Male Infanticide in the Northern Purple-Faced Langur (Semnopithecus vetulus Philbricki) in the Kaludiyapukuna Forest Reserve, Sri Lanka

Infanticide, the killing of conspecific infants, has been observed in many species, including rodents, carnivores, and notably, primates. Although several adaptive and non-adaptive hypotheses have been proposed to explain this phenomenon, most cases to date appear consistent with the sexual selection hypothesis, particularly in primates. According to this hypothesis, males increase their reproductive success by eliminating unrelated unweaned infants, causing females to resume cycling earlier and allowing infanticidal males to mate and sire offspring sooner during their tenure. Here, we document the first confirmed cases of male infanticide in the Northern purple-faced langur, an endangered Sri Lankan colobine living in polygynous groups where outside males challenge resident males for reproductive access, resulting in a “takeover.” Following one male takeover, we documented infanticidal attacks on all three infants present, resulting in the wounding and killing of two and the disappearance of the third, all within the first 2 months of the male's tenure. We also describe: (1) changes in group composition following the male replacement, (2) the age and sex of the victims and perpetrator/s; (3) the effect of infant loss on female interbirth interval; (4) infant defense; and (5) mating access to the victim's mothers after the infanticide. We conclude that despite anthropogenic disturbance at some study sites, infanticide within this species appears to align with the sexual selection hypothesis. Nonetheless, genetic analyses on infants killed and born after the takeover are needed to provide conclusive evidence.