Seasonal Trends in Sperm Quality in Denmark and Florida

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Most studies have reported peak sperm motility in winter months, although findings vary by country. This study investigated whether the quality of semen varies seasonally in Denmark and Florida, and whether ambient temperature contributes to any seasonal trends. Design This retrospective observational study included data from 15,125 candidate sperm donors collected between 2018 and 2024. The cohort comprised 10,670 men from four Danish cities (Aarhus, Aalborg, Odense, and Copenhagen) and 4,911 men from Orlando, Florida. Participants were aged 18–45 years and resided near the collection sites. All ejaculates were analysed within one hour of collection using the same computer-assisted semen analysis (CASA) system and standardised temperature-controlled protocols. Semen parameters assessed included ejaculate volume, sperm concentration, concentrations of progressively motile sperm (‘grade a’ and ‘grade b’), and total motile sperm count (TMSC). The effects of monthly mean outdoor temperature during the month of collection and two months earlier (representing early spermatogenesis) on sperm parameters were also modelled. We used nonlinear statistical methods (GAMs) to accurately display the month-to-month seasonal variation, while accounting for the nonlinear effects of male age, monthly mean temperature, and year of study on semen parameters. Results Strong and consistent seasonal variation was observed in the concentration of progressively motile (‘grade a’ and ‘grade b’) sperm in both Denmark and Florida. The concentration of ‘grade a’ sperm was highest from May to July and lowest between October and March in both countries. Despite climatic and demographic differences, seasonal trends were remarkably similar. No evidence of seasonal variation was found in ejaculate volume or total sperm concentration, indicating that the rate of spermatogenesis did not vary seasonally. TMSC also varied seasonally, even after controlling for average daily temperature in the contemporaneous and preceding months, suggesting that other seasonal factors, possibly related to lifestyle or other environmental factors, may affect sperm motility. Conclusion Semen quality, particularly sperm motility, exhibits clear and consistent seasonal variation in both temperate and subtropical climates. These findings highlight the importance of accounting for seasonality when assessing semen quality for fertility evaluation and indicate that seasonal variation persists even in warm climates such as Florida. Trial registration number: Not applicable. Semen quality Sperm motility Total Motile Sperm Count Seasons Temperature Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Introduction It is well known that many human physiological processes vary across the influenced by seasonal variation in environmental factors such as temperature, daylight duration, and lifestyle year ( 1 ). However, while numerous factors such as age, lifestyle, and environmental conditions are known to influence semen quality that affect male fertility, seasonal variation in sperm parameters (e.g., motile sperm concentration and count, semen volume) remains less clearly understood. Emerging evidence does suggest that semen parameters may exhibit seasonal fluctuations. For instance, studies have reported peaks in sperm concentration and motility during specific months, alongside seasonal variations in semen volume and sperm quality ( 2 – 5 ). These patterns may be influenced by factors such as ambient temperature, which can affect testicular function, or shifts in lifestyle behaviors, including diet and physical activity, which often vary across seasons ( 6 – 9 ). A comprehensive understanding of seasonal effects on semen quality could have significant clinical and practical implications. It may inform the timing of fertility treatments, optimise assisted reproductive protocols, and provide valuable guidance to couples trying to conceive. This study aims to investigate seasonal variations in sperm concentration, motility, and volume, within two large cohorts of candidate donors, examining key semen parameters across different seasons. By exploring the underlying trends and contributing factors, this study aims to enhance our understanding of male fertility and improve reproductive outcomes. Methods Data Collection The study used retrospective data on men residing in either Denmark or Florida who applied to become sperm donors at Cryos International between 2018 and 2024. These men lived in or near Denmark's four largest cities (Copenhagen, Aarhus, Odense, or Aalborg), or in or near Orlando, Florida US. Cryos has facilities in each of these 5 cities where semen samples were collected and analysed. The recruitment process at Cryos, recently detailed by Pacey et al. (2023), required men to be between 18 and 45 years old for them to be invited to provide a semen sample for analysis ( 10 ). Semen Analysis Semen samples were in sterile plastic cups (Sarstedt, Hounisen Laboratory equipment A/S, Skanderborg, Denmark) following a recommended period of 2–5 days of abstinence time. After collection, ejaculates were held at controlled room temperature (21°C) to liquefy. Ejaculate volume was estimated by weighing the samples (assuming a density of 1.0 g/mL) in accordance with guidelines from the World Health Organisation ( 11 ). Within 30–60 minutes of production, an aliquot from each sample was analysed using Computer-Assisted Sperm Analysis (CASA) to assess sperm concentration and motility. The CASA system employed was the Sperm Class Analyzer® (SCA® CASA, Microptic, Barcelona, Spain), utilising a CX-41 upright light microscope (OLYMPUS, Microptic, Barcelona, Spain) and a Makler Counting Chamber (Sefi Medical Instruments Ltd, Haifa, Israel). Sperm motility was categorised into four grades based on WHO (2021) definitions: grade a (rapidly progressive, ≥ 25 µm/s), grade b (slowly progressive, 5– 0–<5 µm/s), and grade d (immotile). All equipment, procedures and CASA set-up were standardised across all sites. Datasets The dataset for each ejaculate included the collection date and city, the man’s age, ejaculate volume (mL), sperm concentration (10⁶/mL), total sperm count (millions per mL), total motile sperm count (TMSC), and concentrations of ‘grade a’ and ‘grade b’ sperm (millions of spermatozoa per mL). Following WHO (2021) definitions, motile sperm (grade a and grade b) were combined to determine TMSC (concentration of motile sperm in an ejaculate) and motile sperm concentration (per mL of semen). Monthly mean temperatures (averaged from daily highs, means, and lows; Fig. 1 ) were obtained from the Danish Meteorological Institute (Copenhagen, Denmark) and Climate Data Online (National Centers for Environmental Information) and used to account for potential ambient temperature effects on sperm quality, as reported in previous studies ( 12 – 14 ). Although Danish law exempts secondary analysis of anonymised data from ethical approval requirements, ethical clearance was secured from The University of Manchester (ref: 2023-18428-31578). Statistical Analyses We used R version 4.5.13 for all analyses, with the function gam from the added package mgcv (v. 1.9-1) for Generalized Additive Models (GAMs) ( 15 , 16 ). These models were used to predict ejaculate volume, total sperm count, sperm concentration, and sperm motility metrics (motile sperm concentrations and TMSC), accounting for and testing the effects of the man’s age, monthly mean temperature, city of donation, and semen production date as predictor variables. In GAMs, we included time (year + (month-0.5/12) and temperature as smoothed effects (at the default parameters), and month (with smoothing parameters bs = cc and k = 12) in each model. In each model, we used the Gamma family with the log-link function to help normalise residuals and satisfy other statistical assumptions (see Supplementary Material Figure S1 ). As the P-values associated GAMs are considered to be approximations, we have avoided using the term ‘significant’ and instead refer to the strength of evidence (see also as ‘weak’ (0.05 < P < 0.10), ‘moderate’ (0.01 < P < 0.05), ‘strong’ (0.001 < P < 0.01), or ‘very strong’ (P 0.10 indicating no evidence, given the data). For this study we used GAMs (Supplementary Material Figure S1 ) rather than the more traditional Generalized Linear Models (GLMs) to model the seasonal variation in semen parameters. GAMs provide several advantages for the analysis of annual and seasonal trends: (i) they can model nonlinear trends; (ii) they provide a more flexible framework when modelling complex relationships that are not constrained by strict assumptions about the distributions of residuals; (iii) they allow us to incorporate different smoothing functions for each continuous predictor; and (iv) like GLMs, they allow us to evaluate the contribution of each predictor to the response variables (e.g., semen parameters) while accounting for variation in the other predictors. Thus, unlike previous studies of seasonal variation in semen parameters, we do not assume that the effects of predictors are linear and we are able to accurately model and display the month-to-month seasonal variation in those parameters. We present the results of GAM analyses here as predicted values from the models. Because the low, mean, and high monthly temperature data were highly correlated (r > 0.85 in each case), we included only mean monthly temperature in each model (Fig. 1 ). Exploratory analyses showed that it was rare for any one of those temperature variables to result in a statistically better-fitting model (based on the Akaike Information Criteria, AIC) given the data. In addition to the contemporaneous monthly temperature, we also modelled the mean daily temperature two months prior as spermatogenesis in humans is reported to take 74 days ( 17 ). Results Study Population The study included semen quality data