What's the point of cannibalism?

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BBC Nature editor Matt Walker has a long meditation on the possible benefits of cannibalism, centered around two bits of newly published research. First, a recently recorded case of a female tamarin monkey killing and eating her own infant. Second, a study documenting behavior in locust swarms.

Cannibalism is emotionally disturbing, he writes, and it also seems logistically nonsensical—if you eat the opposite sex, you limit your chances of mating; if you eat your offspring, you destroy your own genes' hard work; if you eat your neighbors, your lose potential allies; and, eventually, you might encourage others to eat you. So why does cannibalism happen at all? It's a question science doesn't have a solid answer for, especially when it comes to the kind of cannibalism that involves murder, as opposed to simply eating individuals who died in other ways.

But the case of the cannibalistic monkey is especially interesting; the mother moustached tamarind, which lives in the Amazon rainforest in Peru, intentionally killed her young son, by biting and eating its head. That makes it only the third recorded case of maternal infanticide recorded in wild non-human primates.

Researchers can of course only speculate about her motivation ... They suspect the mother killed her baby because she knew it had a low chance of survival anyway. Tamarinds rely on other adults to help raise their young, and there were few of these around when the mother made her fateful decision. So the primatologists think she terminated the investment in her offspring due to the low availability of helpers. The baby was simply born at a bad time, and as tamarinds ovulate relatively quickly after giving birth, the mother, in terms of reproductive economics, made a cost effective decision. That doesn't explain why she then began to consume her young. In this instance, the mother only ate part of her infant's corpse; the head, brain and a small part of his shoulder and neck. So she didn't kill her offspring for its meat. But once dead, she probably gained some nutritional benefit by eating its brain, offsetting some of her costs in producing the baby.

But cannibalism can be about more than just individual survival ... Eating your own may be the driver behind the mass migration, and swarming, of locusts, researchers have just announced. Cannibalistic interactions have been shown before to be the driving force behind the collective mass movement of Mormon crickets and Desert locusts. The basic idea here is that locusts combine into swarms because they are frightened of being eaten by each other. But researchers have now provided the first evidence that cannibalism has an adaptive benefit for desert locusts, which form "bands" as they migrate en mass.

Australian plague locusts cannibalise other vulnerable locusts to compensate for a lack of protein in their diet. Individuals move forward to find new food, and avoid being eaten by each other, producing an advancing swarm. But the scientists, led by Matthew Hansen of the University of Sydney, Australia, also show that locusts that are given the opportunity to eat each other on average survive longer and move further. They call it the "lifeboat mechanism"; locusts actually have a better chance of surviving longer and travelling further if they all jump into a swarm together and become cannibals.

BBC Nature: Cannibalism—What is it good for?

Via John Rennie

Image: Some rights reserved by Lazurite

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