It started off as an enigma. Biologists at field sites around the world reported that frogs had simply disappeared. Costa Rica, 1987: the golden toad, missing. Australia, 1979: the gastric brooding frog, gone. In Ecuador, Arthur’s stubfoot toad was last seen in 1988.
By 1990, cases of unexplained frog declines were piling up. These were not isolated incidents; it was a global pattern – one that we now know was due to chytridiomycosis, a fungal disease that was infecting and killing a huge range of frogs, toads and salamanders.
Our research, published today in Science, reveals the global number of amphibian species affected. At least 501 species have declined due to chytrid, and 90 of them are confirmed or believed extinct.
When biologists first began to investigate the mysterious species disappearances, they were at a loss to explain them. In many cases, species declined rapidly in seemingly pristine habitat.
Species declines typically have obvious causes, such as habitat loss or introduced species like rats. But this was different.
The first big breakthrough came in 1998, when a team of Australian and international scientists led by Lee Berger discovered amphibian chytrid fungus. Their research showed that this unusual fungal pathogen was the cause of frog declines in the rainforests of Australia and Central America.
However, there were still many unknowns. Where did this pathogen come from? How does it kill frogs? And why were so many different species affected?
After years of painstaking research, biologists have filled in many pieces of the puzzle. In 2009, researchers discovered how chytrid fungus kills frogs. In 2018, the Korean peninsula was pinpointed as the likely origin of the most deadly lineage of chytrid fungus, and human dispersal of amphibians suggested as a likely source of the global spread of the pathogen.
Yet as the mystery was slowly but surely unravelled, a key question remained: how many amphibian species have been affected by chytrid fungus?
Early estimates suggested that about 200 species were affected. Our new study reveals the total is unfortunately much larger: 501 species have declined, and 90 confirmed or suspected to have been killed off altogether.
These numbers put chytrid fungus in the worst league of invasive species worldwide, threatening similar numbers of species as rats and cats. The worst-hit areas have been in Australia and Central and South America, which have many different frog species, as well as ideal conditions for the growth of chytrid fungus.
Large species and those with small distributions and elevational ranges have been the mostly likely to experience severe declines or extinctions.
Together with 41 amphibian experts from around the world, we pieced together information on the timing of species declines using published records, survey data, and museum collections. We found that declines peaked globally in the 1980s, about 15 years before the disease was even discovered. This peak coincides with biologists’ anecdotal reports of unusual amphibian declines that occurred with increasing frequency in the late 1980s.
Encouragingly, some species have shown signs of natural recovery. Twelve per cent of the 501 species have begun to recover in some locations. But for the vast majority of species, population numbers are still far below what they once were.
Most of the afflicted species have not yet begun to bounce back, and many continue to decline. Rapid and substantial action from governments and conservation organisations is needed if we are to keep these species off the extinct list.
In Australia, chytrid fungus has caused the decline of 43 frog species. Of these, seven are now extinct and six are at high risk of extinction due to severe and ongoing declines. The conservation of these species is dependent on targeted management, such as the recovery program for the iconic corroboree frogs.
Importantly, there are still some areas of the world that chytrid has not yet reached, such as New Guinea. Stopping chytrid fungus spreading to these areas will require a dramatic reduction in the global trade of amphibians, as well as increased biosecurity measures.
The unprecedented deadliness of a single disease affecting an entire class of animals highlights the need for governments and international organisations to take the threat of wildlife disease seriously. Losing more amazing species like the golden toad and gastric brooding frog is a tragedy that we can avoid.
Lee’s research identified the cause of mysterious and devastating mass frog extinctions that spread across the world starting in the 1970s: it was a skin fungus.
With her colleague Lee Skerratt, here she describes the work that led to her prize, and what is still to be achieved for frog and wildlife conservation in Australia and across the world.
Combating fungi in frogs – why is this important?
