Competitive regimes and female bonding in two species of

squirrel monkeys (Saimiri oerstedi and S. sciureus)

Hypotheses

• Between group competition over resources is the selective factor in determining female relationships. When resources are in discrete patches, females form alliances.

• Gregarious primates live in groups in response to predation risks. The trade off is increased within-group competition. Within-group competition determines social relationships among females.

• If resources are distributed in discrete patches, within-group competition is strong. Females benefit from dominance and alliance formation. They will be reluctant to emigrate alone.

What they looked for

Compared the size distribution of food patches, agonistic behavior associated with feeding, and the nature of between-group relations of both populations.

Saimiri sciureus

Saimiri sciureus• Female bonded species.• Dominance hierarchy with stable kin-based affiliative

alliances.• Mature and breed in natal group (philopatric).• Successional primary forest with variable larger fruit

patches.• Large feeding parties (17-18 members).• Food-based agonism observed 70 times more often.• Formed coalitions especially with bigger fruit patches

worth fighting over, except in a HUGE fruiting tree where they did not need to compete for feeding space.

Saimiri oerstedi

Saimiri oerstedi• Slightly smaller.• Less non-mating season sexual dimorphism.• No dominance hierarchy nor long term affiliative

bonds.• Disperse from natal groups.• Second growth habitat with small uniform fruit

patches.• Small feeding parties (3-4 members).• Food-based agonism almost non-existant, except for

with insects.• Never formed coalitions.

Conclusion

• Both eat similar things… plants, bugs, small lizards and frogs, and fruit. But S. sciureus ate more diverse fruits.

• They live together in groups in response to predation (by mostly raptors), not between-group competition. Between group competition unimportant.

• Differences in social relationships are based on variation of resource distribution where they live. Aggression and coalition are economic strategies.