Citation: Gielen, K.; Louwerse, A.L.; Sterck, E.H.M. The Older the Better: Infanticide Is Age-Related for Both Victims and Perpetrators in Captive Long-Tailed Macaques. Biology 2022, 11, 1008. https://doi.org/10.3390/ biology11071008 Academic Editor: Martine Meunier Received: 29 May 2022 Accepted: 1 July 2022 Published: 4 July 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). biology Article The Older the Better: Infanticide Is Age-Related for Both Victims and Perpetrators in Captive Long-Tailed Macaques Karlijn Gielen 1, *, Annet L. Louwerse 2 and Elisabeth H. M. Sterck 1,2 1 Animal Behaviour and Cognition, Department of Biology, Utrecht University, 3508 TB Utrecht, The Netherlands; e.h.m.sterck@uu.nl 2 Biomedical Primate Research Centre, 2288 GJ Rijswijk, The Netherlands; louwerse@bprc.nl * Correspondence: k.gielen@uu.nl; Tel.: +31-30-253-5304 Simple Summary: In captive primates, new males have to be introduced regularly to prevent inbreed- ing. Sometimes, these males commit infanticide, i.e., the killing of young infants. More knowledge of the risk factors that are associated with infanticide may lower the incidence of infanticide during male introductions. We used explanations of infanticide from wild data and the anti-infanticidal strategies of females to predict these risk factors. Next, we tested these factors using demographic data collected on captive long-tailed macaques over a long period. The ages of both infants and new alpha males are important: infants under the age of 215 days are at risk of being killed, and typically young males (≤8 years of age) commit infanticide. Therefore, to lower the risk of infanticide during male introductions in captivity, we advise introducing only males in their prime age (≥9 years of age), preferably in periods with no infants younger than 215 days of age. Abstract: 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. Keywords: infanticide; social housing; sexual selection; aggression; captivity; management; husbandry 1. Introduction In captive groups of primates, new males have to be introduced regularly to prevent inbreeding. These introductions can be risky since they are often accompanied by an increase in aggression [1–3] that may lead to injuries. Moreover, new males sometimes commit infanticide, a phenomenon in which young infants are killed by a conspecific. Male infanticide in wild populations has been described in many species of primates [4–6]. It is typically observed when an immigrant male enters the group and takes over the alpha position or when a low-ranking resident male takes over the alpha position [7,8]. Yet, not all males commit infanticide [7,9–11], and not all young infants are killed during an alpha male take-over [10,12]. Male benefits and female counterstrategies to infanticide Biology 2022, 11, 1008. https://doi.org/10.3390/biology11071008 https://www.mdpi.com/journal/biology