It may not have particularly elegant table manners, but despite chewing with its mouth open, the surprising sophistication displayed by the ocellate river stingray in the video below has left scientists with their jaws agape.
Until now, it had been thought that chewing as a form of food processing was performed exclusively by mammals, yet the discovery that stingrays also engage in mastication represents a whole new kettle of fish for evolutionary science.
According to a paper published this week in the Proceedings of the Royal Society B, chewing – which the authors characterize as “shearing jaw motions and high-crowned molars” – is thought to have allowed the dietary diversification of mammals, which in turn facilitated the evolution of increasingly complex cognitive and physiological capacities.
However, given that fish lack the complex dental apparatus required for chewing, marine biologists had always assumed that they were unable to chow down on their food in the same way.
In spite of this, stingrays are known to hunt complex prey species such as insects with tough exoskeletons, leading the researchers to wonder how the pancake-shaped creatures process their food before digesting it. To find out, they placed four rays belonging to the species Potamotrygon motoro in a glass-bottomed tank, and used high-speed video cameras to film their mouths from below as they fed on dragonfly larvae.
The researchers discovered that, despite lacking the intricate molars of mammals, the stingrays did indeed chew their food before swallowing it, with “complex, asymmetrical jaw motions” allowing them to compress and shear their prey. Interestingly, the study authors note that the stingrays’ mouths are used exclusively for this purpose, and are not deployed as a means of catching prey in the first place. Instead, this is achieved by “rapid uplift of the pectoral fins,” which sucks prey beneath the ray’s body.
Based on this finding, the study authors propose that the ability to use their fins to capture food is what allowed the rays to develop this chewing behavior, as it freed up their mouths for this sole function, resulting in the evolution of “a highly kinetic feeding apparatus.” This, in turn, enables stingrays to eat a wide variety of food, thereby helping them thrive in a range of marine and freshwater habitats.