" class=\\\"colspec\\\" colname=\\\"c10\\\" colnum=\\\"10\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"char\\\" char=\\\".\\\" class=\\\"colspec\\\" colname=\\\"c11\\\" colnum=\\\"11\\\"\\u003e\\u003c/div\\u003e\\u003cthead\\u003e\\u003ctr\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u0026nbsp;\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u0026nbsp;\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e1950s\\u003c/p\\u003e\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e1960s\\u003c/p\\u003e\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e1970s\\u003c/p\\u003e\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c6\\\"\\u003e\\u003cp\\u003e1980s\\u003c/p\\u003e\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c7\\\"\\u003e\\u003cp\\u003e1990s\\u003c/p\\u003e\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c8\\\"\\u003e\\u003cp\\u003e2000s\\u003c/p\\u003e\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c9\\\"\\u003e\\u003cp\\u003e2010s\\u003c/p\\u003e\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c10\\\"\\u003e\\u003cp\\u003e2020s\\u003c/p\\u003e\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c11\\\"\\u003e\\u003cp\\u003eTotal\\u003c/p\\u003e\\u003c/th\\u003e\\u003c/tr\\u003e\\u003c/thead\\u003e\\u003ctbody\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\" morerows=\\\"1\\\" rowspan=\\\"2\\\"\\u003e\\u003cp\\u003eImported from the wild or foreign zoo\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003eMale\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e7\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e12\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e15\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c6\\\"\\u003e\\u003cp\\u003e7\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c7\\\"\\u003e\\u003cp\\u003e1\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c8\\\"\\u003e\\u003cp\\u003e0\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c9\\\"\\u003e\\u003cp\\u003e1\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c10\\\"\\u003e\\u003cp\\u003e0\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c11\\\"\\u003e\\u003cp\\u003e43\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003eFemale\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e9\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e14\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e47\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c6\\\"\\u003e\\u003cp\\u003e23\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c7\\\"\\u003e\\u003cp\\u003e0\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c8\\\"\\u003e\\u003cp\\u003e0\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c9\\\"\\u003e\\u003cp\\u003e2\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c10\\\"\\u003e\\u003cp\\u003e0\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c11\\\"\\u003e\\u003cp\\u003e95\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\" morerows=\\\"1\\\" rowspan=\\\"2\\\"\\u003e\\u003cp\\u003eExported to foreign zoo\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003eMale\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e0\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e2\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e3\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c6\\\"\\u003e\\u003cp\\u003e7\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c7\\\"\\u003e\\u003cp\\u003e6\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c8\\\"\\u003e\\u003cp\\u003e2\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c9\\\"\\u003e\\u003cp\\u003e0\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c10\\\"\\u003e\\u003cp\\u003e0\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c11\\\"\\u003e\\u003cp\\u003e20\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003eFemale\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e0\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e1\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e4\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c6\\\"\\u003e\\u003cp\\u003e10\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c7\\\"\\u003e\\u003cp\\u003e5\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c8\\\"\\u003e\\u003cp\\u003e2\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c9\\\"\\u003e\\u003cp\\u003e0\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c10\\\"\\u003e\\u003cp\\u003e0\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c11\\\"\\u003e\\u003cp\\u003e22\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003c/tbody\\u003e\\u003c/colgroup\\u003e\\u003c/table\\u003e\\u003c/div\\u003e\\u003c/p\\u003e\"},{\"header\":\"Discussion\",\"content\":\"\\u003cp\\u003eThis