Social dynamics of intergroup encounters in the capped langur (Presbytis pileata).

Data from a 15-month field study of the capped langur (Presbytis pileata) in moist deciduous forest in Bangladesh show that during intergroup encounters males respond more aggressively to extra-group males and to the resident males of unfamiliar one-male groups than they do toward familiar males. Lone males followed established one-male groups and attempted to interact with group females. During intergroup encounters males responded to the approach of unfamiliar males with significantly more displays than were given to familiar (frequently encountered) males. The frequency of displays given to approaching groups showed a negative correlation with the frequency of encounters with those groups. In 50% of intergroup encounters males pushed and bit their group females if the females strayed from the immediate proximity of the group male. Further observations suggest that males obtain females that have emigrated from established groups or attempt to interact with them in established onemale groups. Males had a tenure from at least 13 months to at least 26 months, and between 1986 and 1988 no male changes were seen. Capped langur males defended females from other males, but females did not defend resources or participate in intergroup encounters. The implications of this sex difference in behavior during encounters is discussed with regard to ecological models of female-bonded primate groups.