The confounding, beautiful lives of Colombia’s mountain-dwelling harlequin toads

[Read in Portuguese] [Read in Spanish]

On the world’s tallest coastal mountain, with the beautiful symphony of a tropical, thriving forest and the dense vegetation of a place yet unspoiled by humanity, the most transformative encounter is with an animal that at first might not appear remarkable at all: 

A toad, light brown with pale yellow ridges down either side of his back, recognizable by the missing tip of a toe, perched on a large leaf drooping toward the earth under his weight. 

Known as ‘El Viejito’ or ‘Little Old Man’ to the researchers who have seen him on every monitoring trip for the last 11 years in Colombia’s Sierra Nevada de Santa Marta, this harlequin toad, and all of the other toads of the Atelopus laetissimus species, are extraordinary. Unlike the more than 80 percent of high-elevation harlequin toads that are threatened with some level of extinction, toads of this species are alive and seemingly well here, in this majestic place, up to 1.8 miles above sea level. They appear to somehow be coexisting with the deadly chytrid fungus Batrachochytrium dendrobatidis (Bd), which has decimated high-altitude species elsewhere in Latin America.

And it is not just Atelopus laetissimus. Sierra Nevada de Santa Marta is harboring three additional high-altitude Atelopus (or harlequin toad) species--the Guajira Harlequin Toad (Atelopus carrikerri), the San Lorenzo Harlequin Toad (Atelopus nahumae) and the Starry Night Harlequin Toad (Atelopus arsyecue). All four species seem relatively abundant, though whether their populations are in fact stable is one of many mysteries researchers are working to solve. 

“It’s difficult to say what is happening, and we are seeing this with other amphibians here, too,” said Beto Rueda, a herpetology professor at the University of Magdalena and the researcher who first started monitoring ‘el viejito.’ “All of the species from Sierra Nevada de Santa Marta are doing okay, even though chytrid fungus is here. We haven’t seen mass mortality. Harlequin toads are okay, our glass frogs are okay, our rain frog species are okay. It’s strange.”

Questions abound about why these four species seem to be living with the disease, chytridiomycosis, caused by the fungus, in their environment, even infecting some of the animals.

Are these mountains so remote that a more deadly strain of the pathogen hasn’t yet made it here or were these populations once hard hit and we’re now seeing the toads after they have already rebounded? 

Have these species developed some kind of special immune defenses to survive with chytrid or do they have special antifungal properties in their natural bacteria that is protecting them? 

Is the prevalence of the disease low in their environment and if so, is there something in the environment that slows the spread of disease? 

Is the transmission rate of the disease low enough that it is outpaced by the toads’ reproductive rates? 

Is there something unique about the dynamics between these toads and Bd? 

Or is there something about the way the Indigenous communities are managing the habitat of these animals--land that they share--that is giving the toads a unique advantage?  

Whatever the answer, one thing is for sure: something is happening in the Sierra Nevada de Santa Marta. And it may be the key to preventing the extinctions of the remaining harlequin toad species across Latin America. 

“When you go to Sierra Nevada de Santa Marta, you don’t see just one individual harlequin toad here or there, but dozens of individuals that belong to one of the most threatened groups of amphibians in the world,” says Dr. Lina Valencia, Atelopus Survival Initiative founder, co-coordinator of the IUCN SSC Amphibian Specialist Group Atelopus Task Force and Andean countries coordinator for Re:wild. “Because of this there is a sense among harlequin toad experts that maybe Sierra Nevada de Santa Marta holds the key, that it is the holy grail, that maybe this is where we will find the answers that we have been looking for.”

Bacteria for the win? 

Conservationists and researchers have been looking for answers since the mid-80s, when Bd started its fatal wave across the world, including in South and Central America, Australia and the western United States, shocking everyone witnessing the unprecedented and sudden die-offs. Today, of the 94 harlequin toad species that have been assessed by the IUCN, 83 percent are threatened with extinction, while about 40% of Atelopus species have disappeared from their known homes and have not been seen since the early 2000s, despite great efforts to find them. Four harlequin toad species are already classified as Extinct, according to the IUCN Red List of Threatened Species, but this number is likely higher. 

One of the researchers trying to determine why the Santa Marta toads seem to be faring better is Dr. Vicky Flechas. In 2007, Flechas started working with harlequin toads in the Colombian lowlands, on Gorgona Island. What she found there was that the Elegant Harlequin Toad (Atelopus elegans) was not falling neatly into the general understanding of how Bd affected highland versus lowland harlequin toads. Lowland habitats are usually warmer, which seemed to give toads living here a distinct advantage: Bd is sensitive to warmer temperatures. But in Flechas’s research, she found that the Elegant Harlequin Toads had a surprisingly high prevalence of Bd--but were still surviving. 

“At the beginning, I was really surprised to see what was happening in Gorgona,” Flechas says. “I never imagined that Atelopus that are so susceptible and disappearing across their whole range could end up coexisting with this pathogen.”

It turned out that the Elegant Harlequin Toad has antifungal bacteria on its skin that kills Bd in a petri dish. In 2018, Flechas worked with research student Maria Camila Llanos and biologist Beto Rueda, who rediscovered the Guajira Harlequin Toad in 2008, to see if the Santa Marta harlequin toads, too, had special antifungal bacteria helping to keep them safe. Preliminary results show that there is some bacteria that may be assisting these individuals, but no definitive findings yet. 

