It looks like we've lost another one. The weird and unusual Chile Darwin's frog (Rhinoderma rufum), whose tadpoles grew inside the vocal sacs of adult males, appears to be extinct: a four-year quest failed to turn up any evidence that the species still exists. The frogs were last seen in 1980.

As you might guess from its name, the Chile Darwin's frog was discovered by naturalist Charles Darwin while he was traveling through South America in 1834. More than a century later scientists reclassified the mouth-brooding frogs—the only ones with this unique reproductive strategy—into two species, including the now-lost species that was native to northern Chile. The more simply named Darwin's frog (R. darwinii) still exists farther south in ever-shrinking regions of Chile and Argentina.

Claudio Soto-Azat, a biologist with Andrés Bello University in Santiago, Chile, led a team of Chilean and British researchers to find out how the two Rhinoderma species fared. They first studied 2,244 museum specimens along with the accounts of where they were originally collected to develop historical distribution maps for the two species. They then set out into the wild, searching 223 sites for the frogs. The northern R. rufum species never revealed itself. They found R. darwinii frogs at just 36 fragmented southern locations, each with an average population size of just 33.2 frogs. Their research was published June 12 in PLoS One.

The researchers now recommend that the southern species be reclassified as "endangered" from its current status of "vulnerable." As for the Chile Darwin's frog, the researchers say the lack of evidence suggests the species is extinct, but they haven't completely given up hope. Although most of the frog's historic range has either been destroyed or thoroughly searched, the species could still exist in some forgotten corner of northern Chile, the scientists say. They recommend the species be reclassified from "critically endangered" to "critically endangered (possibly extinct)." The authors suggest that this change could help focus attention on the need to conserve the remaining Darwin's frog species, which still faces a range of threats, including deforestation, invasive species and possibly the chytrid fungus, which has been driving amphibians into extinction around the globe.

The researchers suggest keeping a close eye on the southern Darwin's frog, warning that if its population continues to decline, it may qualify for "critically endangered" status in a few years. Let's hope it doesn't come to that.

Photo: Two southern Darwin's frogs captured in 2009 and 2011, via PLoS One

Previously in Extinction Countdown: