To an unknowing predator, the poisonous, fingerprint-size frogs of the American tropics may look as tasty as their fruity strawberry and banana coloring suggest. But in fact, these frogs are so unappetizing at first bite - some are potentially deadly if swallowed - that most animals spit them out like Brussels sprouts and avoid all future contact.
Researchers are gaining new insight into the frogs' ingenious defense against hungry birds and snakes. By examining the frogs' diet, scientists believe they have solved at least part of the decadeslong mystery about how these brilliantly colored amphibians get their toxic-coated skin.Several studies published in the last few years provide detailed evidence that frogs accumulate their poison by eating ants and other rain forest arthropods that are rich in toxins called alkaloids. Born defenseless and shy, the frogs gradually build up a sickening suit of armor.
"There is strong evidence that frogs are getting at least some of their toxicity from their diet," said Dr. John Daly, the leader of a team from the National Institutes of Health that first published the link between frog poison and diet in 1994. "If you've developed a toxin, you can occupy a niche that other frogs cannot, like hopping around the forest during the day," Daly said. "You can also become brightly colored to advertise `Don't eat me!' "
Daly's research began after he noticed that wild frogs raised in captivity on fruit flies and crickets slowly lost their chemical defense. It was clear that something in the frogs' environment played a role in their poison secretion system.
To narrow the possibilities, Daly experimented with lighting and introduced snakes to frighten the frogs into producing poison. Nothing worked until he fed the frogs invertebrates collected from leaf litter in the rain forest.
Scientists soon focused on ants, which contain alkaloids used to mark trails, repel enemies and attack prey. Studies by Catherine A. Toft, an evolutionist and ecologist at the University of California at Davis, and Maureen A. Donnelly, a herpetologist at Florida International University, show that ants are a favorite prey of many poisonous frogs species.
One of the most detailed investigations of the subject was done by Janalee P. Caldwell, a zoologist at the University of Oklahoma and associate curator of amphibians at the Oklahoma Museum of Natural History. Using a microscope, she examined the stomach contents of nine frog species from the Den-drobatidae family in the Nicaraguan and Ecuadorean jungle.
Caldwell found that the higher the frogs were on the poisonous spectrum, the more ants they ate. Her research was published in the December issue of the Journal of Zoology.
At the extreme, the iridescent green and black Dendrobates aura-tus devoted 73 percent of its diet to ants. At any one time, specimens' stomachs contained 400 ants and a large helping of mites.
In contrast, nonpoisonous frogs like the brown Colostethus mar-chesianus had a more varied and less toxic diet that included spiders, termites, flies and insect larvae. Ants made up only 12 percent of their dinner.
The frogs that Caldwell collected consumed nearly 100 different species of prey that inhabit the trees and leaf litter in the Latin American rain forest.