Exploring Bear Responses to Encounters with Humans Through an Evolutionary Lens

Exploring Bear Responses to Encounters with Humans Through an Evolutionary Lens | Authorea try { document.documentElement.classList.add('js'); } catch (e) { } var _gaq = _gaq || []; _gaq.push(['_setAccount', 'G-8VDV14Y67G']); _gaq.push(['_trackPageview']); (function() { var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true; ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js'; var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s); })(); Skip to main content Preprints Collections Wiley Open Research IET Open Research Ecological Society of Japan All Collections About About Authorea FAQs Contact Us Quick Search anywhere Search for preprint articles, keywords, etc. Search Search ADVANCED SEARCH SCROLL This is a preprint and has not been peer reviewed. Data may be preliminary. 17 April 2025 V1 Latest version Share on Exploring Bear Responses to Encounters with Humans Through an Evolutionary Lens Authors : Tom Smith 0000-0001-8384-2454 [email protected] , Thomas Sharp 0000-0001-6190-5928 , Vincenzo Penteriani , and Russell Van Horn Authors Info & Affiliations https://doi.org/10.22541/au.174490977.73718841/v1 488 views 228 downloads Contents Abstract Information & Authors Metrics & Citations View Options References Figures Tables Media Share Abstract Human encroachment into natural habitats is increasing encounters with wildlife, particularly large carnivores, leading to conflicts and sometimes attacks on humans. Understanding the correlates of these attacks is crucial for enhancing safety. This study examines how evolution has shaped the behavior of the eight extant bear species in human-bear interactions, focusing on ultimate and proximate factors. We review species-specific responses, such as distance-dependent reactions and predatory behaviors, and discuss how evolutionary histories can inform bear safety strategies. Bear behavior is influenced by genetics, cultural learning, individual experience, as well as a host of biotic and abiotic factors. Evolutionary factors like habitat type, competition, predation pressure, and escape options have helped shape species' threat responses. Bears in open habitats tend to adopt aggressive-defensive strategies, while those in forested habitats lean towards avoidance. These adaptations, essential for survival, now pose conservation challenges by increasing human-bear conflict potential. Exploring Bear Responses to Encounters with Humans Through an Evolutionary Lens Tom S. Smith 1 , Thomas Sharp 2 , Vincenzo Penteriani 3 and Russ Van Horn 4 1 Wildlife and Wildlands Conservation Program, Brigham Young University, Provo, Utah, USA. 2 Wildlife SOS-USA, Salt Lake City, Utah, USA. 3 Department of Evolutionary Ecology, National Museum of Natural Sciences (MNCN), Spanish National Research Council (CSIC), Madrid, Spain. 4 Institute for Conservation Research, San Diego Zoo Global, Escondido, CA Abstract Human encroachment into natural habitats is increasing encounters with wildlife, particularly large carnivores, leading to conflicts and sometimes attacks on humans. Understanding the correlates of these attacks is crucial for enhancing safety. This study examines how evolution has shaped the behavior of the eight extant bear species in human-bear interactions, focusing on ultimate and proximate factors. We review species-specific responses, such as distance-dependent reactions and predatory behaviors, and discuss how evolutionary histories can inform bear safety strategies. Bear behavior is influenced by genetics, cultural learning, individual experience, as well as a host of biotic and abiotic factors. Evolutionary factors like habitat type, competition, predation pressure, and escape options have helped shape species’ threat responses. Bears in open habitats tend to adopt aggressive-defensive strategies, while those in forested habitats lean towards avoidance. These adaptations, essential for survival, now pose conservation challenges by increasing human-bear conflict potential. Key Words: bear attacks, bear safety messaging, evolution, human-bear interactions, human-wildlife conflicts, phylogeny Running Title: Role of evolution in human-bear interactions Data Sharing: This article is a synthesis of information from published literature which has been provided in this manuscript. No data were collected for this article that were not already present in those manuscripts in the Literature Cited section. Introduction Evolution explains the diversity of life on Earth (Dawkins 2009). Darwin (1859) proposed that evolution requires genetic variation, differential reproduction and survival (selection), and heredity. Evolution affects both morphology and behavior, with natural selection continually honing species’ fit to their environment (Duckworth 2009, Wcislo 2021). Adaptive behaviors enhance survival and are selected for, while maladaptive ones are removed by natural selection (Emlen et al. 1991). Behaviors exhibited today are the result of both natural selection and mutation, but other forces also shape animal behavior. Genetically encoded behaviors are modified by individual and social learning (Laland and Janik 2006). Interaction with the environment optimizes fitness (Whitehead et al. 2019), affecting foraging behaviors, predator avoidance, mate