Cannibalism - what is it good for?
Cannibalism – what is it good for?
Food is the obvious answer, but there appear to be significant issues associated with eating your own kind. Eating your offspring is a bit pointless, if you’ve put in the energy to raise them in the first place. Devouring members of the opposite sex limits your ability to find a mate. And gobbling up your neighbours can be self defeating – for these good neighbours can lead you to food and water, warn about predators and provide more sociable creatures with company. And if you start a cannibalism trend, the odds are you may end up a victim.
Those reasons help explain why most animals aren’t cannibals. But it doesn’t elucidate why some do eat their own.
However, a few recent and new bits of research shed a little light on the matter (if you’re squeamish, it may be better not to read on).
Earlier this month, the BBC reported the discovery that male wolf spiders cannibalise older females.
In this unusual case of role reversal (among scorpions, spiders and mantises, cannibalism usually takes the form of females eating smaller males, or babies eating their parents), the ladykilling spiders seem driven to their compulsion by the harsh habitats in which they live, in South America's sand dunes along riverbanks and the Atlantic Ocean coast.
Strong winds and extreme temperatures buffet the dunes, which provide scarce refuges for the wolf spiders and an unpredictable abundance of prey. That means the males often decide to eat low quality females rather than mate with them. A straightforward decision between eating and surviving to mate another day, or mating now and possibly dying.
A more uncomfortable case of cannibalism is reported in the journal Primates.
Uncomfortable because it involves one monkey eating another, even more so because it was a mother eating her baby.
I’ve reported on cannibalism in primates before.
In 2009, I wrote about the cases of two female orangutans seen cannibalising the bodies of their recently deceased babies, the first report of such behaviour in great apes.
Then in 2010, a wild bonobo cannibalised her own recently deceased two and a half-year-old infant – the first record for that species.
But the case of the cannibalistic monkey is especially interesting; the mother moustached tamarin, 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. But the scientists that witnessed the act, and had been following and studying the tamarin troop to which she belonged for years, can make some educated guesses.
They suspect the mother killed her baby because she knew it had a low chance of survival anyway. Tamarins 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 tamarins 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. The compulsion it seems, can lead to one of the most impressive, complex and difficult to understand phenomenon among all animals.
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.
When times are tough, it seems, cannibalism can become a rather attractive option.