A Lifeboat to London for Darwin’s Frogs

The folk song “Frog Went a-Courtin’” recounts the efforts of a sword- and pistol-toting frog to woo a mouse, who warns him that without the consent of her Uncle Rat she “wouldn’t marry the president.” The courtship rituals of Darwin’s frogs, in the cool, temperate rainforests of southern South America, are not nearly as conditional. What sets their hookups apart are the child custody arrangements. Darwin’s frogs, named after Charles Darwin, who spotted them in 1834 while exploring Chiloé Island in Chile, take an approach to parenting that is unique among amphibians: Males rear their young in their vocal sacs until the juveniles are ready to fend for themselves, a reproductive strategy known as mouth brooding. Adult Darwin’s frogs are about an inch long and weigh less than a couple of paper clips. Although the frogs are thought to live as long as 15 years in the wild, they are highly susceptible to chytridiomycosis, a virulent amphibian disease caused by the chytrid fungus. Since the 1990s, the pathogen, which invades the surface layers of skin, has been implicated in the mass die-offs of roughly 400 amphibian species and has killed off at least 90 more.