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Cannibalism - what is it good for?

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Matt Walker Matt Walker | 10:26 UK time, Tuesday, 26 April 2011

Locust (Image: Ingo Arndt / NPL)

Cannibalism is a driving force for locusts (Image: Ingo Arndt / NPL)

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.

Male wolf spider eats female (Image: L Watson)

A male wolf spider devours a female (Image: L Watson)

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.Orangutan (Image: Anup Shah / NPL)

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.

Locust swarm (Image: BBC)

 

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.

Comments

  • Comment number 1.

    Fish too. Especially in an aquarium context it's common (and perfectly normal) for fish to eat eggs and even live fry, even ones they've just that moment given birth to. I don't know the explanation, or just that they don't have sufficient intelligence to differentiate between offsping and food. Certainly in the wild currents and the amount of space mean eggs and live young get swept away from danger pretty quickly unlike an aquarium where they hang around.

  • Comment number 2.

    Great blog entry! Perhaps it would be worth pointing out that the term 'cannibal' is a little tricky, being as it conjures up images of human cannibalism, which - as far as we know until now - is a rather different matter than any of the examples Mr Walker mentions. (On the other hand, I strongly assume that most readers are well aware of all this.)
    Behaviours in the wild are sometimes difficult to evaluate; I would presume that there is a good biological reason for any behaviour. All the more interesting, then, to investigate the development of human behaviour. I wonder how much of our 'cultural' behaviour is in reality based on some (currently, or at one time) sound biological reason?

  • Comment number 3.

    Fascinating post, except that I think you mean that it was a tamarin that ate her baby. A tamarind is a tropical tree.

  • Comment number 4.

    Yes, thanks msoniak, I did mean tamarin - and have corrected the text in the blog. Specifially the moustached tamarin Saguinus mystax.

  • Comment number 5.

    It seems that cannibalism may make sense in a case of pure survival - a being will only survive if it eats another. Take the case of the Uruguayan Air Force Flight 571, where in 1972, the survivors of this plane crash in the Andes mountains could only survive if they ate from those who died in the crash. Of course, this was repulsive to them but they did it knowing it was the only source of food. In this case, to answer the question of this article, it is good for staying alive when all other options are exhausted. Looking at this macroscopically, it seems like an act of a community for the good of the community. Of course, in this case, nobody was killed purely for the sake of becoming food - rather it was an act of resourcefulness and necessity.

    Regarding the moustached tamarin mentioned in the article, I wonder that since this is so rare a documented thing, perhaps it was mental illness? Is that not possible in the animal world?

    Good food for thought. Tasty thoughts...

  • Comment number 6.

    Thanks for the information. I´m not a scientist and I hope I´m not influenced by so many zombies movies, but if this happens in primates, could it happen in humans?

  • Comment number 7.

    Many, many years ago, my junior school had a tank full of gerbils in the entrance hall.

    Then came the day we arrived at school to find just one, rotund individual...

  • Comment number 8.

    In re. to rajmataj: Necessity will probably be a large part of it, but there are also examples of human cannibalism for social reasons such as wanting to take power from them. Of course with other animals it would be difficult at best to prove that they share that motivation.

  • Comment number 9.

    Cannibalism is disgusting of course. However, we also have our human species which also kills off large numbers of its own for a wide variety of reasons, and sometimes for no reason. Equally repulsive !!!

  • Comment number 10.

    Cannibalism is a sensible option for herbivorous swarm insects. Vegetation is relatively low in nutrition compared to meat. If a swarm of insects devours all available vegetation to turn it into meat then, say, half eat the other half, each surviving insect has benefited from the consumption of twice as much vegetation than could be eaten by a single insect in the same lifetime. All available vegetation ultimately is used to produce the surviving cannibals which grow more quickly and have a better chance of reproductive success than if they stayed herbivorous.

  • Comment number 11.

    My understanding is that a prime reason for an organism not to be a cannibal was simply that eating your own kind meant ingesting bacteria and viruses that infect your species. A lion eating a goat is less likely to catch anything harmful from it than if he ate another lion. I don't know if this is less of an issue with insects than with warm blooded animals but it seems possible.

  • Comment number 12.

    As far as I know, chickens are quite vicious in terms of cannibalising each other - not because they need to, just because they have an opportunity. Quite strange and possibly worthy of a contrast - perhaps it has to do with weeding out the weak and promoting a healthier species in general (chickens probably not realising that sprained ankles aren't hereditary).

  • Comment number 13.

    if humans Eat each regularly, it can mess you up. Both physically and Mentally. Howcome animals don't suffer from that?

  • Comment number 14.

    I must say Matt that the reasons you provide as issues for cannibalism aren't very Darwinian. Let us go though each in turn to see why...

    1. "Eating your offspring is a bit pointless, if you’ve put in the energy to raise them in the first place."
    Agreed, but as you have pointed out there can be good evolutionary reasons for a parent to recoup its losses by consuming offspring with poor chances of survival. What's more cannibalism is a far broader category than just infanticide.

    2. "Devouring members of the opposite sex limits your ability to find a mate."
    By the same logic then, at least from a males perspective, one should devour as many of the same sex as possible to have sole access to the greatest number of those of the opposite!

    3."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."
    This may be true for certain species with particular niches and life histories, but it is by no means the general case. Conspecifics are competitors, they are using resources that would otherwise be available to you and your offspring and so there are plenty of opportunities for them to be regarded as bad neighbours!

    4. "If you start a cannibalism trend, the odds are you may end up a victim."
    This is group selection think of the worst order! Evolution has no foresight! If it is advantageous to eat members of your own species (i.e. you'll have more babies by doing it) and you don't do it some one else will and you'll be eaten regardless!

    In the comment above the most sensible and general answer for cannibalisms’ "supposed" rarity is given by kwnewton who points out that parasites could be easily transmitted in cannibalistic feeding interactions (indeed, recall BSE or Kuru).

    But all this is really by the by, and the reason that I used the word "supposed" above is that, contrary to what you state in your article, cannibalism is actually very very common in nature and not just in stressed environments. It is especially prevalent in aquatic communities where one estimate has it that 90% of consumer species are cannibalistic! And has been shown in many studies to have important consequences for population and community dynamics.

    Let me conclude by saying that the examples you gave in your article are very interesting indeed, but please be more careful with your reference to ultimate causes in the future.

 

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