selection and a host of other choices animals must make to survive. Cultural learning further modifies behaviors (Whiten 2019). Therefore, an animal’s behavioral responses are a combination of genetics, culturally transmitted information, individual learning, as well as time of year, nutritional and reproductive status, and other factors influencing individuals. An animal’s responses to humans are the result of both ultimate and proximate factors (Figure 1). In this paper we explore the way in which natural selection has shaped the eight extant bears’ responses to threats as well as predatory tendencies, both of concern to humans. While recognizing that an individual bear’s behavior at any point in time is also influenced by its nutritional status, gender, age, reproductive status and other proximate factors, we are focusing on how each species’ evolutionary history has shaped their lineage today. And each extant species of bear does have a suite of routinely observed behaviors towards humans (Herrero 2018). Each bear species has a suite of predictable reactions when threatened due to natural selection (Kurten 1968). For example, grizzlies ( Ursus arctos horribilis ) explosively charge threats, whereas American black bears ( U. americanus ) flee from them (Herrero 2018). However, individual responses vary within and between populations due to proximate forces that individual bears have been subjected to. For example, resource abundance affects brown bear tolerance towards humans (Smith et al. 2005). Food-conditioned American black bears in Yosemite National Park, California USA exhibit different activity patterns than wild bears (Ayers et al. 1986). Species-specific behaviors arise from genetic templates (Herrero 2018, Dugatkin 2020) and then are modified by experience. Additionally, individual status at the time of an encounter, such as nutritional and reproductive status, influence a bear’s response. Human-bear conflict includes various interactions (Hopkins et al. 2010, Krofel et al. 2021, Can 2021). This essay focuses on predatorial and distance-dependent defensive attacks on humans. ’Distance-dependent’ refers to far (low-intensity) and close (high-intensity) responses. For example, interior grizzly bears charge perceived threats at greater distances than coastal brown bears (Smith et al. 2005). Sloth bears’ ( Melursus ursinus ) low-intensity responses occur at distances >10 m (Sharp et al. 2024). Each species has far and close responses, which will be discussed. This essay offers a perspective on human-bear interactions by considering the evolutionary histories of bear species. It discusses: (1) how evolution shapes behavioral responses; (2) the interplay of ultimate and proximate factors; (3) species-specific responses to humans; and (4) how evolutionary histories inform bear safety and management. The Evolution of Modern Bears and Their Behavioral Changes Several authors have addressed ursid evolution, including Stirling and Kurten (1968), Herrero (1972), Stirling and Derocher (1990), McLellan and Reiner (1994), Gittleman (1999), Krause et al. (2008), Kumar et al. (2017), and Zou et al. (2022). Herrero (1972) was the first to relate bears’ behavioral adaptations to human-bear interactions, linking differences in grizzly/brown and American black bears to their habitats and evolutionary histories. The work by Zou et al. (2022) is of particular interest in that reconstructed phylogenies are based on mitochondrial DNA, allowing estimates of both lineage genesis and the timing of speciation for seven of the eight extant bear species (Figure 2). Zou et al. (2022) divided the extant bear species, sans Giant pandas ( Ailuropoda melanoleuca ), into three groups based on latitude and degree of relatedness: northern, intermediate, and southern. The northern group includes the American black bear, brown bears ( Ursus arctos ), and polar bears ( U. maritimus ). The intermediate latitude group included only the Asiatic black bear ( U. thibetanus ), and the southern group included the sloth bear, and sun bear ( Helarctos malayanus ). The Andean bear ( Tremarctos ornatus ) was used as an outgroup having separated this group of six species before formation of those discussed in their study. Additionally, the giant panda was completely excluded from analysis, having separated from all other species of bears long before formation of those discussed in their study. To complete the cladogram, we have included the Andean bear and giant panda in the southern group (Figure 2). A summary of each group’s characteristics is presented in Table 1. Modern bears evolved from small tree-climbing mammals in the family Miacidae about 60 million years ago (Kurten 1968). These ancestors had features of both dogs and bears. In the Miocene Period (~20 MYA), the first identifiable bear , Ursavus elmensis (dawn bear), emerged. About the size of a fox, the dawn bear lived in forests and gave rise to today’s bear species (Jiangzuo and Flynn 2020). This discussion covers the unique evolutionary histories of today’s bear species and their behaviors in human-bear interactions, from the oldest extant ursid (giant pandas) to the most recently evolved (polar bears, Ursus maritimus ; Figure 2). We also discuss the special case of the European brown bear ( U. arctos arctos ) which illustrates how a species’ behavior