Basic Science

Cannibalism in the animal kingdom

In the deserts of the American Southwest, spadefoot toad tadpoles hatch in tiny oases. Until they develop into toadlets, they can’t survive outside of water, but these ponds are transient and quickly evaporate. The tadpoles are in a race against the clock to grow up before their nurseries disappear.

So nearly overnight, some of the brood explode in size. They use their jack-o-lantern teeth and huge jaw muscles to devour their smaller pond mates. Nourished by this extra fuel, they develop quicker, leaving the pond before it can dry out. The spadefoot toad is far from the only animal to eat members of its own species as a normal part of its life cycle. All of these animals do.

If that surprises you, you’re in good company. Until recently, scientists thought cannibalism was a rare response to starvation or other extreme stress. Well-known cannibals, like the praying mantis and black widow spider, were considered bizarre exceptions. But now, we know they more or less represent the rule. While it may seem counterproductive for members of the same species to eat each other, cannibalism can promote the survival of the species as a whole by reducing competition, culling the weak, and bolstering the strong. Some species, like the spadefoot toad, cannibalize in response to environmental pressures.

Their situation is precarious, but cannibalism for them isn’t a last-ditch attempt to avoid starvation. Rather, it’s a way to more quickly outgrow a stage where they’re especially vulnerable to predation or dangerous environmental conditions. Other species, including many fish, indiscriminately cannibalize each other during foraging behavior. Fish produce large numbers of tiny young, and adults exhibit about as much individual recognition of their offspring as humans do for a handful of raisins.

Fish eggs, larvae, and juveniles are easily available, nutrient-rich meals, and with thousands of eggs in a clutch, plenty are still available to hatch after the adults have snacked. Baby fish aren’t just at risk of being cannibalized by adults— siblings eat each other too. Sand tiger shark eggs develop and hatch inside their mother’s oviducts at different times. When the hatchlings run out of yolk from their own eggs, they eat the other eggs and hatchlings until one baby shark from each oviduct remains. When they emerge, the young sharks are well-nourished, experienced predators who stand a better chance of surviving.

Even when they aren’t consumed for nutrition, young animals are especially vulnerable to cannibalism. Hamsters, rats, and other rodent mothers will eat some of their young if they’re sick, dead, or simply too numerous to feed. In other mammals, including bears and lions, males will kill offspring sired by another. That’s because childless females become receptive to mating more quickly than if they were caring for a cub. Rather than waste nutritious meat, the males then eat the dead cubs.

Meanwhile, cannibalism is less common in birds than in other groups, but certain species will eat diseased or dead hatchlings as a way of disposing of the bodies before they can attract maggots. When adults eat each other, males are cannibalized more often than females, usually during mating and generally because they’re smaller. Male Australian redback spiders mate with much larger females.

Rather than scrambling away after mating, the tiny male does a somersault, bringing his abdomen into contact with his mate’s mouthparts. The female showers him with enzyme-rich gut juice and consumes his abdomen. Males not killed in the initial mating crawl back into the fray, often half-eaten, to mate again, after which they’re dispatched to the spider pantry.

 

So not only does the male provide the female with his sperm, but he also provides her with a nutritious meal to better ensure that she’ll survive to pass on his genes. All in all, it’s clear that cannibalism is as much a part of life in the animal kingdom as other, better-recognized behaviors. As we sink our teeth into the evidence of cannibalism in nature, we might ask ourselves, what else have we missed by applying human standards to the natural world?

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