When considering the ever-growing list of species driven to extinction by human activity, certain creatures inevitably tend to stand out in the mind. Dodos are the most obvious example, having come to symbolise the Sixth Mass Extinction; the Passenger pigeon is another, plummeting from millions to none over a single human lifespan; the Thylacine would be a third, living on as a spectral icon of Tasmania’s threatened wildernesses.
Rabbs’ fringe-limbed tree frog seems an unlikely candidate for the pantheon of iconic human-induced extinctions. It was neither large, averaging no more than four inches long, nor particularly colourful, being primarily mottled-brown. To human eyes, it might seem small, slimy and strange.
Despite this, it would be entirely wrong to adopt a dismissive attitude towards this now vanished species. It bears repeating that few treated Dodos, Passenger pigeons or Thylacines with any respect whilst they still lived. Even more importantly, an anthropocentric approach to assessing the worth of species is at the root of today’s extinction crisis.
Here then will be written a tribute to Economiohyla rabborum, the Rabbs’ fringed-limbed tree frog.
A mere eleven years separate the species’ discovery and its extinction. This bleak fact was not lost on Joseph R. Mendelson III of the Department of Herpetology at Zoo Atlanta. Mendelson was part of the expedition to central Panama which first bought Rabbs’ tree frog to the attention of science. He has described himself as a “Forensic Taxonomist”, since the amphibians which he works on describing are dying off almost as fast as he can name them. Reading his reflections upon the morbid aspect which his scientific career has taken on, Mendelson’s sense of loss can be acutely felt.
The Panama expedition was launched as a direct response to extinction, specifically to the ongoing spread of chytridiomycosis amongst amphibian populations. This fungal infection can inflict a one-hundred per cent mortality rate amongst infected amphibians, letting it destroy entire species. Its origins, at least in its most virulent form, remain unclear. The introduction of fungus-infected African clawed frogs to the Americas, in combination with climate change, is currently the prime suspect.
In 2004, the chytrid pathogen was confirmed present in central Panama. The following year, Mendelson and other scientists headed to the region to find amphibians and take them into captive safe-keeping before chytridiomycosis could eradicate them.
There within the cloud forests of the mountains above El Valle de Antón, Rabbs’ tree frog was discovered. Unlike so many of the species which human activity has condemned to death, Rabbs’ tree frog was observed in the wild. As with other members of its genus, Rabb’s tree frog could use the webbing on its hands and limbs to glide through the air from the canopy down to the forest floor. Uniquely amongst known amphibians, it was the males who took the responsibility for feeding the young. Wild males were seen staying with the developing eggs and occasionally inserting themselves into the mass of growing tadpoles, letting their offspring feed off scraps of their skin.
Several dozen Rabbs’ tree frogs, adults and tadpoles alike, were loaded into crates and sent from Panama to US facilities in the hope of saving the species via captive breeding. Among the adult frogs was a young male, later known as ‘Toughie’. He was destined to become the very last of his kind.
Toughie spent most of his life at the Atlanta Botanical Garden, ensconced inside a converted shipping container known as the FrogPOD. This biosecure structure is strictly off-limits to visitors and was designed to serve as an ark for critically endangered amphibians, including Rabbs’ tree frogs.
At first he had company. The captured tadpoles were the first to die, failing to metamorphose in their new surroundings. They were followed by Toughie’s own offspring, born in captivity and dying as tadpoles. In 2008, Rabb’s tree frog was scientifically described. By the end of the following year, the species was functionally extinct. The last known female, one of Toughie’s FrogPOD companions, died taking any hope of the frogs’ survival with her. For the next seven years, Toughie would live alone. After three years, his loneliness became absolute, when the only other Rabbs’ tree frog known to exist was euthanized at another US zoo.
Around this time, Toughie came by his name. It was given him by the young son of the Garden’s Amphibian Conservation Co-ordinator. The boy’s explanation for the frog’s moniker was simple: ‘Because he’s the only one that made it!’
Unfortunately for Toughie and his carers, “making it” can seldom have been a more hollow triumph. Despite the best efforts of concerned scientists, all attempts to eke out a future for the Rabbs’ tree frog had ended with nothing more than an old male, sitting alone in a hollow log with nothing ahead of his species but oblivion.
Whilst comfortable, Toughie’s final home was not large enough for him to glide as he might have done in the Panamanian cloud forests. Judging by the accounts of those who knew him best, he was a wilful frog who disliked being handled, frequently pinching the hands of those who tried to hold him. Thus, apart from his weekly weighings, he was left largely undisturbed.
For most of his life, Toughie was very much the strong and silent type, living on grouchily as his species faded from existence around him. In the wild, male Rabbs’ tree frogs were heard calling, even after chytridiomycosis crept into their forest refuge. In captivity, Toughie fell mute. None of his kind would hear his voice again.
On December 15th 2014, something remarkable happened. The last Rabbs’ tree frog on Earth began to call out from his tank in the FrogPOD. His deep barking was recorded by the Amphibian Conservation Co-ordinator at the Botanical Garden. Once, such sounds might have been considered a mating call. From Toughie, they are the death rattle of a species.
Conservative current estimates suggest any given species of terrestrial vertebrate might expect to exist for one million years. How long Rabbs’ tree frog had lasted prior to September 2016 is as yet unknown. Had they managed merely one per cent of the aforementioned million years though, the frogs would still comfortably pre-date the entire written history of the species whose activities eradicated them.
More than seven thousand amphibian species share our planet. At least two thousand are in imminent danger of extinction. To get a sense of the scale of this crisis, consider that the last common ancestor of all of modern amphibians lived two-hundred-and-fifty million years ago. Staggeringly this means that the cumulative individual years of evolution which went into producing today’s IUCN Red List of amphibians runs well into the billions. This may seem unbelievable yet the great apes alone, with just seven living species, have well over thirty million years of individual evolution between us.
Should we lose all those amphibians which until so recently kept company with Toughie on the endangered list, we stand to lose a living record of evolutionary time greater than the age of the Earth itself. That is what mass extinction means.
Monolithic as the above figures are, they are arguably too big to effectively convey what species loss represents. For that, Toughie’s first and last recorded calls may serve far more eloquently.
(The call of the last Rabbs’ fringe-limbed tree frog, recorded by Mark Mandica, Amphibian Conservation Co-ordinator at Atlanta Botanical Garden)
Dedicated to the memory of Toughie and all his kind before him.
By Matthew Stanfield