Vigilance, predator detection and the presence of supernumerary males in vervet monkey troops

To determine whether adult males provide females with a service, in this case anti-predator behaviour, in return for inclusion in a group a free-ranging troop of vervet monkeys, Cercopithecus aethiops, was observed for 9 months. Males were more vigilant than females and the discrepancy increased as the breeding season approached. Higher levels of vigilance by males were associated with their spending a greater proportion of time either isolated or exposed at the tops of trees. Males, however, were not vigilant because they were exposed but moved into such positions in order to scan the surroundings. Despite this, males were no more efficient than females at detecting predators. The top-ranking male was the most vigilant and active against predators. It is argued from these and other data that male vigilance is directed at other males and is not a quid pro quo for inclusion. Although females benefit from this vigilance they are not capable of controlling male numbers, and troops, as a consequence, contain a number of supernumerary males who may well be the beneficiaries of the vigilance of others.