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Loud calls play an important function in regulating the use of space and structuring social groups and mating systems in a wide range of taxa. In pair‐living territorial animals, where encounters with neighbors and solitary conspecifics are common, these calls are mainly associated with resource defense or mate guarding behaviors. Owl monkeys (Aotus azarae) live in groups of one pair of reproducing adults and 1–4 younger, non‐reproducing individuals. Both sexes disperse when they are around 3 years of age; they become solitary floaters who compete to replace same‐sex adults from other groups. Here, we examined the behavioral responses of Azara's owl monkey pairs toward calls of unpaired and unfamiliar males and females to better understand if the competition between floaters and groups is in relationship to the defense of their territory, their mates, or both. We collected behavioral data from six groups, before, during, and after the playing back of unfamiliar male and female loud calls and of a control stimulus at the center and border of their home ranges. Overall, our results showed that the playback location did not elicit differential responses in the monkeys and that both sexes were more reactive to male than female unfamiliar calls, as evidenced by higher rates of sociosexual and vocal responses, movement toward the playback, and intergroup encounters during and after the experiments. Our study indicates that paired male and female owl monkeys mainly defend their partners toward intruders and emphasizes the need of including the role of sexual competition on both sexes in models about the evolution of pair‐living social organizations and sexual monogamy mating systems. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Animals may behave aggressively towards neighbours. For several bird species it has been shown that males react more vigorously towards calls of neighbours from the centre of their home range than from the edge. This is usually explained by assuming that the centre of a home range is of higher value to a male than the edge. To test these ideas for a primate species, we conducted experiments with playbacks of loud calls on wild male and female Thomas langurs (Presbytis thomasi). These animals may defend resources in their range or protect their offspring from infanticide and males may defend their females. In natural situations between-group encounters in the centre of a home range by the intruding neighbour are more often accompanied by infanticidal attempts than encounters at the edge. Loud calls of neighbours played back from the centre of the home range caused a more vigorous reaction from the resident male than calls from the edge. However, males in food-containing trees did not respond differently to edge playbacks from males in non-food-containing trees. Although males seem to defend resources and not mates, male behaviour may be best explained by defence of infants against infanticide. Female behaviour is best explained by resource defence. [ABSTRACT FROM AUTHOR]

A number of studies on birds and mammals have shown that individuals respond differently to neighbour and stranger call playbacks. This response is generally thought to be adaptive, because differentiating calls from neighbours and strangers can prevent the costs of unnecessary contests. In addition, it has recently been suggested that female lions use call recognition to avoid infanticidal males. In this paper we show that Thomas langur (Presbytis thomasi) males react more vigorously towards calls from strange than neighbouring males. It is hypothesised that, although differentiating between calls from different males can be useful to reduce unnecessary contests between the males, discriminating between individual calls is important because it might reduce the risk of infanticide. [ABSTRACT FROM AUTHOR]