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About 40% of amphibians - a group that includes frogs, salamanders and caecilians - are at a heightened risk of extinction.
The mountain chicken frog is among them. Since 2002 their wild population has plummeted, leaving the species teetering on the brink of extinction.
Hunting, invasive species and habitat destruction may all have contributed to the mountain chicken's decline, but it was the spread of an amphibian-killing fungus that sent the species spiralling towards extinction.
The future of these critically endangered frogs is uncertain, but conservationists are hopeful that there may be a way to help the mountain chicken survive.
Mountain chickens (Leptodactylus fallax) are one of the world's largest species of frog, sometimes growing to over 20 centimetres long and weighing up to a kilogram. They vary in colour but are usually reddish-brown with dark splotches and a cream belly, which allows them to blend in with leaf litter in their humid forest homes.
Mountain chickens used to be found on several islands in the Lesser Antilles, but now only occur on Dominica and Montserrat. The species was once abundant on these two islands, commonly hunted and eaten, and was even formerly an unofficial national dish in Dominica.
Until hunting seasons and bans came into force, it's estimated that 18,000-36,000 mountain chickens were caught per year on Dominica. While there are no official records for Montserrat's historical population, unpublished data shows that 1,043 frogs were harvested there in 1979, and 1,680 in 1980.
Also known as the Dominican white-lipped frog, giant ditch frog and crapaud, mountain chickens are culturally important, appearing in local folk songs, calypsos, poems and jokes. Their likeness is also included on Dominica's coat of arms.
There are several theories for the origin of the unusual common name, 'mountain chicken'. It may be a reference to their large size or that they have been a food. It could also be due to their chicken-like, squawking alarm call.
Mountain chickens are carnivores with voracious appetites. They eat almost anything they can swallow, from crickets, millipedes and tarantulas to small vertebrates including geckos and other frogs.
They are also diligent parents with a unique reproductive strategy. Using a trilling call, a male mountain chicken will entice a female into a nesting burrow, where they mate and a foam nest is produced into which the female lays her eggs.
Tadpoles hatch in the nest and the males and female remain close by, aggressively defending their young as they develop. The female also produces thousands of unfertilised eggs that her young feed on. When the tadpoles metamorphose into juvenile frogs, they leave the nest to fend for themselves. Mountain chickens have a lifespan of about 12 years.
In Dominica, mountain chickens were once widespread, mostly on the west side of the island at low elevations.
Unsustainable hunting may have been impacting mountain chickens for some time, however it was the arrival of Batrachochytrium dendrobatidis (Bd) on the island that initiated their catastrophic decline. Bd is a chytrid fungus that only infects amphibians and can cause the fatal disease chytridiomycosis.
This disease attacks the outermost layer of an amphibian's skin, affecting its ability to regulate water and electrolytes, and it can seriously affect tadpoles' mouthparts. Bd represents one of the most destructive panzootics (the animal equivalent of a pandemic) and has caused amphibian declines and extinctions globally.
Bd was discovered in 1998, but by then it had already spread around the world. The international amphibian trade may have been a key factor in its spread - animals that were infected (unbeknownst to people) were being transported around the world, such as for human consumption and use in laboratories.
In 2002, Dominican officials began to receive reports of large numbers of dead and dying mountain chickens, signalling that Bd had arrived on the island. Within 18 months of the first confirmed case, the mountain chicken population had shrunk by about 85%.
Efforts to track the remainder of the ailing population continued until 2008, at which point no mountain chickens were detected at survey sites, leading experts to presume that the species may have gone extinct on the island.
Nearly 60% of Montserrat is an exclusion zone, implemented after the Soufrière Hills volcano in the south of the island became active in 1995 after centuries of dormancy. Continued volcanic activity notably resulted in Montserrat's capital, Plymouth, being abandoned. More than half the population have since left the island.
The volcano's eruptions also took their toll on the mountain chicken. The frogs were once seen from sea level to the top of the Soufrière Hills at 1,000 metres, but pyroclastic flows and ash falls likely destroyed several populations and their habitat, ultimately restricting the species to a small portion of the island's Centre Hills reserve.
With the devastation of the species in Dominica, the best hope for the mountain chicken's survival seemed to be in Montserrat, but by 2009 Bd was identified there, too.
Forestry officials and a local hunter observed large numbers of the typically nocturnal mountain chicken congregating around water in the daytime, as well as finding dead frogs. Sick-looking individuals displayed the clinical signs of chytridiomycosis, which can include lethargy, redness of the belly and legs, and muscle tremors.
