Frog eyes shed light on their UV vision

Our eyes have been opened to the extraordinary world of amphibian eyesight.

An international team of researchers, led by scientists at the Museum, found that diurnal frogs, which are active during the day, tend to have evolved lenses that filter out UV light to provide sharper focus.

Nocturnal climbing frogs have also experienced stronger natural selection for this trait, likely giving them an advantage when traversing branches and jumping between trees.

Meanwhile, frogs active on the ground or in water at night are more likely to have less UV filtering, helping them to see in the dark.

Dr David Gower, co-author of the paper and a researcher at the Museum, says that studying the vision of these animals gives insights into their overall evolution.

'Studying vision might help us understand the diversity and evolution of frogs and toads,' he says, 'because they are a very large and diverse group with approximately 7,500 living species. Investigating their sensory biology allows a fresh perspective on these animals, looking at the extent to which their vision has constrained or promoted different evolutionary possibilities.

'In addition, studying frog and toad vision will likely improve our overall understanding of vision across all vertebrate life.'

The findings of the study were published in the journal Functional Ecology. 

What's special about amphibian vision?

Eyesight is one of the most important senses in the animal kingdom, allowing organisms to interpret the world around them rapidly. The importance of eyesight to a species' survival can take them down many different evolutionary paths.

Eyes can have very different forms, such as the compound eyes of insects or the camera eye of vertebrates. Their shape can also vary, with deep-sea fish such as the barrel-eyed fish (Opisthoproctus soleatus) having tubular eyes that allow them to pick out the silhouettes of other marine life above them.

Amphibians are unusual because their eyes have adapted very differently to other living vertebrates.

Co-author Dr Jeff Streicher, Senior Curator in Charge of Amphibians and Reptiles at the Museum, explains, 'Amphibians have something interesting going on with their visual systems. They've got a unique rod photoreceptor class which is associated with vision in low light. As they have two different types of rods, there is the potential that they can do things many other vertebrates can't, like detect colour differences in low light conditions.

'The other thing that makes the group unique compared to many other vertebrates is that many amphibians have a biphasic lifecycle, living part of their life underwater and then metamorphosing into generally terrestrial adults.

'This means they're using their eyes in really different light environments in the course of their life, compared with most other vertebrates which spend their whole lives in a very similar optical medium.'

Because some amphibians are not biphasic, they also offer the opportunity to understand how ecology influences the evolution of the visual system.

In particular, the researchers wanted to look at the properties of frog lenses in this study. While the shortest wavelengths of UV light, such as UVB and UVC, are always absorbed by the structure of the eye itself, some animals are able to perceive wavelengths of light further into the UV range than others.

However, this specialist sense can come at a cost. UV can damage the light-sensing retina of the eye, and is prone to issues such as greater scattering and chromatic aberration. These issues reduce the ability of the eye to focus short wavelengths, leading to blurred vision.

In frogs, blocking UV likely improves the sharpness of vision and protects the retina, but reduces night vision ability by reducing the overall amount of light reaching the back of the eye. Climbing frogs active at night would likely need elements of both to navigate the canopy and catch prey.

Dr Katie Thomas, a Postdoctoral Researcher at the Museum and the study's lead author, says, 'We lack detailed ecological data for most species of frogs and toads, but it is likely that those frogs which are climbing and jumping around in in trees at night require sharper vision than those in other environments.'

'While we don’t yet have data from behavioural and morphological studies to confirm this, our results do suggest that even though they are active in low light conditions, climbing frogs generally block UV from reaching their retinas, probably to equip these frogs with sharper vision.'

Bringing clarity to frog eyesight

Building on the previous work of a co-author, eye specialist Professor Ron Douglas, the team shone light through the lenses of 85 frogs and salamanders to measure their spectral characteristics.

They found that diurnal frogs tend to have UV-blocking pigments in their lenses, while the lenses of nocturnal relatives allow more UV to reach the retina. However, nocturnal climbing frogs were an exception, having lenses with UV-blocking characteristics more like the diurnal group.

While this study alone cannot confirm it, the researchers believe this might mean sharper vision is important to these animals and could explain some unusual behaviours in particular species.

'One of the diurnal frog species we were able to include in this study, Hylodes phyllodes, has a very distinctive behaviour where it waves its foot to signal to other members of its species,' Jeff says. 'They live near fast-flowing streams where the noise of the water would block out vocal calls.

'We found that their lenses block quite a lot of short wavelength light, and that this might have evolved alongside the foot-waving behaviour that would benefit from sharper vision.'

Another unusual amphibian, Wallace's flying frog (Rhacophorus nigropalmatus), may also need sharper vision to navigate while gliding from trees in the rainforests of Southeast Asia. Though that species wasn't included in this study, similar research conducted by David found that the lenses of diurnal gliding snakes block out UV.

The next step in this research on amphibians is to begin to bring together the work of different scientific disciplines to develop a more thorough understanding of their vision. For example, the team expect that frogs and toads have visual pigments in their retina that are tuned to detecting UV light depending on the level of UV blocking in their lenses. They are currently analysing physiological and genetic data to test this idea.

The scientists also hope their work encourages more detailed studies on ecology and behaviour in frogs and toads to better test the hypothesis that species with UV-blocking lenses do indeed have sharper eyesight.

'We're looking at the visual system in a variety of contexts and the big goal is to bring that all together,' Jeff says. 'For instance, as well as having lenses that block UV, we have also learned that nocturnal climbing frogs tend to have relatively larger eyes, which are perhaps required to let more light reach the retina overall.'

'Not a lot of work has been done on frog vision, so as soon as you start making preliminary discoveries it prompts many more questions,' David says. 'This study goes part of the way to clarifying some of them.'