Sibling Age and Intragroup Aggression in Captive Saguinus midas midas

Different species of Callitrichidae show a consistent pattern in intragroup aggression. Aggression is more frequent between same-sex individuals, with males and older siblings being more aggressive. These findings have been associated with reproductive competition and dispersal events. However, the few studies dealing with intragroup aggression have not fully explored the effects of food availability on aggression or the relationship between age and aggression. I observed 3 family groups of Saguinus midas for 50 weeks and recorded all occurrences of aggression. A total of 138 agonistic events occurred at an average of 1.16 aggressive acts/hour per group, and 52.2% of aggressive acts occurred during a food shortage condition, which accounted for about 12% of all meals in the groups. During the food shortage, aggression was significantly higher, which suggests that food competition promoted an increase in aggression levels. Aggression amongst siblings accounted for 65.2% of all aggressive acts. Aggression from brothers to sisters was significantly more frequent than aggression between brothers. Aggression rates were significantly different between age classes. Young siblings were significantly more aggressive toward old ones than the reverse. Age and aggression received followed a linear relationship, while age and aggression initiated tended to follow a curvilinear relationship. Higher rates of aggression in young siblings probably were prompted by puberty and associated hormonal changes.