SYDNEY: Poison frogs may get much of the cocktail of toxins they use to fend off predators, from snacking on mites. The find offers clues to novel human medicines, say experts.

These often brightly coloured frogs are known to accumulate toxic compounds, called alkaloids, in thousands of skin glands on their back. They secrete these vile tasting poisons to repel predators when threatened.

Frogs were first suspected of getting alkaloids from their diet when it was discovered that they lost their toxicity in captivity. Researchers predicted that the amphibians sequestered these chemicals from ants and other insects, but the primary source was unknown.

Mitey recyclers

Now, a team led by biologist Ralph Saporito of the Florida International University in Miami, U.S., has identified more than 80 alkaloids in various species of oribatid mites in Panama and Costa Rica. Of these compounds, 42 were matched to secretions of the strawberry poison-dart frog, Oophaga pumilio, found throughout Central America.

They report their discovery – which suggests that mites are a major source of toxins for many poison frogs – today in the U.S. journal Proceedings of the National Academy of Sciences

Oribatid mites are typically just 1 mm in length, and live in the top layers of soil and leaf litter of forests, where they decompose organic material and recycle nutrients.

"This finding is a large breakthrough in our understanding of which arthropods are the dietary sources for most alkaloids in poison frogs," said Saporito. "they represent one of the few examples of this phenomenon among vertebrates."

The next question he plans to work on is how frogs sequester the alkaloids without succumbing to the poison themselves, he said.

"Promise for human ailments"

The find may also have human medical implications, said co-author and biochemist John Daly of the National Institutes of Health in Bethesda, Maryland. "Natural products often come from surprising sources… about 80 per cent of current pharmaceuticals trace their origin back to a natural product."

Alkaloids from frogs have already led to new insights into the functions of local anaesthetics and anticonvulsants, said Daly. These compounds also led to the discovery of epibatidine, a painkiller 200 times more potent than morphine, which is now being tested as a treatment of chronic pain.

Australian frogs have been shown to sequester so-called pumiliotoxins, many of which may also originate in mites, said Daly. These have been used to stimulate the heart and may even have potential as a treatment for autoimmune diseases such as myasthenia gravis, arthritis and diabetes.

"Who knows what discoveries will come as we investigate the further novel substances now discovered in mites," said Daly.

The discovery that mites are the primary source of alkaloids in frogs is significant said Deborah Hutchinson, a herpetologist of Old Dominion University in Virginia, U.S.. "The first step in producing [many] new pharmaceutical drugs is the discovery of novel compounds in nature... like all newly discovered natural products, these compounds may hold promise for the treatment of human ailments."

Sequestering toxins from the diet is known in many species, particularly insects, but also in others such as birds, worms and snakes (see, Snake's poison comes from toads, Cosmos Online).