This Chewing Stingray Sheds Light on the Evolution of How Animals Eat

This Amazonian stingray chews its food like a cow or goat.

Sep 15 2016, 3:11pm

Video: Matthew Kolmann/YouTube GIF by Kate Lunau

When a great white shark gets hungry, it doesn't nibble daintily at its meal. Instead, it rips off a massive chunk of prey and swallows it whole, said Matthew Kolmann, a biologist at the University of Toronto's Scarborough campus, who studies how animals hunt and feed. "Then they let the animal bleed out and shake it apart," before eventually consuming it, he told me.

It sounds terrifying, but there's a point here: Unlike humans, a lot of animals don't chew their food. In fact, for a long time, scientists believed that only mammals broke down food by chewing. "There's been this idea that [chewing] was really important to the diversification of mammals" after the mass extinction that killed the dinosaurs 66 million years ago, Kolmann told me. Even scientists who studied hunting and feeding behaviour tended to ignore "the processing phase," he said—how an animal breaks down food so it can be digested.

"It grew into this dogma that only mammals chew," he continued.

An X-ray of the Potamotrygon motoro stingray. Image: University of Toronto Scarborough

That's part of what makes Kolmann's latest study so surprising. He's interested in stingrays specifically, and in a new paper published in the journal Proceedings of the Royal Society B, he and co-authors describe how some freshwater stingrays that live in the Amazon chew their food—working their jaws back and forth to shear it apart, like cows and goats do.

In the study, Kolmann looked at the ocellate river stingray (Potamotrygon motoro). This fish eats aquatic insects that have shells made of chitin, he said, a tough biomaterial that, like plastic, tends to deform as it's squished instead of breaking. "You don't break plastic by crushing it, but by pulling it apart," he said—suggesting these rays might chew the bugs they eat.

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Using high-speed video to film the rays as they feasted on different kinds prey—fish, somewhat tougher shrimp, and extremely chewy little dragonfly nymphs—Kolmann saw that the rays used an asymmetrical jaw motion, like a cow chewing its cud, as they moved their jaws from side to side. CT scans also revealed that these rays have simple teeth.

"The motion of the jaws is what you'd see in [animals] like goats," he said.

Image: Matthew Kolmann

The idea of a chewing stingray is pretty freaky. But we humans should accept that mammals aren't the only ones to munch on their meals this way. Scientists are starting to catalogue the diversity of creatures who do it: An earlier study, from 2012, found that a New Zealand reptile called the tuatara also chews its meals, with teeth like a "steak knife." Even some dinosaurs chewed, like the Leptoceratops of Western North America.

"Honestly, chewing is a human idea. It's just a word," said Kolmann, who's moving to the University of Washington, to take a new job as postdoctoral researcher at Friday Harbor Laboratories. Chewing is done in a variety of ways, by a variety of creatures.

This tells us something fascinating about evolution, and how certain "physical laws and engineering principles," as Kolmann puts it (including the relative chewiness of certain prey), drove the evolution of a similar type of food processing behaviour in stingrays and in humans—and in lots of other animals, too.

"Drastically different groups of animals, that are only distantly related, are using the same behaviours to tackle the same problems. That's really cool," he said.

Think about that next time you're eating lunch.

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