Bd infections don't affect all amphibians in the same way. Some, like the mountain chicken, are highly susceptible. Others are more resistant and can act as disease vectors, moving the fungus from one area to another. On Montserrat and Dominica, the fungus is also known to be carried by eleutherodactylid frogs and cane toads.
Much of Montserrat's fresh produce is imported from Dominica. It has been suggested that Bd-infected stowaway frogs could have been a way the fungus made the leap to the island.
As was the case in Dominica, the arrival of Bd in Montserrat caused the mountain chicken population to crash. The last male and female were moved closer together with hopes of them breeding, but they were last recorded in 2016, leading the Montserratian population to be considered functionally extinct.
The devastating effects of Bd brought mountain chickens dangerously close to extinction. International organisations, the governments of Dominica and Montserrat, and local NGOs have formed the Mountain Chicken Recovery Programme.
For now, the species' survival is safeguarded by captive, safety-net populations, established with the aim of providing mountain chickens for release efforts. But keeping and breeding these frogs in captivity hasn't been without challenges.
When Bd was detected in Dominica in 2002, 12 mountain chickens were moved from the wild to a biosecure facility at ZSL, but these frogs never successfully bred. In 2005, a breeding centre was established on Dominica, but breeding efforts there were abandoned after this facility was severely damaged by Hurricane Maria in 2017.
In 2009, 50 mountain chickens were evacuated from by then-Bd-positive Montserrat and moved to biosecure facilities in Europe, where they successfully bred in captivity. Between 2011-2013, a series of experimental releases of these frogs were conducted in Montserrat. The last of the 121 released frogs was detected in November 2013, but experts were able to study behaviours and the factors that could affect the mortality of the species in the wild, allowing conservationists to determine how to conduct successful releases in the future.
For instance, by tracking the frogs, potential hotspots for Bd transmission were identified. In the dry season, the mountain chickens were found to congregate at water sources, often in proximity to other amphibians that are known to carry Bd. This might suggest lower chances of disease transmission if releases were conducted in the wet season, when the frogs are known to disperse farther.
There is no known way of entirely removing Bd from an environment, and there are relatively few ways of treating affected amphibians.
In 2016, a successful treatment using baths containing the antifungal drug itraconazole was reported. This lowered the probability of infection in the treated frogs. While the long-term survival of the animals wasn't observed, modelling suggests that the treatment could extend a population's time to extinction, and so could be a way of buying conservationists more time.
One of the latest efforts to help mountain chickens has been through the creation of safe havens for the species in Montserrat, using innovative environmental manipulation techniques.
Bd thrives in wet forest floor environments that are between 17-25°C. The fungus can't survive above 30°C, but mountain chickens can. To exploit this weakness, conservationists have developed semi-wild refuges for the frogs that are uninhabitable for the fungus. These areas have reduced canopy cover, making them generally warmer, but they also include solar-powered pools that keep the water above 30°C. With other species on Dominica and Montserrat still carrying Bd, these heated frog spas may allow mountain chickens to co-exist with the fungus.
There is also some hope that mountain chickens may be able to attain a level of coexistence with the fungus on their own.
Surveys of mountain chickens in Dominica ceased in 2008, when the island's population was presumed extinct. But in 2011 came new reports of the frogs' recognisable calls being heard at night. New surveys located adult, recently metamorphosed and juvenile frogs at multiple sites. The wild population was still holding on and breeding.
While things may be looking up, the mountain chicken isn't out of the woods just yet. With an estimated wild population of under 200 individuals, captive breeding and release is likely to remain a vital part of this species' story for now.
Bd may currently be one of the greatest threats to mountain chickens and many other amphibian species around the world, but there are several others that these animals face, including habitat destruction, pollution and the introduction of invasive species.
As a group, amphibians are also highly sensitive and can act as indicators of environmental change. They can be considered 'a canary in the coal mine' as they warn us of changes that may impact us in the future.
In the video below, watch as Dr Jeff Streicher, the Museum's Senior Curator of Amphibians and Reptiles, Ben Tapley, Curator of Amphibians and Reptiles at the Zoological Society of London (ZSL), and British Academy policy advisor Charise Johnson discuss the worrying trend of amphibian decline and what can be done to turn things around. Actor and environmentalist Katherine Waterston leads the conversation.