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TENURE RELATED CHANGES IN WILD THOMAS'S LANGURS II: LOUD CALLS

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In this paper, we investigate how the number and context of male loud calls of Thomas's langurs (Presbytis thomasi) change over a male's tenure, in relation to changes in the intensity of male mate competition and relative male strength. We also investigate how the calls' impact on the behavior of receivers varies over tenure phases. Thomas's langurs live in one-male multi-female groups; only males produce loud calls; 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. We hypothesised that the tendency to answer another male's calls decreases with distance because a male will invest less when answering becomes less relevant. The tendency to respond to a loud call by an extra-group male indeed decreased with distance, which suggests that males invested less in (costly) calling behavior when the chance of an interaction with that male was low. Extra-group males seemed to recognise males of new groups: they did not discriminate between medium and far distances in answering calls from (relatively unfamiliar) early tenure males. We further hypothesised that an increase in male mate competition would result in more call bouts per day and a higher tendency to answer calls, which was not found. Males with a relatively low strength were expected to keep signalling their presence, but because this low strength includes a higher risk for females and infants, we expected females to avoid loud calling extra-group males. Males with a declining strength continued to signal their presence, as was expected, but they did reduce participation in dawn call bouts, which might be a particularly sensitive measure of their decreased strength. Extra-group males answered calls by males during their late tenure phase more often at medium and far distances, which shows that males recognised calls from late tenure males. Females' avoidance of calling extra-group males remained constant during the early and middle phase but increased during the late tenure phase, as was expected. AMB males clearly avoided males from bisexual groups: they never participated in dawn call bouts, they rarely started or answered calls and they travelled away from calling males. AMB males only answered a call bout in the case of a betweengroup conflict, when their position was already known. Hence, in Thomas's langurs, loud call behavior influenced male mate competition, and it varied in relation to changes in relative male strength.

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