The evolutionarily unique frogs of Cameroon’s Lake Oku have no tongues, claw-tipped toes and 12 full sets of chromosomes. What the Lake Oku clawed frogs (Xenopus longipes) don’t have, however, is a lot of habitat in which to live. They only exist in a tiny mountaintop lake more than 2,200 meters above sea level. Although they face little predation there, the species did encounter an as-yet-unexplained mass mortality event between 2006 and 2010 that killed off dozens, if not hundreds, of these critically endangered frogs.
“The species seems to be doing okay in its natural habitat at the moment,” says Ben Tapley, the head of the reptile and amphibian team for the Zoological Society of London (ZSL). That doesn’t mean they’re safe, though: “Due to living in just one area, they are incredibly vulnerable to the threats of invasive species or a disease outbreak,” he says.
That’s where the ZSL comes in. This year the organization’s London Zoo successfully bred the frogs in captivity for the first time, an important step that could help to create a “captive assurance population,” a safety net in case any further dangers hit the only population in the wild.
Getting the frogs to breed thousands of kilometers from their native habitat wasn’t easy. Before conservationists could get the frogs to reproduce, they needed to figure out how to mimic the natural ecosystem of Lake Oku. “We replicated all of the water parameters and temperatures in the lake,” Tapley says. That included water hardness and pH and well as adapting the lighting to the night-and-day patterns on top of sky-high Mount Oku. The zoo also experimented with different foods, again replicating what would be available in Cameroon, which may have served as breeding triggers.
Understanding the frogs’ competitive breeding behavior also helped. For instance, the zoo observed that its two males tried to dislodge one another when each mated with a female. Keeping them separated improved the odds of successful mating.
Tapley thinks the optimized water conditions didn’t make much of a difference to the adult frogs but they did help the eggs and resulting tadpoles. “The tadpoles seemed more receptive and did much better in the system that replicated the lake,” he says. A total of 13 tadpoles have now metamorphosed into juvenile frogs at the zoo.
London Zoo plans to share its results with conservation teams in Cameroon and in other zoos in hopes of increasing the captive assurance population. It’s a necessary step because the deadly amphibian-killing chytrid fungus has recently been observed on the frogs in Lake Oku. It hasn’t killed any of them yet but it’s too early to say if it will be a problem in the future. “Some species are more susceptible to the disease than others and we currently do not know if the Bd infection is an issue with this species,” Tapley says.
Photos by Ben Tapley, courtesy of ZSL London Zoo