Chytridiomycosis might be the worst disease in history. In a matter of decades, the illness cut a swathe through hundreds of species of frogs, causing mass extinctions as it spread out of Asia into Australia and the Americas.
Our research was the first to identify the cause – a novel chytrid fungus called Batrachochytrium dendrobatidis – but finding ways to combat the disease requires a lot more work.
Here in Australia, six species have already been driven extinct, and another seven are on the brink. Fortunately, in Australia we also have the unique expertise and perspective to prevent further losses if we devote adequate resources to the problem.
The keys to our research success have been a cross-disciplinary approach and a focus on delivering conservation outcomes.
Frog declines had been seen around the world from the late 1970s on, but it wasn’t until 1998 that we identified the chytrid fungus as the cause.
Why had nobody else figured this out before?
Discovering the fungus on the skin of frogs was not rocket science, but rather applying the methods from one discipline to a problem in another. An outbreak investigation approach – using the tools of medicine for frog conservation – allowed us to diagnose the cause of the frog deaths.
The main reason this approach was tried in Australia was the broad knowledge and interest of the late Rick Speare, an extraordinarily eclectic scientist, medical doctor and vet. (Like the prize’s namesake Frank Fenner, he was comfortable using his medical expertise for the environment.)
Rick’s help was sought by Keith McDonald, a chief ranger of Queensland and a herpetologist. Keith was concerned about the health of North Queensland frogs after witnessing major declines in the south.
After looking at the pattern of declines the pair thought they saw the trail of an unknown infectious, waterborne disease. They applied for funds to search for a disease.
The idea that an infectious disease might be responsible for frog declines met resistance because of the belief that a pathogen can never cause extinction, because hosts will evolve resistance. So while Rick and Keith did obtain funding to tackle this urgent global mystery, it was only enough to support a single PhD student.
That PhD student was me, Lee Berger. To cut a long story short, my work in pathology and disease transmission experiments in frogs led to our conclusion that a novel and unusual fungus in the frogs’ skin caused a fatal disease and the mass amphibian deaths seen in North Queensland. As this was the first fungus from the phylum Chytridiomycota found to cause disease in a vertebrate, I had to develop many new methods to be able to further study the disease.
Now we are focused on understanding immunity to improve survival rates of the most threatened species of frogs in the wild.
This work has only been possible due to the extraordinary dedication of our students and staff and the collaboration with specialist scientists such as herpetologists, molecular biologists, immunologists, physiologists and others who have lent their expertise.
What does this mean for Australia’s wildlife?
Our research has clearly shown that introduced diseases can have catastrophic impacts for conservation, much like the arrival of feral predators. In fact, disease can cause extinction much more quickly than predators, within months rather than years. The catastrophe of invasive species is a cost of globalisation that will be ongoing unless we respond.
The responsibility for wildlife lies with environment departments, but because health expertise is in other institutes, wildlife health can fall between the cracks.
We argue that continued support for bodies such as Wildlife Health Australia (WHA) is important. We also need a centre of expertise for outbreak investigation and strategic research to develop new tools for wildlife health management.
Biodiversity will miss out unless we support research that promises no direct and fast commercial return but benefits our nation in the longer term. In particular, and most urgently, Australia must save its frogs before it is too late.
We’ve arranged to meet in a gravel car park at the foot of Mt Majura, a darkening wedge above us in the dusk. My daughter and I wait in the car. It’s winter. A woman passes along the nearby pavement, guiding her way by torchlight. Canberra’s streets are kept dim, I learned recently, for the sake of astronomers at nearby Mt Stromlo observatory. In the decade I’ve lived here, I’ve had an ambivalent relationship with Canberra, but the idea of a city that strikes bargains with stargazing scientists to restrict light pollution leaking skyward is endearing.
