Factors Affecting the Evolution and Behavioral Ecology of the Modern Bears Author(s): Ian Stirling and Andrew E. Derocher Source: Bears: Their Biology and Management, Vol. 8, A Selection of Papers from the Eighth International Conference on Bear Research and Management, Victoria, British Columbia, Canada, February 1989 (1990), pp. 189-204 Published by: International Association of Bear Research and Management Stable URL: http://www.jstor.org/stable/3872919 Accessed: 03/01/2009 19:54 Your use of the JSTOR archive indicates your acceptance of JSTOR's Terms and Conditions of Use, available at http://www.jstor.org/page/info/about/policies/terms.jsp. JSTOR's Terms and Conditions of Use provides, in part, that unless you have obtained prior permission, you may not download an entire issue of a journal or multiple copies of articles, and you may use content in the JSTOR archive only for your personal, non-commercial use. Please contact the publisher regarding any further use of this work. Publisher contact information may be obtained at http://www.jstor.org/action/showPublisher?publisherCode=iba. Each copy of any part of a JSTOR transmission must contain the same copyright notice that appears on the screen or printed page of such transmission. JSTOR is a not-for-profit organization founded in 1995 to build trusted digital archives for scholarship. We work with the scholarly community to preserve their work and the materials they rely upon, and to build a common research platform that promotes the discovery and use of these resources. For more information about JSTOR, please contact [email protected]. International Association of Bear Research and Management is collaborating with JSTOR to digitize, preserve and extend access to Bears: Their Biology and Management. http://www.jstor.org FACTORSAFFECTING THE EVOLUTION AND BEHAVIORAL ECOLOGY OF THE MODERNBEARS1 IAN STIRLING,Canadian Wildlife Service, 5320 122 St., Edmonton, Alberta, T6H 3S5, and Department of Zoology, University of Alberta, Edmonton, Alberta. T6G 2E9 ANDREW E. DEROCHER, Department of Zoology, University of Alberta, Edmonton, Alberta. T6G 2E9 Abstract: The present distribution and abundance of the ursids is but an ephemeral reflection of an evolutionary path that began with the first identifiable bear, the dawn bear (Ursavus elmensis), 20 million years ago in the early Miocene epoch. Although the dawn bear was only the size of a fox terrier, by the Pleistocene its descendents had evolved into some of the largest terrestrial carnivores the world has known. Most bear species evolved in the northern hemisphere although some dispersed and reached South America, Africa and SoutheastAsia. Each species had to cope with ecological changes that affected interspecific competition or the availability of food. Apparently the black bear was sufficiently adapted to have survived largely unchanged from what it was like a million years ago. Numerous species went extinct, leaving only the 8 still present today. Some understand- ing of the evolutionarypressures that the modem bearshave evolved throughmay help us to understandtheir behavioralecology. During the Pleistocene, bears at higher latitudes grew large and ecologically plastic while those closer to the equator remained small and became ecological specialists, as predicted by Geist's (1987) dispersal theory. Adaptations of the teeth of ancestral bear species allowed them to be both herbivores and carnivores. This allowed them to develop large size and broad ecological plasticity. Large body size enabled bears to conserve heat, capture large prey, defend carrion, travel great distances, and, as vegetation increased in the diet, to survive on qualitatively poorer food. Quantity and quality of available food and the degree of sexual dimorphism influenced the size of the home range and the evolution of social behaviorin each species. Bears show a great deal of individual variation in behavior and may exploit different subniches as a result of learned behavior. Slight differences in phenotype may also influence exploitation of subniches. Recent literature indicates that some terrestrial bear species are more active predators than previously thought and some evidence suggests a degree of scaling between the size of bears and the size of their prey. Social signalling appears to have been influenced by life in forest habitats but is not well understood. We give a preliminary interpretation of the social organization of the present day bears through the interactive framework of proximate ecological pressures, phylogenetichistory, and learning. There are likely few populations of bears anywhere in the world whose behavior has not been significantly influenced by man. This may confound our understanding of their behavior and ecology. Remaining populations of bears may not be able to adapt successfully to the combined effects of human predation, disappearing habitat, and climatic change. Int. Conf. Bear Res. and Manage. 