When someone says people eat with their eyes, they usually mean that we assess and appreciate food by sight well before it ever hits our tongues. (What’s more, the appearance of food and even the container it’s in can actually change our perception of its taste.) If they’re talking about frogs, though, they mean it in a much different, more literal way. 

Scientists and nature lovers have often pointed out that frogs’ and toads’ eyes shut tight and sometimes seem to sink into their heads when they swallow food. This is because, scientists proposed, the amphibians’ eyes play a role in moving the food along. Specifically, they push food down the throat like a trash compactor. As biologist Mary Dickerson put it in 1906, “Strange as it may seem, the large eyes of the the toad can be pressed down into the mouth as far below its roof as they rise above the head, and the movement aids effectually in swallowing.” 

That does seem strange, but also really cool. The catch is that no one really tested the idea until 2004, when a team of biologists at the University of Massachusetts set out to see just what a leopard frog’s eyes are up to when it eats.

X-ray videos of the animals taken during meal time revealed that the eyes did move into the mouth, “came in contact with the prey and appeared to force it toward the pharynx.” Recordings of the electrical activity in the eye retractor muscles, meanwhile, showed that the eye retractions were active and on purpose, and that the eyes weren’t just sinking due to low pressure in the mouth below them during swallowing. Finally, an experiment where the researchers cut the nerves to the retractor muscles showed that frogs that couldn't retract their eyes were still able to swallow, but had to swallow almost twice as many times as usual to get each piece of food down. 

All this suggests, the researchers say, that frogs' eyes do help them swallow by aiding the tongue in pushing food into the throat, and probably contribute more to the process when their meals are larger. 

[h/t Io9]