It could be the opening of a children’s story: in a great forest on a mountaintop lies a tiny lake, and in that tiny lake lives a tiny frog. But this isn’t just any frog. No, this frog is different. Very, very different. It has big webbed feet, no tongue at all, and (here’s where we leave typical children’s book territory) a whole bunch of chromosomes. The vast majority of the world’s animals, including humans, have two sets of chromosomes. But the Lake Oku clawed frog has twelve sets, which is a high number even for chromosome abundant organisms like plants.

The Lake Oku clawed frog is “[one] of the most genetically unusual creatures in the world,” said Carly Waterman, the Programme Manager for the Zoological Society of London’s (ZSL) EDGE group.



The chromosome-happy frog is a member of an ancient lineage of amphibians known as pipids, which are only represented today by about 30 species that lack tongues and stick solely to water. Pipids first split off from other frogs 130 million years ago during the age of dinosaurs, but still some 60 million years before Tyrannosaurs Rex came on the scene. Scientists think the frog attained so many chromosomes through hybridising with similar species over millions of years.



Scientists get their first look at the tadpoles of the Lake Oku clawed frogs. Photograph: Ben Tapley/ZSL

The EDGE currently lists the Lake Oku clawed frog (Xenopus longipes) as number 35 out of some 7,000 amphibian species in terms of its evolutionary distinctness (or how far they are separated from other species on the tree of life) and its proximity to extinction. This means the frog is not only extremely distinct, but also highly threatened.



The frog’s fairytale setting – on the top of rain forest-covered Mount Oku in Cameroon – has put it in grave danger. Surviving in just a single lake means the critically endangered frog could be wiped off the face of the Earth by any number of events, including a deadly disease or the introduction of hungry fish. This is why the announcement earlier this month that ZSL’s London Zoo had succeeded in rearing the first ever captive Lake Oku clawed frog tadpoles and juveniles is so important.

“We are absolutely delighted to be the first zoo in the world to have successfully bred the Lake Oku clawed frog,” said Ben Tapley, the Team Leader of Herpetology at the London Zoo.

The story of how conservationists achieved the goal actually begins in 2008. Then an intrepid student named Thomas Doherty-Bone – armed with a grant from the Royal Zoological Society of Scotland – headed to Cameroon to study the country’s unique amphibians, including our story’s hero. Several weeks later – including mulitiple interminable bus rides and an unplanned stay in the hospital for salmonella – Doherty-Bone returned to the UK with a piece of unusual baggage: Lake Oku clawed frogs. In order to carry them abroad, he had to follow a few regulations, including making certain that removing a few frogs would not further endanger the total population.



It was a total career highlight seeing the frogs lay their eggs and watching the tadpoles develop Ben Tapley

“Then I had to discuss with the local community the need to bring frogs from their sacred lake into captivity, so we could have their consent. Then I had to get official permission from the [government] to be allowed to take them out of the country,” Dohery Bone, who is currently studying for a PhD at the University of Leeds, explained. “This did not stop there. I had been measuring the temperature and other environmental conditions of the lake and had sent these details to the amphibian keepers at London Zoo who were preparing to receive the frogs.”

Doherty-Bone’s data, which was consistently updated by a local team in Cameroon, proved crucial in getting the frogs to breed at the London Zoo.

“The challenge was...to mimic these conditions in captivity,” said Tapley. “We obtained water parameters from Lake Oku and the seasonal temperature and pH profile of the lake. We recreated these conditions and mimicked seasonal changes in temperature and pH.”

Six years after they arrived, the frogs finally began to reproduce. For the first time ever, scientists came face-to-face with a Lake Oku clawed frog tadpole. And they did not disappoint.

The Lake Oku clawed frog is a muppet-looking amphibian with a lovely orange hue. Photograph: Ben Tapley/ZSL

“The tadpoles are bizarre-looking, transparent filter feeders – the blood vessels, gills, brain and gut are all visible through the body wall,” Christopher Michaels with ZSL wrote in an EDGE blog. “They take more than six months to transform into frogs and metamorphosis is dramatic, even in frog terms. The widely spaced eyes of the tadpoles migrate to the top of the head, the mouth and internal organs change shape, lungs form, gills diminish and the wide-finned tail is absorbed after legs grow.”