from 15,581 men attending the Cryos International sperm banks in Denmark and Florida. A total of 10,670 men were resident in Aarhus, Aalborg, Copenhagen, and Odense in Denmark, and 4,911 men from Orlando in Florida (Table 1). All participants were 18–45 years old. Ejaculate Volume and Sperm Concentration There was no evidence that either ejaculate volume or sperm concentration varied across months in either Denmark (Fig. 2 ) or Orlando (Supplementary Material Figure S2B,F), suggesting that these parameters remained relatively stable throughout the year. There is, however, very strong evidence from both populations that both parameters varied across the years of study and the men’s ages (Fig. 2 ). There was little evidence that either parameter varied in relation to mean monthly temperatures in the contemporaneous months, except for a negative effect on ejaculate volume in Orlando (Supplementary Material Table S1 ). Rapidly Progressive Sperm (grade a) In both Denmark and Orlando, there is very strong evidence that ‘grade a’ sperm concentration varied seasonally (Fig. 3 , Table 2), with the predicted means being lowest in early winter (November-February) and highest in early summer (May-July). In both countries, there is very strong (Denmark) to moderate (Orlando) evidence that ‘grade a’ sperm concentration was positively related to mean monthly temperature (Fig. 5 ) such that the predicted highest means shifted to spring (March-May) and the lowest means to autumn (September-November; Fig. 3 B,E). The predicted patterns, controlling for contemporaneous monthly mean temperatures at the two countries, were virtually identical (Fig. 5 A, E). Slowly Progressive Sperm (grade b) There is moderate (Denmark) to strong (Orlando) evidence that ‘grade b’ sperm concentration also varied seasonally (Table 2, Supplementary Material Figure S3). In Denmark, the concentration of ‘grade b’ sperm was highest in spring and fall and lowest in summer, whereas in Orlando ‘grade b’ sperm concentrations were highest in summer and lowest in winter (Table 2; Supplementary Material Figure S3). There is also weak evidence that ‘grade b’ sperm concentrations in both countries varied seasonally even when controlling for the non-significant effects of mean temperatures in the contemporary months (Supplementary Material Figure S4). Non-Progressive Sperm Concentration (grades c + d) There is very strong (Denmark) to moderate (Orlando) evidence that the concentration of non-progressive sperm (grades c + d) also varied seasonally (Table 2), with the highest values in winter (November-December) and lowest in summer (May-August). Controlling for contemporaneous monthly temperature, there was still moderate evidence for seasonal variation in the concentrations of ‘grades c + d sperm at both countries (Table 3). Motile (grades a + b) Sperm Concentration In Orlando, there was strong evidence for seasonal variation in the concentration of progressive sperm (grades a + b) with the highest concentrations in early summer (May-July) and the lowest in winter (November-January); there was no evidence to support a seasonal pattern in Denmark (Table 2, Supplementary Material Figure S5). Controlling for temperature in the contemporaneous month provided only weak evidence for a seasonal patten in Orlando (Supplementary Material Figure S6). Total Motile Sperm Count (TMSC) There is weak (Denmark) to moderate (Orlando) evidence that Total Motile Sperm Count (TMSC, grades a + b sperm) varied seasonally (Fig. 4 , Table 2), with high and low values in the same months (May-June and October-November, respectively) in both countries. Controlling for contemporaneous monthly temperature, there was still moderate evidence for seasonal variation in Orlando but not in Denmark (Fig. 5 ). In both countries, there was very strong evidence for an effect of date (Fig. 4 C,G) and the man’s age (Fig. 4 D,H) on TMSC. The effect of age is very similar in both countries being highest between ages 25 and 35–40. The effect of date, however, is remarkably different between the two countries with TMSC in Denmark declining from 2019–2022 (as also reported in Lassen et al. 2024) and in Orlando increasing from 2018–2024. TMSC was positively related to contemporaneous monthly mean temperatures but the relationship for Orlando was not significant. Controlling for that temperature there was still a seasonal pattern in Orlando (Fig. 5 G) but not Denmark (Fig. 5 C). Effects of Temperatures During Spermatogenesis To examine the relationship between ambient temperatures during spermatogenesis, we also modelled the effects of daily mean temperatures two months previously on sperm parameters (Table 3). For the concentrations of ‘grade a’, ‘grade b’, and ‘grades a + b’ sperm there was evidence for a seasonal pattern in both countries when controlling for temperatures two months previously (Supplementary Material Figure S7a-c), although there was little evidence for an effect of those temperatures (Table 3). There was also strong evidence for a seasonal pattern in TMSC in both countries when controlling for those temperatures (Supplementary Material Figure S8), with the highest values for TMSC remaining in summer and the lowest in winter, even though there was no evidence for the effects of those temperatures (Table 3). Discussion Our analyses of semen quality in a large number of men between 2018 and 2024 in Florida and Denmark show clear seasonal patterns, as measured by the concentrations (Table 2) and number of progressively motile sperm (TMSC) in ejaculates (Figs. 3 B,F, 4 B,F, 5 A,E; Table 2; Supplementary Material Figure S3). The seasonal pattern in the concentration of the most progressively motile (grade a) sperm is supported by the strongest evidence (Fig. 3 B,F, Table 2), and is almost identical in both Denmark and Florida, being highest during the summer months and lowest in winter (Figs. 3 B,F, 5 A,E). The similarities between these male populations persist in spite of large climatic differences (Fig. 1 ) and likely differences in ethnicities, lifestyles, and exposure to environmental contaminants that might influence sperm quality. These populations also differed in the year-to-year patterns in sperm quality (e.g., Figs. 3 C,G and 4 C,G; Supplementary Material Figures S3-S8), as well as in the effects of environmental temperatures on sperm quality (Fig. 5 ; Table 3; Supplementary Material Figures S6-S8). By constructing Generalized Additive Models, we have been able to accurately visualise the month-to-month seasonal patterns in semen parameters (Figs. 2 B,F, 3BF, 4B,F, 5A,C,E,G) while accounting for the nonlinear effects of other variables (man’s age, monthly mean temperature, year) on those parameters. This has the advantage of allowing meaningful comparisons between populations in two different climatic environments, revealing that those seasonal patterns, in Denmark and Florida, are very similar. In addition, these statistical models have also revealed similar nonlinear variation in the effects of male age (Figs. 3 D,H, 4 D,H) and temperature (Fig. 5 B,D,F,H) on semen parameters in these two populations, but very different year-to-year changes (Fig. 3 C,G, 4 C,G) in those parameters. The pattern of year-to-year changes in sperm concentrations (Figs. 3 C, Supplementary Material Figure S3) and TMSC (Fig. 4 C) corroborates our previous finding of declines in those parameters in Denmark from 2017–2023 ( 18 ). The clear differences in those year-to-year patterns in Florida deserve further study. As there was no evidence for seasonal variation in ejaculate volume or sperm concentration in either country (Fig. 2 B,F; Supplementary Material Figure S2), the rate of spermatogenesis does not appear to have varied seasonally. However, the quality of spermatozoa produced did (Fig. 3 B,F and 4 B,F; Table 2). Whilst there were strong seasonal patterns in the concentrations of ‘grade a’ and ‘grade b’ sperm, the concentration and total number of progressively motile sperm (grades a + b) showed less pronounced seasonal patterns (Fig. 4 B,F, Table 2). This suggests seasonal variation in the relative numbers of sperm of different motilities, but further work will be needed to investigate the nature of that apparent trade-off. As in the present study, a large-scale analysis of 21,715 semen samples from a southern province of China reported peak progressive motility in February and March, with a subsequent decline in the summer ( 14 ). Similar seasonal trends have been found in other studies, where sperm motility was highest from December to April and decreased in the summer months ( 19 – 22 ). In contrast, a retrospective study conducted in Italy reported that progressive sperm motility was highest during the summer, aligning with our results ( 23 ). Other studies support this finding which suggests that regional and environmental factors, such as differences in temperature, humidity, or lifestyle behaviors may influence how sperm motility varies seasonally ( 24 – 26 ). In our study, there was relatively little evidence for a relationship between sperm quality and temperatures in either the contemporaneous month or two months previously (Table 3). For temperatures in the contemporaneous month, there was evidence only for an effect on grade a sperm concentration (Fig. 5 , Table 3) in Denmark (strong) and Orlando (moderate), and moderate evidence for an effect on TMSC in Denmark (Table 3). As spermatogenesis is reported to take 74 days, some effect of ambient temperatures on sperm quality during spermatogenesis might be expected ( 17 ). However, there was only weak to moderate evidence for an effect of temperature two months prior to ejaculation and the concentrations motile sperm (grades a + b) in both countries, ‘grade a’ sperm in Denmark, and ‘grade b’ sperm in Orlando Table 3). While those statistical effects of outdoor mean temperatures in our study are intriguing, we hesitate to suggest a causal relationship for two reasons. First, it has been well established that