By Karl Gruber
Strawberry poison frogs (Oophaga pumilio) of Costa Rica give their newborn tadpoles a built-in weapon against predators: alkaloids.
Various animals and plants use alkaloids—naturally occurring, bitter-tasting chemical compounds—as a first line of defense.A strawberry poison frog in Guayacan, Costa Rica, in 2011. Photograph by Robert Pickett, Visuals Unlimited/Corbis
For instance, adult strawberry poison frogs get the chemicals from their diets of ants and mites, “which essentially makes the frogs unpalatable to many potential predators,” said Ralph Saporito of John Carroll University in Ohio and leader of a new study on the species. (See more pictures of rain forest animals.)
But “prior to this study, most of what we knew about alkaloids in these frogs came from adults. So, we decided to fill this void by looking at alkaloids in all life stages of the strawberry poison frog,” Saporito said.
The team already knew that strawberry poison frog mothers feed their babies unfertilized eggs. But their new research revealed the eggs are also spiked with alkaloids—the first time an animal has been found to pass on such chemical defenses to its offspring.
Watch a video of another species of strawberry poison frog caring for its tadpoles.
For their study, the researchers measured alkaloid content in strawberry poison frogs during different stages of development. (See more pictures of poison dart frogs.)
In one group, tadpoles were reared and fed by their mothers, and a second group was reared by the researchers and fed with eggs from another species of frog not known to harbor alkaloids.
As the tadpoles from both groups developed, the team analyzed their alkaloid contents. The results were clear-cut: Tadpoles reared by mom contained alkaloids in most stages, whereas tadpoles from the second group showed no sign of these chemicals, according to the study, published November 12 in the journal Ecology.
Researchers also tested whether the presence of these chemicals helped the tadpole defend itself.
Bullet ants were given the choice of attacking two groups of tadpoles: one consisting of strawberry poison frogs and another from a frog species without alkaloids. The bullet ants were nine times more likely to attack the non-toxic species—showing that the alkaloids protected the tadpoles.
Still, some key questions remain.
“At this point, we do not know how females are able to provision their eggs with alkaloids, nor how tadpoles store these alkaloids in their bodies,” Saporito noted.
He also hopes to explore whether other frog species supply their babies with alkaloids, beginning with other members of the egg-eating genus Oophaga.