Why Are Poison Dart Frogs So Colorful? Warning Colors Explained

Warning colors: what aposematism means

Most small animals try not to be seen. Poison dart frogs do the opposite: many of them hop about on the forest floor by day in colors that stand out against the brown leaf litter. This works because they are protected by chemicals. Their skin contains alkaloid toxins, picked up from ants, mites and other prey, that can sicken or kill a small predator.

A conspicuous color helps both sides. A predator that remembers “bright frogs taste terrible” avoids a nasty or deadly meal, and the frog avoids being bitten or grabbed, which could injure it even if the predator spits it out.

The idea goes back to the naturalist Alfred Russel Wallace, who suggested in 1867 that some caterpillars are brightly colored to advertise that they are distasteful. In 1890 the English zoologist Edward Poulton named this aposematism, from Greek words meaning “away” and “sign.” Poison dart frogs are among the most famous aposematic animals of all, alongside wasps, ladybugs and monarch butterflies; see are monarch butterflies poisonous?.

How predators learn to avoid bright frogs

Birds are thought to be the most important visual predators of poison frogs. Birds have excellent color vision, including sensitivity to ultraviolet light, so a red, orange or blue frog stands out clearly to them.

Field experiments support the idea that the colors protect the frogs. In Costa Rica, researchers placed clay models of frogs on the forest floor: some painted like the local red-and-blue strawberry poison dart frog and some plain brown. The brown models were attacked by birds more often than the brightly painted ones, as the warning-color theory predicts.

Predators can avoid bright prey in two ways. Many learn by experience: one unpleasant taste can be enough to put a bird off similar prey for a long time. Some birds also seem naturally wary of new, brightly colored food, and this caution may have helped warning colors get started in the first place.

Are brighter frogs more poisonous?

Often, but not always. In the strawberry poison frog, which has many differently colored populations in Panama’s Bocas del Toro islands, a 2012 study found that the more conspicuous populations, as seen through a bird’s eyes, tended to have more toxic skin. In other words, the colors were “honest” signals of how dangerous each population was.

The pattern is not universal. A study of the granular poison frog in Costa Rica found that less conspicuous populations were more toxic, and comparisons across the whole family suggest trade-offs. Making toxins and bright pigments both cost energy, and a very toxic frog may gain less from advertising loudly.

At the extreme end is the golden poison frog of Colombia, which comes in yellow, orange and pale mint-green forms and carries enough batrachotoxin to make it one of the most toxic animals known. It features in our list of the most poisonous animals.

Toxins and colors also have different origins. Colors are inherited, while toxins come from the diet, which is why captive-bred dart frogs are just as colorful as wild ones but are generally not toxic.

Why strawberry poison frogs come in so many colors

The strawberry poison frog shows how warning colors can split into many forms. In the Bocas del Toro Archipelago, populations on different islands and stretches of coast were isolated as sea levels rose after the last ice age. Today there are roughly 15–30 color morphs: red, orange, yellow, green, blue, white and nearly black, with or without spots.

Several forces probably worked together. Random genetic changes in small, isolated populations can shift colors. Local predators and backgrounds may favor different colors on different islands. Mate choice adds a further push: female strawberry poison frogs can see color well and tend to prefer males that look like their own population, which helps keep the morphs distinct.

Copycats: mimicry among poison frogs

Where a warning signal works, other species can borrow it. Two kinds of mimicry occur among poison frogs.

Müllerian mimicry is when several toxic species share the same warning pattern, so predators learn it faster. The mimic poison frog (Ranitomeya imitator) of Peru is a famous example. It is toxic itself, and different populations closely copy different local poison frogs: in one area it is striped like R. variabilis, in another banded like R. summersi, and in another it resembles R. fantastica.

Batesian mimicry is when a harmless species copies a toxic one. Allobates zaparo of Ecuador and Peru has no skin toxins but resembles colorful poison frogs of the genus Ameerega. Surprisingly, it most closely copies a moderately toxic species rather than the most toxic one living nearby. Experiments on predator learning in 2006 suggested why: the way predators learn and generalize can make the less toxic model the better one to imitate.

Mimicry by shape and pattern is common across the animal world; read about why atlas moth wings look like snake heads.

Not all poison frogs are colorful

The poison dart frog family, Dendrobatidae, contains many species that are brown, gray or black with pale stripes. These frogs are generally not toxic and rely on camouflage, like most frogs. Bright colors and toxicity appear to have evolved at least four separate times within the family, each time in groups that specialize in eating alkaloid-rich ants and mites.

Being colorful and toxic also changes how a frog can live. Protected by their chemicals, aposematic poison frogs can forage openly in daylight, call from exposed perches and carry tadpoles across the forest floor, activities that would be far riskier for a well-camouflaged but defenseless frog.