Frogs Have Huge Eyes Compared To Body Size, And We Now Know Why

The big-eyed tree frog Leptopelis vermiculatus uses its enormous eyes to assess its leafy environment. Monica Martinez Do-Allo/Shutterstock

Frogs come in all sorts of wacky shapes and sizes, but certain groups within them share exaggerated traits, be it vibrancy or potency of poison. Tree frogs have some of the biggest eyes compared to body size of the vertebrates, while cave frogs have tiny eyes, and new research points as to why. The study, published in the journal Proceedings of the Royal Society B, has discovered that a frog's habitat dictates its eye size, explaining why some sport tiny peepers while other's are huge.

Kate Thomas of the Natural History Museum in London set to work with a team of researchers to investigate fresh and preserved frog specimens across 220 species of anurans representing all 55 currently recognized families. They measured the frogs’ body length, cornea size, and total eye size to see how the ratios changed across species.

The frogs measured in the study had a diverse range of eye sizes. © 2020 Thomas et al

Their results showed that among the big-eyed and small-eyed frogs, there was a running theme: shared habitats. Frogs living in caves where conditions are dark and largely monochrome had smaller eyes, while frogs from forest environments had the biggest eyes of all.

Using our eyes is expensive energetically speaking, and so the researchers hypothesize that frogs have adapted to spend big on giant eyes only when totally necessary. Tree frogs were among those with the biggest eyes in Thomas’ collection, which fits the theory. These animals need to take in lots of detail and notice changes in the lay of the land rapidly to avoid predation. They also need to find food, shelter, and other frogs, which if you’re green in a leafy forest is a bit like finding a needle among a stack of moving needles.

The aptly named Budgett's frog spend less energy on its comparatively tiny eyes. Shutterstock/Takayuki Ohama

“Tree frogs have the biggest eyes, and they need to climb and jump and make quick decisions while jumping,” said Thomas to New Scientist. “It’s not that they see any better than we do but, compared to us, they’re investing a lot more of their total energy budget on vision.”

Giving up on massive eyes to focus on senses that are better suited to say, dark murky waters makes sense if they’d be more beneficial to your environment. What remains unclear, however, is what senses the frogs with the largest eyes have sacrificed to enable such a flashy optical budget.

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