Some frogs have the largest eyes of all vertebrates relative to their body size, a significant evolutionary reversal that has long puzzled zoologists. Now, a team of scientists has discovered that the size of the eyes of these animals seems to scale depending on their environment.
Frogs are a diverse group of animals that live on every continent except Antarctica, in a wide range of habitats, from the tops of treetops to the bottoms of rivers and lakes. They have adapted to live in most terrestrial and freshwater environments, hence they are so numerous and widespread.
But what about his huge eyes?
Sight is a very expensive tool or sense in biological terms because it requires a lot of energy to function, which explains why animals that live in dark environments such as caves tend to evolve into smaller and insignificant eyes. That means big-eyed frogs clearly need the opposite: an enormous ability to see contrasts in colors, light, details …
This study demonstrated how frog eyes are closely linked to the habitats in which they live. Thus, tree frogs have huge eyes compared to the body size of vertebrates, while cave frogs have tiny eyes. The work, published in the journal Proceedings of the Royal Society B , reveals that a frog’s habitat dictates the size of its eyes, which explains the substantial difference in size.
Researchers at the Natural History Museum in London examined fresh and preserved frog specimens from 220 species of anurans representing the 55 currently recognized families of frogs. They measured the frogs’ body length, corneal size, and overall eye size to see how the proportions changed between species.
“We wanted to know how eye size evolved in frogs and if that is related to their way of life,” explains Katie Thomas, leader of the work. “This is important, because optically, if you have a bigger eye, you can have better vision, but there is a balance between seeing the contrast and seeing the spatial details. These two aspects are fighting each other, because often to improve one there is than to sacrifice the other “.
Each environment in which a species of frog lives demands something different from its sight
So the darker and gloomy the environment, the smaller their eyes are, which happens both in caves and in amphibians that lead a completely aquatic life; and, by contrast, living frogs have larger eyes. Eyes as large as those seen by tree frogs are like this because they need to capture a lot of detail and notice changes in the layout of the terrain quickly to avoid predation. They also need to find food, shelter, and their other companions.
” Mating habits, habitat, and activity pattern had significant effects on eye size, but the biggest difference was between habitats,” Katie argues. “The main thing we found was that cave species have smaller eyes, which is not unexpected, but so do those that are completely aquatic.”
It is not that by having bigger eyes they see better compared to human beings, but that they are sacrificing more “energy budget” than we do for vision. What price they are paying exactly, we do not know at the moment.
Animals that move into an underground world tend to have their eyes reduced in size. This trend is seen in most major groups of vertebrates, including mammals, fish, lizards, and snakes.