There are other endearing things. One of them is the amount of bushland interspersed throughout the urban landscape. You can be in the middle of suburbia one minute and bushwalking on nearby Black Mountain, Mt Majura or Mt Ainslie ten minutes later. This kind of mixed landscape is ideal for the citizen science project we’re about to launch into this evening, as soon as the co-ordinator of the ACT and Region Frogwatch Program, Anke Maria Hoefer, arrives for our first training session.
The program runs a community-based annual Frog Census framed against a rapid global decline in frog numbers over the past four decades and the extinction of many frog species. The census began in 2002, and the resulting long-term dataset on the abundance and distribution of local frogs has enabled additional research activities including a climate change project. We’ll take part in the latter, which monitors behavioural shifts in frogs through recording their calls at particular sites each week from June until October.
We’re here for a few reasons. One is to get a lived sense of climate change in our immediate urban surroundings. Plus, I want to make a contribution, however small, to the huge dilemma of climate change and its impacts; give my 13-year-old daughter a taste of scientific fieldwork in case it appeals to her; get to know our local surroundings better; and, as a writer, to think about practices that don’t simply observe or contemplate place but also participate in constructive activities at those same locales.
Numerous commentators have observed that the vast and intangible scale of climate change may be an impediment to more people taking action over our warming atmosphere. We know through the science that climate is shaped by the working of the entire planetary system – the earth’s interactive ocean, atmosphere, land and ice systems all linked to human activity. Depending on where you live, (but not in the Pacific Islands, the deltas of Bangladesh, Arctic Canada, or drought-stricken rural Australia), its impacts can seem far-removed from our own lives and the places we know best and care most about. With care, often, comes action. What can seem an amorphous, far-fetched threat is brought closer to home through studies such as Frogwatch.
The project studies the impact of climate change on phenology, or seasonal behaviour. Most frogs only call during the mating season, which is triggered by temperature and rainfall. Different species mate at different times and volunteers record the onset of mating calls from winter breeders (whistling tree frog and common eastern froglet), early and mid-spring breeders (spotted grass frog, plains froglet, striped marsh frog and smooth toadlet), and late spring to summer breeders (eastern banjo frog and Peron’s tree frog).
Frogs are known as an “indicator species” for water quality and local ecosystem health. With their permeable, membranous skin, through which respiratory gases and water can pass, and their shell-less eggs laid in water, they are sensitive to even low concentrations of pollutants in water and soils. In this study, frogs give a different kind of warning – as they begin calling earlier in the season, they reveal and give voice to the warming climate we now all inhabit.
The project is fortunate enough to be able to build upon weekly counts of calling frogs by ecologist Will Osborne during the 1980s and 1990s in the Canberra region. Effects of climate change can be incremental. They can also be non-linear, as scientist Pep Canadell explained to me in a recent interview. “Climate change expresses itself through extremes. It’s not a linear relationship of impacts,” he said.
This mixture of incremental change and unpredictable “expressions” can be difficult to record in the short term. With this in mind, the Frogwatch project builds on Osborne’s historical data along with the Frog Census data to chart changing trends. A preliminary comparison reveals that the breeding season of some local frog species might be commencing up to six weeks earlier than 40 years ago.
A sonic world
Headlights sweep into the car park and Anke Maria arrives with a visiting German student who is also researching frogs. Anke Maria is a whirlwind of talk and activity, honing in on my daughter as we zip our down jackets, pull on beanies and gloves, switch on torches and head up a gravel fire trail toward the first dam, known as FMC200. Only metres later we stop at the base of a narrow drainage gully. It’s been a dry winter, but with a patch of recent rainfall a miniature sump-like drainage area at the base of the gully is alive with frog calls.
“That’s Crinia signifera,” Anke Maria explains, making what seems a perfect imitation of its repetitive call. “How would you describe it?” she asks. My daughter turns to me. The call is repetitive, creaking. We struggle to think of descriptions. It’s like trying to put a flavour into words.
