Why fight? Selective forces favoring between-group aggression in a variably pair-living primate, the white-faced saki (Pithecia pithecia)

Between-group aggression is a common feature of many group-living animals. Yet aggressive behaviors are often costly, involving risk of injury, increased energy expenditure and the potential to reduce feeding time. For aggression to be evolutionarily advantageous these costs must be outweighed by the benefits gained from exclusive access to resources and/or mates, or through committing infanticide. However, the dynamics favoring aggression in species living in small groups may differ from those in larger groups since mating exclusivity is higher and the potential to numerically dominate opponents is lower. We examined the selective factors influencing between-group aggression in a primate that lives in both pairs and small groups, the white-faced saki monkey (Pithecia pithecia). Data were collected on three free-ranging groups at Brownsberg Naturepark (Suriname) over 17 months. Intergroup encounter frequency and intensity of aggression during encounters were compared to temporal changes in diet, variation in ovarian hormones, frequency of copulations and the presence of dependent infants. Participation in between-group aggression was heavily male-biased and the presence of cycling females was a significant predictor of aggression frequency. Percentage of mesocarp in the diet also had a significant effect on the frequency and the intensity of aggression, with high mesocarp consumption corresponding to increased aggression. Presence of dependent infants did not affect between-group aggression. Our results support both the male mate defense and male resource defense hypotheses. suggesting that male reproductive interests are the principle selective pressures acting on between-group aggression in white-faced sakis. White-faced sakis' rigid conformity to traditional expectations of male intrasexual intolerance appears to be unique among primates living in small groups with variable mating systems.