MATE GUARDING AND RISK ASSESSMENT BY MALE MOUNTAIN BABOONS DURING INTER-TROOP ENCOUNTERS

Aggressive herding of females is a frequent but not invariant response by male savannah baboons, Papio cynocephalus, to encounters with othe r troops. While males in some troops are consistently more likely to h erd than those in others, not all inter-troop encounters result in her ding, even within particular troops. This suggests that males assess t he risk of male invasion posed by each encounter and respond according ly. We used data from baboon troops in the Drakensberg mountains to de termine the rules males follow in deciding whether to herd. Consistent differences between troops were explained only by the adult sex ratio . Males were more likely to herd if the sex ratio of their own troop w as female biased, a finding that is concordant with the observation th at males are more likely to immigrate into troops where the sex ratio is more female biased than the population average. Differences within troops were a consequence only of encounter distance, with herding bei ng more likely at closer distances. We found a negative correlation be tween the angle of approach to the other troop and the subsequent angl e of deflection. We interpret this to mean that herding functions to i ncrease the distance between the interacting troops, thereby curtailin g opportunities for strange males to inspect the troop and determine i ts sex ratio. In this way, possibly unlike those in other populations, the decision rules of these male baboons are geared to protecting lon ger-term reproductive prospects. (C) 1998 The Association for the Stud y of Animal Behaviour.