“Who do you think is calling? The male or female?” Anke Maria asks. My daughter pauses, pondering. “The female,” she hazards a guess. “Good try,” says Anke Maria, “but only the male frog calls. Except when the female makes a warning call.” She imitates this staccato warning sound. “And why do you think the males are calling?” Again my daughter pauses to consider.
We continue walking up the gravel slope amid shadowy shapes of eucalypt trees, a tangle of gorse and acacia undergrowth, a row of looming metal electricity pylons strung along the lower contour lines of Mt Majura.
“They could be hungry or they found food,” my daughter replies.
“Good thinking, but they’re calling to attract a girlfriend. And do you know, scientists think that each frog species can only hear the calls of their own species. It’s like tuning into a radio station. There are many different stations, but we can only tune into one at a time. A female whistling tree frog can only hear a male whistling tree frog, a female corroboree frog can only hear a male corroboree frog.”
They recognise the frequency and intensity or pitch of the call, she explains, and also the pattern of the call or its pulse structure. “This helps the female find a mate from their own species and not get confused by other frogs.”
We ponder this sonic world where one species can be deaf to another, turn left down a narrow walking track, torchlight bobbing along with our footsteps, illuminating tussocks of grass, fallen branches, shrubs, stones, until we reach the dam. “This is for you,” Anke Maria passes a thermometer. “You do it,” she tells my daughter. First we record the ambient temperature then my daughter squats at the edge of the water, waving the thermometer gently through the shallows. We note the weather: light cloud cover, low breeze. We estimate the dam’s surface area and depth. Then our small group falls silent as Anke Maria switches on her phone audio-recorder.
For three minutes we hold still and listen. There’s the low hum of the city below, an ambulance siren swells and recedes, distant traffic, the shuffle of our down jackets as we try not to move, someone sniffs in the chill winter air – and the frogs. You can hear them interspersed across space, some close, some farther away, among vegetation rather than water. Because of Anke Maria’s explanation, I understand now these are not call-and-response sounds. They are invitations, serenades, statements of presence, lures. Sometimes the calls come in a cluster, other times at staggered unpredictable intervals. There are at least two species here, I guess. In the distance, a mopoke calls.
When Anke Maria switches off her phone, we relax into movement again. As we walk towards FMC210, our second dam, she tells us we’ve just heard a whistling tree frog (Litoria verreauxii). “How would you describe his call?” Anke Maria asks.
My daughter decides on a stick dragged across a rough, hollow surface. Anke Maria makes the call. Her imitations are pitch perfect, an art form. She will be the one who checks the recordings that non-specialist volunteers send in weekly, uploaded to the Frogwatch website. We will make our guesses at species we’ve heard, but she will verify with her trained ear, a labour-intensive task.
In our information pack is a CD of local frog species. When we get home we lie on the carpet and listen, the house filled with frog noise.
A new frog
A week later, on our first trip into the dark alone, the evening is silvered and rigid with frost, as if everything is held together in some different, more metallic way. It’s three below zero and falling. Our breath steams, our boots crunch, the bush is still. I sense something in a dead tree ahead before I see it, a tawny frogmouth, grey, motionless, an outcropping like a broken limb. We pause several steps away and it regards us, head half swivelled, a little tuft of feathers at the base of its beak.
The following week, on our descent from the dams, once again a frogmouth is in the same tree. A second bird perches a few metres away. They are bound together in some mute, still business. They survey us. We move on with subdued steps. Beyond the birds, the first row of suburban houses begins. We thread our way back to the car with a sense of secrecy and adventure, past back fences, patches of bright window, catching fugitive glimpses of other people’s lives through a half-open door, a crack in a curtain, the blue flicker of TV light.
At the dams we make our recordings. Air temperature, water temperature, ascending over the weeks. On the far side of Mt Majura lies the airport. Often early into a sound recording, a plane takes off, blotting out all other sound. Ecologist Will Osborne tells me he has observed that the aeroplane sound seems to overlap the call parameters (pitch and pulse structure) of the Common Eastern Froglet. Whenever a plane goes over, the froglet stops calling while other species continue – machine and creature competing on the airwaves.
