a201-11f-16-SocialityPredatorsGroups

a201-11f-16-SocialityPredatorsGroups - Introduction to...

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Introduction to Biological Anthropology: Notes 16 Primate sociality: Predators and living in groups Copyright Bruce Owen 2011 - Most haplorrine, and many strepsirrhine, primates are social: they live in groups - Why? - many, maybe most, mammals do not live in groups - although there are many, obvious exceptions - small groups like dogs and lions - herds like bison or horses - but think of cats, squirrels, raccoons, mice, otters, leopards… - the fact that many mammals do not live in groups suggests that being social must have some costs to reproductive success - more competition for food, since members of the same group are always nearby - more vulnerable to infectious disease - danger to offspring by infanticidal males - but there must be some benefits to sociality for primates. - Two main suggestions of possible benefits of sociality: - Resource defense theory of sociality : being in a group improves access to resources compared to being alone - Note: this is a theory about sociality (forming groups), which is different from the similarly-named theory about resource-defense territoriality that we looked at earlier - being in a group improves the ability of females to defend resources against other competitors (usually their own species) - which they would not be able to do as well if they were alone with their infants - females are the focus because females’ reproductive success is more strongly affected by access to food than is males’ reproductive success - this is likely to be true if: - food is relatively scarce and high in value - that is, the benefit of controlling access to it is high - food sources are in patches small enough that they can be defended - food patches are large enough to support several individuals in a group - these conditions are often true of fruits, a common part of primate diets - evidence that suggests that the resource defense theory is correct in some cases: - where ranges overlap, larger groups generally can defeat smaller groups of the same species for access to food patches - females in the larger groups tend to have higher reproductive success than those in the smaller groups of the same species - so the tendency to live in larger groups would be selected for… - evidence against the resource defense theory - true, larger groups get more access to food patches, but individuals within those groups face greater competition for food within the patches from their fellow group members
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Intro to Biological Anthro F 2011 / Owen: Predators and groups p. 2 - Charles Janson’s study of brown capuchin monkeys showed that individuals within groups varied by 37% in the calories of food they ate, while the average calorie intake of entire groups only differed by 3% - that is, there is much more competition within groups than between them - which argues against the resource defense model - there is not as neat a correlation between sociality and resource patchiness as we might hope - many highly social primates are folivores, which eat leaves that are not very patchy
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a201-11f-16-SocialityPredatorsGroups - Introduction to...

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