study revealed that the population of zoo-housed male giraffes in Japan has increased rapidly since 2012, suggesting that a significant surplus of males is developing, even without culling. Moreover, our survival analyses provide the first clear evidence that survivorship has improved for both sexes in Japanese zoos since the 2010s, with disproportionately greater improvements in males. Although females continued to outlive males overall, the survival gap steadily narrowed and effectively disappeared in the most recent cohort. Thus, male-biased improvements in survival appear to have shifted the adult sex ratio toward parity, ultimately accelerating the emergence of surplus males.\\u003c/p\\u003e\\u003cp\\u003eThe timing of the marked survivorship improvements coincides with nationwide changes in husbandry practices, following the translation of the EAZA Giraffe Husbandry and Management Guidelines into Japanese in 2010 [\\u003cspan citationid=\\\"CR10\\\" class=\\\"CitationRef\\\"\\u003e10\\u003c/span\\u003e]. During this period, Japanese zoos substantially revised their nutritional protocols, reducing reliance on grass hay and low-fibre pellets and shifting toward alfalfa hay and browse [\\u003cspan citationid=\\\"CR11\\\" class=\\\"CitationRef\\\"\\u003e11\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR12\\\" class=\\\"CitationRef\\\"\\u003e12\\u003c/span\\u003e], thereby improving nutritional quality. Previous studies have shown that such nutritional improvements contribute to longer lifespans in giraffes [\\u003cspan citationid=\\\"CR13\\\" class=\\\"CitationRef\\\"\\u003e13\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR14\\\" class=\\\"CitationRef\\\"\\u003e14\\u003c/span\\u003e]. At the same time, the widespread adoption of long-term hormone monitoring allowed better estimation of conception and parturition in giraffids and other ungulates [\\u003cspan citationid=\\\"CR11\\\" class=\\\"CitationRef\\\"\\u003e11\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR15\\\" class=\\\"CitationRef\\\"\\u003e15\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR16\\\" class=\\\"CitationRef\\\"\\u003e16\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR17\\\" class=\\\"CitationRef\\\"\\u003e17\\u003c/span\\u003e]. This facilitated more effective pregnancy and neonatal management, which likely improved juvenile survival. Although the precise mechanisms remain unclear, these reforms appear to have conferred relatively greater benefits to males, which previously suffered higher baseline mortality.\\u003c/p\\u003e\\u003cp\\u003eSince 1960, the sex ratio of newborns in Japan has remained at approximately 1:1, indicating no significant bias (see Table\\u0026nbsp;\\u003cspan refid=\\\"Tab3\\\" class=\\\"InternalRef\\\"\\u003e1\\u003c/span\\u003e). After 2000, the proportion of males increased slightly to 52.9%, although the value remained within a similar range compared to that of European zoos (52%) [\\u003cspan citationid=\\\"CR5\\\" class=\\\"CitationRef\\\"\\u003e5\\u003c/span\\u003e]. Although higher male birth ratios have been reported in wild populations (around 60%) [\\u003cspan citationid=\\\"CR18\\\" class=\\\"CitationRef\\\"\\u003e18\\u003c/span\\u003e], no indication existed of a significantly higher male birth rate relative to female births in the zoo-housed population.\\u003c/p\\u003e\\u003cp\\u003eFurthermore, international introductions were clearly biased toward females, with the frequency of female imports exceeding that of males by more than twofold. This is considered one factor contributing to the formation of a population with an excess of females prior to 1990. In recent years, due to advances in domestic breeding, the number of individuals introduced from overseas has been extremely low, and most individuals introduced in the past died by around 2000. This is considered another reason for the disappearance of female dominance in the domestic population after 2000.