Lifestyles of the bright and toxic overlap
They’re bright, beautiful, and dangerous to eat. But until now, no one has known about the home lives of Madagascar’s toxic frogs. Now, a researcher has observed these domestic dramas in the wild.
The behavior of the frogs–in the Mantella genus–appears strangely familiar, reports Heather Heying of the Museum of Zoology at the University of Michigan in Ann Arbor. Her observations reveal a pattern of social and sexual habits rarely seen in frogs, that is, except among poison-dart frogs on the other side of the globe. Yet the poison frogs of Madagascar and the Americas don’t seem closely related, Heying says.
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Apparently, the two toxic groups independently evolved a bold daytime lifestyle defined by elaborate courtships, the defense of territories, and a form of parental care not often seen in amphibians, Heying says.
Also, some 100 species of the poison-dart frogs, in the family Dendrobatidae, pick up toxins or their precursors from the environment and release the poisons when disturbed. The 13 or so Mantella species ooze some of the same defensive compounds from their skin, albeit the milder ones. “If you’ve been handling a frog and touch your lips, they’ll go numb,” says Heying.
In two seasons, she clocked nearly 1,000 hours watching Mantella laevigata on the Madagascan island of Nosy Mangabe. As the first scientist to study the species in the wild, she had to develop basic techniques, including the use of a portable tattooing machine to mark individual animals. Also, she says, “it took a long time to figure who was male and who was female.”
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Unlike most frogs, the poisonous Mantella hop about during the day, Heying reports in the March Animal Behaviour, which is the current issue. The animals also have bright warning colors, as do poison-dart frogs and other toxic animals.
The Mantella frogs cluster in bamboo stands, and their lives revolve around the tribulations of real estate. Heying found that males often violently defend territories of up to 2 square meters around a well, such as a rain-filled stalk of broken bamboo. When a female answers the call of a chirping male, she hops toward him. Then he closes the distance, shifting to a quieter one-note call. When they are together, he rests his chin against her. “Then he leads her, calling her softly, to a well,” Heying says.
If she accepts the site–by staying in the water–the male clasps her and fertilizes the sole egg she attaches to the side of the well above the water line. This species represents only the third frog known to lay eggs singly, Heying reports.
If the female hops out of the well, however, the male leads her to another one. A male without a well on his territory, or with a well that doesn’t appeal to the female, invades the territory of another male.
If rains don’t drown the egg, it hatches in about 10 days and the larva plunks into the well’s water. About 10 times, Heying saw females visit a well with a tadpole and provide the little occupant with lunch in the form of an unfertilized egg.
She also often noticed Mantella males leading females to pools harboring tadpoles. The resulting embryo either develops into a new tadpole or becomes food for the older ones. Either way, the male wins because the effort supports his offspring, Heying explains.
These details give “a remarkable example of convergence,” says dart-frog specialist Kyle Summers of East Carolina University in Greenville, N.C. As both groups of frogs have evolved to accumulate toxins, they’ve also been freed to explore pools, flirt, and fight in daylight.