may change due to different predator pressures and environments (i.e. European brown bear vs. North American brown bear). Giant Panda The giant panda diverged from the common ancestor of all Ursidae, Ursavus elmensis , in the Miocene Era (~16-18 MYA; Kumar 2017), making it the oldest extant member of this family (Yu et al. 2007). They prefer primary and secondary forests in rugged terrain, with moderate to high bamboo densities (Hull et al. 2014). While giant pandas primarily subsist on bamboo (99% bamboo stems, shoots, and leaves), they also ingest small animals and carrion (Servheen et al. 1999), thus making them omnivorous. Before their range contracted, giant pandas coexisted with Asiatic black bears throughout large portions of their range, and they still overlap in Changqing National Nature Reserve, China (Li et al. 2023). However, Wang et al. (2015) did not observe any negative interactions between giant panda and Asiatic black bears. Giant pandas also evolved alongside large predators such as the tiger (e.g., Hu et al. 2023). How giant pandas coexisted with large predators is not known. However, a few aspects of their biology and ecology may provide clues as to how they coexisted over time. First, they use dense and extensive bamboo thickets which are difficult for tigers and other large predators to move through and hunt in. Secondly, although not considered to be expert climbers such as American black, Asiatic black and sun bears, they are more agile climbers than either sloth bears or brown bears. Third, they are known to move cubs between multiple maternal dens, which likely assists in predator avoidance. And finally, their unique coloration has been cited as an adaptation which reduces predator detection (Caro et al. 2017, Nokelainen et al. 2021). Giant pandas avoid humans but have been known to charge when threatened (Servheen et al. 1999). There have been no documented attacks, defensive or predatorial, of wild giant pandas on people. The possibility of encounters and attacks is limited by the giant panda’s currently low population levels, distribution in dense bamboo stands scattered over rugged terrain, and a minimal human presence where they persist. Andean Bear Estimates of the divergence time of the Andean bear ( Tremarctos ornatus ) from other bear species range from 4.1 MYA (Mitchell et al. 2016) to 12.9 MYA (Krause et al. 2008). The Andean bear descended from North American short-faced bears ( Arctodus spp.; Mitchell et al. 2016, Schubert et al. 2019, Soibelzon et al. 2008), but are less than one-fourth their size. The earliest members of Tremarctos evolved alongside large carnivores like the American lion ( Panthera atrox ) and saber-toothed cats ( Smilodon spp.), as well as various wolf species ( Canis spp., Tedford et al. 2009). These predators likely posed threats to juvenile and possibly adult bears. They presently coexist with pumas ( Felis concolor ) and jaguars ( Panthera onca ). Jaguars occasionally prey on Andean bears, perhaps explaining why these species rarely overlap, such as on the east slopes of the Andes (Peyton 1999, Vickowski and Van Horn 2024). Andean bears are excellent climbers and spend much of their time in trees foraging, resting and for refuge from perceived threats (Penteriani and Melletti 2021). The Andean bear depends on forest cover, occupying a range of forests from low elevation tropical to high elevation cloud forests. They also use alpine grasslands (Velez-Liendo et al. 2013, Aurich-Rodriguez et al. 2022, Rojas-VeraPinto et al. 2022). Though there is little known about Andean bear maternity dens, they use ground dens on steep slopes, possibly to avoid discovery by pumas (Van Horn et al. 2021). Andean bears are primarily herbivorous though they occasional ingest animal matter and are capable of killing cattle, which they occasionally do (Goldstein et al. 2006). Andean bears rarely make physical contact with humans (Penteriani and Melletti 2021). There are no reports of predatorial attacks, which is unexpected as they occasionally prey on animals much larger than humans, such as cattle (Velez-Liendo et al. 2021). We found only one defensive attack involving humans. This occurred when an adult woman surprised a bear in a cornfield and the bear bit the woman on the face before fleeing (Servheen and B. Peyton 1999). However, this species appears to wholly avoid humans (Servheen and Peyton 1999). Asiatic Black Bear Asiatic black bears diverged from the other bear species approximately 3.6 to 5.8 million years ago (Zou et al. 2022). They have a hybrid origin that is the result of ancestors from both the northern species of bears (i.e., polar, brown, American black) and the more southern species (i.e., sun and sloth bears; Zou et al. 2022). Asiatic black bears inhabit dense tropical forests, subtropical rainforests, and montane forests. As omnivores, they feed on fruits, seeds, honey, insects, eggs, and vertebrates such as birds, reptiles, and deer. Their expert climbing ability helps mitigate predation pressures. Over time, Asiatic black bears shared habitats with numerous large predators similar to those faced by other Asian bear species. Tigers are the most capable extant predator, and various reports verify that they prey on Asiatic black bears wherever they coexist. Brown bears have also been documented attempting to