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Lecture 15

Lecture 15.docx

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Shawn Lehman

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ANT333 Lecture #15 – Primate Socioecology & Behavioural Ecology Social Structure Defined  “A dynamic system expressing the interactions of a number of factors within both the ecological and social milieux that influence the spatial dispersion and grouping tendencies of populations” (Crook, 1970:198).  Socioecology seeks to explain the evolution and maintenance of social groups and mating systems. Eisenberg (1972)  Focused on correlation between diet categories & group size.  Proposed variety of possible phylogenetic pathways to explain evolution of different social structures.  For example, cohesive multi-male groups could have evolved originally from solitary individuals and gone through phases as pair-bonded groups and uni-male groups with extensive contact with females.  First attempt at doing multi-species study Clutton-Brock & Harvey (1977)  Compared ecological and behavioral factors (diet, home range, body size, activity cycle, and social structure) in 100 primate species.  Congeneric species in different ecological categories showed different social structure.  Frugivores of related species live in larger groups than folivores (e.g., Lemur catta and Lemur fulvus rufus).  Other correlations demonstrated between morphology and habitat type.  Hallmark of behaviour ecology  Species in the same genera did not exhibit the same patterns  Some of the data was falsified, no longer used Problems with Early Models  Many exceptions exist that do not fit any of the models (e.g., Mandrills)  Parallel evolution of social structures of equal complexity can be seen in each primate radiation. Social structures of equal complexity can also be demonstrated from strepsirhines to Old World monkeys.  Many frugivores are found in small monogamous groups as well as large groups. Correlation ≠ Causation  Subtle differences in social structure and mating systems develop in congeneric species living in similar ecological conditions.  Thus, subtle ecological differences can impose more important constraints on social behavior than gross ecological correlates. Fission-Fusion Sociality  Independent individuals in the group will vary from time to time.  Fission – when food is scarce, group will break up  Chimpanzees, pygmy chimpanzees & Ateles. 1  Ecological explanation: species exploit large, unpredictable, & dispersed patches.  Alternative perspective: result of larger body size. Large animals no longer have to depend on group living to avoid predation and can feed in smaller sub-groups. Ecology ≠ Social Structure?  Wrangham (1980): intrasexual competition influences behavioral strategies which in turn affects social relationships within groups.  Assumption: males primarily compete for females whereas females compete for food. Ecology ≠ Social Structure?  Explains fission-fusion sociality for chimpanzees and orangutans but not the maintenance of cohesive groups of gorillas.  Other possible influences are predation, cultural influences, and phylogenetic inertia. Energy and Social Structure Revisited  Re-emphasize familiar themes: predation pressure, food distribution, and food density.  Social structure is outcome of individual meeting basic requirements by avoiding predator, obtaining food, and ultimately reproducing viable offspring.  Variation in these selection pressures can then influence the evolution of different social structures. Ecology & Social Structure Revisited  Environmental variables influence certain group characteristic & become main links between ecology & social behavior.  Extent of group cohesion & group size then determines social structure.  Example: smaller group sizes selected for if animal chooses to be cryptic to avoid predation. Smaller groups use smaller home ranges which accommodate fewer individuals. Patch size and density also influence social structure.  Increase in eyes for predators = decrease in food and mating for some Model of New Socioecology Van Schaik & van Hooff’s (1983) Hypotheses on Evolution of Social Structure 1. Primate ancestors were nocturnal and solitary 2. Evolution of group living coincided with evolution of diurnal behavior 3. Because a majority of diurnal primate live in polygynous groups, monogamy is a secondarily derived feature Benefits of Living in Large Group  More “eyes and ears” are available for predator detection.  Individuals can share more information about the location of food patches and patch quality.  Patches can be easily defended by larger groups than by individuals in small groups.  More mates to select from and assistance from relatives can be expected. 2 Costs of Living in Large Group  Greater visibility to predators.  Must travel further to find patches large enough to support more individuals.  In areas with less abundant food, subordinate animals may be expelled or forced to wait until other animals have fed. Patch Size  In some patches, fewer individuals can feed simultaneously, and small patches are more rapidly depleted than large patches.  Small, low density patches would be costly for larger groups because they would be depleted rapidly and groups would have to travel further and more frequently between patches.  Primate species relying on foods that are found in small, evenly scattered patches usually live in small groups. – may be spurious because it is measured from a point of view Terborgh and Janson (1986) Social structure comes down to three things: 1. PREDATION -- If enhanced protection from predators is principal selection advantage of group living in primates, then situations in which primates are relatively free of threat of predators should lead to reduced mean group size and/or increased variance in group size. 2. RESOURCES – Conditions that intensify or relax intragroup competition for food should decrease or increase mean group size. Moreover, increased travel costs and reduced net food intake due to within-group competition set the upper limit to viable group size. 3. REPRODUCTIVE SUCCESS – Females should enjoy maximum reproductive success in groups that are average in size for the species. - No data to back it up, based on “expert wisdom” Another Way to See Model  Variations in group size are a result of trade-offs between within-group competition for food resources and protection from predators But What About Folivores?  Some folivores live in small groups despite presumed low costs of feeding competition.  They experience significant predation risk; fitness maximised in larger groups where increased competition balanced by improved survival.  Upper limit to group size by social factors, su
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