Is Fatter Sexier? Reproductive Strategies of Male Squirrel Monkeys (Saimiri sciureus)

Sexually dimorphic traits can evolve through male-male competition or female choice. Squirrel monkeys (genus Saimiri) live in large multimale, multifemale groups and are seasonal breeders with concealed ovulation. In several species of the genus, females are dominant to males. Males show weight gain preceding and during the mating season, which produces a fattened appearance in the upper arms, shoulders, and torso. Although much is known about the physiology of fattening, the evolutionary function(s) of this sexually dimorphic trait, including possible benefits and costs, remain(s) unknown. This unusual reproductive physiology of males is suggestive of sexual selection. Here I present data on wild Saimiri sciureus studied in Brazil to describe male reproductive investment in the species and to examine the hypothesis that male fattening is a product of sexual selection. I observed at least nine adult males via focal animal sampling and ad libitum observations during four mating seasons and during an additional 10 nonbreeding season months for comparison. Compared to less robust males, fatter males spent significantly more time near females and less time alone. These males also spent more time engaged in sociosexual activities and less time feeding/foraging, suggesting a trade-off between maintenance and reproductive behaviors. The 2-mo mating season accounted for 62% of all male-male agonism observed over one 12-mo period. These results are suggestive of male-male competition for females. However, males did not coerce females to mate and females often rejected males; this pattern suggests female choice. It is possible that male fattening in Saimiri is a product of both intra- and intersexual selection. Males experience intense reproductive costs related to agonism with other males, and related to time and energy invested in the pursuit of females. Although fattening may mitigate some of these costs by aiding in male-male competition and in female preference, this phenomenon is likely not without physiological costs to adult males.