Chewing might not seem like a particularly special skill, but it was long thought to be a strictly mammalian ability. Only a few non-mammals do it – and you might be surprised to find out that a freshwater stingray from the Amazon basin is one of them. 

Using high-speed cameras and a glass-bottom enclosure, a team of researchers led by University of Toronto PhD candidate Matt Kolmann found that the ocellate river stingray (Potamotrygon motoro) actually chews its food.

Kolmann first became interested in the feeding strategies of freshwater rays after learning that some of them feed on insects. The rays' prey of choice has an exoskeleton made largely of chitin (the same substance that gives beetles and sea mice their brilliant shine). Besides its light-bending ability, chitin is known for its toughness – and yet ray skeletons lack the calcium-rich bone mammals rely on to break down hard food. 

"[So] how do you break tough stuff? Hard stuff? Flexible stuff?" Kolmann asked back in June in a guest post about his research at Southern Fried Science. "These rays are serving as an example of how animals tackle problems in materials science." 

Rays don't actually use their mouths to nail a target. Rather, they create suction by lifting up the edges of their wide discs, which pulls everything towards the mouth (you can recreate this kind of manoeuvre by first pressing down and then arching your back while lying in the bath). This move away from using jaws for snatching possibly sparked the evolution of a new, more complicated function for them, Kolmann suspects. 

Just like you do, these stingrays have loose jaw joints, and the team found that those joints are capable of complex motion: forward-backward to pinch prey in place, and left-right to tear it to bits. Sure, lots of aquatic animals can shred – just look at sharks and crocodiles – but these animals all employ the bite-and-shake method of shearing flesh: 

"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," says Kolmann. 

Stranger still, another freshwater ray, the smooth back ray (Potamotrygon orbignyi) can essentially change the shape of its teeth for a more successful chew. By flexing the jaws, the smooth, simple teeth can be reoriented, creating a "pointier" bite. Unlike its cousin, the smooth back ray is an opportunistic hunter, and it's possible that this dental switch-up helps the fish tackle even harder grub like crustaceans. 

The team hopes to find out just how many of the 30 or so freshwater ray species are chewers, and in the process, learn more about how the various feeding strategies of these animals evolved.

Freshwater rays didn't seek life away from the sea, but rather found themselves stuck in "sweet water" after the rise of the Andes Mountains changed the flow of regional estuaries. Isolated from the Caribbean, the rays' environment became less and less saline, and over millennia, they adapted to the change.

"Freshwater rays are a great study system for investigating how biodiversity arises," writes Kolmann. "Instead of extinction, these rays flourished in new habitats, with new diets, and against new competitors."