Why some animals turn cannibal | Science

Jay Rosenheim had no idea 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 ravenous green insect that sucks sap from crops, leaving behind moldy waste and a slew of deadly viruses—he and his colleagues decided to sick another group of insects on them: a stout group of native aphid assassins known as big-eyed bugs.

It worked—for a while. Then, as space became scarce on the plants, something unexpected happened: The big-eyed bugs stopped attacking the aphids and began to hunt one another, devouring hordes of their own eggs. They “became wildly cannibalistic,” says Rosenheim, an entomologist at the University of California, Davis.

Eating your own kind is fairly common throughout the animal world, from single-celled amoebas to salamanders, he and his colleagues report in a new review in Ecology. But not as many species snack on their brethren as one might expect—and the team has detailed the reasons why.

First off, cannibalism is risky. If you’ve got dangerous claws and teeth, so do your comrades. Female praying mantises are notorious for biting the heads off of much smaller males during mating, for example, but they also occasionally go toe to toe with an evenly matched female. “I’ve seen one female chew the leg off another,” Rosenheim says, “and then the female who lost the leg somehow manages to kill the other one.”

Cannibalism is also dicey from a disease perspective. Many pathogens are host specific, so if a cannibal devours an infected companion, it risks picking up the same disease. Different populations of humans have found this out the hard way multiple times. One of the most famous examples is the spread of a rare and fatal brain disease called kuru that ravaged the Fore people of New Guinea in the 1950s. Kuru raged across the Fore community through a cannibalistic funerary ritual in which families cooked and ate the flesh—including contaminated brain tissue—of deceased relatives. Once the Fore phased the ritual out, the spread of kuru was stopped in its tracks.

Finally, cannibalism is a terrible way to pass down one’s genes. “From an evolutionary perspective, the last thing you want to do is eat your offspring,” Rosenheim says. That’s a major reason big-eyed bugs limit their population sizes by snacking on their own offspring. If they grow too numerous—as happened with the aphid experiments—they deposit eggs all over the place. And because they can’t recognize their own eggs, they end up devouring their own brood.

Although cannibalism is far from ideal, certain conditions appear to make the risky behavior worthwhile. Even if you’re eating a friend—or an heir—if you’re starving, you’ve got to protect your survival, says Erica Wildy, an ecologist at California State University, East Bay, who was not involved with the study. In her own work, Wildy has found that hunger makes long-toed salamander larvae more likely to nibble on—and occasionally eat—one another.

In their review, Rosenheim and his colleagues pinpoint specific hormones—octopamine in invertebrates and epinephrine in vertebrates—that appear to be linked to increasing rates of cannibalism. As conditions become crowded and food becomes scarce, the amounts of these hormones spike and “hangry” animals attack whatever they can snatch with jaws, legs, or pincers.

The study also highlights how certain conditions make some young amphibians such as tiger salamanders and spadefoot toads turn into supercannibals. When a pond is crowded with larvae, some tadpoles transition into a “cannibal morph” by bulking up and sprouting gaping jaws studded with pseudofangs. Similar cannibal morphs crop up 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, such as the highly invasive cane toad, take the opposite approach. When hungry cannibals are lurking, vulnerable toad larvae accelerate their growth and development, tacking on mass to become too big to scarf down.

In most cases, the end result of rampant cannibalism is positive: an uncrowded, healthier population. For that reason, Rosenheim shies away from viewing cannibalism as barbaric. “When we think of cannibalism in human populations, we recoil,” he says. “But cannibalism is one of the key contributors to balancing out nature.”

Jay Rosenheim had no idea 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 ravenous green insect that sucks sap from crops, leaving behind moldy waste and a slew of deadly viruses—he and his colleagues decided to sick another group of insects on them: a stout group of native aphid assassins known as big-eyed bugs.

It worked—for a while. Then, as space became scarce on the plants, something unexpected happened: The big-eyed bugs stopped attacking the aphids and began to hunt one another, devouring hordes of their own eggs. They “became wildly cannibalistic,” says Rosenheim, an entomologist at the University of California, Davis.

Eating your own kind is fairly common throughout the animal world, from single-celled amoebas to salamanders, he and his colleagues report in a new review in Ecology. But not as many species snack on their brethren as one might expect—and the team has detailed the reasons why.

First off, cannibalism is risky. If you’ve got dangerous claws and teeth, so do your comrades. Female praying mantises are notorious for biting the heads off of much smaller males during mating, for example, but they also occasionally go toe to toe with an evenly matched female. “I’ve seen one female chew the leg off another,” Rosenheim says, “and then the female who lost the leg somehow manages to kill the other one.”

Cannibalism is also dicey from a disease perspective. Many pathogens are host specific, so if a cannibal devours an infected companion, it risks picking up the same disease. Different populations of humans have found this out the hard way multiple times. One of the most famous examples is the spread of a rare and fatal brain disease called kuru that ravaged the Fore people of New Guinea in the 1950s. Kuru raged across the Fore community through a cannibalistic funerary ritual in which families cooked and ate the flesh—including contaminated brain tissue—of deceased relatives. Once the Fore phased the ritual out, the spread of kuru was stopped in its tracks.

Finally, cannibalism is a terrible way to pass down one’s genes. “From an evolutionary perspective, the last thing you want to do is eat your offspring,” Rosenheim says. That’s a major reason big-eyed bugs limit their population sizes by snacking on their own offspring. If they grow too numerous—as happened with the aphid experiments—they deposit eggs all over the place. And because they can’t recognize their own eggs, they end up devouring their own brood.

Although cannibalism is far from ideal, certain conditions appear to make the risky behavior worthwhile. Even if you’re eating a friend—or an heir—if you’re starving, you’ve got to protect your survival, says Erica Wildy, an ecologist at California State University, East Bay, who was not involved with the study. In her own work, Wildy has found that hunger makes long-toed salamander larvae more likely to nibble on—and occasionally eat—one another.

In their review, Rosenheim and his colleagues pinpoint specific hormones—octopamine in invertebrates and epinephrine in vertebrates—that appear to be linked to increasing rates of cannibalism. As conditions become crowded and food becomes scarce, the amounts of these hormones spike and “hangry” animals attack whatever they can snatch with jaws, legs, or pincers.

The study also highlights how certain conditions make some young amphibians such as tiger salamanders and spadefoot toads turn into supercannibals. When a pond is crowded with larvae, some tadpoles transition into a “cannibal morph” by bulking up and sprouting gaping jaws studded with pseudofangs. Similar cannibal morphs crop up 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, such as the highly invasive cane toad, take the opposite approach. When hungry cannibals are lurking, vulnerable toad larvae accelerate their growth and development, tacking on mass to become too big to scarf down.

In most cases, the end result of rampant cannibalism is positive: an uncrowded, healthier population. For that reason, Rosenheim shies away from viewing cannibalism as barbaric. “When we think of cannibalism in human populations, we recoil,” he says. “But cannibalism is one of the key contributors to balancing out nature.”