Primate Group Size and Interpreting Socioecological Models: Do Folivores Really Play by Different Rules?
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Evolutionary Anthropology 16:94–106 (2007) ARTICLES Primate Group Size and Interpreting Socioecological Models: Do Folivores Really Play by Different Rules? TAMAINI V. SNAITH AND COLIN A. CHAPMAN Because primates display such remarkable diversity, they are an ideal taxon tion and group size. We conclude by within which to examine the evolutionary significance of group living and the ec- suggesting future refinements of both ological factors responsible for variation in social organization. However, as with empirical inquiry and theoretical any social vertebrate, the ecological determinants of primate social variability models that we hope will improve are not easily identified. Interspecific variation in group size and social organiza- our ability to adequately characterize tion results from the compromises required to accommodate the associative and the competitive regime and social or- dissociative forces of many factors, including predation,1–3 conspecific harass- ganization of folivorous primates. ment and infanticide,4–6 foraging competition1,7 and cooperation,8 dominance interactions,9 reproductive strategies, and socialization.10–12 Causative explana- tions have emerged primarily through the construction of theoretical models that EXPLANATORY MODELS OF organize the observed variation in primate social organization and group size rel- PRIMATE SOCIAL ORGANIZATION ative to measurable ecological variation.1,2,13–16 Early assessments of primate soci- oecology relied primarily on correla- In the first portion of this paper, tional analyses to examine the rela- Tamaini Snaith is a Ph.D. candidate in we take a historical perspective, Biology and Anthropology at McGill Uni- tionships between ecological and versity in Montreal. Her research has reviewing the development of exist- social variation, and generally cate- dealt with the behavior, ecology, and con- ing models of primate socioecology. gorized primates according to group servation of orangutans in Borneo and moose in Nova Scotia, and she worked We pay particular attention to the size, the number of males per group, as a conservation biologist and advocate manner in which these models have or broad ecological categories based for the Alberta Wilderness Association. been applied to folivores because, Colin Chapman is a Canada Research on diet, locomotion, and habi- 1,3,18,19 Chair Professor of Anthropology and the recently, contradictions have em- tat. This work provided impor- McGill School of Environment. His current erged between empirical data and tant insights into the variation in pri- research is focused on the ecological determinants of primate behavior and longstanding assumptions about mate behavior and the ecological abundance, emphasizing novel means of food competition in folivores, which conditions associated with various promoting primate conservation. Cur- challenge conventional interpreta- behavioral traits. The research of be- rently, both authors conduct field work in Kibale National Park, Uganda. tions of their competitive regime. In havioral ecologists working on other Departments of Anthropology and Biology, the second part of the paper, we taxa8,9,20–22 provided a strong theo- McGill University, 855 Sherbrooke St. examine how traditional assump- West, Montreal, Canada, H3A 2T7. Phone: retical basis for the development of 514-398-1242, Fax: 514-398-7476, E-mail: tions have led to what has been primate-specific qualitative models [email protected] called the folivore paradox.4,17 We grounded in evolutionary theory. In Department of Anthropology and McGill 14 School of Environment, McGill University, explicitly examine the context within a seminal paper, Wrangham shifted Wildlife Conservation Society, 2300 which this paradox was proposed the focus of primate socioecology in Southern Boulevard, Bronx, NY 10460, and critically examine whether foli- USA. E-mail: [email protected] two ways. First, rather than relying vores experience food competition. on correlational analyses, he used an Throughout the paper, we focus spe- evolutionary approach to generate cifically on ecological variation and hypotheses about the adaptive signif- the consequent expressions of food icance of social organization. Sec- Key words: folivore paradox; group size; social competition. While we recognize the ond, he focused primarily on female organization; socioecology; scramble competition potential importance of predation, relationships as determined by food infanticide, and social factors, no competition. This brought primate VC 2007 Wiley-Liss, Inc. attempt is made to exhaustively socioecological theory in line with DOI 10.1002/evan.20132 Published online in Wiley InterScience review these phenomena or to dis- the basic premise that, for most spe- (www.interscience.wiley.com). cuss their effects on social organiza- cies, female behavior is affected by ARTICLES Folivore Group Size and Socioecology 95 Box 1. Types of Food Competition2,31 Between-group contest (BGC) sity. The effects of BGS are pre- take the form of aggression, dis- occurs when territories or food patches sumed to be independent of group placement, or avoidance. can be cooperatively defended by size and to have little effect on Within-group scramble (WGS) group members. This type of competi- social behavior. occurs due to the limited nature of the tion should favor larger or more aggres- Within-group contest (WGC) food supply, which must be shared sive groups that can supplant or occurs when food is distributed so among group members. Competition exclude smaller groups from feeding that it can be monopolized or increases in intensity with group size sites. defended, resulting in the develop- and smaller groups are favored. Be- Between-group scramble (BGS) ment of dominance hierarchies, dif- cause feeding opportunities are re- results from the common use of ferential access to resources, and duced equally for all group members, food resources by all groups or skewed energy gains. Behavioral WGS is unrelated to the development individuals, and increases in inten- consequences include direct con- of dominance hierarchies and does not sity with increasing population den- tests over access to food and may lead to skewed resource acquisition. ecological variables and food com- ability to defend a home range of vided conceptual clarity in subse- petition, while males are primarily appropriate size (in territorial spe- quent models. The type and intensity affected by mating competition and cies) will ultimately constrain group of food competition have important the distribution of receptive fe- size. Non-female bonded species implications for group size, social males.8,20,23 Female food competi- were classified into two categories. behavior, dominance relationships, tion, as the ultimate evolutionary One category includes species that and dispersal patterns. Depending on force influencing primate social orga- rely almost exclusively on high-qual- the distribution and abundance of nization, has provided the funda- ity, patchy resources and display food resources, individuals in groups mental starting point for all subse- short-term variation in group size so will experience either contest or quent models. that individuals can respond to fluc- scramble competition, or both. When Wrangham14 proposed an ecologi- tuations in food availability and individuals or groups can exclude cal mechanism for the formation of competition intensity, as do chim- others from resources, contest com- female-bonded primate groups; spe- panzees (Pan troglodytes) and spider petition results in differential access cifically, females will live in groups monkeys (Ateles spp.). The second to food and ultimately leads to when the benefits of cooperative category includes those whose pre- skewed fitness. Scramble competition resource defense outweigh the costs ferred foods occur in low-quality uni- occurs because all individuals must of within-group feeding competition. form patches with a large number of forage from the same limited Females will form bonds with their feeding sites (i.e., leaves, particularly resource base and results in equally relatives to cooperatively defend mature leaves), and thus live in co- reduced feeding opportunities for all access to food resources. Large hesive groups with little or no feed- individuals. Because the cost of groups will out-compete smaller ing competition. Many non-female scramble competition intensifies as groups and obtain greater fitness by bonded folivores were placed in this groups grow larger, within-group excluding neighboring groups from category. scramble competition is expected to food sources. This requires that By highlighting the social conse- impose a limit on group size, while high-quality food is distributed in quences of food competition, Wrang- contest competition between groups discrete, defensible patches, and that ham14 refocused the attention of pri- may favor larger groups.14,31 fallback foods (those eaten when matologists. Empirical work has sup- Various authors built on Wrang- preferred high-quality foods are un- ported the importance of between- ham’s work to make predictions about available), occur in large, uniform group competition, among other fac- the outcome of competition on group patches that minimize within-group tors, in determining group size and size and social relationships.2,15,16,32 competition. When feeding sites social organization among prima- van Schaik2 modified Wrangham’s within food patches are limited or tes,24–26 birds,22 carnivores,27 and model, suggesting