Why Do Mammals Kill Members of Their Own Species?

Why Do Mammals Kill Members of Their Own Species?
(AP Photo/David Zalubowski)
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In January 2016, millions were touched by a viral photo apparently depicting a male kangaroo 'hugging' his mortally injured female companion in front of her young joey, seemingly trying to pick her up off the ground. Photographer Evan Switzer, who captured the image, said, "It was a pretty special thing, he was just mourning the loss of his mate."

Dr. Derek Spielman, a senior lecturer in veterinary pathology at the University of Sydney, knew better. Rather than mourning the incapacitated female, the male was likely trying to mate with her. What's more, he had likely caused her injuries in the first place while pursuing his ravenous sexual appetites.

“Competition between males to mate with females can be fierce and can end in serious fighting,” Spielman told The Guardian. “It can also cause severe harassment and even physical abuse of the target female, particularly when she is unresponsive or tries to get away from amorous males. Pursuit of these females by males can be persistent and very aggressive to the point where they can kill the female. That is not their intention but that unfortunately can be the result, so interpreting the male’s actions as being based on care for the welfare of the female or the joey is a gross misunderstanding...”

When it comes to killing members of their own species, kangaroos are one of the worst mammalian offenders, according to a review recently published in the journal Proceedings of the Royal Society B. The Spanish scientists behind the research found that even-toed ungulates (hoofed animals), primates, shrews, and wallabies were other mammals that frequently murder their own.

As humans (and mammals), we're intimately aware of the various reasons we kill each other, but why do non-human mammals commit murder? That's what the researchers wanted to find out.

For this endeavor, they collected information on intra-species adult killings in more than a thousand mammals, finding evidence for the act in 280 of them. Examining the data, they found that males were by far the most frequent killers.

"Adulticide between males was reported in at least 232 species, males killing females in 42 species, females killing males in 30 species and females killing females in 35 species," they wrote.

As for why males are so lethally violent, one reason leapt out of the scientific literature: competition for mates.

"When adulticide occurs in this context, it is mostly incidental, death happening from two males fighting so intensely that they may fatally wound each other," the researchers wrote.

Females, on the other hand, killed almost exclusively to protect their offspring from members of their own species seeking to harm them.

Now, don't let all this nefarious talk ruin your impression of our furry mammalian friends – not all of them are murderers! According to the researchers, the heinous act is virtually absent amongst bats, whales, dolphins, and rabbits.

Source: Gómez JM, Verdú M, González-Megías A. 2021 Killing conspecific adults in mammals. Proc. R. Soc. B 288: 20211080. https://doi.org/10.1098/rspb.2021.1080

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