We humans aren’t so surprised when creepy-looking animals cannibalize each other. Black widow spiders or praying mantises munching on their mates make sense somehow—they look a little monstrous to begin with—but did you know that some seahorses are cannibals? That’s right, the peaceful seahorse, denizen of Disney movies and little girls’ sticker books, occasionally vacuums up its own young with its cute little snout. I can also ruin butterflies and squirrels for you, and if you have ever raised bunny rabbits, well, you may already know.
I learned my own hard lesson about cute animal cannibals some years ago when I worked at a nature center. We had a mating pair of adorable little Eastern screech owls on loan from a zoo. One day, I discovered the male missing. Panicked about an animal escaping, I searched high and low. Then I noticed a lump on the female’s chest. Sure enough, her owl pellets later revealed she had eaten her mate, and we returned just one of the borrowed owls to the zoo. Cannibalism, it turns out, is not uncommon in animals stressed by changes like a new environment.
Geologist David Soulsby, author of a new book on animal cannibalism, was first inspired by a swarm of cannibalistic ground crickets on a road in Zimbabwe. “Many of the insects had been killed by passing vehicles, others were cannibalistically scavenging upon the bodies,” he says in an interview with thepoultrysite.com. “This set me wondering about the phenomenon of animals eating their own kind in nature.”
He scoured the scientific literature on animal cannibalism. The resulting book, Animal Cannibalism: The Dark Side of Evolution, is aimed mainly at biologists, plus animal breeders who contend with cannibalism as a bottom-line problem. The level of scientific detail would be overkill for most general readers, unless you happen to be the Gory Details blogger or another such superfan of animal oddities. But for us superfans, there are some real treasures inside. Here are a few surprises from Soulsby’s book.
Sometimes mom eats her babies, and sometimes babies eat mom.
Several spider species practice matriphagy. Here, baby ground spiders filmed in southwest France eat their dead mother.petepage/YouTube
Mothers eating their young is pretty common, and in fact the most common victims of cannibalism are eggs and newborns. But kids can be cannibals too.
Matriphagy, or mother-eating, is found in some species of insects, scorpions, nematode worms and spiders. One remarkable case of self-sacrifice taken to the limits occurs in crab spiders (Diaea ergandros). The mothers provide their spiderlings with unfertilized “nurse” eggs to eat. The young eat the eggs and also, slowly, their mother. Over the course of weeks, she is eaten away until she falls immobile and is consumed entirely. At least it’s not for nothing: Spiderlings generally do quite well in cases of matriphagy, with higher weights and survival rates than young that don’t eat mom.
Luckily for humans, evolutionary forces dictate that our moms are safe. Only young that are born ready to care for themselves can pull off matriphagy. Even if human fetuses had teeth (and aren’t you glad now that they don’t?), cannibalistic babies would chew their way out of mom only to find themselves totally helpless and without a source of milk.
Perch and their cousins are big-time cannibals.
Cannibalism is most common, Soulsby says, in beetles, spiders and fish in the order Perciformes, a large and common group that includes perch and cichlids. Cichlids are known to be aggressive (I had a green terror cichlid that would leap from the aquarium to nip at a hovering bag of fish flakes), but who knew perch had a dark side? Many of these fish are the kind that little kids catch with a bobber, but they’re big-time cannibals, with parents and siblings often gobbling up the young.
Walleyes, one member of this group, eat each other tail-first. Sometimes one will start to eat another and then get eaten itself, and Soulsby notes that chains have been reported with of up to four fish simultaneously cannibalizing each other.
Some individuals within a species specialize in cannibalism.
In some amphibians, there can be two larval forms: a normal and a cannibalistic type, called a cannibal morph. The cannibals grow larger and, at least in tiger salamanders, have a broader head with a wide mouth and jutting lower jaw. A cannibal morph’s teeth can be up to three times longer than in a normal salamander. Studies of tiger salamanders and the Asian salamander Hynobius retardatus have found that cannibal morphs develop when larvae are crowded in large numbers and are mostly unrelated (same species but from different parents). It makes sense that natural selection would favor sparing siblings that share genes, much the same way that many animals tend to preferentially help siblings over unrelated animals.
Salamanders that develop big crushing jaws specifically to eat their own kind may seem like a really unfair evolutionary invention, but in the big picture they can be good for the species. If a pond dries up, a cannibal morph might be the only survivor, thanks to having grown bigger and fatter on its brothers and sisters. It’s the ultimate adaptation for taking one for the team.