Why do this frog’s thighs glow blue-green?

This effect is known as iridescence: the surface appears to shift between blue and green because microscopic structures within the tissue reflect light in a particular way. This is well known in butterflies, beetles and birds, but such cases are rarely described in amphibians.

The new study has been published in the journal *Austral Ecology*.

Scientists studied the green-and-gold bell frog (Ranoidea aurea) — a well-known Australian species that is considered to be at risk. The brightly coloured patches of skin are usually hidden but become visible when the frog moves. The researchers suggest that this sudden burst of colour may help the frog to distract or confuse predators.

Details

At first glance, this frog does not look particularly unusual: a green back, golden spots – a typical amphibian found in Australian marshes and water bodies. But its most interesting feature is not on its back, but on the inner thighs.

When the frog is sitting still, these patches are barely visible. But if it jumps, turns round or tries to hide, a predator might catch a glimpse of a bright blue patch for a split second. It has now been discovered that this patch is not simply blue: it can shimmer and appear green depending on the angle of view.

For a predator, such a flash can act as a visual distraction. One moment there was an animal blending into the grass and water right in front of it — and suddenly, as it leaps, a bright, shifting colour appears. This could distract the predator, compromise the accuracy of its attack, or give the frog a fraction of a second to escape.

What exactly did the scientists find?

A team led by biologist John Gould from Newcastle University has documented one of the clearest examples of iridescence ever described in amphibians in Ranoidea aurea. The photographs show that the skin on the inner thigh changes colour from blue to green as the viewing angle changes.

This is significant because, until now, the bright blue colour in frogs was most often explained by the scattering of light within the skin. However, true iridescence usually requires a more ordered microstructure — much like that found in butterfly wings or beetle shells.

Put simply, this frog’s blue colour is probably not caused by ordinary pigment or random ‘cloudy’ tissue. It may be created by a complex system of reflective plates in the skin, which act as natural optics.

Why is blue such an unusual colour in animals?

In animals, the colour blue is often produced differently from red, yellow or brown. In many cases, it arises not from a blue pigment, but from the structure of the surface.

Microscopic layers, plates or fibres reflect and scatter light in such a way that the eye perceives a blue, light blue, green or iridescent hue. This is known as structural colour. It depends not only on the chemical composition of the tissue, but also on its internal structure.

This is precisely why the feathers of some birds, the wings of butterflies or the shells of beetles can appear metallic and change colour when in motion. In this Australian frog, scientists observed a similar phenomenon — but in the amphibian’s skin.

Why does the frog have such thighs?

The most likely explanation is protection from predators. In biology, this is known as ‘flash colouration’. The animal conceals its bright colour whilst sitting still, then suddenly reveals it when moving.

This tactic is found in various animals. Whilst they are still, they are difficult to spot. But when fleeing, they display a bright signal that can distract a predator. When the animal freezes again and the bright patch is hidden, it is harder for the predator to work out exactly where it has gone.

In the case of the green-and-gold bell frog, its blue thighs were previously thought to be a possible defensive signal. This new finding adds another layer to this: if the patch is not just bright but also iridescent, it may be more noticeable when the frog leaps.

Why this matters

At first glance, this is a minor detail: so the frog’s skin shimmers. But for biologists, it is an important clue to just how complex amphibian colouration can be.

Frogs often appear to have simple colouration: green, brown, or spotted. In reality, their skin can function as a multi-layered optical system, involving pigment cells, reflective structures and the microscopic architecture of the tissue.

This study shows that even in a well-known species, it is possible to discover features that had previously gone unnoticed. This means that other amphibians may also possess hidden optical effects — they simply haven’t yet been examined from the right angle.

Background

Ranoidea aurea, or the green-and-gold bell frog, is an Australian species associated with wetland habitats. Females can reach around 10 cm in length, whilst males are usually smaller. The species is known for its bright green colouration with golden patches, although when cold or inactive, the frogs may darken to blend in better with their surroundings.

Like many amphibians, this species is vulnerable to habitat loss. In Australia, special conservation programmes are in place, including the restoration and creation of water bodies, so that isolated groups of frogs can breed and maintain their populations.

This new research is therefore interesting not only as a fascinating story about unusual colouration. The better scientists understand the behaviour, appearance and defence mechanisms of rare species, the more accurately they can assess their ecology and conservation needs.

Source

Study: John Gould, “Shifty Frogs: Evidence of Iridescence Among Amphibians”, Austral Ecology, 2026.