In September 2020, the New Scientist magazine reported on research to determine why frogs have evolved big eyes.
Some frogs have the biggest eyes of all vertebrates (animals with backbones), in relation to their body size, and zoologists did not know why. Now researchers have found that the size of the eyes of these vertebrates seems to depend upon their environment.
Eyesight requires a lot of energy to function – focusing, adjusting peripheral vision, calculating distance, determing what the object is, and so on. There is a lot of things the eye must do quickly to ‘see’ what is in front and around it. Scientists think this is why animals living in dark environments, such as caves, often evolve to have smaller eyes.
Big-eyed frogs need the ability to see contrasts, details, colours, shapes, and rapid changes so that they can survive and thrive in their environment, says Kate Thomas at the Natural History Museum in London, England. She says, ‘It’s not that big-eyed frogs see better than humans, but compared to us, they are investing a lot more of their total energy budget on vision.’
Kate Thomas and her research colleagues measured the body length, total eye size, and cornea size of thousands of fresh and conserved specimens of frogs, representing 220 species in all 55 families of frogs worldwide.
The results showed that three factors affect the size of the frogs’ eyes: 1) their active hours – whether they are more active during the day (diurnal) or at night (nocturnal), 2) how they reproduce, and 3) their habitat. The greatest factor is their habitat.
Arboreal (tree) frogs have the biggest eyes, because they need to climb and jump and make quick decisions while jumping, says Kate Thomas. Also, their bright colours might also be a critical form of sexual signalling that other tree frogs need to be able to see.
In contrast, Budgett’s frogs (Lepidobatrachus laevis), from South America, have very small eyes. This is because they usually live in stagnant, murky freshwater and rely more on their other senses for survival, such as smell, touch, hearing, and taste, than on their sight.
Journal reference: Proceedings of the Royal Society B, DOI: 10.1098/rspb.2020.1393
Photographer: Martina Nicolls
Martina Nicolls: SIMILAR BUT DIFFERENT IN THE ANIMAL KINGDOM