It’s clear to see that species like tree frogs have gigantic eyes, but the visual systems of most frogs have gone largely unstudied by scientists.
An international, interdisciplinary team of researchers is starting to change that. In the first study to come out of the collaboration, the team examined museum specimens representing all 55 frog families to test hypotheses about the evolution of frog eye size and its relationship to different aspects of their lifestyles. The results show that, overall, frogs are investing a lot of energy in maintaining their eyes and that vision is likely important to their survival and reproductive success.
Rayna Bell, assistant curator of herpetology at the California Academy of Sciences and one of the new paper’s authors, says that although frogs were an early model for studying vision at the turn of the 20th century, the research was limited to a small number of easily accessible species.
“It was not reflective of the enormous diversity of frogs that we know exist today,” she says. “Our understanding of vision in frogs has lagged behind such research in other vertebrates, such as fishes, birds, and mammals.”
There are more than 7,000 frog species living in a variety of different habitats and ecosystems: Some spend their entire lives underwater, others live in the treetops, and others burrow underground.
The first goal of Bell and her colleagues was to broaden out from those initial frog species whose vision had been characterized historically and document the diversity in eye size among frogs. To do this, her team depended primarily on specimens preserved in natural history collections around the world. They measured eye size in 220 frog species representing all 55 families.
The study revealed that frogs have relatively large eyes for their body size, with certain species of tree frog coming out on top. Bell says that relative eye size is an indication of how much of an organism’s energy budget is invested in eye tissue.
“Eyes are metabolically expensive to maintain, so if you have large eyes, that suggests you are relying pretty heavily on vision and investing energy in maintaining that eye tissue,” she says.
The team also tested whether eye sizes were correlated with life history traits like where the frogs lived or whether they are active at day or night. As is seen in other animals, there was a strong association between habitat type and eye size, with species that live underground or in murky water having reduced eyes. It’s likely that in these light-deficient environments, animals are not relying as much on vision and it’s not worth it to invest in growing large eyes.
Bell says this study is an important first step in understanding the diversity of eye size in frogs. Next, she and her colleagues are looking at the genetic underpinnings of this variation and characterizing photoreceptor sensitivity in different frogs. She’s also interested in how frogs’ visual systems change as they develop from aquatic tadpoles into adults who might live in a very different habitat.
“It’s interesting from an evolutionary perspective but also from a behavioral perspective, in terms of understanding how frogs are sensing and interacting with their environment,” says Bell.
“We know that they have big eyes. We don’t know specifically why but we’re working on it.”
Thomas KN, Gower DJ, Bell RC, Fujita MK, Schott RK, and Streicher JW. (2020). Eye size and investment in frogs and toads correlate with adult habitat, activity pattern and breeding ecology. Proceedings of the Royal Society B 287: 20201393. Doi: 10.1098/rspb.2020.1393.