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Parasites encourage cannibalism in some animals, say scientists

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University of Leeds

Parasites may be responsible for encouraging some animals to engage in cannibalism, claims a new study published in the journal Royal Society Open Science. Scientists investigating the effects of the common parasite Pleistophora mulleri found that adult shrimps infected with the creature were more likely to eat their young than uninfected adults were. The shrimp were already known to be occasional cannibals, but the presence of parasites significantly increased the chances of this happening.

the parasites invade hosts in their millions

The parasite in question is incredibly small (about the size of a human red blood cell) and moves into the shrimp's muscles like a spore. These parasites don't make much of a difference in small numbers, but eventually millions can flood into the host. At this point, the shrimp's muscles become damaged and the animal is forced to eat more food to meet the extra energy demands of its tiny freeloaders.

"The parasite is quite debilitating. It takes over huge areas of the muscle, so instead of a nice transparent shrimp you get quite a chalky appearance because of muscles packed with the parasite," said Alison Dunn, a senior author on the paper, in a press release.

Dunn told Discovery News that turning to cannibalism helps the shrimp "deal with the cost of the infection as it gains more food." She added that earlier studies had found that the muscle damage caused by the parasites could stop the shrimps from catching their normal prey. "So perhaps cannibalism of smaller shrimp is the only way these sick animals can survive," she said.

cannibalism can act as a lifeboat mechanism

The teams of scientists from the UK and South Africa noted that cannibalism has been recorded in more than 3,000 species (including humans) and that it is often thought to act as a "lifeboat mechanism," giving organisms easy access to food in situations of dire need. Cannibalism also has the benefit of reducing competition and providing a bonus to an individual's growth and survival.

Parasitism is similarly widespread, and scientists have already found numerous examples of parasites changing their hosts' behavior to suit their needs. Phys.org cites the examples of a tapeworm that makes fish swim faster or slower depending on whether it wants them to be eaten, and a parasitic wasp that half-paralyzes ladybugs to stand guard over its larvae. It's conceivable, said the scientists examining the infected shrimp, that inducing cannibalism is just another strategy parasites are using to survive.

"Our research does not suggest any link between parasites and human cannibalism," said Dunn. "Cannibalism for the shrimp, unlike in humans, is a significant source of food even in uninfected animals. It seems unlikely that a parasite would be under evolutionary pressure to influence cannibalism in humans."