Infanticide in a colonial cooperative bird is not associated with direct reproductive benefits
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Abstract
Infanticide, the killing of dependent young by adult conspecifics, is an extreme form of conflict that remains poorly understood. Previous studies suggest that it is associated with reproductive benefits, either by making breeders newly available for reproduction or by increasing offspring care and reducing competition for resources. In addition, infanticide may result from a reproductive trade-off, where parents kill their offspring if environmental conditions compromise their reproductive value. Here, we combine video, genetic and individual-level reproductive data collected over 12 years to describe infanticide in a highly social, cooperative bird, the sociable weaver Philetairus socius and to establish whether direct reproductive benefits or life-history trade-offs are possible motivators for this behaviour. We documented 50 cases of infanticide and identified 27 aggressors with known histories. These were mostly born outside the colonies where they committed infanticide (63%) and there was a bias towards younger individuals. Aggressors were usually not part of the victim’s breeding group (74%), but in two cases infanticide was committed by the breeding female, and in five by helpers. In most cases, aggressors were not reproducing at the time of the infanticide, were not recorded mating with any of the breeders or helpers from the attacked nest, nor occupied the nest from where the chicks were evicted. Only 13 of the known aggressors were confirmed as breeders after the attacks, and these mostly reproduced at the colony where the attacks were committed. Overall, we found no evidence for direct reproductive benefits of infanticide in sociable weavers. However, the younger age of aggressors, lack of genetic relatedness to the victims and future breeding at the colony suggest that competition between breeding groups, and hence indirect reproductive benefits, could provide motivation for infanticide in this population. Additional work over longer periods is needed to identify these possible indirect fitness benefits of infanticide.
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Given the high costs, animal parents generally have diverse strategies to protect their offspring from infanticidal attacks of other adults. Rare observations of infanticide might therefore make it appear like an opportunistic behaviour, but systematic studies have been able to show that it is generally specific sets of individuals who kill offspring in particular situations. Bringing together such observations is challenging, but it can reveal the likely adaptive reasons for why the behaviour occurs in a given population. In this study, Covas and colleagues (2025) made use of detailed videos recorded at the nests of a bird species, the sociable weaver Philetairus socius, to show that the killers are mostly younger, unrelated individuals who subsequently breed in different groups in the same colony, suggesting that competition between …
Given the high costs, animal parents generally have diverse strategies to protect their offspring from infanticidal attacks of other adults. Rare observations of infanticide might therefore make it appear like an opportunistic behaviour, but systematic studies have been able to show that it is generally specific sets of individuals who kill offspring in particular situations. Bringing together such observations is challenging, but it can reveal the likely adaptive reasons for why the behaviour occurs in a given population. In this study, Covas and colleagues (2025) made use of detailed videos recorded at the nests of a bird species, the sociable weaver Philetairus socius, to show that the killers are mostly younger, unrelated individuals who subsequently breed in different groups in the same colony, suggesting that competition between groups might explain the occurrence of infanticide in this study.
This study provides important first insights into a a behaviour that is hard to study. Even though the study is based on a high-quality dataset collected over 12 years, only 50 infanticide attempts were observed. The authors are careful in not overinterpreting the inferences that can be made from these observations. Their findings though convincingly show that individuals do not always get immediate benefits from their actions. The authors also lay out additional avenues for future studies in this and other studies to build on their findings. More broadly, this study highlights the value of long-term studies, showing how these can provide opportunities for systematic studies of even rare behaviours as well as the in-depth understanding to derive relevant context for the results.
References
Rita Covas, Liliana R. Silva, Andre C. Ferreira, Pietro B D'Amelio, Rita Fortuna, Delphine DuVal, Matthieu Paquet, Claire Doutrelant (2025) Infanticide in a colonial cooperative bird is not associated with direct reproductive benefits. bioRxiv, ver.7 peer-reviewed and recommended by PCI Ecology https://doi.org/10.1101/2025.05.27.656097
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