Science

Some freshwater stingrays chew their food like goats, researchers find

Researchers at the University of Toronto have observed an unusual trait amongst some variety of stingrays: they can chew their food in much the same way as mammals.

Two species of stingrays can shred and tear their prey apart, allowing them to eat aquatic insects

Freshwater stingray chews its food like a goat (courtesy of Matthew Kolmann)

6 years ago
Duration 0:28
Freshwater stingray chews its food like a goat (courtesy of Matthew Kolmann)

Researchers at the University of Toronto have observed an unusual trait amongst some variety of stingrays: they can chew their food in much the same way as mammals.

These findings suggest chewing may be more common among non-mammals than previously thought.

Matthew Kolmann used high-speed video cameras to capture footage of stingrays eating a small insect.

The stingrays catch their prey by lifting the front of their wide, disc-like bodies and letting water and prey flow into their mouths on their undersides, sort of like a suction cup.

They then extend their upper and lower jaws outward, quickly moving them up, down and side-to-side in a chewing motion, grinding up the insects' tough exterior.

A freshwater stingray chows down on an aquatic insect larvae. This chewing motion is rarely seen in non-mammal species. (Matthew Kolmann)

"It's pretty extraordinary when you think about it — here's this bizarre-looking fish from the Amazon that evolved these behaviours separately from mammals, but chews its food just like a cow or a goat," Kolmann said in a press release.

The chewing motion allows the stingrays, with soft bodies mostly made out of cartilage and teeth that are much duller than a human's, to chow down on shellfish and insects.

Other animals in their family of species, like sharks, can bite down on prey, but are unable to replicate the more complicated side-to-side chewing motion.

​The study was published this week in the journal Proceedings of the Royal Society B.

Mammals likely developed this ability 65 million years ago, allowing them to grind down and digest tough prey that would otherwise be too difficult to eat.

"Both mammals and these stingrays — two groups that have little to do with each other — developed a similar solution to tackling a bio-materials problem, and that is how to break down tough prey," Kolmann said.​

Researcher Matthew Kolmann studied the eating habits of freshwater stingrays after he discovered two species native to the Amazon feed on aquatic insects. (Ken Jones)

Kolmann and his team of researchers observed two freshwater stingray species: Potamotrygon motoro, which also eats other fish, and crustaceans like crabs and prawns, and Potamotrygon orbignyi, which only eats insects.

P. motoro and P. orbignyi developed this ability largely because they had no other choice. The species invaded the Amazon between 20 and 40 million years ago, and had to evolve a method to break down the tough outer membrane of insect larvae in order to properly digest it.

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