8:189-204 The bears are a fascinating group to analyze in an and, in the northernhemisphere, the biomass and diver- evolutionarycontext because of the remarkablevariation sity of plants suitable for ungulates reached its zenith in their adaptations. The largest species are about 10 (Guthrie 1984). Savannas and steppes became wide- times heavier than the smallest. The degree of sexual spread. At higher latitudes and altitudes the climate dimorphismranges from species with none to those in amelioratedand vegetation became abundant during inter- which males are twice the size of females. Bears have glacial periods. Mammalgroups such as ungulatesdis- evolved specializedniches as carnivores,herbivores, and persed,speciated, and becamelarger (Geist 1978, 1987). myrmecophages,while retainingthe functionalability to During this period, the first identifiablebear species of be omnivores. Pandas,which normallyeat only 2 species the genus Ursus, U. minimus,appeared in the late Plio- of bamboo will still eat meat, while polar bears, which cene deposits of Europe. It probablygave rise to the specialize in hunting seals, also eat berries. The home present day members of the genus as well as several ranges of different species can range from less than 10 extinct species. km2to over 100,000 km2. Coincidentwith this largeincrease in the biomassand The present day distributionand abundanceof the diversity of prey species, there was an explosion in the ursids is but an ephemeralreflection of an evolutionary diversityof predators.From the late Pliocenethrough the path that began with the appearanceof the first identifi- Pleistocene,the bearsspeciated and evolved into some of able bear,the dawnbear, 20 million yearsago in the early the largest terrestrialcarivores known (Kurten 1968, Miocene epoch (Kurten 1976). Consequently,a brief Kurtenand Anderson1980). Duringthis period,several examinationof the evolutionaryhistory of moder bears species of bearsevolved, dispersed,and disappeared. An may help in interpretingthe available informationon importantpoint is that many more species once existed theirpresent day behavioralecology. The dawnbear was thando today. Forexample, severalspecies of the genus only about the size of a fox terrier,but even after 10 Tremarctoslived in southeasternNorth America (e.g., million years, its descendent Protursus, from the mid- the Floridacave bear,T.floridanus) and SouthAmerica. Miocene of Europe,was only modestly larger. Today only the spectacledbear (T. ornatus) survives in About 5 million years ago in the mid-Pliocene the the northernAndes. Similarly, the large short-faced worldclimate became drier (Kurten 1976, Guthrie1984) bears(Arctodus spp.) of NorthAmerica, the cave bearsof Eurasia Ursus in Invited paper (e.g., spelaeus Europe),various popula- 190 BEARS-THEIR BIOLOGY AND MANAGEMENT tions of brown bears (U. arctos) in Europe, Asia, and THE SIZE OF BEARS NorthAfrica, and the Langebaanwegbear (Agriotherium The modem bears are all medium- to large-sized africanum)of South Africa (Kurten 1966, 1967, 1968; mammals,ranging from 27 kg for the female Malayan Hendey 1977) all thrivedand subsequentlywent extinct. sun bear (U. malayanus)(Nowak and Paradiso1983) to In generalthough, the bearswere most successful where over 800 kg for an exceptionallylarge male polarbear in they originated,in northtemperate areas. primecondition (DeMaster and Stirling 1981). The latter Each species had to cope with a series of climatic representsa remarkableincrease in body size, relativeto changesthat in turnaffected interspecific competition or their small Miocene ancestor,the dawn bear,20 million alteredfood availability. A few, such as the American years ago. About 15 million years later, when Ursus black bear(U. americanus),were sufficientlyadapted to minimusfirst appeared,it weighed approximately50 kg, survive changingecological conditions and exist today, similarto the maximumweight of the male Malayansun largely unmodifiedfrom what they were like a million bear,the smallestof thepresent day ursids. Kurten(1976) yearsago. Thebrown bears and polar bears (U. maritimus) noted thatits canines were thin and sharpbut the molars of todayare morphologically similar to, thoughsmaller in had already become enlarged and better adapted for size, than their Pleistocene ancestors(Kurten 1976). feeding on vegetation, adaptationsthat together may An importantconsideration when analyzing the be- have madeit possible for the bearsto evolve a largebody. havioralecology of the presentday bears is the possible Even when the smallerspecies of bearsare