Why Do Red-Eyed Tree Frogs Have Red Eyes? Startle Coloration

What is startle coloration?

Startle coloration, also called a deimatic display, is a defense in which an animal hides bright colors at rest and shows them suddenly when threatened. The aim is not to warn of poison but to shock or confuse a predator for a fraction of a second. Many animals use it: some moths and butterflies have hidden eyespots on their hind wings, some mantises spread colorful forelegs, and certain frogs flash bright patches on their thighs.

The red-eyed tree frog (Agalychnis callidryas) is one of the best-known examples among frogs. Its whole color scheme is arranged so that the vivid parts can be switched off. During the day, it rests on the underside of a leaf with its legs folded under its body, its blue flanks pressed against the leaf and its eyes closed. Only the green back shows.

How the red-eyed tree frog’s startle display works

When a snake, bird or other predator touches or approaches a resting frog, the frog’s calm green outline can change in an instant:

1. The eyes open, showing two large red disks with black slit pupils.
2. The frog shifts its legs, exposing the blue-and-yellow flanks and the bright orange feet.
3. It jumps or climbs away, often flashing more color as it moves.

A predator that has locked onto a green, leaf-shaped target is suddenly faced with large, staring red eyes and contrasting colors. Even a short pause may be enough for the frog to escape. Field observations fit this idea well, and it is the leading explanation offered by herpetologists. It is still a hypothesis rather than a fully tested fact: controlled experiments on how real predators react to the display are limited, and the colors may do more than one job.

Other possible reasons for red eyes

Scientists have suggested some additional or alternative roles for the frog’s bright colors:

• Camouflage at rest. With the eyes closed and legs tucked in, the frog is very well camouflaged. The bright colors cost little because they are hidden most of the day.
• Confusing a chase. A frog that flashes color as it jumps and then folds up again may leave a predator searching for a bright target that has vanished. This “flash coloration” is also used by some grasshoppers and moths.
• Signals between frogs. Males have regional differences in flank color, and females in some areas seem to prefer males with the local pattern. Whether eye color plays a role in this is not established.

None of these ideas rules out the others. The startle effect remains the most widely cited explanation for the eyes in particular.

The patterned third eyelid

A resting red-eyed tree frog does not cover its eyes with ordinary skin. It draws up a thin, translucent lower eyelid, often called the third eyelid or nictitating membrane. In this species it is marked with a fine net of golden lines, which makes the closed eye look greenish gold rather than red.

Because the membrane lets some light through, the frog can probably still detect shadows and movement while it rests. That would help it react quickly if a predator comes close, so that the startle display and escape can begin before the attacker strikes.

How do red-eyed tree frogs see at night?

The red color is not known to help the frog see better. What matters for vision at night is the structure of the eye:

• Large eyes gather more light, important for an animal that hunts insects in a dark forest.
• Vertical slit pupils can open very wide in darkness and close to a narrow slit in daylight, protecting the sensitive retina. This pupil shape is common in night-active animals.
• Forward and sideways placement of the bulging eyes gives a wide field of view for spotting both prey and predators.

With these eyes, the frog can find moths, crickets and flies by night and keep watch for snakes and other hunters.

Red eyes, bright flanks and poison: what is the difference?

Bright colors in frogs often mean poison, but not here. In poison dart frogs, bold colors are a constant warning, called aposematism: the frogs are toxic, visible all the time and move around by day. In the red-eyed tree frog, the colors are hidden most of the time and shown only for a moment. The frog relies on surprise and escape, not on tasting bad. The full story is in are red-eyed tree frogs poisonous?, and you can compare the warning colors of their relatives in why are poison dart frogs so colorful?.

Myths and facts about red-eyed tree frog eyes

• Myth: bright red eyes mean the frog is dangerous. The frog is harmless to people; its colors work as a bluff, not a warning.
• Myth: the eyes glow in the dark. The eyes reflect a flashlight beam, as many animals’ eyes do, but they do not produce light.
• Fact: young frogs do not have red eyes. Froglets start out with yellowish eyes and a brown body and only gain the red eyes as they mature.

Learn more about the species in the red-eyed tree frog profile.