Despite having been some of the first animals used to study vision, surprisingly little is known about how frogs see © hehaden/Flickr CC BY-NC 2.0

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Frogs have some of the biggest eyes among all vertebrates

Despite being some of the first animals which were used in the study of vision, surprisingly little is known about how frogs see.

A new project looking into how frog vision differs across species is showing how the eyes of the animals are closely linked to the habitats in which they live. It has also revealed that frogs have some of the biggest eyes relative to their body size across all vertebrates.

Frogs are a diverse group of animals that live on every continent except Antarctica, in a wide range of habitats from the tops of rainforest canopies to the bottom of rivers and lakes. They have adapted to live in most freshwater and land-based environments.

Each of these require the amphibians' eyesight to function in a specialised way, depending on whether they are exposed to strong sunlight, dappled shadows or pitch blackness. How frogs have adapted to these conditions is still not fully understood.

Dr Katie Thomas has been working on a project at the Museum to investigate how the eye size of frogs differ, publishing the results in the journal Proceedings of the Royal Society B.

'We wanted to know how eye size evolved across frogs, and if that is correlated to how they live,' explains Katie. 'This matters, because optically if you have a bigger eye, you can have better vision, but there is a balance between seeing contrast and seeing spatial detail.

'These two aspects are fighting against each other, because often to improve one you have to sacrifice the other one.'

An brown, aquatic frog with a large flat body and very small eyes on a plain black background.

Some frogs, such as this Cameroon clawed frog, are fully aquatic and live their entire lives at the bottom of rivers and lakes © Brian Gratwicke/Flickr CC BY 2.0

Katie found that a frog's eye size is related to the environments it lives in, with the most dramatic differences seen in those species that burrow underground and those that live a fully aquatic life.

She also discovered just how big frogs' eyes are when compared to their body size - showing vision is pretty important to these amphibians. 

Life in the dark

Each environment that a species of frog lives in demands something different from their eyesight. Eyes also need to be adapted to a frog's lifestyle, taking into account what time of day they are active and how they find mates.

Katie and her colleagues examined how the eyes of frogs varied by measuring them on 220 different species across all 55 families of the amphibians. In addition, they collected data on the body size of the frogs, as well as associated information on where and how they live.

'Mating habits, habitat and activity pattern all had significant effects on eye size, but the biggest difference was among habitats,' explains Katie. 'The main thing we found was that species that are burrowing have smaller eyes than everything else, which is not unexpected, but so do those that are fully aquatic.'

Animals which move into a subterranean world often reduce the size of their eyes. This trend is seen across most major groups of vertebrates, including mammals, fish, lizards and snakes.

A very wrinkly burrowing frog, with tiny reduced eyes, sits on a brown piece of bark on the forest floor.

Animals which live underground or in the dark, such as this burrowing frog from Thailand, often have reduced eyes © Rushen/Flickr CC BY-SA 2.0

This is partly because burrowing animals rely less on vision in the darkness, but also because large sensitive eyes could be easily damaged by moving through the soil.

The discovery that eyes of burrowing frogs are most similar to those which were living underwater was something of a surprise to the researchers, but is likely explained by where in the aquatic environment these amphibians are living. 

'For the aquatic species, we think their relatively small eyes are probably because a lot of the frogs live in pretty murky habitats and there is not a lot of light available,' says Katie. 'Some frogs have even evolved lateral line systems, which are super-sensitive touch-based systems, so maybe they are not relying on vision a lot.

'Interestingly, there are some frogs that were aquatic but still had big eyes, and those are species live at the surface of the water and often stick their eyes out to look above.'

This suggests that even within the aquatic habitat, there may be variations in eye size of frogs depending on where in the water column they are living.

A frog with huge eyes is peaking over the edge of a green leaf, with a black background.

It turns out that frogs have really big eyes, up there with the birds © Sue Cro/Flickr CC BY-NC 2.0

Really big eyes

It has been assumed that a frog's vision is one of its key senses because large eyes are energy-intensive for the body to produce and maintain.

So Katie and her colleagues also compared relative frog eye size to that of fish, reptiles, mammals and birds.

'We looked at how eye size for frogs scaled with body size, and then compared that across vertebrates in general,' explains Katie. 'We found that frogs just have really big eyes. 

'I think it was surprising because we knew that birds have huge eyes compared to other animals, and it turns out that frogs are up there with the birds.

'Frogs were an early model for visual physiology when people were figuring out how eyes worked, but then their vision has really been neglected among vertebrate vision researchers for several decades. So it is it kind of interesting that they have such presumably important eyes.'  

As scientists continue to study frog vision, we can expect to gain even clearer insights in the world of these diverse amphibians.