Tapley said “it was a total career highlight seeing the frogs lay their eggs and watching the tadpoles develop and knowing that no one else had seen the process before.”

Once grown, the frogs have big appetites. Since they don’t have tongues, they suck in their prey with their broad mouths using their arms to help facilitate entry.



Doherty-Bone, who has fed them banana pieces, said the frogs “really can be voracious.” He once watched seven Lake Oku clawed frogs tear apart a praying mantis that had been unlucky enough to fall into the lake.



“The one time I didn’t have my camera on me,” he bemoaned.



On hearing that the captive frogs had finally bred, Doherty-Bone said he felt “very, very relieved...the effort was not for nothing.” But he added that he is “also concerned managers in Cameroon might attempt to use this as an excuse to harm the frog’s habitat with impunity – such as attempting introduction of exotic fish.”

Locals had proposed introducing fish in the past, but every time wildlife managers and environmentalists had struck it down, according to Doherty-Bone.

Lake Oku clawed frog tadpole with little limb visible. Photograph: Ben Tapley/ZSL

“It only has to succeed once for it to become a disaster though,” he added. Lake Oku clawed frogs are tiny – averaging a little over 30 millimetres – and would be easy pickings for predatory fish.

“The most important conservation action is to continue to work with local communities to ensure that the lake continues to be protected, and that predatory fish are never introduced,” noted Waterman with EDGE.

The other major threat – disease – may prove more unmanageable though. Indeed, the worst was feared when frogs were observed dying off en masse in both 2006 and 2010, but researchers haven’t pinpointed the cause.

“These die-offs are still a mystery,” said Doherty-Bone.

Lake Oku’s clawed frogs have tested positive for chytrid (short for chytridiomycosis), a virulent disease caused by the fungus, Batrachochytrium dendrobatidis. Scientists blame chytrid, which kills amphibians by seeping in through their skin and leads to heart failure, for wiping out dozens of species and putting hundreds, maybe thousands, at risk.



A male Lake Oku clawed frog at the zoo. Photograph: Ben Tapley/ZSL

But Topley said chytrid “has not been linked to any [Lake Oku clawed frog] mortalities.”

Whereas chytrid has decimated amphibians across the Americas, researchers have discovered that some species show resilience to chytrid – especially in parts of Asia and Africa – where they have probably lived with the disease for centuries.



“The frogs are still there in apparently large numbers,” said Doherty-Bone. “We are however still trying to work out what the causes and consequences of [chytrid] are for the frog population – all the more reason to have a backup colony away from the lake.”

Lake Oku in Cameroon. Photograph: Public Domain

Backup colonies, insurance populations, or captive breeding troupes – whatever your preferred term – have increasingly become the norm in amphibian conservation. On top of chytrid, amphibians also face deforestation, habitat destruction, air and water pollution, pesticides, invasive species, a sometimes predatory pet trade and climate change. Still, in some cases the reasons why particular species are vanishing remains complex, contentious, and even downright baffling. But everyone agrees that amphibians are especially vulnerable given that most spend a part of their life in freshwater and another part on land.

The IUCN Red List currently considers 31 percent of the world’s amphibians threatened with extinction, which is much higher than for other well-surveyed groups like mammals and birds. Although shocking, the estimate is also quite conservative. Today, a growing number of amphibians survive only behind glass, but at least they survive; it’s thought as many as 159 species have vanished forever in recent times.



For now, the Lake Oku clawed frog is one of the lucky ones, surviving both in the wild and behind glass. But, hopefully, the insurance population will never have to be used, hopefully these bizarre beings will continue swimming in their little mountaintop lake for eons to come, gobbling up whatever gets too close and beguiling us with their many chromosomes.



“Lake Oku is an astonishingly beautiful lake,” said Doherty-Bone. “You usually see [the frogs] if you wait for dusk, then they start swimming out, free of fear of predation by birds. They are then dotted around the lake.”

Most children’s stories have happy endings. Let’s hope this one does too: with lots of frogs peering up at the stars from a mountaintop lake.