prey on Asiatic black bears while denned in trees (Seryodkin et al. 2003, Seryodkin et al. 2018). Asiatic black bears are sympatric with sun bears in Southeast Asia, brown bears in eastern Russia, giant pandas in China, and to a limited degree, sloth bears in India and Nepal (Steinmetz et al. 2020). Asiatic black bears are shy and yet easily excitable bears. They tend to run or climb trees when confronted by people but have also been known to attack defensively in response to surprise encounters or vulnerable young. Nonetheless, throughout most of their range defensive attacks are rare. However, in Kashmir, India, a clear outlier, over 2,300 Asiatic black bear attacks were reported to the local government between 2000 and 2020, with 114 (4.8%) resulting in death (Mir et al. 2023). Predatory attacks appear to be exceedingly rare but may go unreported due to being masked by defensive attacks. Yamazaki (2017) documented six attacks (4 resulting in death) from May-June 2016, in the northern part of Honshu Island, Japan that, although not witnessed, appeared to be predatory due to several of the bodies being partially consumed. It was unknown whether the same bear or multiple bears were responsible for these attacks. Asiatic black bear defensive attacks are much more common than for other arboreal bears such as the American black bear (Penteriani and Melletti 2021). Possible causes for this may include: (1) strong predation pressure from a host of competitive arboreal predators including other bears, leopards and primates, throughout large parts of their range, and (2) the constant threat of predation by large forest predators, namely the tiger (Seryodkin et al. 2018). The fact that predatorial attacks are exceedingly rare is unexpected given their diet and size. It is possible, however, that misreported defensive attacks were predatorial in nature. Sun Bear The sun bear evolved on the Asian continent, splitting from other bear species 4-6 million years ago (Kumar et al. 2017, Zou et al. 2017). They presently overlap with Asiatic black bears in Southeast Asia. Throughout their evolution, they have been sympatric with numerous large predators such as the Asiatic black bear, tigers, leopards ( Panthera pardus ; Kawanishi and Sunquist 2004, Vonkhemheng 2011, Naing et al. 2019), and pythons ( Python spp., Fridriksson 2005). Dhole ( Cuon alpinus ) are potential threats but do not actively prey on sun bears (Naing et al. 2019). Sun bears are the smallest extant bear species and are expert climbers. They mostly respond to threats by climbing (Humphrey and Bain 1990). Presently, sun bears inhabit dense tropical and subtropical rainforests, montane forests, and swamp habitats (Steinmetz et al. 2011). They feed on fruits, seeds, honey, insects, eggs, and vertebrates like birds, reptiles, and deer (Steinmetz et al. 2013). Sun bears are shy, reclusive and rarely attack humans unless provoked and when escape is not an option (Woods 1885, Servheen and Salter 1999, Penteriani et al. 2021). Predatory attacks on humans do not occur, presumably due to their small size. Sloth Bear Sloth bears evolved on the Indian subcontinent, diverging from other bears 4–6 million years ago (Yu et al. 2007, Nyakatura and Bininda-Emonds 2012, Kumar et al., 2017, Zou et al., 2022). They inhabit dry and wet tropical forests, savannahs, scrublands, and grasslands (Joshi et al. 1995, Sreekumar and Balakrishnan 2002, Akhtar et al. 2004, Yoganand et al. 2006, Seidensticker et al. 2011, Ramesh et al. 2012). They are an obligate myrmecophage, uniquely adapted to habitats with low fruit abundance but high termite density (Steinmetz et al. 2021). As such, termites and ants are dietary staples, hence their claws are adapted for digging rather than climbing (Sharp et al. 2024). They also consume fruits and occasionally scavenge meat. Sloth bears co-evolved with the same host of large predators as the Asiatic black bear and sun bears did. The sloth bear’s only extant predator is the tiger (Sharp et al. 2024). Unlike the Asiatic black bear and sun bear, sloth bears do not climb trees to escape predation, but become aggressive, standing bipedally, vocalizing, and charging to dissuade tigers. Mothers carry their young on their backs for 6-9 months, a unique behavior among bears, presumably as a hedge against predation (Joshi et al. 1999). This allows mothers to protect cubs during intense encounters with tigers, or from leopards which may opportunistically attack and kill a cub (Sharp et al. 2024). Sloth bears are responsible for more attacks on humans than any other bear species, and all of their attacks are defensive (Sharp et al. 2020, Sharp et al. 2022). There has never been a documented predatorial attack on a human, undoubtedly due to the fact they are not carnivores. However, their aggressive behavior towards people may have evolved from co-existing with large predators and being adapted for digging, not climbing. Without the ability to use trees for refuge, they became aggressive to deter predation (Sharp et al. 2024). American Black Bear The genus Ursus evolved in Eurasia, giving rise to species like Ursus minimus , a small bear that likely led to modern black and brown bears. Fossils suggest that bears related to modern black bears migrated into North America from Eurasia via the Bering Land Bridge approximately