Zoologger is our weekly column highlighting extraordinary animals – and occasionally other organisms – from around the world

It’s a tadpole eat tadpole world (Image: Superstock)

Species: Lithobates sylvaticus

Habitat: much of the north half of North America, from Canada to the Appalachians – where they must survive being frozen solid

Wood frog tadpoles look innocent as they swim in pools or streams in early spring. But they have a ruthless taste for flesh. When the pool gets crowded, these tadpoles don’t just push competitors aside. They eat them.

“Tadpoles are like water balloons; if you puncture their body cavities, they’re dead,” says Dale Jefferson of the University of Saskatchewan in Saskatoon. The cannibal starts to eat the victim alive, and other tadpoles join in the feast.


Why do they do it? Maybe because they are in a race to grow up. The winners transform into frogs and hop into the outside world – the losers become lunch.

Hungry hungry tadpoles

Many species of tadpole have been known to resort to cannibalism occasionally, but it is not clear what drives them to it. To find out, Jefferson studied captive wood frog tadpoles.

One idea is that cannibalism might be the ideal diet to help tadpoles grow rapidly, because it supplies exactly the mix of nutrients found in their bodies. But when Jefferson tried feeding lone swimmers a diet of ground-up tadpoles, they refused to eat it until they were nearly starving. Like humans, they only turned cannibal as a last resort.

But when Jefferson kept the tadpoles in dense groups, he found that they readily ate their ground-up compatriots. That suggests the tadpoles were competing with each other, trying to reach adulthood first.

However, cannibals did not grow as fast as tadpoles that were fed a protein-rich mixture of brine shrimp, suggesting that the cannibal diet was far from ideal.

Tough world

Jefferson says the tadpoles are responding to a difficult situation. The small pools of water they live in are inherently short-lived, so they must grow fast or die when the water dries up. Eating each other might not be the best diet, but it gets rid of the competition.

Cannibalism has its risks, particularly disease transmission. But chemical cues build up if lots of tadpoles are excreting waste into the water, and these make the tadpoles more competitive. Jefferson suggests that the tadpoles are making a subtle decision and turning cannibal when these cues reach a critical threshold.

“The neat part is that the tadpole can assess the situation and make appropriate evaluations,” says Ronald Altig of Mississippi State University.

Wood frog tadpoles aren’t the only ones with a taste for blood – tadpoles of New Mexico spadefoot toads eat plants when they first hatch, but sometimes transform into voracious predators.

Journal reference: Naturwissenschaften, DOI: 10.1007/s00114-014-1156-4