By Rebecca Morelle
Science reporter, BBC News, Costa Rica
The golden toad has vanished from Costa Rica's rainforests
Scientists are set to begin a hunt for the some of the world's rarest frogs in Costa Rica, including the iconic golden toad, last seen some 20 years ago.
A team from Manchester University and Chester Zoo are in Costa Rica to track down the highly endangered creatures.
BBC News will follow their trek deep into the cloud forests of Monteverde.
Amphibian numbers around the world have crashed, in part because of a deadly fungus. Costa Rica has been particularly badly hit.
Expedition leader Andrew Gray, from the University of Manchester's Manchester Museum, said: "Costa Rica's highlands used to be major biodiversity hotspots - but in many areas, amphibian populations have been completely decimated."
In the late 1980s, herpetologists around the world found that amphibian populations were suffering unprecedented declines, but they struggled to understand exactly why.
A decade later, researchers isolated a previously unknown fungus, Batrachochytrium dendrobatidis, which was infecting amphibians, effectively suffocating them by making it impossible for them to breathe across their skin.
Recently, the scientists working on the Global Amphibian Assessment estimated that one-third of all amphibians were threatened with extinction and about 120 species had already become extinct since the 1980s.
Many believe the disease caused by the chytrid fungus is a key factor for this crash. Other causes are thought to include habitat destruction and changes in climate.
Countries in Central America have been particularly badly affected by the deadly chytrid fungus, which is widespread there. A great deal of effort is now being put into place to safeguard any remaining species.
To find this species last year that was thought to be extinct at the same times as the golden toad was incredible
Andrew Gray said: "For the last 10 years, I've been working with others to ensure the future for frogs that have so far escaped extinction.
"One of the main things I have been doing is establishing breeding populations in Manchester Museum for a number of very, very rare species - including the splendid leaf frog (Cruziohyla calcarifer), the yellow-eyed leaf frog (Agalychnas annae) and the lemur leaf frog (Hylomantis lemur).
"I've also been working with the Costa Rican authorities and scientists to put conservation measures into place at the sites where any rare frogs are found."
Last year, Mr Gray caught a glimpse of the Ithsmohyla rivularis in the cloud forests of Monteverde - a frog that was thought to have gone extinct about 20 years ago.
Manchester Museum has a splendid leaf captive breeding population
He said: "To find this species last year that was thought to have become extinct at the same time as the golden toad was incredible - it is the rarest tree frog in the world."
He has now been given special permission by the Costa Rican authorities to collect some of the frogs to take back to Manchester.
He told the BBC: "We are returning to thoroughly search the site in the hope of finding more specimens.
"It's not going to be easy - they live deep in the Monteverde rainforest, they are only a couple of centimetres in size and they only come out in the dead of night - and while the males do call, the females don't make a sound."
'Never say never'
The rediscovery of Ithsmohyla rivularis has spurred the team on to also try to seek out a golden toad (Bufo pereglines).
This colourful amphibian, which scientists only discovered in 1966, became the iconic symbol of amphibian decline. In 1987 there were approximately 1,500 of the toads, but just two years later it had vanished from the face of the rainforest.
Mr Gray said: "We are going to be trekking through an area where the golden toad used to thrive. It is very unlikely we will find one - but as last year's discovery showed us, never say never."
The Manchester team has been looking at the properties of frogs' skin
While in the rainforest, the team will also be trying to track down the miniature red-eyed tree frog (Duellmanohyla uranochroa) - a species on the brink of extinction - to investigate how some frogs may be able to prevent the chytrid fungus from taking hold.
This is a unique opportunity to study the frogs in their natural habitat
Previous research has shown that some species of tree frog have a special pigment in their skin that enables them to reflect light, allowing them to "sunbathe" without drying out.
Physicist Mark Dickinson, from Photon Science Institute at the University of Manchester, will be taking a spectrometer into the field to investigate how different frog species reflect light.
He said: "So far, I've only been able to investigate captive frogs in the lab. This is a unique opportunity to study the frogs in their natural habitat."
The team believes that the ability to sit out in the Sun may allow the frogs' skin to heat up just enough to kill off chytrid - preventing the disease from taking its grip.
Some of the team will also be heading to the last known breeding site of the green-eyed frog (Lithobates vibicarius) where Chester Zoo is helping to support a conservation programme.
BBC News will be keeping track of the researchers' progress over the next two weeks.
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