Male residence and association patterns in Costa Rican squirrel monkeys ( Saimiri oerstedi )

A model is developed to interpret the evolution of the unusual pattern of male residence and social structure in the Costa Rican squirrel monkey ( Saimiri oerstedi ). Observations of a wild population document that males 1) maintain close spatial and social associations with other males, especially other males in their birth cohort, but not females and infants; 2) exhibit negligible within‐troop male‐male aggression, high levels of antipredator vigilance, and frequent predator deterrence; 3) cooperate in aggressive olfactory investigation of females; and 4) maintain residence in their natal troop with their male birth cohort, eventually succeeding to reproductive positions in their natal troop. Less commonly, male age‐cohorts leave the troop well past reproductive maturity and usurp the reproductive male cohort in another troop. We suggest that this divergence from the social structure typical of male squirrel monkeys in Peru (males transient between troops, weak male‐male bonds, high within‐troop male aggression, little investment in anti‐predator behavior) arose because female S. oerstedi in Costa Rica are not philopatric. In Costa Rica, long term reproductive cooperation is likely advantageous to males, because of the frequent movement of female S. oerstedi between troops; the potential costs of inbreeding are reduced. Male S. oerstedi of all ages residing in the same troop, but especially those of the same age cohort, are predicted to have a much higher degree of genetic relatedness than adult females. © 1994 Wiley‐Liss, Inc. This article is a US Government work and, as such, is in the public domain in the United States of America.