Chewing has always been thought to be exclusive to mammals, humans included. It is one of the key innovations that has led to their success in surviving. Chewing allows mammals to fuel their warm-blooded (endothermic) metabolism with tough prey that would otherwise be too costly to ingest without breaking it down first. For example, a goat, a hooved mammal, chews hard and then shears it sideways. On the other hand, sharks, a type of fish, rip off massive chunks of their prey and swallow it whole. Freshwater stingrays are not mammals, but a fish that chews like one.
A type of freshwater stingrays, Potamotrygon motoro, known as the peacock-eye stingray or Ocellate river stingray are native to the basins of the Uruguay, Orinoco, and Amazon Rivers. They are known to grow up to be 3 feet long, and can weigh up to 33 pounds. They have a circular disk with raised eyes from the dorsal surface. They are brown or beige in color with many light orange spots surrounded by dark rings. A close relative of the P. motoro is the Potamotrygon orbignyi. They are known as smooth back river stingrays and can grow to be approximately 35 centimeters or 13.8 inches long and weigh 3 pounds. These marine fish are unique for a family that includes sharks, skates, and other stingrays in that they tend to feed on aquatic insects. Stingrays have fins that encircle their head called a disk. They don’t use their mouth to catch their prey, but use the disk. By lifting the front part of their disk, they are able to draw water and prey underneath, similar to a suction cup. Once they have succeeded in trapping their prey, they grab it by rapidly protruding their jaws, shredding and tearing it apart. While they don’t grab their prey through the use of their mouths, they do have the same chewing motion as a cow or goat.
Both species of freshwater stingrays are found all throughout the Amazon in lakes and rivers. They live from the mouth of the river near the eastern Brazil coast all the way to the foothills of the Andes in Peru. Each type has a different diet, the P. motoro’s diet consists of crustaceans such as crabs and prawns as well as insects, while P. orbignyi only eats insects.
Obviously stingrays are not mammals, but for some reason the bizarre-looking fish has evolved to chew like a goat or a cow. According to Matthew Kolmann, freshwater stingrays evolved to eat insects and chew because they had little to no choice. When they invaded the Amazon River 20-40 million years ago, there was an abundant amount of aquatic insect larvae. While these insects were very nutritious to freshwater stingrays, they were also very difficult to break down, so chewing was necessary in order for digestion. Therefore, leading to the use of chewing among stingrays. Some scientists believe that insect-feeding may have helped the rays avoid competition with other fish when they moved from saltwater to the freshwater of the Amazon River.