The effects of acute crowding on aggressive behavior of Japanese monkeys.

The effect of short-term acute crowding on aggressive behavior in a troop of Japanese monkeys (M. fuscata) was studied. The troop was housed in an 8058 sq. meter corral with free access through an open tunnel to a 187 sq. meter pen. During 3 experimental periods of 4, 6, and 5 days, the entire troop was confined in the pen, or about 2.3% their previous area. Both mild and severe forms of aggression occurred more frequently in the crowding pen. When the monkeys were returned to the corral during control sessions severe aggression disappeared and mild forms decreased to baseline levels. Several considerations led to the hypothesis that the effect on severe aggression was due to removal from a familiar habitat, rather than increased density, and that crowding per se produced the increase in mild aggression. Monkeys of both sexes displayed higher frequencies of aggression when crowded. Males, however, produced and received a greater proportion of the total attacks under crowded conditions. Low-ranking adults, subadult, and juvenile males showed the greatest increase in attacks received. In contrast, females were proportionately less aggressive and less frequently attacked when crowded. These differential effects were related to the spatial structure of wild troops. There was no breakdown in the social structure of the troop under crowded conditions. The dominance hierarchy, in fact, was more predictive of aggressive events in the pen than in the corral. These findings when contrasted to social structure breakdown in large feral troops raise the possibility of species sensitivity to increased numbers, rather than increased density, as part of a mechanism for population control.