The Older the Better: Infanticide Is Age-Related for Both Victims and Perpetrators in Captive Long-Tailed Macaques

In wild primates, infanticide is a risk that is especially prevalent when a new male takes over the alpha position. Insight into risk factors related to infanticide may decrease the incidence of infanticide in captivity during male introductions. We investigated several risk factors of infanticide derived from hypotheses explaining infanticide in the wild and tested this in captive long-tailed macaques (Macaca fascicularis) using demographic data spanning a 25.5-year period. Factors that are related to infanticide in the wild explained a large proportion, but not all incidences, of infanticide in captivity. Consistent with the wild data, infants young enough to decrease the interbirth interval (<215 days) were at risk of being killed. In contrast to studies from the wild, infanticidal males were more than 2.5 years younger than non-infanticidal males. This indicates that captive settings can lead to new risks since relatively young males may gain the alpha position, promoting infanticide. Therefore, we propose the adolescent male risk hypothesis as a captive risk factor in which subadult males pose a risk of infanticide. In conclusion, the ages of both males and infants are related to infanticide in captivity and have to be taken into account during male introductions.