When Stephen Deban saw his first Phrynomantis bifasciatus frog, he knew he was looking at something truly bizarre. The petite red and black frog seemed able to fling its tongue in almost any direction, even trapping insects that had wandered onto it's forelegs. This was completely unexpected. Most frogs only aim their tongues in one direction; straight forward. Somehow the tiny anuran had developed the frog equivalent of manual dexterity. But how Phrynomantis controls its agile tongue wasn't clear. Most frogs simply flip the tongue out, yet Phrynomantis was definitely doing something different; but what? Kiisa Nishikawa decided to put the amphibian through its feeding paces. Working with Jay Meyers, James O'Reilly and Jenna Monroy, the team probed the tongue's inner workings to get to the bottom of the frog's unorthodox feeding habits (p. 21).
By mammalian standards, frogs' tongues aren't particularly versatile, in fact they're backwards. Connected at the front of the jaw, the tongue points backwards in the mouth, so when a frog spots a tasty morsel, it's simply a matter of snapping the mouth open so that the front-attached tongue rolls out and flips forwards. Meyers explains that for most frogs, this is an uncontrolled ballistic movement. But watching Phrynomantisesdeviations from the straight and narrow, it was clear to Meyers that the sideways flick was a controlled, muscular movement.
Teaming up, Meyers and O'Reilly came up with three possible scenarios for the tongue's deviations. Either the animal's jaw flexed as it opened, so that it threw the tongue to one side, or a muscle on one side of the tongue contracted, pulling the tongue in that direction. The third possibility was that the animal controlled its tongue hydrostatically. Meyers explains that when a hydrostatic muscle contracts, its volume doesn't change. So if a muscle is constrained by connective tissue, then a contraction in one direction will cause the muscle to extend in another. Meyers figured that the frog's mobile tongue could be guided hydrostatically so that a contracting muscle on one side would cause that side of the tongue to lengthen, and push the outside edge of the tongue so that it turns away from the contracting side.
Knowing that they could identify the tongue twister by temporarily disconnecting the nerves to various muscles, and then testing the frog's aim,the team set about tempting the animals with termite snacks. First they disconnected the jaw bending muscles on one side to see whether they affected that frog's aim. But its accuracy was unaffected, so the team began focusing on the muscles in the tongue. They disconnected the nerve that controls all of the tongue's flipping muscles on one side, and set the frogs the same termite-trapping task.
Meyers and O'Reilly reasoned that if the tongue was controlled by a contraction that pulled it in the direction of the contracting muscle, then a frog that had lost the use of muscles on that side could only direct its tongue in the opposite direction. But if the tongue was directed hydrostatically, then it could trap termites on the disconnected muscle's side, and not the other.
Challenging the animals to catch termite treats placed on either side,Meyer's patience was rewarded; the frogs captured termites on the same side as the inactive muscles. The tongue was hydrostatically directed. And when the team looked at the muscle structures in the tongue, there were the telltail signs of a hydrostatic muscle, pushing, not pulling, the tongue in the right direction.