three MYA (Kurten and Anderson 1980, Miller and Kellogg 1955). Ursus americanus , the American black bear, first appeared in the North American fossil record around two million years ago. During the Pleistocene Ice Age, black bears coexisted with larger bear species, including the short-faced bear, which was much larger and more carnivorous (Matheus 1995). However, black bears proved more adaptable than short-faced bears, with a diet consisting mainly of vegetation, insects, and small mammals (Pelton 2003). Their smaller size and ability to hibernate allowed them to survive in the Wisconsin Ice Age, while the short-faced bear went extinct around 11,000 years ago (Sorkin 2006). American black bears developed traits for climbing (Herrero 1978): sharp claws, smaller body size, and reduced hind feet. Black bears select habitats with dense underbrush or trees which enable them to evade threats. Cubs remain with mothers for 1.5 years, learning to climb and escape predators. Black bears typically flee from threats rather than confront them, and their range is limited to treed areas wherever grizzlies are sympatric (Herrero 1978). Although black bears usually avoid humans, even when surprised (Herrero 2018), most black bear attacks involve only one or two people, indicating avoidance of larger groups (Herrero et al. 2011). In human fatalities, black bears acted as predators most of the time (Herrero et al. 2011). Most fatal attacks were predatory and carried out by single bears, not females with young. Brown/Grizzly Bear Brown bears evolved from the Etruscan Bear ( Ursus etruscus ) in Asia (McLellan and Reiner 1995, Pérez-Hidalgo 1992). Modern brown bears first appeared in Asia during the Middle Pleistocene Epoch (~0.5 MYA) and spread to North America, Europe, and northern Africa (Herrero 1972, McLellan and Reiner 1995). Their radiation into and across North America was possibly delayed by the short-faced bear (Geist 1987), which dominated North America at the time. However, by 13,000 YBP they spread southward (McLellan and Reiner 1995, Davison et al. 2010). Brown bears thrived on the tundra, using long, stout claws to dig for plant taproots (Herrero 1972). Their omnivorous dentition allowed them to process both meat and plants, thus facilitating large bodies (Geist 1987). Life on the treeless tundra may have led to their explosive, aggressive-defensive behavior, as they could not climb or outrun threats (Herrero 1972). Brown bears prefer meat due to its higher nutritional density, making humans potential prey, though predatory behavior is rare (Penteriani et al. 2017, Smith and Herrero 2018, Bombieri et al. 2019). Brown bears typically avoid conflict with humans (Herrero 2018). However, 40% of attacks on people are due to surprise encounters, triggering a ’neutralize the threat’ strategy (Smith and Herrero 2018). In only a few interactions (4.5%) in Alaska, brown bears were deemed predatory, pursuing people as food (Miller et al. 2024). In summary, these bears are aggressive defenders of their space, resources, and offspring. This behavior evolved with fierce competitors on the tundra and remains a concern for humans today. Polar Bear The polar bear is the most recent bear species, separating from brown bears about 1.05 MYA (Zou et al. 2022). Polar bears radiated into vast Arctic habitats around 0.1 MYA (Stirling and Derocher 1990, Geist 1978). On sea ice, they did not face interspecific, but rather intraspecific predation pressure. Hence, female polar bears aggressively defend intraspecific threats to their cubs and select habitats for foraging and denning apart from other bears (Stirling and Derocher 1990). Polar bears are obligate carnivores, depending wholly on seals for their diet (99%; Amstrup 2003). Herrero and Fleck (1990) stated that polar bears are less aggressive towards humans than brown bears, but more aggressive than black bears. They are, however, the most predatory of the three North American bear species, with 25% of polar bear-human incidents being ascribed to predation (Miller 2024). Predatory events are rare, and most polar bears tend to flee perceived threats (Wilder et al. 2017). Predatory attacks generally involve one or two people, suggesting risk aversion when outnumbered. Polar bear mothers with dependent young charge and/or attack perceived threats (Clark et al. 2012). Eurasian Brown Bear The brown bear and cave bear ( Ursus spelaeus ) diverged in Eurasia approximately 1.2–1.4 MYA (Loreille et al. 2001). Climatic fluctuations during and after the Pleistocene shaped the genetic relationships within Eurasian brown bears, especially during the Last Glacial Maximum (26,000–19,000 years ago; Saarma et al. 2007). Studies in the mid-1990s identified two main lineages, the western and eastern, which diverged approximately 0.514 MYA (Anijalg et al. 2018; de Jong et al. 2023; Tumendemberel et al. 2023). Like their North American counterparts, Eurasian brown bears are omnivores with strong carnivorous tendencies (Schwartz et al. 2003). Eurasian brown bears interact with many other carnivores, including felids and canids. They are dominant scavengers and kleptoparasites in the Holarctic region (Krofel et al. 2012). In interactions with tigers, brown bears are sometimes killed, especially subadults or females (Seryodkin et al. 2018). However, Eurasian brown bears