Ocellate river stingrays munch like some rats Free

Grinding away at our dinners, it is evident how much we have in common with many of our mammal cousins. ‘Chewing is an incredibly important behaviour’, says J. D. Laurence-Chasen, from the University of Chicago, USA, explaining that mastication allows animals to extract nutrition that might otherwise remain enclosed in tough skins and protective cases. Not surprisingly, most studies of chewing have focused on mammals. ‘It was once thought that mammals were the only true chewers,’ he says, but more recently it has turned out that other creatures also munch their food before swallowing, even if the action is out of sight, for instance when the mouth is situated beneath the body. ‘A recent paper by Matt Kolmann and colleagues provided a first look at how the ocellate river stingray, Potamotrygon motoro, chews its food’, says Laurence-Chasen, explaining that the researchers filmed the fish's mouths in action after slurping up a tasty treat. ‘They suggested that the stingray chews its food like mammals’, says Laurence-Chasen, but he and PI Elizabeth Brainerd suspected that there was a lot more going on beneath the stingray's scales than was visible. ‘This seemed like a perfect opportunity for XROMM’, smiles Laurence-Chasen.

X-ray reconstruction of moving morphology (or XROMM) allows scientists to take a peek at what is going on inside an animal's body when moving. So, while an undergraduate at Brown University, USA, in Elizabeth Brainerd's lab, Laurence-Chasen and Jason Ramsay gently inserted up to 20 minute tungsten markers into the jaws of ocellate river stingrays, ready to film their chewing motion as they dined on beef heart and crunchy dragonfly larvae. But Laurence-Chasen admits that catching the stingrays in the act of dining could be tricky. ‘Our X-ray video cameras are limited to 10 s per trial – because of overheating’, he says, adding that some of the fish could be fussy eaters, ‘[although] earthworms were a consistent favourite’, he recalls.

Painstakingly following the motion of each individual cartilage structure in the fish's complex jaws, Laurence-Chasen realised that the fish were unexpectedly alternating between two chewing styles. Most of the time the animals simply opened and closed their jaws, compressing the food between their scaly teeth. However, about once every 10 chews, the stingrays threw in an alternative jaw manoeuvre. On those occasions, the fish clamped their jaws more tightly together while also thrusting the upper jaw forward. ‘The upper jaw toothplate comes down past the lower jaw tooth plate, shearing the prey between the upper and lower jaws, producing what looks like an overbite,’ explains Laurence-Chasen. And when he compared the stingray's overbite jaw action with that of chewing mammals, it was similar to the nibbling motion of some rodents, such as mountain viscacha rats (Octomys mimax) and Chilean rock rats (Aconaemys fuscus).

‘Discovering a new type of chewing motion – overbites – was surprising and exciting’, says Laurence-Chasen, who is keen to understand how the fish control the different jaw manoeuvres. ‘I think an important step would be to figure out what sort of sensory information the stingrays are getting from their mouths, and how’, he says. In addition, he is intrigued by the different selective pressures that have driven such distantly related animals to come up with the same dining style despite their enormously different jaw designs.