\\u003c/p\\u003e\\u003cp\\u003eIn summary, the primary cause of the male surplus of giraffes in Japanese zoos was the improvement in male mortality rates that became pronounced after the 2010s, while the effects of sex ratio at birth and international introductions remained secondary factors. Thus, male surplus has emerged as a side effect of the \\u0026ldquo;success\\u0026rdquo; of improved husbandry. Improvements in zoo environments have been reported to reduce mortality rates among various species of animals other than giraffes [\\u003cspan citationid=\\\"CR19\\\" class=\\\"CitationRef\\\"\\u003e19\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR20\\\" class=\\\"CitationRef\\\"\\u003e20\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR21\\\" class=\\\"CitationRef\\\"\\u003e21\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR22\\\" class=\\\"CitationRef\\\"\\u003e22\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR23\\\" class=\\\"CitationRef\\\"\\u003e23\\u003c/span\\u003e]. Therefore, similar phenomena involving surplus males may occur in other species as well. The findings of this study provide valuable information for developing sex ratio management strategies for zoo-housed populations, not only for giraffes, but also for other species, particularly polygynous ones.\\u003c/p\\u003e\\u003cp\\u003eInterestingly, improvements in husbandry conditions may not only lead to increased survival rates but may also potentially influence the sex ratio of newborns. The hypothesis that improved maternal condition increases the probability of male births [\\u003cspan citationid=\\\"CR24\\\" class=\\\"CitationRef\\\"\\u003e24\\u003c/span\\u003e] has been supported across multiple animal species, including ruminants [\\u003cspan citationid=\\\"CR25\\\" class=\\\"CitationRef\\\"\\u003e25\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR26\\\" class=\\\"CitationRef\\\"\\u003e26\\u003c/span\\u003e, \\u003cspan citationid=\\\"CR27\\\" class=\\\"CitationRef\\\"\\u003e27\\u003c/span\\u003e]. Our data show the sex ratio at birth in Japanese zoos remained close to parity, not supporting this prediction at present; but if husbandry practices continue to improve, such effects could emerge in the future. Therefore, this hypothesis highlights the importance of continuing to monitor changes in population dynamics.\\u003c/p\\u003e\"},{\"header\":\"Materials and Methods\",\"content\":\"\\u003cp\\u003eTo clarify sex-specific demographic changes in Japanese zoo-housed giraffes, we referenced the \\u0026lsquo;2023 Giraffe Studbook Japanese Regional\\u0026rsquo;, which lists almost all individuals kept in Japan. For each sex, we examined the (1) annual population size as of 31 December, (2) number of births, and (3) number of giraffes imported from or exported to foreign zoos.\\u003c/p\\u003e\\u003cp\\u003eFor the survivorship analyses, we calculated the number of days each individual survived based on birth and death dates, or until 31 December 2023 for living individuals. For individuals still alive at the end of the study period (31 December 2023), survival time was right censored at that date. Since few individuals were born before 1980, we focused on those born between 1980 and 2023. Individuals lacking birth or death dates or who became untraceable (i.e., data lacking after transfer overseas) were excluded from the analysis. The final dataset consisted of 1006 individuals (males: 511, females: 495).\\u003c/p\\u003e\\u003cp\\u003eWe conducted a Cox proportional hazard analysis in R [28] using the survival package [29]. In these analyses, coefficients\\u0026thinsp;\\u0026lt;\\u0026thinsp;1 (i.e., the 95% confidence interval excludes 1) indicated reduced mortality risk compared with the reference group. Birth cohorts were defined as four discrete categories: 1980\\u0026ndash;1989, 1990\\u0026ndash;1999, 2000\\u0026ndash;2009, 2010\\u0026ndash;2019. The significance level was set at p\\u0026thinsp;\\u0026lt;\\u0026thinsp;0.05. As in Scherer et al. 2024, survivorship for juveniles was calculated using individuals under the age of four, and survivorship for adults was calculated using all individuals that had reached the age of four. For the adult survivorship analyses (\\u0026ge;\\u0026thinsp;4 years), time-at-risk was measured from the 4th birthday (left-truncated at age 4) until death or censoring on 31 December 2023. Kaplan\\u0026ndash;Meier survival analysis was also conducted, and log-rank tests were applied to compare sexes and birth cohorts.