When I upload the recordings, Anke Maria responds and confirms (or not) my guesses at species. You should soon hear Crinia parinsignifera she emails, so keep your ears peeled for a high pitch narky baby cry!
Her enthusiasm is infectious, her aural sketches vivid, memorable. When we hear the new frog, I know exactly what it is. Everyone on the team, each with sites to attend scattered across Canberra, has been waiting for this particular call.
It might show that an early spring breeder is shifting its season into winter. This minor-sounding alteration has a cascade of flow-on effects. Frogs stagger breeding seasons, giving each species its portion of acoustic space to call, breed, then when eggs hatch into tadpoles to feed (a mode of “time sharing” water and its resources). If seasons shift, merge and overlap, competition for resources intensifies, and survival can be jeopardised.
But this year it’s a cold, dry winter. This telling species, Crinia parinsignifera, is calling two weeks later than last year (when it called early). Meanwhile northern Australia is experiencing its warmest July on record. Non-linear. As the monitoring season progresses, dam levels drop. By the end of October, waters have fallen almost silent.
Will Osborne sends an email around, explaining that cold nights and low water levels will make it hard to interpret this season’s counts. “Most species feel insecure about going out onto that exposed mud and trying to find a call site or searching for mates! It will be a big rush when the weather warms and we get good rains – the calling sequence could be condensed this year which will be interesting…”
Many volunteers join Frogwatch because they want to participate in a hands-on, climate change-related study with real life applications. “They highly value the opportunity to be involved in climate change actions,” Anke Maria says. She captures one of the dilemmas of our times. Many people want to take action but are unsure how. As artist Natalie Jeremijenko observed): “What the climate crisis has revealed to us is a secondary, more insidious and more pervasive crisis, which is the crisis of agency, which is what to do.” Citizen science gives volunteers an opportunity to do something.
Studies that chart the impacts of climate change here-and-now disrupt the assumption that effects will occur in a distant future or at some remote geographic location (melting ice caps, apocalyptic cities under 20 metres of water). Instead, they start to build a picture of measurable effects experienced at the current level of 1°C warming above pre-industrial levels – let alone at 2°C or above, which is what we’re committing to based on current emissions rates. In the Canberra region alone, studies are being conducted into impacts of global warming on urban lizard species (who reside next to the local DFO) and alpine pygmy possums.
At a broader scale, Pep Canadell has observed major ecological transformation in Australia that occurred with a 1.2°C increase during the last El Niño. He calls the El Niños a “window into the future because they bring all this heat and put the world where it may be in 30 or 20 years’ time.”
“These ecological signs are unprecedented, all in this little window of a warmer world that the El Niño brought for us,” said Canadell during our interview. He went on to list even more signs. “For some reason these things don’t go through the media enough because of … whatever,” he added.
The Frogwatch project enables volunteers to dwell in an everyday way with such dispersed ecological signals, which, connected together with other studies, provide a larger picture of both current and future impacts. Volunteers are privileged to make their small citizen science contribution to understanding and recording these signs better.
Unfortunately, just as I completed this article, the Frogwatch Program discovered that its funding from the ACT Government was not renewed in the 2018–19 budget. Without core funding, the organisation and its annual Frog Census will cease. The enthusiasm of volunteers will help to collect another season’s data for the climate change study but it too is under serious threat unless alternative funding can be sourced.
When our monitoring season finished last year, I asked my daughter whether she wanted to do it again. “Yes,” she replied without hesitation. “What did you like most about it?” I asked. “I don’t know,” she said, “it was just fun.” And so, as Canberra’s heavy frosts set in, we have begun again, treading up towards FMC200, waiting for frog calls to begin.
Saskia Beudel’s full interview with Pep Canadell will be published in December 2018 in the journal Weber.