climate, and particularly environmental temperatures, influence many human activities ( 27 ). Thus, it might be lifestyle that is associated with outdoor temperatures and not the temperatures themselves that influence the concentration of progressively motile sperm. Second, the temperature variables that we tested are simple monthly mean values rather than more fine-grained weather measures, like daily wind chill and precipitation and the variability of all variables, that could also influence both spermatogenesis and activities. The potential effects of temperature on sperm quality deserve further study as our measures of ambient temperature in this study were rather coarse grained as we did not know what temperatures each man experienced when samples were provided nor during the entire period of spermatogenesis. Nor did we have any information on the lifestyles and activities of individual men that might have been able to provide even stronger evidence for their effects on quality sperm production. The seasonal pattern in ‘grade a’ sperm concentration, for example, remained when we accounted for temperature either in the contemporaneous month (Fig. 5 A,E) or two months earlier (Fig. 5 ; Supplementary Material Figures S7, S8), suggesting either that something other than temperature also affected the seasonal variation in sperm quality, or our measures of temperature were not fine-grained enough. To the best of our knowledge, only two recent studies have studied a large and longitudinal dataset to examine both seasonal variation and the effects of temperature on semen quality - both from sperm banks in China ( 4 , 5 ). Both studies support the idea that temperature plays a key role. In a comprehensive recent study of 41,689 semen samples from 10,802 men from Wuhan in central China found significant effects of ambient outdoor temperatures on semen quality while accounting for a measure of ambient pollution level as well as the subject’s age, body mass index (BMI), smoking and alcohol use, education level, and recent sexual activity ( 4 ). All measures of semen quality except ejaculate volume decreased markedly as daily mean temperature, averaged over the 90 days prior to semen collection, increased from 10°C to 30°C. Measures of sperm motility tended not to decline at ambient temperatures above 20°C, suggesting a threshold effect. Importantly, some semen parameters (total motile sperm number and total sperm number) were correlated with temperatures 70–90 days before ejaculation, at the start of spermatogenesis. While the sample sizes and mean age of the men (26.8 years) in that study were similar to ours (mean age = 26.0 in Denmark, 27.7 in Orlando), the climate in Guandong province is different, being generally warm and humid with short, relatively dry winters and summers that are long, hot, and wet ( 5 ). A more recent study of 44,564 semen samples from 11,050 men from Guandong province in southern China, also found significant effects of outdoor temperatures on semen quality, while accounting for the same additional predictors in their models ( 5 ). In contrast to the findings from the Wuhan study, all measures of semen quality except ejaculate volume decreased markedly as daily mean temperature, averaged over the 90 days prior to semen collection, increased from 10°C to 30°C. Again, measures of sperm motility tended not to decline at ambient temperatures above 20°C, supporting the idea of a threshold effect. Climate might be partly responsible for the differences from the Wuhan study as Guandong province is further south, where the climate is different, as described above ( 5 ). In addition to the clear seasonal patterns, there was evidence for variation in sperm quality across years and the men’s ages in both countries. In Denmark, sperm quality declined precipitously from 2019–2022 as reported in Lassen et al. (2024) using the same dataset but a less comprehensive analysis ( 18 ). That decline appears to have stopped by 2023 and appears to be starting to increase in 2024 (Figs. 3 C, 4 C), as expected if the decline was due to changes in lifestyle or health during the COVID pandemic that began in late 2019. In Orlando, overall sperm quality (as measured by TMSC) increased gradually from 2018 to 2024 (Fig. 4 G). We have no explanation for this strongly supported trend, and it deserves further study. In both countries there is very strong evidence for sperm quality changing with the age of the men (Figs. 3 D,H and 4 D,H). Sperm quality was lowest at both ends of the age spectrum ( 40 years) and peaked during their thirties. Indeed, there is enough uncertainty in the predicted values to suggest that men between ages 25 and 40 have little variation in sperm quality, another pattern that deserves further study. Lower sperm quality before the age of 25 (Figs. 3 D,H, 4 D,H) could be explained by increased sexual activity / reduced abstinence duration which we did not quantify, whereas lower quality after age 40 is expected as a normal result of aging ( 28 , 29 ). One striking feature of the samples that we analysed is the large amount of variation in sperm parameters. Within the 315 samples from Copenhagen in March 2024, for example, the mean concentration of ‘grade a’ sperm was 6.93 millions/mL with the range from 0–72 millions/mL and a standard deviation of 9.84. Thus, the coefficient of variation (CV) for this sample was 142%. To put this in perspective, the CV for most phenotypic traits under natural selection is usually < 10% for traits under natural selection and 10–20% for those under sexual selection ( 30 , 31 ). The reasons for this extreme variation are unknown but are undoubtedly due to genetic variation, lifestyle (diet, exercise, smoking and alcohol consumption), recent sexual activity and men’s ages. Despite the extreme variation in the traits that we measured we were able to find evidence for a large amount of seasonal variation in mean traits in part by accounting for variation in age, year and month, but also because we had large sample sizes over several years. Conclusion The results of this study indicate a clear seasonal pattern in sperm quality that is almost identical in both Denmark, where the climate is temperate with cold winters and warm summers, and Florida, where the climate is warm-to-hot year round (Fig. 1 ). We also found some evidence for relationships between sperm quality parameters and average monthly temperatures both in the contemporaneous month and two months previously when spermatogenesis began, particularly in the concentration of the most progressively motile (grade a) sperm. Most importantly, however, when controlling for temperature, the seasonal patterns persisted, suggesting that other factors may contribute to these seasonal trends. Declarations Acknowledgements The authors acknowledge the contributions of all laboratory technicians of Cryos International sperm bank for pertinent semen analysis, donor candidate assessment, cryopreservation, and registration of all semen samples. Authors’ contributions All authors took part in conceptualising the study and editing the final manuscript. EL and RM drafted the paper, and RM undertook the data analysis. Funding The study received no external funding. Data availability The data used for all analyses in this study are publicly available at (Lassen et al., 2025). Supplementary Material is available at Reproductive Biology and Endocrinology online. Author details 1 Cryos International - Denmark, Vesterbro Torv 3, 8000 Aarhus, Denmark. 2 School of Medical Sciences, Faculty of Biology, Medicine and Health, Core Technology Facility, 46 Grafton Street, University of Manchester, Manchester, M13 9NT, United Kingdom. 3 Department of Biology, Queen’s University, Kingston, ON K7L 3N6, Canada. Ethics approval and consent to participate Not applicable. Consent for publication Not applicable. Competing interests E.L. and A.-B.S. are employees of Cryos International. AAP is also a member of the Cryos External Scientific Advisory Committee, he also undertakes consultancy for Carrot Fertility, and in the last two years has delivered educational lectures for IBSA Institut Biochemique SA, and Mealis Group but all monies were paid to the University of Manchester. He is also the co-chair of the UKNEQAS Reproductive Sciences Advisory Committee, is a member of the Advisory Boards for the Progress Educational Trust (Charity Number 1139856) and the Science Media Centre (Charity Number 1140827) and Patron of the Fertility Alliance (Charity Number 1206323 (all unpaid). He is a member of the Guidelines Development Groups for the National Institute for Health and Care Excellence, and the World Health Organisation (again all unpaid). None of the authors were directly involved in the collection or physical analysis of semen samples. References Hohm I, Wormley AS, Schaller M, Varnum MEW. 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Environmental factors contributed to circannual rhythm of semen quality. Chronobiol Int. 2017;34(3):411–25. https://doi.org/10.1080/07420528.2017.1280046 . Redźko S, Kuczyński W, Szamatowicz J, Wasilewski T, Mrugacz G, Borawska R, et al. Seasonal changes in donor’s sperm motility: CASA parameters. Ginekologia polska. 1998;69(6):485–9. PMID: 9695368. Spira A. Seasonal variations of sperm characteristics. Arch Androl. 1984;12 Suppl:23–8. PMID: 6535452. Kim SE, Kim Y, Hashizume M, Honda Y, Kazutaka O, Hijioka Y, et al. Positive Association of Aggression with Ambient Temperature. Yale J Biology Med. 2023;96(2):189–96. https://doi.org/10.59249/RXZX5728 . Johnson SL, Dunleavy J, Gemmell NJ, Nakagawa S. Consistent age-dependent declines in human semen quality: A systematic review and meta-analysis. Ageing Res Rev. 2015;19:22–33. https://doi:10.1016/j.arr.2014.10.007 . Carlsen E, Holm Petersen J, Andersson AM, Skakkebaek NE. Effects of ejaculatory frequency and season on variations in semen quality. Fertility Steril. 