Cannibalism is rampant in the animal kingdom, including among some humans in the past. Since germs can sneak from victims to predators, one might suspect diseases linked to cannibalism would prove widespread.
Instead, diseases spread by cannibalism are rare. New calculations suggest this is because cannibals usually dine alone. If cannibals do feast together, germs could begin taking advantage.
"Maybe this is why cannibalism is no longer common among people as it was in the past, because of the strong negative effects it can have when transmitting a disease," ecologist Volker Rudolf at the University of Virginia at Charlottesville told LiveScience.
Cannibalism is "stunningly common in nature," Rudolf explained. "In the past, there were misconceptions that cannibalism was something that only happened in labs or under extreme conditions. But it isn't."
Even though people now typically abhor cannibalism, "it was quite a common practice in many human societies in the past," Rudolf added.
Scientists do know of diseases transmitted solely or mostly through cannibalism. For instance, Kuru is a degenerative brain ailment in Papua New Guinea, akin to mad cow disease, that only afflicts people who eat human flesh.
Since cannibalism is widespread, Rudolf and his colleague Janis Antonovics wanted to see how commonplace diseases spread by cannibalism were. Scientific archives showed that while cannibalism was seen in animals ranging from crustaceans and insects to mammals, it apparently was only the predominant route for transmission of disease in two cases--Kuru in humans, and the protozoan Sarcocystis in lizards, in which cases the reptiles snack on each others tails.
To investigate the prevalence question, the researchers fused calculations used to forecast how diseases spread throughout a population with ones used to estimate how predator and prey levels rise and fall in relationship to one another in the context of cannibalism.
After calculating how diseases jump from predator to prey in cannibalism, the researchers discovered that cannibalism was only an effective means of spread if cannibals dined together rather than solo on victims, findings detailed in the May 7 issue of the Proceedings of the Royal Society B.
"Such group cannibalism is really rare in nature," Rudolf said. "Besides humans, the only other example we could find are with chimpanzees, where males group together to attack and consume other chimps. Cannibalism is very common in nature, but it's usually one-on-one."
In cases where group cannibalism does occur in nature, however, it might prove fruitful to investigate, the researchers noted. For instance, the young of certain insects and spiders are known to devour their own mothers, while male lions are known to eat cubs after they acquire new harems. Rudolf hopes to investigate how cannibalism and group dynamics affect each other with experiments in insects.
"This is exciting work," said population ecologist Nat Holland at Rice University in Houston. It raises the question of whether or not epidemics of sexually transmitted diseases in humans are related to the size of groups in orgies. "Those are social diseases as well," he said.