Familiarity and threat of opponents determine variation in Thomas langur (Presbytis thomasi) male behaviour during between-group encounters

During between-group encounters in primates, males often defend mates, food or infants against other males. Males, however, show variation in contests between opponents. In other taxa, such variation has been attributed to variation in familiarity with or threat of opponents. Here we present the results of analyses of between-group encounter intensity variation in Thomas langurs while controlling for threat when evaluating familiarity and vice versa. Encounter intensity was measured by the proportion of encounters with chases and the number of loud calls per minute during the encounter given by the focal male. The results indicate that both familiarity and threat influence encounter intensity. Less familiar opponents had more intense encounters and opponents that differed in the threat level to each other also had more intense encounters. Thus, Thomas langur males seem to incorporate information on both the level of threat and familiarity of other males to make a decision on how to react during a between-group encounter.

Male monkeys remember which group members have given alarm calls

Primates give alarm calls in response to the presence of predators. In some species, such as the Thomas langur ( Presbytis thomasi ), males only emit alarm calls if there is an audience. An unanswered question is whether the audience's behaviour influences how long the male will continue his alarm calling. We tested three hypotheses that might explain the alarm calling duration of male Thomas langurs: the fatigue , group size and group member behaviour hypotheses. Fatigue and group size did not influence male alarm calling duration. We found that males only ceased calling shortly after all individuals in his group had given at least one alarm call. This shows that males keep track of and thus remember which group members have called.

Female dispersal, inbreeding avoidance and mate choice in Thomas langurs (Presbytis thomasi)

Female social dispersal in primates differs from the general mammalian pattern of locational dispersal. Both nulliparous and parous females may disperse to another group. Several hypotheses can explain female social dispersal: reduction of predation risk, inbreeding avoidance, and offspring protection through mate choice. We tested these hypotheses with an extended data set of Thomas langurs (Presbytis thomasi) and investigated parameters of male behaviour that females may use in their dispersal decisions. Data were collected over a 12.5-year period from a wild population in Sumatra, Indonesia, allowing for some critical tests of the hypotheses. Females dispersed to a group smaller than their original one, thereby refuting the predation risk hypothesis. Maturing nulliparous females only dispersed when their father was resident. Therefore, dispersal by nulliparous females was best explained through inbreeding avoidance. Parous females transferred to young, adult males. These males provided better protection to offspring against predation and infanticide than the old, late tenure males. Therefore, females transfer to better protector males. The male behavioural cues that females use to assess male quality were unclear. Females, however, may use proxies of male age, such as group composition and acoustical characteristics of loud calls, as indicators of male quality. The results suggest that female mate choice is an important function of social dispersal by parous females. Its importance in locational dispersal remains to be investigated.

Thomas Langurs (Presbytis thomasi) Discriminate Between Calls of Young Solitary Versus Older Group-living Males: a Factor in Avoiding Infanticide?

The ability to differentiate among calls from different individuals has been shown for a number of animal species and several functions have been suggested. One hypothesis, developed for lions (Panthera leo), is that the ability to distinguish between calls from neighbour versus strange males is linked to the avoidance of infanticidal ( i.e. strange) males. Since infanticide is widespread among primates, we tested the applicability of this hypothesis to Thomas langurs (Presbytis thomasi). Young adult males, that mostly reside in all-male bands or are solitary (called AMB males), were more likely to be infanticidal than adult, usually older, males that reside in mixed-sex groups (called MSG males). We use playbacks to demonstrate that Thomas langurs are able to differentiate between loud calls from AMB male strangers and MSG male strangers. Thomas langur males responded more vigorously to playbacks of calls from AMB (i.e. more likely to be infanticidal) males than to calls from MSG males. Females showed a more cautious response to the calls from AMB males than to the calls from MSG males. Both these reactions are in accordance with the infanticide hypothesis and suggest that Thomas langurs differentiate between loud calls of AMB and MSG stranger males and incorporate this information in their behaviour to avoid infanticide.