Tenure related changes in wild Thomas's Langurs. I : Between-group interactions

Between-group conflicts are believed to serve two major functions: mate defence and resource defence. In addition, in species where females can disperse, both female choice and male coercion may play a role in between-group interactions. In this paper, I evaluate the resource vs mate defence hypotheses and investigate male and female strategies in between-group interactions in wild Thomas's langurs (Presbytis thomasi). This species lends itself well to study the intertwining of male and female strategies. Thomas's langurs live in one-male multifemale groups; both males and females disperse from their natal groups; female secondary dispersal is also common, and infanticide occurs. The life-span of a group is, as a rule, restricted to the tenure of its reproductive male (median tenure length is 72 months). Male tenure in bisexual groups was divided into three phases: the early phase (no infants yet), the stable middle phase, and the late phase (last year). Because AMBs remained after all females had left a male, they were treated as a fourth phase. I analysed interactions of group members with individuals outside the group using data from a four year field study (1093 observation days) of 15 bisexual and eight all-male groups of Thomas's langurs at Ketambe, Indonesia. Two types of interactions could be distinguished: (1) group encounters: whole groups meet each other, and (2) male provocations: a male silently approaches a group and suddenly attacks the individuals. This study involves 329 group encounters and 265 male provocations. The results support the mate defence hypothesis, but are ambiguous about the role of resource competition: Group encounters during the middle and late tenure phase took place more often in the context 'food patch', than was expected, and they mostly took place in fruit patches, which were a preferred food item. However, the proportion of group encounters that involved aggression did not depend on feeding context or tenure, and fruit availability did not influence group encounter rates. Male aggression during between-group conflicts reflected mate defence, rather than resource defence, although females had the possibility of obtaining resource defence through a male's mate defence. This study suggests that females use the outcome of group encounters and male provocations to assess the relative strength of males, and thus 'chose' a male who is able to defend future offspring. It also suggests that males can use coercion (infanticide and aggression against females) to show the relative weakness of a female's current male and encourage her to transfer: Aggressive behaviour during group encounters was primarily between males. Females only reacted aggressively to extra-group males when their infant was under attack. Male infanticide only occurred during provocations by males from neighbouring groups or AMBs. The intensity of male competition for mates, as measured by male-male aggression and extra-group male interest in a groups' females, was higher during the early and late phases, than during the middle phase of the tenure. It was also higher during the AMB than during the late phase of male tenure. The relative strength of males, as measured by a male's provoking behaviour and his ability to protect his females from aggression by extra-group males, was lower during the late than during the middle tenure phase. Male strength seemed lowest during the subsequent AMB phase. Females avoided extra-group males least during the early tenure phase, before infants were born.