The folk song “Frog Went a-Courtin’” tells the story of a frog attempting to woo a mouse, who insists that her Uncle Rat's consent is necessary for marriage. In contrast, the courtship of Darwin’s frogs, found in the temperate rainforests of southern South America, is characterized by unique parenting practices rather than conditional courtship.
Named after Charles Darwin, who discovered them in 1834 on Chiloé Island in Chile, Darwin’s frogs exhibit a rare reproductive strategy known as mouth brooding, where males carry their young in their vocal sacs until they are ready to survive independently.
Adult Darwin’s frogs measure about an inch in length and weigh less than a couple of paper clips. Although they can live up to 15 years in the wild, they are highly vulnerable to chytridiomycosis, a deadly disease caused by the chytrid fungus. This pathogen has been linked to the decline of approximately 400 amphibian species since the 1990s, resulting in the extinction of at least 90 others.
Chiloé Island, once a refuge for Darwin’s frogs, has seen the chytrid fungus emerge, likely due to climate change that has created more favorable conditions for the pathogen, according to conservation biologist John W. Wilkinson.
Within a year of the fungus's detection, over 90 percent of the island’s Darwin’s frogs were lost. Ben Tapley, a curator at the London Zoo, noted that the frogs succumb to the infection within weeks.
Herpetologist Mark O’Shea emphasized that the arrival of chytridiomycosis, following pollution and habitat destruction, could be catastrophic for already stressed Darwin’s frog populations.
In response to the imminent extinction of the species, the London Zoo initiated a rescue mission in October, establishing a captive breeding program to preserve the frogs. Dr. Tapley remarked on the frogs' unique evolutionary traits.
During the breeding season, male Darwin’s frogs attract females with a rapid, high-pitched whistle. After mating, the female lays up to 40 eggs in leaf litter and departs, while the male guards the eggs for up to three weeks. As the larvae develop, the male collects them with his tongue and transfers them to his vocal sac, where they grow safely.
Approximately two months later, the male “births” the froglets through his mouth. The only other amphibian known to exhibit this behavior is the northern Darwin’s frog, presumed extinct since 1981.
At Parque Tantauco, a nature preserve on Chiloé Island, the decline of the frog population was significant. Conservationists from London aimed to retrieve fungus-free frogs for preservation and potential treatment development against the chytrid fungus.
During a five-day expedition, 55 specimens were collected, and skin swabs were tested for contamination. All but two frogs were cleared for transport to London. The 53 frogs were carefully packed in climate-controlled boxes for their 7,000-mile journey.
Upon arrival in London, the frogs were placed in a biosecure environment that mimicked their natural habitat. They were housed as breeding pairs to enhance genetic diversity, with plans to maintain a stud book for the population.
Eleven males were found to be carrying young, resulting in 33 hatchlings last month, each measuring two-tenths of an inch. If these captive Darwin’s frogs are eventually reintroduced to their rainforest habitat, they will face the risks posed by natural predators in southern Chile.