“Though we have so many questions, that these species still persist in the Sierra Nevada de Santa Marta inspires us to work to protect the species and their habitats,” says José Luis Pérez González, a biologist with Fundación Atelopus, which is leading the research on and conservation of these toads. “By working day by day with indigenous communities, farmers, environmental experts, NGOs and affiliated researchers, we hope to ensure these species can live safely in their home in the Sierra Nevada de Santa Marta for a long time to come.”

Lost and found

Just northwest of Colombia, in Cordillera de la Costa in Venezuela, another researcher made a similarly surprising discovery that both expanded our understanding of chytrid, while raising additional questions. Seventeen years after the last documented sighting of the Rancho Grande Harlequin Toad (Atelopus cruciger), a small population was rediscovered in 2003. Dr. Margarita Lampo, a researcher with the Venezuelan Institute for Scientific Research, started a monitoring program in 2005 that involved a monthly three-hour drive from Caracas, a one-hour walk from the closest town, and then hours of searching for, measuring and photographing every toad. In 2007 her team also started swabbing each animal to test for Bd. The Rancho Grande Harlequin Toad is highly vulnerable to chytridiomycosis--infected frogs die within a few weeks of exposure to the fungus.

In 2014, after engaging in one of the longest mark-recapture experiments ever conducted in a frog population, the work paid off. Lampo found that it was not an antifungal bacteria allowing the toads to survive or even the elevation at which they live. Instead, transmission of Bd was low, while the reproductive rate (recruitment) of the species, which currently lives between .2 and .4 miles above sea level, was outpacing the deaths caused by the disease. The Rancho Grande Harlequin Toad is the only species of Atelopus that has been documented in Venezuela since 2008.

“If recruitment of healthy juveniles is high enough to compensate for the loss of infected individuals, then the population can establish an equilibrium,” Lampo says. “This is exactly what we found. The number of newly recruited individuals is almost as high as the number of adult frogs in the population. Now we need to figure out: are transmission rates in Sierra Nevada de Santa Marta low while recruitment is high, or are these species resistant to Bd, in contrast to the Rancho Grande Harlequin Toad?”

Another important question Lampo hopes to answer is what other threats could affect reproductive success for the toads? Habitat destruction, pollution, and the climate crisis could all lead to fewer tadpoles to replenish the population. 

“If recruitment is decreased, then the population could collapse even with low transmission rates of the fungus,” Lampo says. “So the other key element that we need to understand is how temperature and droughts affect recruitment rates.”

Living the high life

According to Dr. Cori Richards-Zawacki, a professor of biological sciences at the University of Pittsburgh, high-elevation harlequin toads that we once feared had vanished are somehow starting to come back. This includes the Variable Harlequin Toad (Atelopus varius), which Richards-Zawacki rediscovered in the Panamanian highlands in 2012 with fellow biologist Dr. Jamie Voyles. 

“Elevation was a great predictor about where you were going to see these declines happen when the pathogen invaded naive communities of hosts,” Richards-Zawacki says. “So initially elevation was the most important thing to know where you’d have these outbreaks of disease and declines, even extinctions. But I’m optimistic right now that what we’re seeing is that these high-elevation frogs are finding a way, one way or another. To quote Jurassic Park, ‘life is finding a way.’”

In 2018, Richards-Zawacki and Voyles published a paper finding that nine of the 12 amphibian species they studied that had seemingly vanished as the result of chytrid are recovering, including the Variable Harlequin Toad. Yet the pathogen was still present and as strong as it was a decade ago. The results suggest that while the pathogen has not changed, the amphibians have started to evolve better defenses to fight the disease. 

Scientists aren’t sure whether those defenses are related to a change in the antimicrobial peptides in the animals’ mucus, the bacterial communities on the frogs’ skin, or the “turning on” of specific genes related to the animals’ immune systems. Nor are they sure if this is what is going on with the Sierra Nevada de Santa Marta toads. Dr. Vicky Flechas aims to determine when Bd first arrived in Sierra Nevada de Santa Marta by looking at museum specimens, which could help determine whether the populations living there today are rebounding with one of these new defenses after a population crash years ago. 

“I’m super excited to hear what they’re finding in Colombia,” Richards-Zawacki says. “We saw something similar in Panama, but it’s even better to hear about it with these species with teeny tiny little distributions in the middle of absolutely nowhere. So the chances for them to survive this were probably even lower, so it’s amazing that they did.”

The future of El Viejito

For the first time ever, a new network of passionate conservationists across South and Central America is bringing together the resources and expertise to collectively figure out not only what is going on in Sierra Nevada de Santa Marta, but how to solve some of the broader mysteries around chytrid. All of this while also protecting harlequin frogs everywhere they live from other major threats: habitat destruction and degradation, the introduction of invasive species such as the Rainbow trout, pollution, illegal collection for the pet trade, and the effects of climate change.

And while the Atelopus Survival Initiative faces a number of major challenges in preventing the loss of any more harlequin toad species, El Viejito and his friends are still hanging on, even against all the odds.

“When I first saw El Viejito, it was the middle of the night, with a beautiful moon, all of the sounds of the animals, these beautiful toads, and a waterfall,” Valencia says. “It was amazing, certainly from a biological perspective, experiencing this natural beauty. And from a conservation perspective, it was a really wonderful moment that felt like we can do this, we can save this group of amphibians. There is hope.”

(Top photo: Starry Night Harlequin Toad by Jaime Culebras/Photo Wildlife Tours)