It was a remarkably elaborate mating ritual. When a male Chiriquí harlequin frog found its mate, it would climb on to the female’s back, grip its armpits with its forelimbs and hug it. Females of the species were often twice as large as the males, and they would remain in this mating clasp for days or even months – depending on when the female was ready to lay her eggs. During this time, the male might forgo eating and lose up to 30% of its body weight, but it was willing to wait. It has been almost 30 years since a scientist last witnessed this act. In 2019, with little fanfare, the species was declared extinct. Once, the Chiriquí harlequin frog (Atelopus chiriquiensis), which, scientifically speaking, was a toad, could be found in abundance in the wet highland forests of Costa Rica and Panama, usually where mean annual rainfall exceeds 4,000mm. In February 1994, when Erik Lindquist, an American biologist, visited La Amistad international park linking the two countries, he heard them calling from all over, and frantically counted 113 Chiriquí harlequin frogs within a 10-metre stretch of creek. “You kind of realise, oh my goodness, I may be stepping on some, because they’re just everywhere,” he recalls. Despite being classified as a toad, it didn’t look like one. It had smooth, not warty, skin. Its vivid colouration – it could be green, yellow, rust brown or grey, with lavender or red mottling – warned predators of its toxicity. It preferred meandering on dry land to swimming. Lindquist saw some of them “waving” their forelimbs at others, perhaps to avoid fights, or as a mating behaviour. “I’ve also seen females signal to males, like, hey, come get me, kind of thing,” he says, laughing. Yet when Lindquist returned to the area just three months later, the streams were silent. This decline in numbers was also being observed in other protected sites and with other frog species in Central America. In 1996, at the Fortuna forest reserve in Panama, the American herpetologist Karen Lips found 54 dead or dying frogs belonging to 10 species from four streams, when their numbers had been abundant just a year earlier. She described dying frogs expiring in her hands after a short struggle to escape and dead ones being “frozen” in their normal calling positions. That year – 1996 – marked the last year a Chiriquí harlequin frog was ever seen, anywhere in the world. It vanished so suddenly that any last-ditch attempts to save it via captive breeding were impossible. Scientists found this sudden and widespread disappearance of frogs in Central America perplexing, since it was occurring in protected forests and could not be attributed to habitat loss. In 1999, Lips landed on the culprit: a fungal disease that infects a frog’s skin, stopping it from breathing and regulating its water levels, causing its heart to fail. Harlequin frogs are particularly susceptible to the fungus since they favour cool, moist habitat that is conducive to its growth. The disease, native to Asia and, perhaps, unintentionally introduced elsewhere as part of the global wildlife trade, continues to threaten amphibian species. It has reportedly devastated global biodiversity more than any other disease ever recorded, having driven the decline of 501 amphibian species, of which 90 are presumed extinct in the wild. But there is still a sliver of hope. “At least some frog species in Costa Rica have been found again in places that we thought they had disappeared from, and some have been found in new localities,” says Federico Bolaño, a local biologist. Encouraged, scientists continue to visit the known habitats of vanished frogs, hoping they will find survivors. However, according to Jorge Rodríguez-Matamoros, a Costa Rican conservationist: “The new populations tend to be very small, and the species continue to be fragile to extinction.” Experts also think climate change may have compromised the frogs’ immunity or altered how the disease survives and spreads in some way. The loss is personal for biologists like Lindquist. “These frogs are a little bit special for me because they were everywhere,” he says. “And then they weren’t.”