Francois Becker knows his frog calls ― and he knows them well. But in early December, while conducting research near the summit of a remote mountain in eastern Zimbabwe, the ecologist heard a call he could not quite place.
"When I first heard [it], I thought it might be an insect," Becker, a graduate student at the University of Cape Town, told The Huffington Post over email on Monday. "It was a soft, high-pitched whistle repeated several times."
But as he got closer to the sound, Becker determined that it wasn't an insect at all. "The 'texture' of the call, for lack of a better word, confirmed that it was probably a frog," he said. Specifically, a kind of Arthroleptis frog.
Arthroleptis is a genus of frogs endemic to tropical sub-Saharan Africa. They're known for their high-pitched whistling calls and tiny stature (the largest species of the genus, Arthroleptis tanneri, grows to just two inches long). They are also direct breeders, meaning they ― unlike most other frogs — are born as fully formed froglets and not as tadpoles.
"Francois had done a great deal of work on [Arthroleptis frogs] in South Africa, and had paid particular attention to their calls," Becker's research colleague, Robert Hopkins, told The Zimbabwean in an interview last month. "He heard a call which he recognized as that of an Arthroleptis, but did not or could not identify it, so he tracked that call and ultimately found the first specimen."
The source of the unusual whistle turned out to be an unimaginable treasure: a rare cave-dwelling frog that had not been seen in over 50 years.
This is Arthroleptis troglodytes, also known as a "cave squeaker." Becker took the first photos of the species ever.
The cave squeaker was last spotted in the rocky Chimanimani mountains of eastern Zimbabwe in 1962, the year it was first discovered. The minuscule animal measures no more than an inch long, and scientists assumed it was extinct after several unsuccessful searches, including a 2010 expedition.
Hopkins, a 75-year-old researcher with Zimbabwe's Natural History Museum, had been searching for the frog since 1998, to no avail.
In a last-ditch attempt to find the elusive animal, he applied for a grant from the Mohamed bin Zayed Species Conservation Fund last year to conduct another search. He roped in Becker and a Zimbabwean entomologist named Scott Herbst. In December, the group headed to the mountains of Chimanimani, the only place the frog was known to exist.
Hopkins, who has cancer, was not able to participate in the search effort due to his age and illness, he explained in a research report. So he was in Chimanimani village on Dec. 2 when he received a phone call from a "very excited" Becker.
"[H]e had located an Arthroleptis troglodytes," wrote Hopkins in the report. "It was great surprise and release after all these years."
Remarkably, Becker didn't just find one lonesome cave squeaker that day, but several.
He said it had taken him about 45 minutes to track down the first frog after hearing its call. "I was so excited when I saw it that my hands were shaking, and I let it slip away," he recalled. "It hopped into a deep crevice and I could no longer see it. However, by this time I had recorded the call and was playing it back to them, to prompt other nearby males to start calling. It took me about another 40 minutes to find the next one."
Becker and his team, which also included two local guides, collected three males and one female that day.
Hopkins said they gathered a "great deal of data," including DNA samples that have been sent for analysis. Hopkins said the cave squeakers appear to be "breeding well" in the Chimanimani mountains.
"There seems to be a very viable population," he wrote in the report. The exact number of frogs, however, remains unknown.
Scientists are now considering conservation strategies for the frog species. Hopkins told The Zimbabwean that he, together with the Zimbabwe Parks and Wildlife Management Authority, are mulling over the possibility of breeding cave squeakers in his laboratory and releasing them back into the wild.
Hopkins worries that the recent discovery will prompt people to illegally capture and export cave squeakers from Chimanimani. A Parks Authority spokeswoman told The Associated Press this week that experts are devising a management plan to protect the animal.
"We are expecting an influx of scientists looking for it," said Caroline Washaya-Moyo. "We will do everything in our power to protect and conserve the frog."
Amphibians, including frogs, are one of the most threatened groups of animals on the planet. According to the International Union for Conservation of Nature, about 1 in 3 amphibians face the threat of extinction. Climate change, habitat destruction and disease are some of the greatest threats facing frogs and other amphibians.
A 2011 study found that amphibians may not be evolving fast enough to deal with the "enormous" human-induced changes to the environment over the past 100 years.
"With a permeable skin and exposure to both aquatic and terrestrial problems, amphibians face a double whammy," zoologist Andrew Blaustein, co-author of the study, told LiveScience at the time. "Because of this, mammals, fish and birds have not [yet] experienced population impacts as severely as amphibians."
Dominique Mosbergen is a reporter at The Huffington Post covering climate change, extreme weather and extinction. Send tips or feedback to dominique.mosbergen@Twitter.or follow her on