CANNIBALS are rare in the animal kingdom partly because eating your relatives makes you sick, say researchers in the US who have tested the idea for the first time.

For years, biologists have wondered why cannibalism is rare. In theory,
eating your own kind can give nutritional and competitive advantages, although communities of cannibals might also have a tendency to wipe themselves out. One answer, they reckoned, was that animals avoid cannibalism to stop the spread of species-specific pathogens.

To test this theory, a team led by David Pfennig of the University of North
Carolina at Chapel Hill created “diseased” larvae of the cannibalistic tiger
salamander, Ambystoma tigrinum, and of another tadpole species,
Ambystoma texanum, by exposing them to tanks infected with their natural pathogens, such as Clostridium species. Diseased and healthy larvae of both species were then given as food to 24 slightly larger and more mature tiger salamander larvae.

The larvae that ate diseased larvae of their own species grew less quickly
and were less likely to survive metamorphosis than all the other animals,
Pfennig’s team will report in a forthcoming issue of Animal Behaviour.
“Cannibalism may be more rare because you risk getting pathogens or parasites,”

says Pfennig. “They coevolve with their host.”

So the answer is in plain sight. Cannibalism causes disease.