Cannibalism has a bad rap, but the more scientists learn about it, the more they discover it’s a vital part of nature.
The practice of eating one’s own kind is “wildly common across the animal kingdom,” says biologist Bill Schutt, author of Cannibalism: A Perfectly Natural History and a biology professor at Long Island University. It’s most often observed in invertebrates and fish, he says, but cannibalism occurs in every major animal group.
“For a long time the conventional wisdom amongst ecologists was that cannibalism was an aberrant behavior,” borne of the stress of captivity or unnatural lab conditions, adds Jay Rosenheim, an entomologist and nematologist at University of California, Davis.
“Only in recent decades has it been seen as an adaptive strategy for survival and reproduction.”
In the family
Many species cannibalize their young, usually when an infant is sick, deformed, or born into conditions where the mother can’t produce milk or provide other food. If a baby dies or is unable to survive, its body can become a valuable source of nutrients for the parent. This is generally known as filial cannibalism.
Species that engage in filial cannibalism include leopards, African lions, Tonkean macaques, and many fish species.
This form of cannibalism can also happen much earlier, when the offspring are still eggs. Egg cannibalism can be easy, nourishing, and requires little effort.
“If you’re a cod and you’re laying five million eggs, right there in front of you is a food source that is non-threatening and nutritious,” says Schutt.
Female cichlid fish from Central Africa, called Burton’s mouthbrooders, carry their eggs in their mouths. Surprisingly, a recent study showed these mothers sometimes consume more than three-quarters of their own eggs and baby fish.
In other species, males will kill infants unrelated to them, an act known as infanticide. In these instances, such as with the North American red squirrel, males kill and eat pups who aren’t theirs or whose paternity is uncertain. Doing so puts the female back into estrus and ready to mate again quickly. The male can then guard her against other copulation, ensuring the pups are his. Infanticide also occurs in lions and chimpanzees.
In other species, siblings will turn on each other for a chance at life. Spadefoot toads, for example, have been known to lay eggs in ponds that can dry up quickly. In response, some tadpoles grow bigger heads, wider mouths, and sharper teeth than their omnivorous siblings. Then these cannibal morphs eat their kin, which helps them mature at high speed.
Overcrowding is most likely to spur cannibalistic behavior, adds Rosenheim.
“When a species is at a relatively low density … there’s enough food to go around, they’re not bumping into each other, they have their own space, and there aren’t a lot of diseases being transmitted, you don’t often see a lot of cannibals,” he says.
Some species don’t wait until they’re born to start eating their siblings.
Female sharks carry hundreds of eggs in their two uteruses. They mate with multiple males and carry offspring from multiple fathers. Inside the womb, developing sharks will eat the remaining unfertilized eggs and eventually their embryonic siblings, a behavior called intrauterine cannibalism. Eventually only two unborn sharks are left, one in each uterus.
This makes them, in other words, born hunters.
Dying to live on
Spiders employ yet another form of cannibalism known as matriphagy, in which newborns eat their mother.
In her final act of motherhood, a velvet spider will dissolve her own organs and vomit them up for her offspring to eat before they eventually consume her entire body.
The last organs that liquify are her heart and ovaries, a failsafe to ensure that if a male spider eats her brood, she can mate with him and try again. (Watch baby spiders eat their mother alive.)
Many insect or arachnid fathers don’t live to see their offspring. In an act of sexual cannibalism, females consume all or part their male partners during or after mating.
Male Australian redback spiders are often cannibalized by larger and more mature females. Some even twist their bodies to end up on impaled on her fangs, which not only provides a healthy meal for the expectant mom but also increases the chance the offspring he sired will survive.
No less hungry are female sagebrush crickets, which bite males’ hindwing while mating and drink their nutrient-rich blood, known as hemolymph. This is an attempt to make the male unable to mate with another female again. (Watch a video of a male polar bear cannibalizing a cub.)
Famously, European praying mantis females bite off a male’s head before he’s even started to mate. But males have adapted a way around this. Even headless, a male mantis’ sexual movements are controlled by tissue in his abdomen. The male’s sacrifice contributes to the health of both the female and the number of offspring he sires with her.
For many animals, cannibalism has benefits. But Rosenheim notes that it also has drawbacks, such as spreading disease. And eating a relative isn’t the optimal way to pass genes forward.
“All things being equal, avoiding cannibalism is a good idea,” he says.