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.
Chiloé Island was once a sanctuary for Darwin’s frogs, but two years ago monitoring confirmed the presence of the chytrid fungus. “This is likely a result of climatic change that has made local conditions more favorable for it,” said John W. Wilkinson, a conservation biologist based in Britain.
Within a year, a chytridiomycosis outbreak had claimed more than 90 percent of the island’s Darwin’s frogs. “They die within weeks of becoming infected,” said Ben Tapley, a curator at the London Zoo who specializes in critically endangered reptiles and amphibians.
Mark O’Shea, who is a herpetologist and a co-author of the guidebook “Frogs of the World,” noted: “After pollution, habitat alteration, fragmentation and destruction, and predation by invasive species, the arrival of chytridiomycosis could be the final nail in the coffin for many stressed Darwin’s frog populations.”
In October, with the species on the brink of extinction, the London Zoo organized a rescue mission and established a captive breeding program to help save the frog from being wiped out. “There’s nothing quite like Darwin’s frogs,” Dr. Tapley said. “Their evolutionary distinctiveness is really, really striking.”
A breed apart
During breeding season, the advertisement male Darwin’s frogs use to attract females is a rapid, high-pitched whistle — piiiip, piiiip, piiiip. “It’s more of a squeak, like something metallic that needs oiling,” Dr. Tapley said.
When the female appears and the couple mates, she subsequently lays up to 40 eggs in a leaf litter, then hops away. The male stands guard over the frogspawn in a vigil that can last up to three weeks. As the larvae reach the wriggling stage, the father-in-waiting scoops them up with his tongue. The embryos pass into his vocal sac, an expandable fold of skin that reaches back to the groin. Within the membrane’s deep folds, the tadpoles develop in safety.
Some two months later, the dad, who remains silent during the incubation period and transforms from dull brown to bright green, “births” a colony of froglets through his mouth. The only other amphibian that behaves this way is the northern Darwin’s frog (Rhinoderma rufum), whose existence has not been documented since 1981 and which is presumed to be extinct.
A box of frogs
At Parque Tantauco, a private nature preserve on the southern tip of Chiloé Island, the population decline was dramatic. The London conservationists were charged with retrieving fungus-free frogs for safekeeping. Ultimately, they hope to devise treatments to mitigate the threat of the fungus.
Fifty-five specimens were rounded up during a five-day expedition. To screen for possible contamination, skin swabs were flown to a laboratory in Santiago, the Chilean capital. All but two frogs were cleared for the 7,000-mile journey to London. The 53 frogs were individually packed with ice and wet moss in climate-controlled transport boxes — two ounces of frog in a 120-pound box.
After traveling by boat, plane and van to their new digs in London, the frogs were transferred to a biosecure room that recreated the lighting, humidity and foliage of their natural habitat. “They were housed as breeding pairs to maximize genetic diversity,” Dr. Tapley said. “We plan to establish a stud book, like the ones used to maintain the pedigrees of racehorses.”
Eleven of the males turned out to be carrying young. Last month, they spit out 33 hatchlings. Each measured two-tenths of an inch, the size of a pencil eraser.
The frog in the folk song met his end in the gullet of a lily-white duck that “swallowed him up.” If London’s captive Darwin’s frogs are ever reintroduced to the rainforest, they must take their chances with the predators in southern Chile.