by Whit Gibbons

November 15, 2009

I recently received the following question about frogs and toads.

Q. My friend says that some frogs are poisonous. However, he also says that toads do not cause warts. What is the truth?

A. Your friend is correct on both counts. Toads have bumpy skin and some of these bumps are glands that produce toxins, but scientists have no evidence that a person can get warts from touching a toad, despite the common superstition. And although many, perhaps most, frogs and toads have at least some toxins produced by glands in their skin, no frogs inject venom through fangs or stingers. Hence frogs are poisonous but not venomous.

The toxins produced by skin glands of frogs unquestionably serve as defense against some of their predators by making the frogs unpalatable. The toxic-producing glands are all over the body and legs in some species, providing protection no matter where a predator grabs the frog. The toxic substances probably provide an additional service to the frog in some species by discouraging parasites from attaching themselves to the frog's skin and inhibiting the growth of fungus or bacteria.

A pair of paratoid glands is evident on the top of the head in some species of frogs and toads. These large glands produce toxic secretions that can sometimes be seen as a milky liquid if the gland is squeezed. Secretions from the paratoid glands of some species, such as the marine toad, can be extremely toxic and even lethal for some animals that bite them. The poisonous substance is called bufotoxin. The effect it has on one potential predator is evident when a dog bites a common garden toad and then begins to foam at the mouth.

The toxic skin secretion of frogs such as the common gray treefrog and the introduced Cuban treefrog can cause extreme discomfort if it gets into the membranes of the eyes or nose. A few South American species of frogs known as the poison dart frogs in the family Dendrobatidae produce alkaloid toxins on their skin that are among the deadliest poisons known. People have been known to go into a comalike state and almost die after picking up one of these frogs if the toxins enter even a minor cut on their hand. Sufficient amounts of these toxins in the bloodstream can actually kill a human being.

The most poisonous frog, possibly even terrestrial vertebrate, in the world is the golden poison frog, Phyllobates terribilis, of Colombia. Among the toxins produced by the species is batrachotoxin, a powerful alkaloid that can kill large mammals, including people. When natives of a certain region of Colombia rub the skin of golden poison frogs on their blowgun darts, they produce a lethal weapon that can kill an animal almost instantly upon penetration.

The color patterns of poison dart frogs and other Dendrobatidae include brightly colored blues, reds, and yellows, which are presumably warning colors to other animals that might try to eat them. Many species come out in the daytime, instead of at night like most other frogs. Poison dart frogs are presumed to acquire their toxins by eating certain kinds of invertebrates that produce batrachotoxins and other alkaloids. Prey items of poison dart frogs that have been identified include a particular family of beetles, several species of ants, and a species of millipede. The frogs apparently are able to sequester the toxins without being harmed themselves. The level of toxicity varies considerably among species within the family of Dendrobatidae and among other species of frogs worldwide. Interestingly, when poison dart frogs are kept in captivity and fed diets of crickets or mealworms, they eventually lose their skin toxicity. But they retain their brilliant color patterns.

The eggs and tadpoles of certain species of frogs and toads have been found to be unpalatable to some predators, such as fish and salamanders, but whether poisonous substances are involved is unknown for most species that have been examined. Though the tadpoles of the golden poison frog are not poisonous, juveniles become poisonous soon after metamorphosis, when they assume a terrestrial diet.

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