The arrow frog (Phyllobates terribilis), a small bright yellow amphibian that lives in Colombian forests, is so lethal that only by brushing it, the Indians get poisoned darts to use as weapons.

The quantity of batrachotoxins, the poisonous substances of vegetable origin, present on the skin of each of these animals is sufficient to crush 10 human beings weighing 68 kg. The only venom-resistant creature is a species of snake, and the frogs do not seem to produce antidotes to these toxins. How, then, do they not poison themselves?

How the poison works. Batrachotoxins are not produced directly from frogs, but derived from their diet (perhaps, from beetles of which they are fond). The specimens raised outside the original habitat are in fact harmless. These poisonous alkaloids act on the sodium channels (the "access gates") of nerve cells, blocking the transmission of stimuli from the nerves to the muscles and preventing them from relaxing. One of the first symptoms is heart failure.

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The example of fish. Even tetradotoxins - the neurotoxins of puffer fish - act on the sodium channels, but a mutation in the amino acids modifies the sodium channels of these animals, which are so immune to their poison.

Scientists at the State University of New York then studied the amino acids of arrow frogs. In particular, they identified five mutations naturally present in these amphibians and replicated them in mouse muscles: when all five mutations were present, the sample was totally immune to the poison.

Mystery revealed. The researchers then tested one mutation at a time until the secret of frog immunity was identified in the amino acid N1584T. It would therefore be a single genetic mutation to eliminate the lethal effects of poison in amphibians: a shrewd and invisible evolutionary strategy.