2004;82(2). https://doi.org/10.1016/j.fertnstert.2004.01.039 . Alatalo RV, Höglund J, Lundberg A. Patterns of variation in tail ornament size in birds. Biol J Linn Soc. 1988;34(4):363–74. https://doi.org/10.1111/j.1095-8312.1988.tb01969.x . Zajitschek SR, Zajitschek F, Bonduriansky R, Brooks RC, Cornwell W, Falster DS, et al. Sexual dimorphism in trait variability and its eco-evolutionary and statistical implications. Elife. 2020;9. https://doi.org/10.7554/eLife.63170 . Tables Tables 1 to 3 are available in the Supplementary Files section. Additional Declarations Competing interest reported. E.L. and A.-B.S. are employees of Cryos International. AAP is also a member of the Cryos External Scientific Advisory Committee, he also undertakes consultancy for Carrot Fertility, and in the last two years has delivered educational lectures for IBSA Institut Biochemique SA, and Mealis Group but all monies were paid to the University of Manchester. He is also the co-chair of the UKNEQAS Reproductive Sciences Advisory Committee, is a member of the Advisory Boards for the Progress Educational Trust (Charity Number 1139856) and the Science Media Centre (Charity Number 1140827) and Patron of the Fertility Alliance (Charity Number 1206323 (all unpaid). He is a member of the Guidelines Development Groups for the National Institute for Health and Care Excellence, and the World Health Organisation (again all unpaid). None of the authors were directly involved in the collection or physical analysis of semen samples. Supplementary Files Supplementary.docx Tables.docx Cite Share Download PDF Status: Published Journal Publication published 21 Feb, 2026 Read the published version in Reproductive Biology and Endocrinology → Version 1 posted Reviews received at journal 10 Nov, 2025 Reviewers agreed at journal 05 Nov, 2025 Reviewers agreed at journal 04 Nov, 2025 Reviewers agreed at journal 03 Nov, 2025 Reviewers agreed at journal 03 Nov, 2025 Reviewers invited by journal 03 Nov, 2025 Editor assigned by journal 30 Oct, 2025 Submission checks completed at journal 30 Oct, 2025 First submitted to journal 28 Oct, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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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-7971305","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":542725140,"identity":"7e7f3cd8-f69e-4229-a26d-d8d6f4a0afd4","order_by":0,"name":"Emilie Lassen","email":"","orcid":"","institution":"Cryos International","correspondingAuthor":false,"prefix":"","firstName":"Emilie","middleName":"","lastName":"Lassen","suffix":""},{"id":542725141,"identity":"446c9aa9-d513-4257-9d58-72a8287dbe01","order_by":1,"name":"Allan A Pacey","email":"","orcid":"","institution":"University of 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1","display":"","copyAsset":false,"role":"figure","size":168434,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eMonthly average of mean daily temperatures in Denmark and Florida (2018-2024).\u003c/strong\u003e Red solid lines with open circles show Denmark; blue dashed lines with closed circles show Florida (Orlando).\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-7971305/v1/74457d973062819b6dbf0e9a.png"},{"id":95807685,"identity":"ab6ad3e3-fdf8-46a7-8f71-ca99f7c60724","added_by":"auto","created_at":"2025-11-13 08:49:04","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":172506,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eNo seasonal variation in ejaculate volume or sperm concentration in Denmark.\u003c/strong\u003e(A,E) Raw data (means ±95% CLs; log scale). (B,F) Monthly trends, with approximate \u003cem\u003eP\u003c/em\u003e-values from GAM models. (C,G) Trends across 7 years; (D,H) trends by age. Rug plots on x-axes show sample distributions. Similar results for Orlando are in Supplementary Figure S2\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-7971305/v1/34a970bb6cee081106eb85e6.png"},{"id":95807674,"identity":"663f8479-e379-4ba8-a710-4d1e435eeb25","added_by":"auto","created_at":"2025-11-13 08:49:04","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":152656,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eVariation in grade A sperm concentration in Denmark (red) and Orlando (blue).\u003c/strong\u003e (A,E) Monthly means ±95% CLs of raw data. (B–D,F–H) Predicted trends from GAM models: by month (A,B,E,F), date (C,G), and age (D,H). Approximate \u003cem\u003eP\u003c/em\u003e-values indicate predictor effects. See Figure 2 for graph details and Supplementary Material for model details.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-7971305/v1/1b95e526b1dfc5ae87e8136c.png"},{"id":95807861,"identity":"5c312cea-ada5-4373-842b-b8273bf5ce68","added_by":"auto","created_at":"2025-11-13 08:49:13","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":114080,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eVariation in total motile sperm count (TMSC) in Denmark and Orlando.\u003c/strong\u003e\u003cbr\u003e\n(A,E) Monthly means ±95% CLs from raw data across all years. (B–D,F–H) Predicted values from GAM models: by month (B,F), date (C,G), and age (D,H). Approximate \u003cem\u003eP\u003c/em\u003e-values indicate predictor effects. See Figure 2 for graph details and Supplementary Material for model details.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-7971305/v1/2d5f438395c487a8cc14730b.png"},{"id":95807865,"identity":"5f41e030-87ed-4cf3-aa68-b413027c81d7","added_by":"auto","created_at":"2025-11-13 08:49:13","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":171457,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eEffect of monthly mean temperature on sperm concentration and TMSC.\u003c/strong\u003e\u003cbr\u003e\n(A–D) Denmark; (E–H) Orlando. (B,F) Grade A sperm concentration; (D,H) TMSC. (A,C,E,G) Seasonal variation. Predicted GAM trends control for date and age. The dashed blue line in (A) shows Orlando’s seasonal variation from (E), with scale on the right axis. See Figure 2 for graph details and Supplementary Material for model methods.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-7971305/v1/29111e543ab188cadf69e313.png"},{"id":103252477,"identity":"e97c496c-30a6-47ac-a218-2d35ffa91bcb","added_by":"auto","created_at":"2026-02-23 16:14:57","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1533564,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7971305/v1/23a03261-8f75-4ee2-ae13-ded8d2dfde81.pdf"},{"id":95807499,"identity":"95761f24-8eba-4277-89af-0aa61974ecee","added_by":"auto","created_at":"2025-11-13 08:48:47","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":4303389,"visible":true,"origin":"","legend":"","description":"","filename":"Supplementary.docx","url":"https://assets-eu.researchsquare.com/files/rs-7971305/v1/4c82424e32b7cb33a6b336a2.docx"},{"id":95808155,"identity":"13b7f87f-cf57-4b61-9fae-2aef6a4d194e","added_by":"auto","created_at":"2025-11-13 08:49:21","extension":"docx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":2384777,"visible":true,"origin":"","legend":"","description":"","filename":"Tables.docx","url":"https://assets-eu.researchsquare.com/files/rs-7971305/v1/83ed8d0a3c96cfc3e52391b6.docx"}],"financialInterests":"Competing interest reported. E.L. and A.-B.S. are employees of Cryos International. AAP is also a member of the Cryos External Scientific Advisory Committee, he also undertakes consultancy for Carrot Fertility, and in the last two years has delivered educational lectures for IBSA Institut Biochemique SA, and Mealis Group but all monies were paid to the University of Manchester. He is also the co-chair of the UKNEQAS Reproductive Sciences Advisory Committee, is a member of the Advisory Boards for the Progress Educational Trust (Charity Number 1139856) and the Science Media Centre (Charity Number 1140827) and Patron of the Fertility Alliance (Charity Number 1206323 (all unpaid). He is a member of the Guidelines Development Groups for the National Institute for Health and Care Excellence, and the World Health Organisation (again all unpaid). None of the authors were directly involved in the collection or physical analysis of semen samples.","formattedTitle":"Seasonal Trends in Sperm Quality in Denmark and Florida","fulltext":[{"header":"Introduction","content":"\u003cp\u003eIt is well known that many human physiological processes vary across the influenced by seasonal variation in environmental factors such as temperature, daylight duration, and lifestyle year (\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e). However, while numerous factors such as age, lifestyle, and environmental conditions are known to influence semen quality that affect male fertility, seasonal variation in sperm parameters (e.g., motile sperm concentration and count, semen volume) remains less clearly understood.\u003c/p\u003e\u003cp\u003eEmerging evidence does suggest that semen parameters may exhibit seasonal fluctuations. For instance, studies have reported peaks in sperm concentration and motility during specific months, alongside seasonal variations in semen volume and sperm quality (\u003cspan additionalcitationids=\"CR3 CR4\" citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e). These patterns may be influenced by factors such as ambient temperature, which can affect testicular function, or shifts in lifestyle behaviors, including diet and physical activity, which often vary across seasons (\u003cspan additionalcitationids=\"CR7 CR8\" citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eA comprehensive understanding of seasonal effects on semen quality could have significant clinical and practical implications. It may inform the timing of fertility treatments, optimise assisted reproductive protocols, and provide valuable guidance to couples trying to conceive. This study aims to investigate seasonal variations in sperm concentration, motility, and volume, within two large cohorts of candidate donors, examining key semen parameters across different seasons. By exploring the underlying trends and contributing factors, this study aims to enhance our understanding of male fertility and improve reproductive outcomes.