Why so many animals eat their own kind
More than 1,500 animal species have been caught eating members of their own species, herbivores and scavengers included. It sounds like a freak behaviour, yet in lean times it can be a sound survival strategy, trimming the competition and turning a rival into a meal.
The habit becomes more common when food runs short, because a conspecific is a ready source of energy. Female Fletcher's frogs, for example, lay eggs in temporary pools with almost nothing to eat, so tadpoles there survive by feeding on each other. A diet of one's own species can also be converted into body tissue more efficiently than plants, which carry surplus elements an animal must spend energy getting rid of, so well-fed cannibals often grow faster and bigger.
Removing rivals frees up food, shelter and territory, and can open mating opportunities. In confused flour beetles, crowding triggers cannibalism that brings the population back down. Aquatic ecosystems see it most, though even there it is uncommon: in a study of roughly 12,000 wild fish, only about 0.3 percent had eaten their own kind in their latest meal. Sexual cannibalism, in which a female eats a male around mating, is a special case.
The strategy carries costs. Hunting others of your species brings a higher risk of injury and more energy spent searching. Eating the young of your own family defeats the purpose by wiping out shared genes: a male stickleback may mistake his own eggs for a competitor's. Some animals avoid this trap. Spadefoot toad tadpoles that turn cannibal tend to spare their siblings and eat unrelated tadpoles instead. Although individual cannibals benefit, populations where it is common tend to have lower expected survival overall.
Disease is another hazard, though cannibalism can also shrink parasite numbers by removing hosts. Prions have spread this way: kuru, a brain disease, affected tribes in Papua New Guinea that ate the dead in funeral rites, and mad cow disease arose largely from feeding cattle remains to other cattle. Parasitic worms, viruses and tapeworms can pass between reptiles, insects, crustaceans and fish in the same fashion.
Source: Cannibalism