are generally the apical predator in most areas, with intraspecific forces primarily shaping their behavior. The behavior of this bear species has been shaped by evolution in forest habitats and intense human-caused mortality (Allendorf and Hard 2009). Persecution has likely led to the disproportionate removal of bold (risk prone) individuals, resulting in more shy (risk averse) individuals in populations (Wilson et al. 1994, Smith and Blumstein 2008, Ciuti et al. 2012). Consequently, European brown bears may have become more vigilant, risk averse, and learned to avoid human contact during times of persecution (Shivik 2014). The European brown bear has inflicted human injuries and deaths but at low frequencies compared to brown bears in other parts of their range (Penteriani and Melletti 2021). Their behavior likely results from heavy selection pressure against bold individuals by people. When humans enter their personal space, however, they aggressively defend themselves, just like North American brown bears. Connecting Contemporary Bears’ Behavioral Traits with Their Evolutionary Backgrounds The behavior exhibited by a bear confronted by a human is the result of several factors, including genetics, culturally transmitted information, and individual learning (Figure 1). Each species’ evolutionary history has modified their behaviors, with relative frequencies of aggressive-defensive and predatory behaviors varying among the eight extant species (Figure 3). Considering each species’ evolutionary history, including habitat types, predation, competition, and options for escape from would-be predators provides insights into their behaviors towards humans. While intraspecific variation exists, each species has a somewhat predictable response to threats. However, the Asiatic black bear is an exception to this as they sometimes respond to sudden encounters by climbing, yet at other times charge and attack. It may be that this wide variation in responses of Asiatic black bears to threats is a function of their hybrid heritage. Expansion into Open Habitats Associated with Defensive and Aggressive Behaviors The earliest Ursids began as forest animals, but three species (i.e., brown, polar, and sloth bears), successfully expanded into more open, largely treeless habitats, or at least into habitats where the trees are too small to climb for refuge (Jiangzuo and Flynn 2020, Sharp et al. 2024). Brown bears radiated from eastern Russia into the vast tundra habitats of North America and developed an explosive aggressive-defensive response to perceived threats due to the lack of trees for predator evasion (Meachen and Samuels 2012, Stirling and Derocher 1990, Geist 1987). Brown bear cubs can climb trees to escape predators but not as often as American black bears. Unlike black bear mothers which flee from threats, brown bear mothers hold their ground, protect their cubs, and attack threats, an adaptative response to life is treeless habitats (Herrero 1972). Sloth bears left behind dense forests and spread into open habitats such as grasslands and scrub forests, areas which largely lack trees for refuge from predators such as tigers. Their slow climbing ability, a result from having developed long claws for digging, makes them unsuited for escaping predators quickly. Consequently, sloth bears have developed aggressive-defensive behavior, even more so than brown bears. This highly defensive temperament may be due, in part, to their relatively small size with respect to the large predators which confront them (Herrero 1978). Polar bears expanded from boreal tundra out onto Arctic Sea ice. Unlike brown or sloth bears, polar bears did not face interspecific predators out on pack ice, but intraspecific conflict has been ever-present as with most bear species. Their cryptic coloration not only aids in the hunting of seals but also helps them avoid predatory conspecifics. Additionally, females with cubs avoid areas frequented by other polar bears (Smith 2021). This lack of both inter and intraspecific conflict may explain why they never evolved a strong aggressive-defensive behavior as did brown and sloth bears. Thus, several species’ adaptations to open habitats have led to aggressive-defensive behaviors in those bears. Bears that do not use trees for refuge must defend themselves and their cubs against both intra and interspecific predation. This aggressive-defensive trait, which contributed to their success, is now a conservation concern as they confront humans as they would any other ambush predator. Bear predation on humans linked to diet and body size Bear predation on humans is rare (Penteriani and Melletti 2021, Bombieri et al. 2023). Whether a bear species preys on humans depends on its diet and size (Stirling and Derocher 1990). Of today’s bear species, only three (American black, brown/grizzly, and polar), and possibly a fourth (Asiatic black bears), have been documented preying on humans. The giant panda and sloth bear are not carnivorous. Sun bears, though they occasionally hunt vertebrates, are too small to prey on humans. Andean bears, though larger than humans and capable of preying on large vertebrates, have no documented predatory attacks on humans. A summary of extant bear species’ body mass (kg; Penteriani and Melletti 2021) and predatory tendencies towards humans is presented