\u003c/p\u003e"},{"header":"Methods","content":"\u003ch2\u003eData Collection\u003c/h2\u003e\u003cp\u003eThe study used retrospective data on men residing in either Denmark or Florida who applied to become sperm donors at Cryos International between 2018 and 2024. These men lived in or near Denmark's four largest cities (Copenhagen, Aarhus, Odense, or Aalborg), or in or near Orlando, Florida US. Cryos has facilities in each of these 5 cities where semen samples were collected and analysed. The recruitment process at Cryos, recently detailed by Pacey et al. (2023), required men to be between 18 and 45 years old for them to be invited to provide a semen sample for analysis (\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e).\u003c/p\u003e\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003eSemen Analysis\u003c/h2\u003e\u003cp\u003eSemen samples were in sterile plastic cups (Sarstedt, Hounisen Laboratory equipment A/S, Skanderborg, Denmark) following a recommended period of 2\u0026ndash;5 days of abstinence time. After collection, ejaculates were held at controlled room temperature (21\u0026deg;C) to liquefy. Ejaculate volume was estimated by weighing the samples (assuming a density of 1.0 g/mL) in accordance with guidelines from the World Health Organisation (\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e). Within 30\u0026ndash;60 minutes of production, an aliquot from each sample was analysed using Computer-Assisted Sperm Analysis (CASA) to assess sperm concentration and motility. The CASA system employed was the Sperm Class Analyzer\u0026reg; (SCA\u0026reg; CASA, Microptic, Barcelona, Spain), utilising a CX-41 upright light microscope (OLYMPUS, Microptic, Barcelona, Spain) and a Makler Counting Chamber (Sefi Medical Instruments Ltd, Haifa, Israel). Sperm motility was categorised into four grades based on WHO (2021) definitions: grade a (rapidly progressive, \u0026ge;\u0026thinsp;25 \u0026micro;m/s), grade b (slowly progressive, 5\u0026ndash;\u0026lt;25 \u0026micro;m/s), grade c (non-progressive, \u0026gt;\u0026thinsp;0\u0026ndash;\u0026lt;5 \u0026micro;m/s), and grade d (immotile). All equipment, procedures and CASA set-up were standardised across all sites.\u003c/p\u003e\u003c/div\u003e\n\u003ch3\u003eDatasets\u003c/h3\u003e\n\u003cp\u003eThe dataset for each ejaculate included the collection date and city, the man\u0026rsquo;s age, ejaculate volume (mL), sperm concentration (10⁶/mL), total sperm count (millions per mL), total motile sperm count (TMSC), and concentrations of \u0026lsquo;grade a\u0026rsquo; and \u0026lsquo;grade b\u0026rsquo; sperm (millions of spermatozoa per mL). Following WHO (2021) definitions, motile sperm (grade a and grade b) were combined to determine TMSC (concentration of motile sperm in an ejaculate) and motile sperm concentration (per mL of semen). Monthly mean temperatures (averaged from daily highs, means, and lows; Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) were obtained from the Danish Meteorological Institute (Copenhagen, Denmark) and Climate Data Online (National Centers for Environmental Information) and used to account for potential ambient temperature effects on sperm quality, as reported in previous studies (\u003cspan additionalcitationids=\"CR13\" citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e). Although Danish law exempts secondary analysis of anonymised data from ethical approval requirements, ethical clearance was secured from The University of Manchester (ref: 2023-18428-31578).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\n\u003ch3\u003eStatistical Analyses\u003c/h3\u003e\n\u003cp\u003eWe used R version 4.5.13 for all analyses, with the function \u003cem\u003egam\u003c/em\u003e from the added package \u003cem\u003emgcv\u003c/em\u003e (v. 1.9-1) for Generalized Additive Models (GAMs) (\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e). These models were used to predict ejaculate volume, total sperm count, sperm concentration, and sperm motility metrics (motile sperm concentrations and TMSC), accounting for and testing the effects of the man\u0026rsquo;s age, monthly mean temperature, city of donation, and semen production date as predictor variables. In GAMs, we included time (year + (month-0.5/12) and temperature as smoothed effects (at the default parameters), and month (with smoothing parameters bs\u0026thinsp;=\u0026thinsp;cc and k\u0026thinsp;=\u0026thinsp;12) in each model. In each model, we used the Gamma family with the log-link function to help normalise residuals and satisfy other statistical assumptions (see Supplementary Material Figure \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e). As the P-values associated GAMs are considered to be approximations, we have avoided using the term \u0026lsquo;significant\u0026rsquo; and instead refer to the strength of evidence (see also as \u0026lsquo;weak\u0026rsquo; (0.05\u0026thinsp;\u0026lt;\u0026thinsp;P\u0026thinsp;\u0026lt;\u0026thinsp;0.10), \u0026lsquo;moderate\u0026rsquo; (0.01\u0026thinsp;\u0026lt;\u0026thinsp;P\u0026thinsp;\u0026lt;\u0026thinsp;0.05), \u0026lsquo;strong\u0026rsquo; (0.001\u0026thinsp;\u0026lt;\u0026thinsp;P\u0026thinsp;\u0026lt;\u0026thinsp;0.01), or \u0026lsquo;very strong\u0026rsquo; (P\u0026thinsp;\u0026lt;\u0026thinsp;0.001), with P\u0026thinsp;\u0026gt;\u0026thinsp;0.10 indicating no evidence, given the data).\u003c/p\u003e\u003cp\u003eFor this study we used GAMs (Supplementary Material Figure \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e) rather than the more traditional Generalized Linear Models (GLMs) to model the seasonal variation in semen parameters. GAMs provide several advantages for the analysis of annual and seasonal trends: (i) they can model nonlinear trends; (ii) they provide a more flexible framework when modelling complex relationships that are not constrained by strict assumptions about the distributions of residuals; (iii) they allow us to incorporate different smoothing functions for each continuous predictor; and (iv) like GLMs, they allow us to evaluate the contribution of each predictor to the response variables (e.g., semen parameters) while accounting for variation in the other predictors. Thus, unlike previous studies of seasonal variation in semen parameters, we do not assume that the effects of predictors are linear and we are able to accurately model and display the month-to-month seasonal variation in those parameters. We present the results of GAM analyses here as predicted values from the models.\u003c/p\u003e\u003cp\u003eBecause the low, mean, and high monthly temperature data were highly correlated (r\u0026thinsp;\u0026gt;\u0026thinsp;0.85 in each case), we included only mean monthly temperature in each model (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Exploratory analyses showed that it was rare for any one of those temperature variables to result in a statistically better-fitting model (based on the Akaike Information Criteria, AIC) given the data. In addition to the contemporaneous monthly temperature, we also modelled the mean daily temperature two months prior as spermatogenesis in humans is reported to take 74 days (\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e).\u003c/p\u003e"},{"header":"Results","content":"\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\u003ch2\u003eStudy Population\u003c/h2\u003e\u003cp\u003eThe study included semen quality data from 15,581 men attending the Cryos International sperm banks in Denmark and Florida. A total of 10,670 men were resident in Aarhus, Aalborg, Copenhagen, and Odense in Denmark, and 4,911 men from Orlando in Florida (Table\u0026nbsp;1). All participants were 18\u0026ndash;45 years old.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\u003ch2\u003eEjaculate Volume and Sperm Concentration\u003c/h2\u003e\u003cp\u003eThere was no evidence that either ejaculate volume or sperm concentration varied across months in either Denmark (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e) or Orlando (Supplementary Material Figure S2B,F), suggesting that these parameters remained relatively stable throughout the year. There is, however, very strong evidence from both populations that both parameters varied across the years of study and the men\u0026rsquo;s ages (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). There was little evidence that either parameter varied in relation to mean monthly temperatures in the contemporaneous months, except for a negative effect on ejaculate volume in Orlando (Supplementary Material Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\n\u003ch3\u003eRapidly Progressive Sperm (grade a)\u003c/h3\u003e\n\u003cp\u003eIn both Denmark and Orlando, there is very strong evidence that \u0026lsquo;grade a\u0026rsquo; sperm concentration varied seasonally (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, Table\u0026nbsp;2), with the predicted means being lowest in early winter (November-February) and highest in early summer (May-July). In both countries, there is very strong (Denmark) to moderate (Orlando) evidence that \u0026lsquo;grade a\u0026rsquo; sperm concentration was positively related to mean monthly temperature (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e5\u003c/span\u003e) such that the predicted highest means shifted to spring (March-May) and the lowest means to autumn (September-November; Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eB,E). The predicted patterns, controlling for contemporaneous monthly mean temperatures at the two countries, were virtually identical (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e5\u003c/span\u003eA, E).