My office is filled with colorful images of frogs, toads and salamanders from around the world, some of which I have collected over 40 years as an immunologist and microbiologist, studying amphibian immunity and diseases. These jewels of nature are mostly silent working members of many aquatic ecosystems.
The exception to the silence is when male frogs and toads call to entice females to mate. These noisy creatures are often wonderful little ventriloquists. They can be calling barely inches from your nose, and yet blend so completely into the environment that they are unseen. I have seen tropical frogs in Panama and native frogs of Tennessee perform this trick, seemingly mocking my attempts to capture them.
My current research is focused on interactions between amphibians and two novel chytrid pathogens that are linked to global amphibian declines. One, Batrachochytrium dendrobatidis ( abbreviated as Bd), has caused mass frog dieoffs around the world. Recently my lab group contributed to a study showing that some species of amphibians in Panama that had declined due to Bd infections are recovering. Although the pathogen has not changed, these species appear to have developed better skin defenses than members of the same species had when Bd first appeared.
This is very good news, but those who love amphibians need to remain vigilant and continue to monitor these recovering populations. A second reason for concern is the discovery of a closely related chytrid, Batrachochytrium salamandrivorans (Bsal), which seems to be more harmful to salamanders and newts.
Global frog decline
More than a decade ago, an epidemic of a deadly disease called chytridiomycosis swept through amphibian populations in Panama. The infection was caused by a chytrid fungus, Batrachochytrium dendrobatidis. Scientists from a number of universities, working with the Smithsonian Tropical Research Institute in Panama, reported that chytridiomycosis was moving predictably from west to east from Costa Rica across Panama toward Colombia.
I was part of an international group of scientists, funded by the National Science Foundation, who were trying to understand the disease and whether amphibians had effective immune defenses against the fungus. Two members of my lab group traveled to Panama yearly from 2004 through 2008, and were able to look at skin secretions from multiple frog species before and after the epidemic of chytridiomycosis hit.
Many amphibians have granular glands in their skin that synthesize and sequester antimicrobial peptides (AMPs) and other defensive molecules. When the animal is alarmed or injured, the defensive molecules are released to cleanse and protect the skin.
Through mechanisms that remain a mystery, we observed that these skin defenses seemed to improve after the pathogen entered the amphibian communities. Still, many frog populations in this area suffered severe declines. A global assessment published in 2004 showed that 43 percent of amphibian species were declining and 32 percent of species were threatened.
Signs of resistance
In 2012-2013, my colleagues ventured to some of the same sites in Panama at which amphibians had disappeared. To our great delight, some of the species were partially recovering, at least enough so that they could be found and sampled again.
We wanted to know whether this was happening because the pathogen had become less virulent, or for some other reason, including the possibility that the frogs were developing more effective responses. To find out, we analyzed multiple measures of Bd‘s virulence, including its ability to infect frogs that had never been exposed to it; its rate of growth in culture; whether it had undergone genetic changes that would show loss of some possible virulence characteristics; and its ability to inhibit frogs’ immune cells.
As our group recently reported, we found that the pathogen had not changed. However, we were able to show that for some species, frog skin secretions we collected from frogs in populations that had persisted were better able to inhibit the fungus in a culture system than those from frogs that had never been exposed to the fungus.
The prospect that some frog species in some places in Panama are recovering in spite of the continuing presence of this virulent pathogen is fantastic news, but it is too soon to celebrate. The recovery process is very slow, and scientists need to continue monitoring the frogs and learn more about their immune defenses. Protecting their habitat, which is threatened by deforestation and water pollution, will also be a key factor for the long-term survival of these unique amphibian species in Panama.
Salamanders (and frogs) at risk
On a global scale, Bd is not the only threat. A second pathogenic chytrid fungus called Batrachochytrium salamandrivorans (abbreviated as Bsal) was recently identified in Europe, and has decimated some salamander populations in the Netherlands and Belgium. This sister species probably was accidentally imported into Europe from Asia, and seems to be a greater threat to salamanders than to frogs or toads.
