Why Are Poison Dart Frogs Poisonous? Toxins From Their Diet

Where do poison dart frogs get their poison?

Poison dart frogs (family Dendrobatidae) live in the tropical forests of Central and South America. Many species are specialist hunters of very small arthropods, especially ants and oribatid mites, which are soil mites smaller than a pinhead. Many of these little animals carry bitter or toxic alkaloids, either made by themselves or picked up from plants and fungi.

When a frog eats them, the alkaloids pass from the gut into the blood and are moved to granular glands in the skin, where they are stored and released when the frog is attacked. Scientists call this sequestration. Researchers have identified more than 800 different alkaloids in poison frogs, and a single frog often carries a mixture of many.

The link with diet was first proven by a simple observation. Frogs collected in the wild stayed toxic in captivity for some time, but their captive-raised offspring, fed on fruit flies and crickets, had no toxins at all. When scientists later added alkaloids to the food of captive frogs, the frogs took them up and stored them in their skin.

Which prey make dart frogs toxic?

Different toxins come from different prey, and the main sources are still being worked out:

• Ants supply many alkaloids. Toxic dart frogs tend to eat far more ants than their nontoxic relatives.
• Oribatid mites are a major source too. A 2007 study by Ralph Saporito and colleagues found pumiliotoxins, a group of alkaloids common in dart frogs, in mites from the same forests.
• Beetles and millipedes contribute other compounds.
• Batrachotoxin, the extremely potent toxin of the golden poison frog, probably comes from small beetles of the family Melyridae. Beetles of this family in New Guinea carry batrachotoxin, which is also why some New Guinea birds that eat them, such as the hooded pitohui, are poisonous.

Because the diet varies, so does toxicity. The same species can be strongly toxic in one forest and much milder a few valleys away, and the exact cocktail of alkaloids differs between populations.

Why did poison dart frogs evolve to be poisonous?

Toxins are a defense against predators. A frog that tastes foul or makes a predator sick is less likely to be eaten, and the bright colors of many species advertise the danger. This combination of toxin and warning colors is called aposematism, and it means a bird or snake that has one bad experience learns to avoid similar frogs. Read more in why are poison dart frogs so colorful?.

Studies of the family’s evolution suggest that eating ants and mites and becoming toxic went hand in hand. Species that specialized on these small prey gained a chemical defense almost for free, and in several lineages bright coloration evolved alongside.

Some mothers even pass toxins to their young. Female strawberry poison dart frogs feed their tadpoles unfertilized eggs, and research has shown that these “food eggs” contain alkaloids, giving the tadpoles their own early chemical protection.

Why don’t poison dart frogs poison themselves?

Carrying toxins that attack nerves and muscles is risky, so poison frogs have evolved ways to cope, although the details are still being studied:

• Changed target proteins. Some dart frogs have small changes in the proteins their toxins attack, such as receptors in nerve cells, that make those proteins less sensitive.
• Toxin-binding proteins. Some species carry proteins in their blood that grab alkaloids and stop them from reaching sensitive tissues, acting a bit like a sponge.
• Storage in skin glands. Keeping toxins locked away in skin glands also limits how much reaches the frog’s own body.

How the golden poison frog survives batrachotoxin is not fully understood. Earlier research suggested its sodium channels resist the toxin, while more recent laboratory work has questioned this, so a combination of mechanisms is likely.

Are all poison dart frogs equally poisonous?

No. The family has roughly 200 species, and they range from harmless to extremely toxic.

Only three Colombian Phyllobates frogs carry large amounts of batrachotoxin, and the golden poison frog is by far the most toxic of them; see how poisonous is the golden poison frog?. Most other species carry milder alkaloids that taste bitter and can make a predator ill.

Poison dart frogs are not the only animals that borrow toxins from their food. Mantella frogs in Madagascar, monarch butterflies with their milkweed toxins and pitohui birds in New Guinea all use the same trick, a striking example of convergent evolution.

Is a poison dart frog dangerous to people?

Wild dart frogs, and above all wild golden poison frogs, should never be handled. Captive-bred frogs lack the dietary alkaloids and are generally considered nontoxic, but handling still harms their delicate skin. For details, see can you touch a poison dart frog?