Little did Jay Rosenheim know that his team’s plan to protect California’s cotton fields would lead to an explosion of cannibalism. Faced with ever-destructive cotton aphids — a tiny, voracious green insect that sucks sap from crops, leaving behind moldy waste and a slew of deadly viruses — he and his colleagues decided to sicken another group of insects with them: a powerful group of native aphid assassins , which are known as big-eyed beetles.
It worked – for a while. Then, as space on the plants ran out, something unexpected happened: the big-eyed beetles stopped attacking the aphids and began chasing each other, devouring hordes of their own eggs. They “became wildly cannibalistic,” says Rosenheim, an entomologist at the University of California, Davis.
In the animal world, from the unicellular amoeba to the salamander, eating one’s own kind is fairly common, he and his colleagues report in a new report in ecology. But not as many species snack on their brethren as one might expect – and the team have detailed the reasons why.
First of all, cannibalism is risky. If you have dangerous claws and teeth, your comrades will too. Female praying mantises, for example, are notorious for biting off the heads of much smaller males during mating, but they will also occasionally go toe-to-toe with a worthy female. “I’ve seen one woman bite off another woman’s leg,” says Rosenheim, “and then the woman who lost the leg somehow manages to kill the other woman.”
Cannibalism is also tricky from a disease perspective. Many pathogens are host specific. So if a cannibal devours an infected companion, they risk catching the same disease. Various populations of people have found this out the hard way on numerous occasions. One of the most famous examples is the spread of a rare and deadly brain disease called kuru that afflicted the Fore people of New Guinea in the 1950s. Kuru rampaged through the Fore community through a cannibalistic burial ritual in which families cooked and ate the flesh—including contaminated brain tissue—of deceased relatives. When the Fore phased out the ritual, the spread of kuru was halted.
After all, cannibalism is a horrible way to pass on your genes. “From an evolutionary point of view, you definitely don’t want to eat your offspring,” says Rosenheim. That’s one of the main reasons big-eyed bugs limit their population size by feeding on their own offspring. If they become too numerous – as in the aphid experiments – they lay eggs everywhere. And because they can’t recognize their own eggs, they end up eating their own brood.
Although cannibalism is far from ideal, certain conditions seem to make the risky behavior worthwhile. Even if you eat a friend — or an heir — when you’re starving, you have to protect your survival, says Erica Wildy, an ecologist at California State University, East Bay, who wasn’t involved with the study. In her own work, Wildy found that when hungry, long-toed salamander larvae tend to nibble on one another—and occasionally eat.
In their review, Rosenheim and his colleagues pinpoint specific hormones — octopamine in invertebrates and epinephrine in vertebrates — that appear to be associated with increased rates of cannibalism. When conditions get tight and food becomes scarce, levels of these hormones rise and “hungry” animals will attack anything they can snatch with jaws, legs, or pincers.
The study also highlights how certain conditions turn some young amphibians, such as tiger salamanders and spadefoot toads, into super-cannibals. When a pond becomes overcrowded with larvae, some tadpoles transform into a “cannibal morph” by bloating and sprouting gaping jaws tipped with pseudo-fangs. Similar cannibalistic morphs appear in mites, fish, and even fruit flies, whose cannibalistic larvae are armed with 20% more teeth on their mouth hooks than their counterparts.
Other creatures, like the highly invasive cane toad, take the opposite route. When hungry cannibals lurk, vulnerable toad larvae accelerate their growth and development, gaining mass to become too large to eat.
In most cases, the end result of rampant cannibalism is positive: an uncrowded, healthier population. Rosenheim therefore shies away from seeing cannibalism as barbaric. “When we think of cannibalism in human populations, we cringe,” he says. “But cannibalism is a big part of balancing nature.”