Bsal has not yet been detected in North America. I am part of a new consortium of scientists that has formed a Bsal task force to study whether it could become invasive here, and which species might be most adversely affected.
In January 2016 the U.S. Fish and Wildlife Service listed 201 salamander species as potentially injurious to wildlife because of their their potential to introduce Bsal into the United States. This step made it illegal to import or ship any of these species between the continental United States, the District of Columbia, Hawaii, the Commonwealth of Puerto Rico or any possession of the United States.
The Bsal task force is currently developing a strategic plan that lists the most urgent research needs to prevent accidental introduction and monitor vulnerable populations. In October 2017 a group of scientists and conservation organizations urged the U.S. government to suspend all imports of frogs and salamanders to the United States.
In short, it is too early to relax. There also are many other potential stressors of amphibian populations including climate change, decreasing habitats and disease. Those of us who cherish amphibian diversity will continue to worry for some time to come.
South American horned frogs (Ceratophrys) can capture and swallow whole animals up to their own body size, including other frogs, lizards, snakes and rodents. This is possible because they have jaws that can produce an extremely forceful bite.
Just how powerful is the bite was part of our study, published today in Scientific Reports. We found that small horned frogs – with a head width of 45mm – can bite with a force of 30 Newtons (N). That would feel like having three litres of water balanced on the end of your fingernail.
More impressively, the largest horned frogs from Brazil – with a head width of 100mm – are calculated to bite with a force of 500N. That’s like having 51 litres of water balanced on your fingertip.
A bite like this is similar to that of reptiles and mammals with heads of similar size.
Measuring bite force
Bite force was measured using a special device called a force transducer. Unlike most frogs, horned frogs willingly open their mouths and bite objects (or fingers) as a defensive response.
This makes it easy to place the free ends of the device into the frog’s mouth so that it bites forcefully, and the device measures the amount of applied force. The free ends of the device are covered in leather to protect the animal’s jaws and to provide a naturalistic gripping surface for the teeth.
Our study is also unusual in that the relationship between size and bite force was measured using multiple measurements from the same individuals from different points during their growth rather than just using a sample of different-sized individuals.
If just out of reach, the horned frog may lift one or both rear legs over its head and wiggle its toes to attract the attention of the potential meal. Once the victim is in range, the frog will rapidly lunge forwards with a wide open mouth.
The extremely adhesive tongue sticks to the prey and retracts, pulling the prey into the mouth, and the huge jaws clamp shut with great force to prevent escape.
Associated with the impressive bite forces of horned frogs and their ambush lifestyle are several important anatomical traits. They have a heavily built skull in which many of the connections between individual skull bones fuse together as the animal grows.
The horns in horned frogs are small pointed structures above the eyes. Their prominence varies among species, and they may help to camouflage the frogs by looking like the tips of the leaves on the forest floor where they sit in wait for prey.
Horned frogs are not currently considered endangered but some species are considered near threatened. Like many animals they are suffering a loss of natural habitat. They are also often killed by local people because of false beliefs that they are venomous, and collection for the pet trade may also be significant.
A prehistoric frog with a bigger bite
Bite forces of some ancient frogs may have been even more impressive than those of today’s South American horned frogs.
Beelzebufo ampinga is a large heavily built frog from the Late Cretaceous period of Madagascar with a skull at least 150mm wide.
Detailed comparisons with the available skeleton suggest that its closest living relatives might be the South American horned frogs. When the relationship between bite force and size in the South American horned frogs is applied to the skull width of Beelzebufo the value obtained is 2,200N. That’s a massive 224 litres of water balanced on a fingertip – more than three times the weight of an average Australian woman.
A bite of this force is comparable to estimates for mammalian predators such as wolves and female lions, and within the realm of bite forces measured for crocodiles and turtles with similar skull widths.
