Now, we see through a glass frog darkly

There is a kind of army camouflage pattern called “frog skin” which looks like the mottled green, brown and tan colours of a common frog. Army battledress camouflage works in the same way that frog colouration and patterns do – to break up or disrupt the outline of the frog and protect it from predators.

The most iconic example of disruptive colouration is the black and white giant panda. Pandas in a zoo, seen up close and against highly artificial backgrounds, seem obvious to us. When pandas are observed at a distance in their natural habitat, however, the black parts fade into the background rocks and disguise their outline from potential predators.

There are many ways that animals use colour to hide from predators including background matching, disruptive colouration, colour changing and transparency. Which method works best depends on the usual background that the animal is found in.

If the background is consistent and predictable then well colour-matched animals will escape predation more often and live to reproduce, spreading the genes for that colour. Examples include green caterpillars, which blend into the colour of their leaf lunch and are less obvious to birds.

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If the background an animal is found in changes colour with the season, or as the animal moves, however, then colour matching is not so effective as an animal could end up being more obvious if it finds itself on a contrasting background – unless it is transparent.

A Harry Potter-like cloak of invisibility enables transparent animals to escape predators by seamlessly blending with whatever background they find themselves against. Many aquatic animals, particularly invertebrates like shrimp or jellyfish, can be transparent and hard to spot in the water.

A transparent jelly on land, however, would not work so well as light moves differently in the air compared with a squidgy water-filled jelly and so, would make the transparent animal obvious in a different way. This is why transparent land animals are not so common as in the water.

A notable exception to the land transparency problem are a group of frog species called “glass frogs”. These species have fully transparent undersides, through which you can see their internal organs, including beating hearts and blood vessels. Their upper sides are not fully transparent, with scattered green pigment cells obscuring their internal organs.

A fully transparent frog might indeed be more obvious to predators because its internal organs and different tissues would show through its skin, light would bend and refract as it passed through causing a disruption in the background and the hypothetical transparent frog would also lack protection from damaging ultraviolet light.

Glass frogs are more like frosted glass frogs, translucent rather than transparent. What use is a transparent belly if you spend most of your time sitting on a leaf where this remarkable innovation does not seem to see the light of day, though?

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The clue is that the leaves that glass frogs rest on are themselves translucent to different degrees. The light filtering through a leaf passes through the frog’s transparent underside into the frog’s body subtly lightening the frog’s appearance through its sparsely pigmented upper side. On a darker leaf, the frog appears darker.

Fascinating research by James Barnett and colleagues showed that the translucency of glass frogs is a subtle form of camouflage that brightens or darkens the frog depending on the luminance of its background, enabling it to blend in with the different kinds of leaves it is found on.

Glass frogs combine this kind of colour matching with disruptive colouration as their limbs have higher translucence than the body. This means that glass frogs have a gradient of colour matching to their background, which is closest to the leaf at their outline where the limbs are held close to their bodies, thus breaking up the frog’s edges – just like the giant panda.

St Paul in his letter to the Corinthians may not have been talking about glass frogs when he used the phrase “now, we see through a glass darkly”, but he certainly captured the gist of translucent colouration.

Yvonne Buckley is an ecologist, Irish Research Council laureate and professor of zoology at Trinity College Dublin