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\n\u003ch3\u003eSlowly Progressive Sperm (grade b)\u003c/h3\u003e\n\u003cp\u003eThere is moderate (Denmark) to strong (Orlando) evidence that \u0026lsquo;grade b\u0026rsquo; sperm concentration also varied seasonally (Table\u0026nbsp;2, Supplementary Material Figure S3). In Denmark, the concentration of \u0026lsquo;grade b\u0026rsquo; sperm was highest in spring and fall and lowest in summer, whereas in Orlando \u0026lsquo;grade b\u0026rsquo; sperm concentrations were highest in summer and lowest in winter (Table\u0026nbsp;2; Supplementary Material Figure S3). There is also weak evidence that \u0026lsquo;grade b\u0026rsquo; sperm concentrations in both countries varied seasonally even when controlling for the non-significant effects of mean temperatures in the contemporary months (Supplementary Material Figure S4).\u003c/p\u003e\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\u003ch2\u003eNon-Progressive Sperm Concentration (grades c\u0026thinsp;+\u0026thinsp;d)\u003c/h2\u003e\u003cp\u003eThere is very strong (Denmark) to moderate (Orlando) evidence that the concentration of non-progressive sperm (grades c\u0026thinsp;+\u0026thinsp;d) also varied seasonally (Table\u0026nbsp;2), with the highest values in winter (November-December) and lowest in summer (May-August). Controlling for contemporaneous monthly temperature, there was still moderate evidence for seasonal variation in the concentrations of \u0026lsquo;grades c\u0026thinsp;+\u0026thinsp;d sperm at both countries (Table\u0026nbsp;3).\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec12\" class=\"Section2\"\u003e\u003ch2\u003eMotile (grades a\u0026thinsp;+\u0026thinsp;b) Sperm Concentration\u003c/h2\u003e\u003cp\u003eIn Orlando, there was strong evidence for seasonal variation in the concentration of progressive sperm (grades a\u0026thinsp;+\u0026thinsp;b) with the highest concentrations in early summer (May-July) and the lowest in winter (November-January); there was no evidence to support a seasonal pattern in Denmark (Table\u0026nbsp;2, Supplementary Material Figure S5). Controlling for temperature in the contemporaneous month provided only weak evidence for a seasonal patten in Orlando (Supplementary Material Figure S6).\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec13\" class=\"Section2\"\u003e\u003ch2\u003eTotal Motile Sperm Count (TMSC)\u003c/h2\u003e\u003cp\u003eThere is weak (Denmark) to moderate (Orlando) evidence that Total Motile Sperm Count (TMSC, grades a\u0026thinsp;+\u0026thinsp;b sperm) varied seasonally (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e4\u003c/span\u003e, Table\u0026nbsp;2), with high and low values in the same months (May-June and October-November, respectively) in both countries. Controlling for contemporaneous monthly temperature, there was still moderate evidence for seasonal variation in Orlando but not in Denmark (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e5\u003c/span\u003e).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eIn both countries, there was very strong evidence for an effect of date (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e4\u003c/span\u003eC,G) and the man\u0026rsquo;s age (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e4\u003c/span\u003eD,H) on TMSC. The effect of age is very similar in both countries being highest between ages 25 and 35\u0026ndash;40. The effect of date, however, is remarkably different between the two countries with TMSC in Denmark declining from 2019\u0026ndash;2022 (as also reported in Lassen et al. 2024) and in Orlando increasing from 2018\u0026ndash;2024. TMSC was positively related to contemporaneous monthly mean temperatures but the relationship for Orlando was not significant. Controlling for that temperature there was still a seasonal pattern in Orlando (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e5\u003c/span\u003eG) but not Denmark (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e5\u003c/span\u003eC).\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec14\" class=\"Section2\"\u003e\u003ch2\u003eEffects of Temperatures During Spermatogenesis\u003c/h2\u003e\u003cp\u003eTo examine the relationship between ambient temperatures during spermatogenesis, we also modelled the effects of daily mean temperatures two months previously on sperm parameters (Table\u0026nbsp;3). For the concentrations of \u0026lsquo;grade a\u0026rsquo;, \u0026lsquo;grade b\u0026rsquo;, and \u0026lsquo;grades a\u0026thinsp;+\u0026thinsp;b\u0026rsquo; sperm there was evidence for a seasonal pattern in both countries when controlling for temperatures two months previously (Supplementary Material Figure S7a-c), although there was little evidence for an effect of those temperatures (Table\u0026nbsp;3). There was also strong evidence for a seasonal pattern in TMSC in both countries when controlling for those temperatures (Supplementary Material Figure S8), with the highest values for TMSC remaining in summer and the lowest in winter, even though there was no evidence for the effects of those temperatures (Table\u0026nbsp;3).\u003c/p\u003e\u003c/div\u003e"},{"header":"Discussion","content":"\u003cp\u003eOur analyses of semen quality in a large number of men between 2018 and 2024 in Florida and Denmark show clear seasonal patterns, as measured by the concentrations (Table\u0026nbsp;2) and number of progressively motile sperm (TMSC) in ejaculates (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eB,F, \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e4\u003c/span\u003eB,F, \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e5\u003c/span\u003eA,E; Table\u0026nbsp;2; Supplementary Material Figure S3). The seasonal pattern in the concentration of the most progressively motile (grade a) sperm is supported by the strongest evidence (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eB,F, Table\u0026nbsp;2), and is almost identical in both Denmark and Florida, being highest during the summer months and lowest in winter (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eB,F, \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e5\u003c/span\u003eA,E). The similarities between these male populations persist in spite of large climatic differences (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) and likely differences in ethnicities, lifestyles, and exposure to environmental contaminants that might influence sperm quality. These populations also differed in the year-to-year patterns in sperm quality (e.g., Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eC,G and \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e4\u003c/span\u003eC,G; Supplementary Material Figures S3-S8), as well as in the effects of environmental temperatures on sperm quality (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e5\u003c/span\u003e; Table\u0026nbsp;3; Supplementary Material Figures S6-S8).\u003c/p\u003e\u003cp\u003eBy constructing Generalized Additive Models, we have been able to accurately visualise the month-to-month seasonal patterns in semen parameters (Figs.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eB,F, 3BF, 4B,F, 5A,C,E,G) while accounting for the nonlinear effects of other variables (man\u0026rsquo;s age, monthly mean temperature, year) on those parameters. This has the advantage of allowing meaningful comparisons between populations in two different climatic environments, revealing that those seasonal patterns, in Denmark and Florida, are very similar. In addition, these statistical models have also revealed similar nonlinear variation in the effects of male age (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eD,H, \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e4\u003c/span\u003eD,H) and temperature (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e5\u003c/span\u003eB,D,F,H) on semen parameters in these two populations, but very different year-to-year changes (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eC,G, \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e4\u003c/span\u003eC,G) in those parameters. The pattern of year-to-year changes in sperm concentrations (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eC, Supplementary Material Figure S3) and TMSC (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e4\u003c/span\u003eC) corroborates our previous finding of declines in those parameters in Denmark from 2017\u0026ndash;2023 (\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e). The clear differences in those year-to-year patterns in Florida deserve further study.\u003c/p\u003e\u003cp\u003eAs there was no evidence for seasonal variation in ejaculate volume or sperm concentration in either country (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eB,F; Supplementary Material Figure S2), the rate of spermatogenesis does not appear to have varied seasonally. However, the quality of spermatozoa produced did (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eB,F and \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e4\u003c/span\u003eB,F; Table\u0026nbsp;2). Whilst there were strong seasonal patterns in the concentrations of \u0026lsquo;grade a\u0026rsquo; and \u0026lsquo;grade b\u0026rsquo; sperm, the concentration and total number of progressively motile sperm (grades a\u0026thinsp;+\u0026thinsp;b) showed less pronounced seasonal patterns (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e4\u003c/span\u003eB,F, Table\u0026nbsp;2). This suggests seasonal variation in the relative numbers of sperm of different motilities, but further work will be needed to investigate the nature of that apparent trade-off.