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Introducing the Lake Oku Clawed Frog

By on November 12, 2008 in Uncategorized, Amphibians

This is the first blog from Thomas Doherty-Bone, who is carryng out conservation research on amphibians unique to Cameroon. Thomas is being supported by a grant from ZSL‘s Erasmus Darwin Barlow Conservation Expeditions fund. See the previous post for more information.

In the misty hills of Cameroon….

Lake Oku

If you were to go to the North West Province of Cameroon, and wanted to see a scenic lake with a difference, then Lake Oku is the one for you. This lake is hailed as sacred by the local Oku community, and its banks are surrounded by thick evergreen forest. It is a small lake, with an average depth of 32 m, and a shoreline of approximately 5 km. In the mountains at an elevation of about 2,392 meters above sea level, this lake is apparently without fish. However in the place of fish exists a special species of frog, the Lake Oku Clawed Frog, Xenopus longipes

Lake Oku clawed frog

The Family (….of genetic freaks!)

This frog belongs to the family pipidae, a group of highly aquatic frogs, including the common laboratory subject: the African Clawed Frog (X. laevis), used in the past for pregnancy tests. Pipid frogs are thought to have emerged around the Jurassic period, and presently occur in South America and Africa, with the majority of species concentrated in Africa. These frogs are special because of their more established evolutionary history compared to other, more familiar anuran lineages, and because of the variation of chromosome number between species. Some species have two copies of each chromosome, while others have four copies (polyploidy – additional chromosomes). If you are X. longipes, you will have a staggering six copies of each chromosome, matched only by the Ugandan endemic, X. ruwenzoriensis. It has been postulated that such genetic characteristics evolved millions of years ago, through the hybridization of Xenopus species with differing chromosome number. Reasons for such a characteristic to evolve has been linked to parasites (such as helminths), with polyploidy suggested as a way to increase a selective advantage by shedding a long co-evolved parasite that otherwise impinges on the host’s survival and breeding potential. The potential biomedical value of these frogs has barely been addressed.

Most Pipid frogs will commonly be found submerged in water somewhere, eating any bite-size object that moves in front of their grappling arms and snappy jaws (said to be one of the most lethal mastication tools in amphibia, if not the animal kingdom).  Breeding season will typically coincide with the start rainy season for many species. Unlike other Xenopus, considerably little is known about the life history, reproduction or eco-physiology of X. longipes, which was not described by science until 1991. 

Lake Oku clawed frogs

Setting the standard of a biodiversity hotspot

Many members of Pipidae can move about on land, colonising new ponds, rivers, streams, but X. longipes is found to be very inept on land. Based on this, and the absence of fish in the Lake, it makes sense that this frog is unable to make anywhere else its home, apart from Lake Oku. No other lake in Cameroon has been found to hold this species, despite other lakes being examined in past expeditions. There is a second endemic pipid frog in this region, the Volcano Clawed Frog, X. amieti (named after Jean-Claude Amiet, who has significantly contributed to knowledge on the amphibians of Cameroon), which has a broader range and has even been found living in fish ponds on people’s farms. This species of pipid frog has four copies of each chromosome, so not quite as strange as our Lake Oku counterpart. This is testament to the biodiversity generated in the highlands of West Africa: in much the same way that species have evolved in island archipelagos, so have the mountains in Africa nurtured a variety of different species. Instead of oceanic water, it is the receding and recurring grassland (driven by dynamics in precipitation over millennia), isolating and reconnecting these forested mountain “islands”. In this case, the archipelago is the Biafran Highlands, which extend from Bioko Island (in the Bight of Biafra) through Cameroon to southern Nigeria.

On the Edge?

The Lake Oku Clawed Frog has been listed as Critically Endangered following the IUCN’s Global Amphibian Assessment. This is because of the hugely restricted range (to within 243 hectares). There is also the concern that Lake Oku could be one of the many lakes in Africa that have succumbed to well meaning but ill thought-out development projects to improve protein availability to communities – the introduction of fish (e.g. as in Lake Victoria). For these reasons, along with its amazing genetic characteristics, this frog has also been placed in the top 100 of the EDGE programme (at #34), highlighting its evolutionary significance and vulnerability to extinction. The remnants of the montane forests of this region have so far been safe guarded by forest management groups, devolved from the government to the community, a project implemented by Birdlife International. This has partly protected the Lake from degradation through deforestation, but does leave a gap for the specific conservation of amphibians through other factors.

In 2006, an expedition lead by myself to North West Province, observed many Xenopus longipes either dead or dying and sporting lesions on their body. There were many healthy looking frogs also, and we made a preliminary survey over a month to see how many frogs we could count and how many were moribund. Working under biosecurity (disinfected boots and equipment), we captured, marked and released 737 frogs, 28 of which displayed illness (about 4% of population). Tissue samples were collected and taken to the Institue of Zoology, Zoological Society of London, where they were screened for chytrid fungus and ranavirus. Chytrid was found to be absent, while ranavirus results were inconclusive. Meanwhile, the current status of the Lake Oku Clawed Frog is unclear – is this species in decline, or was this a natural phenomena? 

Dead X. longipes

Very little information is available on this lake and its inhabitants, so conclusions cannot be made with any confidence. We must assume the worst, however. Work is being planned to gain more information on this species to help determine the causes and implications of these deaths and morbidities. To learn about this, see the next blog in this series.