For context, the bite force of an adult human male averages only about 25% that of a large Beelzebufo. With a bite like that, Beelzebufo would have been capable of easily overcoming small or juvenile dinosaurs that shared its environment.
At night, the mountain forests of New Guinea come alive with weird buzzing and beeping calls made by tiny frogs, some no bigger than your little fingernail.
These little amphibians – in the genus Choerophryne – would shrivel and dry up in mere minutes in the hot sun, so they are most common in the rainy, cooler mountains.
Yet many isolated peaks, especially along northern New Guinea, have their own local species of these frogs.
So how did localised and distinctive species of these tiny frogs come to be on these isolated peaks, separated from each other by hotter, drier and rather inhospitable lowlands?
Our new study of their DNA, published this week in the open access journal PeerJ, reveals how they achieved this feat. It reveals a dynamic past, and more worryingly it highlights the future vulnerability of tropical mountain forests and their rich biodiversity.
A hotspot of frog diversity
New Guinea has an astounding diversity of frogs: more than 450 known species and counting. This is nearly double the diversity in Australia, a landmass ten times larger.
Remarkably, a majority of these species are in a single species-rich, ecologically diverse group that have dispensed with the tadpole stage.
Instead they hatch out of their eggs as tiny little replicas of the adults. Because they do not depend on still pools of water to breed, they do really well in the incredibly wet, but steep mountains of New Guinea.
One of our group (Stephen Richards) has been collecting DNA from frogs across New Guinea for the past 20 years. This work is at times arduous and painful. Having a leech worm its way into the back of one’s eye, and then stay there for more than a week, is not pleasant.
But these trips are also extremely rewarding. So far we have described more than 70 new species, and discovered many more that await description.
They also provide opportunities to explore some of world’s most wild places. Perhaps the best example is the first scientific expedition to the remote Foja Mountains.
We also found several species of Choerophryne frog. DNA from these allowed our team to test two potential ways that miniature frogs could have come to occupy distant mountain peaks that are separated by inhospitable lowlands.
Across the Fojas by frog
The first way involves mountain-top frogs evolving separately on each isolated peak, potentially from larger frogs capable of surviving in the hotter and drier, nearby lowlands.
If this were the case, the frog on any given mountaintop would be most genetically similar to frogs from adjoining lowlands.
The other way involves exploiting climate change. During past phases of global cooling (glacial periods), the colder, wetter, mountainous habitats of New Guinea expanded downhill, a process termed elevational depression.
If depression was extensive enough, the frogs on one mountain might have been able to travel across tracts of cool, wet lowlands to colonise other mountains.
Later, a warming climate would wipe out the lowland populations, leaving two isolated mountain populations, which might eventually become new species.
If this were the case, we would expect the frogs in different mountains to be genetically related, since they almost literally hopped from one peak to the other.
Our new study of the DNA of the little Choerophryne frogs indicates they used both routes to conquer the peaks of New Guinea.
In the remote Foja mountains, for example, there are three species of Choerophryne. One species has evolved in situ in northern New Guinea from nearby lowland frogs.
The other two are related to frogs from distant mountains of central New Guinea, and presumably moved across the intervening lowlands during cooler glacial periods.
The little frogs and the future
Why does it matter how the tiny frogs moved to their mountain habitats? Because it could be a warning to their future survival.
Tropical mountains have some of the most biodiverse assemblages of plants and animals in the world. Their ecosystems are also far more dynamic than is popularly recognised.
Just like glaciers, the movements of frogs (and other organisms) up and down mountains has tracked global temperatures. As we’ve shown, the global cooling in past glacial periods allowed the mountain-dwelling frogs to move down across the lowlands to find new mountain peaks.
But today, as global temperatures soar to levels not seen for millions of years, their habitable cool zones are heading in the other direction: shrinking uphill.