\\u003c/p\\u003e\\u003cp\\u003eThe studbook categorises the giraffes into six subspecies-based lineage groups: reticulated giraffe (\\u003cem\\u003eGiraffa camelopardalis reticulata\\u003c/em\\u003e), Masai giraffe (\\u003cem\\u003eG. c. tippelskirchi\\u003c/em\\u003e), Cape giraffe (\\u003cem\\u003eG. c. giraffa\\u003c/em\\u003e), Angola giraffe (\\u003cem\\u003eG. c. angolensis\\u003c/em\\u003e), Nigerian giraffe (\\u003cem\\u003eG. c. peralta\\u003c/em\\u003e), and Rothschild\\u0026rsquo;s giraffe (\\u003cem\\u003eG. c. rothschildi\\u003c/em\\u003e). The reticulated giraffe lineage group includes pure reticulated giraffes as well as hybrids between reticulated giraffes and other subspecies (mostly Rothschild\\u0026rsquo;s giraffes). The remaining hybrid giraffes were grouped together in a seventh group. Our analysis did not consider differences between lineages.\\u003c/p\\u003e\"},{\"header\":\"Declarations\",\"content\":\"\\u003cp\\u003e\\u003cstrong\\u003eAcknowledgements\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eWe express our gratitude to JAZA for allowing us to use the 2023 Giraffe Studbook of the Japanese Region. There is no funding to report.\\u0026nbsp;\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003eAuthor contributions\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eM.G. and Y. K. conceived and designed the study, analyzed the data, and wrote the manuscript. S.K. and M.Y. contributed to the study design and part of the discussion. I.S. organized the data and contributed to part of the discussion. All authors reviewed and approved the final version of the manuscript.\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003eCompeting interests\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eThe authors declare no competing interests.\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003eData availability\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eAll data supporting the findings of this study are provided in the Supplementary Information.\\u003c/p\\u003e\"},{\"header\":\"References\",\"content\":\"\\u003col\\u003e\\n \\u003cli\\u003eAsa C.S., Porton I., Plotka E.D. Contraception as a management tool for controlling surplus animals. In: \\u003cem\\u003eWild mammals in captivity: Principles and techniques for zoo management.\\u003c/em\\u003e eds. D. G. Kleiman, K. V. Thompson and C. K. Baer. 2nd ed (Chicago, IL: University of Chicago Press, 1996), 451\\u0026ndash;467.\\u003c/li\\u003e\\n \\u003cli\\u003eConway WG. The surplus problem. In: AAZPA National Conference Proceedings, 20\\u0026ndash;24. Wheeling, WV: American Association of Zoological Parks and Aquariums\\u0026nbsp;(1976).\\u003c/li\\u003e\\n \\u003cli\\u003eLindburg D.G.\\u0026nbsp;Zoos and the \\u0026ldquo;surplus\\u0026rdquo; problem. \\u003cem\\u003eZoo Biol.\\u003c/em\\u003e\\u003cstrong\\u003e10\\u003c/strong\\u003e, 1\\u0026ndash;2 (1991).\\u0026nbsp;(doi.org/10.1002/zoo.1430100102)\\u003c/li\\u003e\\n \\u003cli\\u003eClauss M., Roller M., Bertelsen M.F. et al. Zoos must embrace animal death for education and conservation. \\u003cem\\u003ePNAS\\u003c/em\\u003e\\u003cstrong\\u003e122\\u003c/strong\\u003e(1), e2414565121 (2025).\\u0026nbsp;(doi.org/10.1073/pnas.2414565121)\\u003c/li\\u003e\\n \\u003cli\\u003eEAZA Giraffe EEPs.\\u0026nbsp;EAZA Husbandry and Management Guidelines for Giraffa camelopardalis. 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(2022) https://CRAN.R-project.org/package=survival\\u003c/li\\u003e\\n\\u003c/ol\\u003e\"}],\"fulltextSource\":\"\",\"fullText\":\"\",\"funders\":[],\"hasAdminPriorityOnWorkflow\":false,\"hasManuscriptDocX\":false,\"hasOptedInToPreprint\":true,\"hasPassedJournalQc\":\"\",\"hasAnyPriority\":true,\"hideJournal\":true,\"highlight\":\"\",\"institution\":\"\",\"isAcceptedByJournal\":false,\"isAuthorSuppliedPdf\":false,\"isDeskRejected\":\"\",\"isHiddenFromSearch\":false,\"isInQc\":false,\"isInWorkflow\":false,\"isPdf\":false,\"isPdfUpToDate\":true,\"isWithdrawnOrRetracted\":false,\"journal\":{\"display\":true,\"email\":\"info@researchsquare.com\",\"identity\":\"researchsquare\",\"isNatureJournal\":false,\"hasQc\":true,\"allowDirectSubmit\":true,\"externalIdentity\":\"\",\"sideBox\":\"\",\"snPcode\":\"\",\"submissionUrl\":\"/submission\",\"title\":\"Research