in Table 2. The Unusual Case of the Andean Bear Unlike other species of bears, the Andean bear has rarely, if ever, attacked humans for predation or defense. They are, however, both carnivorous and large enough to do so, yet they do not. The ‘land of coexistence’ theory set forth by Oriol-Cotterill et al. (2015) is an attempt to explain why some carnivores, both apex and meso-carnivores, can coexist closely with humans but not attack them. However, whether living near people or in isolated wilderness, the Andean bear avoids human confrontation. Hence, the force driving the ‘land of coexistence’ phenomenon, human persecution, fails to explain the consistency of response of Andean bears to throughout a range of human densities. The evolutionary history of the Andean bear, however, strongly suggests that their coexistence with larger, more capable, predators has been the force that has given rise to their avoidance strategy. Andean bears that responded to large predators aggressively were culled from the gene pool, leaving only those reclusive and risk averse bears to reproduce. Today, they generalize that response in their interactions towards people, hence the lack of human-bear conflict with this species. The Unusual Case of the Asiatic Black Bear Asiatic black bears are forest bears and excellent climbers, using trees as refuge against large predators. However, they are reportedly a more wary bear than their American counterpart, and thus defensive attacks are comparatively common to other bears that climb very well, including American black bears, Andean bears, sun bears (Penteriani et al. 2020). This wariness could be the result of being a relatively large bear while also overlapping with large forest predators in Asia, exemplified by the tiger. Or perhaps overlapping with a host of potentially aggressive arboreal species such as macaques. Whatever the cause, it exemplifies that even forest dwelling tree climbing bears can be a danger to humans, though usually they tend not to be. Summary of Factors Influencing Bear Behavior Towards Humans Human interaction with bears elicits species-specific responses influenced by several factors: Predatorial Behavior (Figure 4) Dietary niche : Bears’ diet influences predation on humans. Size : Bears need to be similar in size to humans or larger for predation to occur. Defensive Behavior (Figure 5) • Predator pressure : Evolutionary responses to predators shape bear behavior towards humans. • Habitats used : Bears in open spaces tend to be more aggressive, while forest-dwelling bears escape conflict by climbing trees or seeking cover. • Intraspecific competition : Competition with other bears or species influences responses to humans. • Selective removal of aggressive bears : Culling aggressive bears shifts gene frequencies towards wariness and risk avoidance. The role of each of these factors in shaping today’s extant bear species is summarized in Table 3. Using This Knowledge to Improve Human-Bear Coexistence Understanding the evolution of bear behavior towards humans can inform our efforts to minimize conflict (Penteriani et al. 2016). For instance, it is important to know which bear species react aggressively when suddenly encountered and which are capable of predatory behavior. It is reasonable to suggest that the average person walking through bear habitat does not need to understand the ecology and evolution of a particular bear to learn what they should do, or not do, to avoid encounters or survive attacks. However, for those of us who study bear attacks, understanding how a bear evolved to use its habitat and deal with potential threats is key to determining why bears react the way they do and finding reasonable ways to mitigate attacks. Understanding how humans should respond to a specific bear encounter is important, but also understanding the likely response of the bear is also important. For example, understanding that sloth bears only attack defensively allows us to tell people that falling to the ground and covering up is likely your best option. Additionally, a person fighting back during a sloth bear attack will likely cause the bear to fight back harder, since this is what it has done with predators for several million years, whether the extant tiger, or the extinct saber-toothed cat (Meganteron). Of course, one should always avoid surprising bears prone to defensive-aggressive responses and must carry deterrents such as firearms or bear spray when in the habitat of predatory bear species. Additionally, understanding a bear species’ evolutionary history may help explain why some species frequently target the head-neck region of humans when attacking, likely to neutralize a predator’s weapon, their teeth (Smith and Herrero 2018). Shielding the head and neck from bites and paw swats is key to surviving an attack. 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BMC Evolutionary Biology. 7: 198. http://www.biomedcentral.com/1471-2148/7/198 Zhao, S., P. Zheng, S. Dong, X. Zhan, Q.I. Wu, X. Guo, Y. Hu, W. He, S. Zhang, W. Fan, and L. Zhu. 2013. Whole-genome sequencing of giant pandas provides insights into demographic history and local adaptation. Nature Genetics 45: 67-71. doi:10.1038/ng.2494 Zou, T., W. Kuang, T. Yin, L. Frantz, C. Zhang, J. Liu, H. Wu, and L. Yu. 2022. Uncovering the enigmatic evolution of bears in greater depth: The hybrid origin of the Asiatic black bear. Proceedings of the National Academy of Sciences. 