\u003c/p\u003e\u003cp\u003eAs in the present study, a large-scale analysis of 21,715 semen samples from a southern province of China reported peak progressive motility in February and March, with a subsequent decline in the summer (\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e). Similar seasonal trends have been found in other studies, where sperm motility was highest from December to April and decreased in the summer months (\u003cspan additionalcitationids=\"CR20 CR21\" citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e). In contrast, a retrospective study conducted in Italy reported that progressive sperm motility was highest during the summer, aligning with our results (\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e). Other studies support this finding which suggests that regional and environmental factors, such as differences in temperature, humidity, or lifestyle behaviors may influence how sperm motility varies seasonally (\u003cspan additionalcitationids=\"CR25\" citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eIn our study, there was relatively little evidence for a relationship between sperm quality and temperatures in either the contemporaneous month or two months previously (Table\u0026nbsp;3). For temperatures in the contemporaneous month, there was evidence only for an effect on grade a sperm concentration (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e5\u003c/span\u003e, Table\u0026nbsp;3) in Denmark (strong) and Orlando (moderate), and moderate evidence for an effect on TMSC in Denmark (Table\u0026nbsp;3). As spermatogenesis is reported to take 74 days, some effect of ambient temperatures on sperm quality during spermatogenesis might be expected (\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e). However, there was only weak to moderate evidence for an effect of temperature two months prior to ejaculation and the concentrations motile sperm (grades a\u0026thinsp;+\u0026thinsp;b) in both countries, \u0026lsquo;grade a\u0026rsquo; sperm in Denmark, and \u0026lsquo;grade b\u0026rsquo; sperm in Orlando Table\u0026nbsp;3).\u003c/p\u003e\u003cp\u003eWhile those statistical effects of outdoor mean temperatures in our study are intriguing, we hesitate to suggest a causal relationship for two reasons. First, it has been well established that climate, and particularly environmental temperatures, influence many human activities (\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e). Thus, it might be lifestyle that is associated with outdoor temperatures and not the temperatures themselves that influence the concentration of progressively motile sperm. Second, the temperature variables that we tested are simple monthly mean values rather than more fine-grained weather measures, like daily wind chill and precipitation and the variability of all variables, that could also influence both spermatogenesis and activities. The potential effects of temperature on sperm quality deserve further study as our measures of ambient temperature in this study were rather coarse grained as we did not know what temperatures each man experienced when samples were provided nor during the entire period of spermatogenesis. Nor did we have any information on the lifestyles and activities of individual men that might have been able to provide even stronger evidence for their effects on quality sperm production. The seasonal pattern in \u0026lsquo;grade a\u0026rsquo; sperm concentration, for example, remained when we accounted for temperature either in the contemporaneous month (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e5\u003c/span\u003eA,E) or two months earlier (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e5\u003c/span\u003e; Supplementary Material Figures S7, S8), suggesting either that something other than temperature also affected the seasonal variation in sperm quality, or our measures of temperature were not fine-grained enough.\u003c/p\u003e\u003cp\u003eTo the best of our knowledge, only two recent studies have studied a large and longitudinal dataset to examine both seasonal variation and the effects of temperature on semen quality - both from sperm banks in China (\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e). Both studies support the idea that temperature plays a key role. In a comprehensive recent study of 41,689 semen samples from 10,802 men from Wuhan in central China found significant effects of ambient outdoor temperatures on semen quality while accounting for a measure of ambient pollution level as well as the subject\u0026rsquo;s age, body mass index (BMI), smoking and alcohol use, education level, and recent sexual activity (\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e). All measures of semen quality except ejaculate volume decreased markedly as daily mean temperature, averaged over the 90 days prior to semen collection, increased from 10\u0026deg;C to 30\u0026deg;C. Measures of sperm motility tended not to decline at ambient temperatures above 20\u0026deg;C, suggesting a threshold effect. Importantly, some semen parameters (total motile sperm number and total sperm number) were correlated with temperatures 70\u0026ndash;90 days before ejaculation, at the start of spermatogenesis. While the sample sizes and mean age of the men (26.8 years) in that study were similar to ours (mean age\u0026thinsp;=\u0026thinsp;26.0 in Denmark, 27.7 in Orlando), the climate in Guandong province is different, being generally warm and humid with short, relatively dry winters and summers that are long, hot, and wet (\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eA more recent study of 44,564 semen samples from 11,050 men from Guandong province in southern China, also found significant effects of outdoor temperatures on semen quality, while accounting for the same additional predictors in their models (\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e). In contrast to the findings from the Wuhan study, all measures of semen quality except ejaculate volume decreased markedly as daily mean temperature, averaged over the 90 days prior to semen collection, increased from 10\u0026deg;C to 30\u0026deg;C. Again, measures of sperm motility tended not to decline at ambient temperatures above 20\u0026deg;C, supporting the idea of a threshold effect. Climate might be partly responsible for the differences from the Wuhan study as Guandong province is further south, where the climate is different, as described above (\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eIn addition to the clear seasonal patterns, there was evidence for variation in sperm quality across years and the men\u0026rsquo;s ages in both countries. In Denmark, sperm quality declined precipitously from 2019\u0026ndash;2022 as reported in Lassen et al. (2024) using the same dataset but a less comprehensive analysis (\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e). That decline appears to have stopped by 2023 and appears to be starting to increase in 2024 (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eC, \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e4\u003c/span\u003eC), as expected if the decline was due to changes in lifestyle or health during the COVID pandemic that began in late 2019. In Orlando, overall sperm quality (as measured by TMSC) increased gradually from 2018 to 2024 (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e4\u003c/span\u003eG). We have no explanation for this strongly supported trend, and it deserves further study.\u003c/p\u003e\u003cp\u003eIn both countries there is very strong evidence for sperm quality changing with the age of the men (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eD,H and \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e4\u003c/span\u003eD,H). Sperm quality was lowest at both ends of the age spectrum (\u0026lt;\u0026thinsp;25 and \u0026gt;\u0026thinsp;40 years) and peaked during their thirties. Indeed, there is enough uncertainty in the predicted values to suggest that men between ages 25 and 40 have little variation in sperm quality, another pattern that deserves further study. Lower sperm quality before the age of 25 (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eD,H, \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e4\u003c/span\u003eD,H) could be explained by increased sexual activity / reduced abstinence duration which we did not quantify, whereas lower quality after age 40 is expected as a normal result of aging (\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e, \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eOne striking feature of the samples that we analysed is the large amount of variation in sperm parameters. Within the 315 samples from Copenhagen in March 2024, for example, the mean concentration of \u0026lsquo;grade a\u0026rsquo; sperm was 6.93 millions/mL with the range from 0\u0026ndash;72 millions/mL and a standard deviation of 9.84. Thus, the coefficient of variation (CV) for this sample was 142%. To put this in perspective, the CV for most phenotypic traits under natural selection is usually\u0026thinsp;\u0026lt;\u0026thinsp;10% for traits under natural selection and 10\u0026ndash;20% for those under sexual selection (\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e, \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e). The reasons for this extreme variation are unknown but are undoubtedly due to genetic variation, lifestyle (diet, exercise, smoking and alcohol consumption), recent sexual activity and men\u0026rsquo;s ages. Despite the extreme variation in the traits that we measured we were able to find evidence for a large amount of seasonal variation in mean traits in part by accounting for variation in age, year and month, but also because we had large sample sizes over several years.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eThe results of this study indicate a clear seasonal pattern in sperm quality that is almost identical in both Denmark, where the climate is temperate with cold winters and warm summers, and Florida, where the climate is warm-to-hot year round (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). We also found some evidence for relationships between sperm quality parameters and average monthly temperatures both in the contemporaneous month and two months previously when spermatogenesis began, particularly in the concentration of the most progressively motile (grade a) sperm. Most importantly, however, when controlling for temperature, the seasonal patterns persisted, suggesting that other factors may contribute to these seasonal trends.