Square\",\"twitterHandle\":\"researchsquare\",\"acdcEnabled\":true,\"dfaEnabled\":false,\"editorialSystem\":\"\",\"reportingPortfolio\":\"\",\"inReviewEnabled\":false,\"inReviewRevisionsEnabled\":true},\"keywords\":\"giraffe, longevity, studbook, surplus, zoo\",\"lastPublishedDoi\":\"10.21203/rs.3.rs-7858970/v1\",\"lastPublishedDoiUrl\":\"https://doi.org/10.21203/rs.3.rs-7858970/v1\",\"license\":{\"name\":\"CC BY 4.0\",\"url\":\"https://creativecommons.org/licenses/by/4.0/\"},\"manuscriptAbstract\":\"\\u003cp\\u003eSurplus animals exceeding housing capacity pose a major challenge for zoo population management. In polygynous species such as giraffes (\\u003cem\\u003eGiraffa camelopardalis\\u003c/em\\u003e), where fewer males than females can be accommodated per facility, male surpluses are particularly problematic. Although the proportion of males among zoo-housed giraffes has increased globally in recent decades, the demographic drivers remain unclear. Here, we analysed Japanese zoo studbook data (1980\\u0026ndash;2023) to examine population trends, birth sex ratios, and sex-specific survival to elucidate factors influencing the population sex balance. Our analysis showed that the number of males rose sharply after 2012 and surpassed females for the first time in 2021, indicating the emergence of a pronounced male surplus. Survival analyses showed that both sexes experienced higher survival following husbandry reforms around 2010, but the improvement was disproportionately greater in males. Although females still exhibited higher survivorship, the survival gap between sexes narrowed and disappeared in the most recent cohorts, shifting the adult sex ratio toward parity and accelerating the male surplus. This study demonstrates that male surplus can emerge as an unintended consequence of successful husbandry improvements rather than biased birth ratios or reduced culling. These findings provide key implications for proactive sex ratio management in zoos.\\u003c/p\\u003e\",\"manuscriptTitle\":\"Survival improvements in zoo-housed giraffes lead to surplus males: A long-term analysis of longevity and mortality in Japanese zoos\",\"msid\":\"\",\"msnumber\":\"\",\"nonDraftVersions\":[{\"code\":1,\"date\":\"2025-10-15 05:27:57\",\"doi\":\"10.21203/rs.3.rs-7858970/v1\",\"editorialEvents\":[{\"type\":\"communityComments\",\"content\":0}],\"status\":\"published\",\"journal\":{\"display\":true,\"email\":\"info@researchsquare.com\",\"identity\":\"researchsquare\",\"isNatureJournal\":false,\"hasQc\":true,\"allowDirectSubmit\":true,\"externalIdentity\":\"\",\"sideBox\":\"\",\"snPcode\":\"\",\"submissionUrl\":\"/submission\",\"title\":\"Research Square\",\"twitterHandle\":\"researchsquare\",\"acdcEnabled\":true,\"dfaEnabled\":false,\"editorialSystem\":\"\",\"reportingPortfolio\":\"\",\"inReviewEnabled\":false,\"inReviewRevisionsEnabled\":true}}],\"origin\":\"\",\"ownerIdentity\":\"4c8d3392-2793-43d9-8e8a-859bdc27d3b6\",\"owner\":[],\"postedDate\":\"October 15th, 2025\",\"published\":true,\"recentEditorialEvents\":[],\"rejectedJournal\":[],\"revision\":\"\",\"amendment\":\"\",\"status\":\"posted\",\"subjectAreas\":[{\"id\":56315100,\"name\":\"Biological sciences/Ecology\"},{\"id\":56315101,\"name\":\"Earth and environmental sciences/Ecology\"},{\"id\":56315102,\"name\":\"Biological sciences/Evolution\"},{\"id\":56315103,\"name\":\"Biological sciences/Zoology\"}],\"tags\":[],\"updatedAt\":\"2025-12-17T10:10:02+00:00\",\"versionOfRecord\":[],\"versionCreatedAt\":\"2025-10-15 05:27:57\",\"video\":\"\",\"vorDoi\":\"\",\"vorDoiUrl\":\"\",\"workflowStages\":[]},\"version\":\"v1\",\"identity\":\"rs-7858970\",\"journalConfig\":\"researchsquare\"},\"__N_SSP\":true},\"page\":\"/article/[identity]/[[...version]]\",\"query\":{\"redirect\":\"/article/rs-7858970\",\"identity\":\"rs-7858970\",\"version\":[\"v1\"]},\"buildId\":\"8U1c8b4HqxoKbykW_rLl7\",\"isFallback\":false,\"isExperimentalCompile\":false,\"dynamicIds\":[84888],\"gssp\":true,\"scriptLoader\":[]}","source_license":"CC-BY-4.0","license_restricted":false}