119(31):e2120307119. Table 1. A summary of extant bear species’ characteristics, response to predation and to human interactions. 5.91 Ma Northern High Latitude 20-75 o N (Circumpolar Region) Large Bodies Ecologically Plastic 2.02 Ma American Black Bear 95-165 kg (F to M) Mothers run, send cubs up a tree, return later for them Submissive 1.05 Ma North American Brown Bear 207-389 kg (F to M) Climbing not an option in open habitats so they face down threats (aggressive-defensive) Aggressive-defensive 1.05 Ma European Brown Bear 207-389 kg (F to M) Climbing an option but brown bears don’t; largely submissive in response to threats Submissive 1.05 Ma Polar Bear 197-322 kg (F to M) Nothing to climb so they stand down predators Aggressive-defensive 5.66 Ma Intermediate Intermediate Latitudes 6-34 o N in SE Asia Intermediate Sized Bodies 5.66 Ma to Present Asiatic Black Bear 90-150 kg (F to M) Usually climb, but sometimes charge Aggressive-defensive 5.91 Ma Southern Low Latitudes 21 o to 30 o SE Asia and S. America Smaller Bodies Ecological Specialists 4.39 Ma Sun Bear 50 kg (smallest bear; F=M) Able to climb trees to escape predators Submissive (CI = 2.47-5.53 Ma) Sloth Bear 55-145 kg (F to M) aggressive charge Aggressive-defensive Spectacled Bear 120-175kg (F to M) Able to climb trees to escape predators Submissive 16-18 Ma Southern Low to Mid Latitudes 26 – 42 o N Niche specialist Giant Panda Bear 89-107 kg (F to M) Able to climb trees to escape predators Submissive Table 2. A summary of extant bear species’ body mass (kg; Penteriani and Melletti 2021) and predatory tendencies towards humans. Ursus maritimus 800 kg (M) Carnivore (99% of diet is animal protein) Highest of the extant bears, approximately 44% in documented incidents. Ursus arctos 500 kg (M) Omnivore (both carnivory and herbivory) Lowest of North American species, 5%; n = 9 in Russia and n = 6 in North America. Ursus americanus 409 kg (M) Omnivore (both carnivory and herbivory) Moderate level of predation in North America, 12% of documented incidents. Ursus thibetanus 200 kg (M) Omnivore (both carnivory and herbivory) < 1% of documented incidents. Ursus ursinus 192 kg (M) Insectivore (insects, fruits; no meat) None documented Tremarctos ornatus 175kg (M) Herbivore (plant matter, some animal protein) None documented Ailuropoda melanoleuca Ursus malayanus 125 kg (M) 65 kg (M) Herbivore (99% bamboo) Omnivore (insects, plant matter, some animal protein) None documented. None documented. 1 Christiansen, P. 2007. Evolutionary implications of bite mechanics and feeding ecology in bears. J. Zoology 272:423-443. Table 3 . A summary of factors influencing bear behavior towards humans. Bear Species Dietary Niche 1 Bear’s Body Size 2 Predator Pressure Habitats Used 3 Predation on Humans 4 Selective Removal Giant Panda - + +/- - - - Andean Bear + + + + - - Asiatic Black Bear + + + + +/- - Sun Bear - - - - - - Sloth Bear - + + + - - American Black Bear + + + + + - Brown Bear + + + + + - Polar Bear European Brown Bear + + + + + + + + + - - + 1 Carnivorous = +; Not carnivorous = - 2 Equal to or larger than humans? (Yes = +, No = -) 3 The habitat used by a given species can have an influence on their response to other species. A plus (+) means no impact whereas a minus (-) means that habitat does have an influence on their behavior towards other species. 4 Yes = +, No = -, Possibly = +/- Figure legends Figure 1 . The process of behavioral evolution for both people and bears. Figure 2 . Phylogeny of the eight extant bear species, shown descended from the common Miacid ancestor. Figure 3 . The relative frequency of bear attack types on humans for the eight extant bear species (American black bear picture: from https://www.123rf.com/corporate/; image ID 33366834, copyright belizar). Figure 4 . Factors influencing the evolution of predatory behavior in bears (sun bear picture: from https://www.123rf.com/corporate/; image ID_24096359, copyright_ Anan Kaewkhammul). Figure 5 . Evolutionary factors influencing human-bear encounter outcomes. Figure 1 Figure 2 . Figure 3 Figure 4 Figure 5 Information & Authors Information Version history V1 Version 1 17 April 2025 Copyright This work is licensed under a Non Exclusive No Reuse License. Keywords evolutionary ecology experimental evolution selection analysis terrestrial vertebrate Authors Affiliations Tom Smith 0000-0001-8384-2454 [email protected] Brigham Young University View all articles by this author Thomas Sharp 0000-0001-6190-5928 Wildlife Institute of India View all articles by this author Vincenzo Penteriani Instituto Nacional del Carbon View all articles by this author Russell Van Horn San Diego Zoo Wildlife Alliance View all articles by this author Metrics & Citations Metrics Article Usage 488 views 228 downloads .FvxKWukQNSOunydq8rnd { width: 100px; } Citations Download citation Tom Smith, Thomas Sharp, Vincenzo Penteriani, et al. Exploring Bear Responses to Encounters with Humans Through an Evolutionary Lens. Authorea . 17 April 2025. DOI: https://doi.org/10.22541/au.174490977.73718841/v1 If you have the appropriate software installed, you can download article citation data to the citation manager of your choice. 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