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Certain frogs have some of the biggest eyes of all vertebrates, relative to their body size, which is a significant evolutionary investment that has puzzled zoologists. Now, researchers have found that the eye size of these animals seems to scale depending on their environment.

Eyesight is costly in biological terms because it requires a lot of energy to function, which explains why animals living in dark environments like caves often evolve to have smaller eyes. That means big-eyed frogs clearly need the ability to see contrasts, details, colours and rapid changes in order to thrive in their environment, says Kate Thomas at the Natural History Museum in London.

鈥淚t鈥檚 not that they see any better than we do,鈥� she says. 鈥淏ut compared to us, they鈥檙e investing a lot more of their total energy budget on vision.鈥�

Thomas and her fellow researchers measured the body length, total eye size and cornea size in thousands of fresh and conserved specimens representing 220 species and all 55 families of frogs worldwide. Their results showed that frogs鈥� active hours and how they reproduce affect eye size moderately, but the greatest factor linked to eye size is habitat.

鈥淭ree frogs have the biggest eyes, and they need to climb and jump and make quick decisions while jumping,鈥� Thomas says. Their bright colours might also be a critical form of sexual signalling that other tree frogs need to be able to see.

By contrast, Budgett鈥檚 frogs, which usually live in stagnant, murky waters and rely more on other senses for their survival, have very small eyes.

This still leaves the question of what frogs are giving up in order to have large eyes, says Thomas. If they balance out their energy budget by reducing other senses, such as hearing, researchers have yet to identify which ones. Frogs generally hear very well, she says.