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors acknowledge the contributions of all laboratory technicians of Cryos International sperm bank for pertinent semen analysis, donor candidate assessment, cryopreservation, and registration of all semen samples.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthors\u0026rsquo; contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll authors took part in conceptualising the study and editing the final manuscript. EL and RM drafted the paper, and RM undertook the data analysis.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe study received no external funding.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe data used for all analyses in this study are publicly available at (Lassen et al., 2025). Supplementary Material is available at \u003cem\u003eReproductive Biology and Endocrinology\u0026nbsp;\u003c/em\u003eonline.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor details\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003csup\u003e1\u003c/sup\u003eCryos International - Denmark, Vesterbro Torv 3, 8000 Aarhus, Denmark.\u003c/p\u003e\n\u003cp\u003e\u003csup\u003e2\u003c/sup\u003eSchool of Medical Sciences, Faculty of Biology, Medicine and Health, Core Technology Facility, 46 Grafton Street, University of Manchester, Manchester, M13 9NT, United Kingdom.\u003c/p\u003e\n\u003cp\u003e\u003csup\u003e3\u003c/sup\u003eDepartment of Biology, Queen\u0026rsquo;s University, Kingston, ON K7L 3N6, Canada.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthics approval and consent to participate\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent for publication\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eE.L. and A.-B.S. are employees of Cryos International. AAP is also a member of the Cryos External Scientific Advisory Committee, he also undertakes consultancy for Carrot Fertility, and in the last two years has delivered educational lectures for IBSA Institut Biochemique SA, and Mealis Group but all monies were paid to the University of Manchester. He is also the co-chair of the UKNEQAS Reproductive Sciences Advisory Committee, is a member of the Advisory Boards for the Progress Educational Trust (Charity Number 1139856) and the Science Media Centre (Charity Number 1140827) and Patron of the Fertility Alliance (Charity Number 1206323 (all unpaid). He is a member of the Guidelines Development Groups for the National Institute for Health and Care Excellence, and the World Health Organisation (again all unpaid). None of the authors were directly involved in the collection or physical analysis of semen samples.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eHohm I, Wormley AS, Schaller M, Varnum MEW. Homo temporus: Seasonal Cycles as a Fundamental Source of Variation in Human Psychology. Perspect Psychol Sci. 2024;19(1):151\u0026ndash;72. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1177/17456916231178695\u003c/span\u003e\u003cspan address=\"10.1177/17456916231178695\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eCentola GM, Eberly S. Seasonal variations and age-related changes in human sperm count, motility, motion parameters, morphology, and white blood cell concentration. 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Elife. 2020;9. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.7554/eLife.63170\u003c/span\u003e\u003cspan address=\"10.7554/eLife.63170\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"},{"header":"Tables","content":"\u003cp\u003eTables 1 to 3 are available in the Supplementary Files section.\u003c/p\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"reproductive-biology-and-endocrinology","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"rbej","sideBox":"Learn more about [Reproductive Biology and Endocrinology](http://rbej.biomedcentral.com)","snPcode":"12958","submissionUrl":"https://submission.nature.com/new-submission/12958/3","title":"Reproductive Biology and Endocrinology","twitterHandle":"@BioMedCentral","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"BMC/SO AJ","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"Semen quality, Sperm motility, Total Motile Sperm Count, Seasons, Temperature","lastPublishedDoi":"10.21203/rs.3.rs-7971305/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7971305/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003e\u003cstrong\u003eBackground\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eSpermatogenesis is a temperature-sensitive process, but previous studies on seasonal variation in semen quality have produced conflicting results, often due to differences in sample size, methodology, or local climate. Most studies have reported peak sperm motility in winter months, although findings vary by country. This study investigated whether the quality of semen varies seasonally in Denmark and Florida, and whether ambient temperature contributes to any seasonal trends.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDesign\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis retrospective observational study included data from 15,125 candidate sperm donors collected between 2018 and 2024. The cohort comprised 10,670 men from four Danish cities (Aarhus, Aalborg, Odense, and Copenhagen) and 4,911 men from Orlando, Florida. Participants were aged 18–45 years and resided near the collection sites. All ejaculates were analysed within one hour of collection using the same computer-assisted semen analysis (CASA) system and standardised temperature-controlled protocols. Semen parameters assessed included ejaculate volume, sperm concentration, concentrations of progressively motile sperm (‘grade a’ and ‘grade b’), and total motile sperm count (TMSC). The effects of monthly mean outdoor temperature during the month of collection and two months earlier (representing early spermatogenesis) on sperm parameters were also modelled. We used nonlinear statistical methods (GAMs) to accurately display the month-to-month seasonal variation, while accounting for the nonlinear effects of male age, monthly mean temperature, and year of study on semen parameters.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eResults\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eStrong and consistent seasonal variation was observed in the concentration of progressively motile (‘grade a’ and ‘grade b’) sperm in both Denmark and Florida. The concentration of ‘grade a’ sperm was highest from May to July and lowest between October and March in both countries. Despite climatic and demographic differences, seasonal trends were remarkably similar. No evidence of seasonal variation was found in ejaculate volume or total sperm concentration, indicating that the rate of spermatogenesis did not vary seasonally. TMSC also varied seasonally, even after controlling for average daily temperature in the contemporaneous and preceding months, suggesting that other seasonal factors, possibly related to lifestyle or other environmental factors, may affect sperm motility.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConclusion\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eSemen quality, particularly sperm motility, exhibits clear and consistent seasonal variation in both temperate and subtropical climates. These findings highlight the importance of accounting for seasonality when assessing semen quality for fertility evaluation and indicate that seasonal variation persists even in warm climates such as Florida.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTrial registration number: \u003c/strong\u003eNot applicable.\u003c/p\u003e","manuscriptTitle":"Seasonal Trends in Sperm Quality in Denmark and Florida","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-11-13 08:16:25","doi":"10.21203/rs.3.rs-7971305/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"editorInvitedReview","content":"","date":"2025-11-10T13:47:17+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"316399005390445348978616894139319927316","date":"2025-11-05T11:15:42+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"224964235089692839048644064041502624244","date":"2025-11-04T21:57:22+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"216557525250689612644670931570036390652","date":"2025-11-03T12:30:28+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"155507803847612334382784603137456413589","date":"2025-11-03T10:23:14+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-11-03T09:16:29+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-10-30T15:17:02+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-10-30T15:13:02+00:00","index":"","fulltext":""},{"type":"submitted","content":"Reproductive Biology and Endocrinology","date":"2025-10-28T10:37:13+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"reproductive-biology-and-endocrinology","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"rbej","sideBox":"Learn more about [Reproductive Biology and Endocrinology](http://rbej.biomedcentral.com)","snPcode":"12958","submissionUrl":"https://submission.nature.com/new-submission/12958/3","title":"Reproductive Biology and Endocrinology","twitterHandle":"@BioMedCentral","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"BMC/SO AJ","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"31f81fed-f00d-4a56-b9ac-ef8ca6098346","owner":[],"postedDate":"November 13th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2026-02-23T16:13:32+00:00","versionOfRecord":{"articleIdentity":"rs-7971305","link":"https://doi.org/10.1186/s12958-026-01537-w","journal":{"identity":"reproductive-biology-and-endocrinology","isVorOnly":false,"title":"Reproductive Biology and Endocrinology"},"publishedOn":"2026-02-21 15:58:41","publishedOnDateReadable":"February 21st, 2026"},"versionCreatedAt":"2025-11-13 08:16:25","video":"","vorDoi":"10.1186/s12958-026-01537-w","vorDoiUrl":"https://doi.org/10.1186/s12958-026-01537-w","workflowStages":[]},"version":"v1","identity":"rs-7971305","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7971305","identity":"rs-7971305","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}