Weird Animal Question of the Week

Friends For Dinner: Why Some Animals Become Cannibals

Motivations for eat your own kind range from mind-controlling parasites to starvation.

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A male polar bear eats a cub in Churchill, Manitoba. The behavior may be becoming more common, experts say.


An exclusive video of a male polar bear cannibalizing a cub ran on National Geographic last week—the first known instance of the behavior caught on film.  

Tough to watch, we know, but cannibalism has long been known to occur in polar bears during times when seals, their main prey, are hard to find. (See "4 Ways Polar Bears Are Dealing With Climate Change.") 

Weird Animal Question of the Week took the author’s prerogative to ask “Why do animals cannibalize?” 

Baby Eaters

It isn’t always desperation that drives animal to eat their own. (See "The Flesh-Eaters: 5 Cannibalistic Animals.") 

Take the African lionAlison Dunn, an evolutionary biologist at the University of Leeds in the U.K., says when a young male lion takes over a pride from an older male, it will kill and sometimes cannibalize some of the cubs. 

There's a strategy behind this: The females go back into estrus, so the males “get to mate with them and have their own babies," Dunn says. (Also see "Rare Picture: Hippo Seen Eating Hippo—and More Cannibals.") 

World's Deadliest: Lion vs. Lion This alpha male lion has territory and females to spare. But when two bachelor males invade his turf, this lion king will have to fight them both to keep his crown.

Cannibalizing young is normal among the tiny shrimp Gammarus duebeni—especially when they're infected with a mind-controlling parasite called Pleistophora mulleri.

The parasites steal food from the shrimp, causing the ravenous hosts to eat twice as many of their own kind as usual, says Dunn, who co-authored a study on the bizarre phenomenon

The infected shrimp also turn white and sluggish, making them look even more like zombies. (Related: "Meet 5 'Zombie' Parasites That Mind-Control Their Hosts.") 

Another freeloader with a cannibalistic connection is the twisted wing parasite, says Katy Prudic, an entomologist at University of Arizona.  

Female twisted wing parasites—which are eyeless and legless—live their whole lives inside their host, such as a bee or a wasp.  

After being inseminated, the mother lay eggs “basically inside herself,” which also hatch inside her and are thought to "eat her body from the inside out" before escaping into the next host, Prudic says.  

You Could At Least Be My Dinner First

Many of us have heard about sexual cannibalism among spiders and praying mantises, which is a sort of “glorified nuptial gift,” Prudic says. (Related: "Surprise! Male Spiders Eat Females, Too.") 

World's Weirdest: Deadly Praying Mantis Love Sure, love can break your heart. But if you're a male praying mantis, it can literally eat you alive. During mating, the female bites off his head... and then devours his corpse for nourishment.

“The female needs more energy than the environment can provide. So the male, in exchange for siring her offspring,” sacrifices himself as a snack. 

But sweet little butterflies? 

Usually they don't eat their own kind, but it does happen.  

For instance, caterpillar larvae that hatch and don't find enough food “chow through anything, and that includes eggs of their siblings,” she adds. (Also see "Praying Mantises Falling Victim to Sex Cannibal.") 

So for butterflies, cannibalism is the result of a “low-resource, high-density situation.”  

Ch-ch-ch-ch-changes

Tiger salamander larvae can become cannibals in crowded conditions. But it’s not for lack of food. 

Larvae can come in different varieties, or "morphs"—including one that's born cannibalistic. This morph is bigger than the other types: “It’s got bigger jaws, bigger body, bigger everything —it’s a serious investment. They are committed to being cannibals,” Prudic says. 

Interestingly, the cannibal morphs will avoid eating their kin

“If you can imagine you ate a complete stranger, you'd get a meal and you'd remove a competitor from the situation,” says the University of Leeds' Dunn. 

"But if you accidentally ate your siblings or offspring, you had just killed a relative carrying your own genes. That would be a really daft idea.”  

That’s one reasons humans don’t do it…right? Err...  

Weird Animal Question of the Week answers your questions every Saturday. If you have a question about the weird and wild animal world, tweet me, leave me a note or photo in the comments below, or find me on Facebook.

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