The Kids Are Not OK: Causes of Infanticide in Female Mammals

The Bonnet Macacque, one of the 89 species in which females have been shown to commit infanticide (Image Credit: Vino Rex, CC BY 2.0, Image Cropped)

The evolution of infanticide by females in mammals (2019) Lukas & Huchard, Philsophical Transactions of the Royal Society – Biological Sciences, https://doi.org/10.1098/rstb.2018.0075

The Crux

The practice of male mammals killing their rival’s cubs has been well-documented by wildlife biologists. The image of a male lion striding away from a pride with a dead cub in his mouth is quite haunting (spare a though for Scar’s kids when Simba takes over again). But infanticide by female mammals has received less attention.

Whilst males generally only kill young to ensure they have more access to mates, the motivations behind infanticide in females are more complex. It ultimately comes down to resource competition, but the resources themselves are myriad – milk, availability of space, care from more than one ‘parental’ figure (allocare), and social status. These four resources make up the competing hypotheses as to why females commit infanticide. This week’s researchers wanted to know what factors of a species biology increased the likelihood of a mother to kill an infant of the same species.

Did You Know: Anthropomorphism

Jokes about the Lion King aside, we’re talking about a fairly serious subject here – females murdering children. But it’s important not to project too many human emotions onto other animal species (even the cute and cuddly ones). We’ve written about Anthopomorphism before on EcoMass, and whilst we may see this behaviour as inherently cruel, most other species have no way to comprehend the values we may instill upon them. They’re just engaging in the behaviour that has allowed them to evolve and persist in their surroundings.

What They Did

The study involved the synthesis of data on almost 300 different species, 89 of which female infanticide has been recorded in. They then compared the presence of infanticide to a range of different species’ traits, including social structure, average litter size, where the litter is raised, whether or not the father is involved in care, to name a few. They also tested the aforementioned hypotheses for infanticide based on these traits.

What They Found

Species in which infanticide is present generally invest more energy into maternal care and are found in harsher environment. This confirms that resource competition is the primary motivation to kill other young. However the tendency for a species to commit female infanticide varied across the different types of social structures.

If a species received allocare, female infanticide was more likely to occur, with the dominant female more likely to be the killer. In pair breeders, the mother only killed other young when the father was also involved in caring for the young. Competition over breeding space seems to be a factor for many species, with almost all species that kill outside of their own home range keeping their offspring in burrows.

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Species in which both high maternal energy is required in raising children and that also exist in harsher environmental conditions are more likely to commit female infanticide, making these cubs likely candidates to be killed by their mother (Image Credit: Nick Jonsson, CC BY 2.0)

Problems?

A synthesis likes this necessitates a somewhat superficial approach. It’s taking a fairly binary look at every species – do their females commit infanticide or not? What I would love to see is a study within a species showing the likelihood of infanticide within a population across an environmental (say harshness of conditions) or social (size of group/tendency to disperse) gradient. That is of course, not the scope of this paper.

So What?

The results and suggestions of this study were many, and as usual we’ve only managed to touch on a few in the above section. What that suggests is that although female infanticide is a much more complex issue than male infanticide. However the results do indicate that it has generally evolved as a means of maximising the resources females can provide their own young with.

We’re increasingly starting to discover ways that species deal with limiting population growth in areas where high densities can detract from the overall population’s health. Further investigation into these methods is crucial, as it allows us to better understand the mechanisms behind population abundance and density fluctuations.

And it gives us a raft of new mammalian Disney villains.

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