Vigilance in wild Thomas's langurs (Presbytis thomasi): the importance of infanticide risk

This study examines vigilance as a behavioural indicator of the importance of infanticide risk by comparing the infanticide avoidance hypothesis with the predation avoidance and mate defence hypotheses for wild Thomas's langu rs (Presbytis thomasi) in Sumatra. We found that all individuals were more vigilant in situations of high predation risk, i.e. lower in the trees and in the absence of neighbours. Females were also more vigilant on the periph ery of the group. However, there were variations in vigilance levels that c ould not be accounted for by the predation avoidance hypothesis. Males with out infants showed higher levels of vigilance in areas of home range overla p than in non-overlap areas during the early phase of their tenure, strongl y suggesting mate defence. In these areas of home range overlay where Thoma s's langur groups can interact, males may attack females and infants, and s o the infanticide risk for males and females with infants is likely to be h igh in these areas. Only females with infants, but not males with infants o r females without infants, showed higher vigilance levels in overlap areas than in non-overlap areas; in addition, in over-lap areas, females with an infant were more vigilant than females without an infant, while this was no t the case in non-overlap areas. Both females and males with infants were m ore vigilant high in the trees than at medium heights in overlap areas but not elsewhere. These findings can only be explained by the infanticide avoi dance hypothesis. In contrast to predator attacks, infanticidal male attack s come from high in the canopy, and only occur in overlap areas. There was a significant sex difference in vigilance, but males were only more vigilan t than females without an infant, and not more vigilant than females with a n infant. We conclude that vigilance varied mainly in relation to the risk of predation and infanticide. Mate competition only played a role for males during the early phase of their tenure. Predation risk seems to offer the best explanation for vigilance for all individuals in the absence of infant s. Both predation risk and infanticide risk played a role for females and m ales with infants.