Study finds good reasons to go after your comrades—and some bad ones, too
Jay Rosenheim had no clue his team’s strategy to defend California’s cotton crops would spark a cannibalism epidemic. Faced with the ever-destructive cotton aphids—a tiny ravenous green insect that sucks sap from crops, leaving moldy waste and a slew of deadly viruses in their wake—he and his colleagues decided to infect them with another group of insects: a stout group of native aphid assassins known as big-eyed bugs.
It was effective—for a time. When room on the plants grew short, something unexpected happened: the big-eyed bugs stopped attacking the aphids and started hunting each other, consuming swarms of their own eggs. According to Rosenheim, an entomologist at the University of California, Davis, they “grew outrageously cannibalistic.”
He and his colleagues explain in a recent review in Ecology that eating your own kind is quite frequent across the animal world, from single-celled amoebas to salamanders. However, not as many species feed on their relatives as one might expect—and the team has explained why.
To begin with, cannibalism is dangerous. Your allies have vicious claws and fangs, just like you. Female praying mantises are infamous for biting the heads off much smaller males during mating, yet they will also go toe to toe with an evenly matched female on occasion. “I’ve seen one female gnaw the limb off another, and then the female who lost the leg kills the other,” Rosenheim adds.
Cannibalism is also risky in terms of disease transmission. Because many viruses are host specific, a cannibal who consumes an infected companion risks contracting the same sickness. Various human cultures have discovered this the hard way several times. One of the most well-known examples is the outbreak of kuru, a rare and lethal brain illness that destroyed the Fore people of New Guinea in the 1950s. Kuru raged over the Fore society as a cannibalistic funeral ceremony in which families cooked and ate the flesh of departed relatives, including tainted brain tissue. The spread of kuru was halted after the Fore phased out the rite.
Finally, cannibalism is a bad way to pass down genetic information. “Eating your children is the last thing you want to do from an evolutionary standpoint,” Rosenheim adds. This is one of the main reasons big-eyed bugs regulate their population growth by eating their own progeny. If they become too numerous, like in the aphid trials, they lay eggs all over the area. And because they can’t distinguish their own eggs, they eat their own offspring.
Although cannibalism is not ideal, certain circumstances appear to justify the dangerous activity. If you’re starving, you have to safeguard your survival, says Erica Wildy, an ecologist at California State University, East Bay who was not involved in the study.
Wildy discovered that long-toed salamander larvae are more inclined to nibble on—and occasionally eat—one another when they are hungry.
Rosenheim and his colleagues identify two hormones—octopamine in invertebrates and epinephrine in vertebrates—that appear to be associated to rising rates of cannibalism in their review. As circumstances deteriorate and food becomes scarce, the levels of these hormones rise, and “hangry” animals attack everything they can snare with their jaws, legs, or pincers.
The study also shows how specific environmental factors cause some juvenile amphibians, such as tiger salamanders and spadefoot toads, to become supercannibals. When a pond becomes overcrowded with larvae, certain tadpoles bulk up and develop gaping mouths studded with pseudofangs, transforming them into a “cannibal morph.” Similar cannibal variants have been discovered in mites, fish, and even fruit flies, whose cannibalistic larvae have 20% more teeth on their mouth hooks than their counterparts.
Other species, such as the invasive cane toad, adopt a different strategy. When hungry cannibals are nearby, fragile toad larvae increase their growth and development, packing on bulk until they are too large to eat.
In most circumstances, rampant cannibalism has a beneficial outcome: a less congested, healthier population. As a result, Rosenheim avoids labeling cannibalism as barbarous. “We shudder when we think about cannibalism in human communities,” he says. “However, cannibalism is a significant component to nature’s balance.”
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