A specimen of Du Toit's torrent frog in a glass jar on a self.

There are only a handful of specimens of Du Toit's torrent frog held in collections, such as this one at the Museum of Comparative Zoology, Harvard University ©Breda Zimkus/Museum of Comparative Zoology, Harvard University

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Extinction of rare frog is a huge blow to the diversity of life

A species of frog not seen for over 60 years is likely to be extinct. As one of the few remaining species of an ancient lineage of amphibians, its loss is a huge blow to the diversity of life.

The frog, once found in the rainforest streams of Mount Elgon in Kenya, is thought to have disappeared due to the logging of the forests and the conversion of its habitat to farmland. 

Warm, humid mountains tops like this are often referred to as 'sky islands'. Set adrift among the drier lowlands they provided a refuge for many rainforest species that would not otherwise survive in this region.

These islands contain a range of different amphibians, including a group of frogs known as torrent frogs. Specifically adapted to the fast-flowing rivers that surge through the forest and down the mountains, they are evolutionarily distinct, having split off from their nearest living relatives over 70 million years ago.

One of these species known as Du Toit's torrent frog was last seen in the 1960s, and despite countless attempts to find this little amphibian on the slopes of Mount Elgon, it is now thought that it is likely extinct.

Dr Simon Loader is the Principal Curator in Charge of Vertebrates at the Natural History Museum and an expert on east African frogs. He has helped author a paper looking into whether it is likely that Du Toit's Torrent frog still clings on.

'If we lose this frog, we lose a large amount of evolutionary history and that is what makes them important and why projects were initiated to find Du Toit's torrent frog in the first instance,' explains Simon.

'It's not just losing a species, it is losing a distinctive branch of the evolutionary tree.'

The tragic conclusion that the species is now likely to have become extinct has been published in the African Journal of Herpetology. 

A veiw across the hills of Kenya, showing mountain tops covered in rainforest and shrouded in cloud.

The wet, forested tops of the mountains have given them the name 'sky islands' as they allow rainforest species to survive in an otherwise very hot and dry environment  ©Jacob Mueti/National Museum of Kenya

Evolutionarily distinct

Du Toit's Torrent frog is one of 13 species in a family known as Petropedetidae.

These frogs are known as torrent frogs, due to the fact that they live in fast-flowing streams high up in the mountains. To survive there, they have evolved a number of adaptations to prevent them from being swept downstream.

'It is quite challenging for frogs to live and breed in these torrent rivers,' says Simon. 'One particular feature with their tadpoles is their sucker-like mouthparts which allows them to attach themselves to rocks, preventing them being washed down the rivers.

'As adults, they also have t-shaped fingertips, which gives them more surface area to grasp onto slippery rocks.' All these adaptations make these frogs rather interesting and special.

The Petropedetidae family of amphibians is ancient, having diverged from close relatives during the Cretaceous Period around 70 million years ago. At this time, plesiosaurs were still swimming the oceans, pterosaurs were riding thermals in the skies and dinosaurs were roaming the land. 

A waterfall pours off the top of a mountain and down a canyon. It is surrounded by lush green vegetation.

The torrent frogs are adapted to live in the fast-flowing rivers that gush down the sides of the mountains ©Jacob Mueti/National Museum of Kenya

Within the Petropedetidae family, Du Toit's torrent frog is part of a distinct genus with just two other species of frog, both of which are found in the mountain streams of Tanzania.

'These frogs belong to a group which are quite distantly related to each other and they all reside in East African mountain rainforests,' explains Simon. 'Their nearest relatives are actually a radiation of torrent frogs found in mountains in central Africa.'

Losing part of this evolutionarily distinct lineage of frog is significant blow not only to amphibians, but to the history of all life on this planet.

Preserving what remains

Simon and his colleagues were able to assess whether the torrent frog is likely to survive by looking into historic accounts of when the frog was first found and scientifically described in the 1930s, when an expedition collected specimens in the 1960s (and when it was last seen), and then at more recent attempts to search for it.

They used these records to assess the amount of time and effort that went into looking for the amphibians, and then worked out the probability of its survival given recent attempts to find the frog.

Over the last 20 years there have been numerous attempts to find Du Toit's torrent frog driven by colleagues at the National Museums of Kenya, including the first author of the scientific paper Jacob Mueti Ngwava.

Simon explains, 'Colleagues at the National Museums of Kenya have carried out a phenomenal number of surveys on Mount Elgon to try and find it.

'Unfortunately, and very sadly, they haven't found a single specimen.' 

Two men balancing on a log that is over a rapidly flowing river, surrouned by high cliffs.

Despite decades of searching by Jacob Mueti Ngwava and his colleagues at the National Museums of Kenya, no living Du Toit's frog has been seen for 60 years ©Jacob Mueti/National Museum of Kenya

This has led to the conclusion that Du Toit's torrent frog no longer survives in the mountains of Kenya, but Simon is still careful not to make sweeping assumptions about its extinction.

'The only caveat is that we don't really know what is happening on the Ugandan side where the species might be found and where surveys haven’t been conducted as intensively,' says Simon. 'There might be some remnant forest patches that exist and where Du Toit's torrent frog might still be found.'

This lack of herpetological surveys in Uganda is largely due to the paucity of research on amphibians and reptiles in this part of the world. But if the poor state of the forests in Uganda is anything to go by then it is unlikely to be good news for Du Toit's torrent frog.

'The main finding from the paper is that we think Du Toit's Torrent frog is likely to be extinct but this still needs to be fully assessed by IUCN Red List team who determine extinction of animal and plant species,' explains Simon.

While it is upsetting that Du Toit's torrent frog is probably lost forever, there is still hope in the other two surviving members of its genus, both of which are found further south in Tanzania.

Protecting and preserving these two amphibians could save an entire branch of the evolutionary tree from being completely wiped out and sustain what diversity remains in these remarkable 'sky island' forests.