Seasonality in squirrel monkeys (Saimiri sciureus): social facilitation by females.

The extent to which social living arrangements influenced seasonal changes in physiology and behavior was examined in adult squirrel monkeys (Saimiri sciureus). Data were collected over 20 months (encompassing two breeding seasons) from animals that were housed in three mixed-sex social configurations that varied in the number of heterosexual and isosexual social partners. For both sexes, the presence of multiple females was found to facilitate reproduction. Females housed with other females were more likely to exhibit seasonal ovarian cyclicity and tended to have higher conception rates. Social facilitation of reproduction was particularly prominent for subordinate females. The presence of same-sex companions also resulted in reduced adrenocortical output in females during the first nonbreeding season. In males, the availability of multiple females increased plasma testosterone levels, except in the presence of more dominant males. Seasonal increases in male weight coincided with increased cortisol levels and were most prominent in social groups containing multiple females. The influence of multiple females on male seasonality occurred despite the finding that male-female interactions were infrequent and, in fact, occurred less frequently when isosexual partners were available. Unexpectedly, affiliative social interactions between same-sex and opposite-sex partners occurred less frequently during the breeding season. As expected, behaviors associated with sex tended to increase during the breeding season. Very little agonism was observed during the course of the study and there was no evidence of interanimal competition for mates.