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Tracking the tiny: Harmonic direction finders aid study of small amphibians  

  • Research into small amphibians has been stymied by limited means of tracking their movements, hindering conservation efforts.
  • Harmonic direction finding technology, adapted from avalanche rescue systems, is being used to track some of the world’s smallest amphibians.
  • It has helped improve scientists’ understanding of chytridiomycosis, a disease causing massive decline of amphibians around the world.

Around one third of amphibian species are considered globally threatened by the IUCN, due primarily to habitat loss, overexploitation for the wildlife and pet trades, climate change, and disease. Adding to their precarious state is the fact that we simply don’t know much about many of these species, which makes planning conservation strategies difficult. Many of the smaller amphibians remain particularly enigmatic, in part because their tiny bodies, which often weigh less than half a teaspoon of sugar, make them difficult to track using conventional methods. But a piece of equipment called a harmonic direction finder is enabling scientists to begin unravelling the complex world of small-amphibian behavior in the hope that better understanding will improve conservation.

“[F]or many species, little is known about their habitat requirements, including the type and amount of habitat needed to maintain populations,” Elizabeth “Betsy” Roznik, a biologist with the University of South Florida who studies tiny frogs using harmonic direction finders, told Mongabay.

Radiotelemetry has long been used on larger amphibian species. But the smaller creatures are too slight to strap a cumbersome radio tag onto.

Harmonic direction finders overcome this hurdle. They arose from a device originally designed to help rescue people after an avalanche. Inventor Magnus Granhed came up with the idea after being involved in a rescue incident, developed a prototype in 1980, and commercialized the design as the RECCO system. Skiers and snowboarders the world over now often wear small, passive reflector tags in their clothing or gear. In the event that they are buried under an avalanche, rescue personnel using a handheld RECCO detector can locate them by sending out a radio signal that bounces back off the tag.

A common mist frog with a harmonic direction finder tag tied around its waist. The technology is cheap and lightweight, allowing scientists to learn about small amphibians. Photo by Betsy Roznik.

The technology is known in scientific research as harmonic direction finding. In the field scientists use a hand-held transceiver, often a RECCO device, that sends out radio waves. The waves bounce off a small diode tag attached with an antenna to the study subject. The transceiver picks up this reflected signal, allowing scientists to track the location of their target.

The technology made its tracking debut on insects, with some of the first studies emerging in the 1990s. Starting around ten years ago, it has increasingly been used to study tiny frogs in tropical rainforests across the world.

Betsy Roznik (right), armed with a RECCO tracking device to locate tagged frogs, pauses in the forest with a colleague. Photo by Betsy Roznik.

Tracking frogs with harmonic direction finding is helping scientists to understand why the lethal disease chytridiomycosis is pushing some amphibian species to the brink. Roznik’s study subject, the common mist frog (Litoria rheocola), which lives tucked away in the remote rainforest of northeastern Australia, is one of those affected.

The disease causes an overload of keratin production in frogs’ skin cells, hardening the skin and reducing its permeability. Since frogs absorb both water and essential electrolytes through their skin, they can die as a result. Stream-dwelling frogs are often the worst affected because the fungal pathogens spread through contact with contaminated water.

“Chytridiomycosis is responsible for the greatest loss of biodiversity caused by disease in recorded history,” Roznik said. “This disease has caused catastrophic declines or extinctions in over 200 amphibian species around the world.”

Prior to her study, published in the journal PLOS ONE, little was known about the common mist frog, which Roznik describes as “very small and secretive.” Roznik’s study sought to discover how the frog’s behaviour during different seasons can make it more or less prone to contracting chytridiomycosis.

“They can be seen perching on streamside vegetation and rocks at night, but… nothing was known about where they go during the day, how far they move, or how far they can be found away from the stream,” she said.

With the harmonic direction finder Roznik and her team tracked the common mist frog in Wooroonooran National Park in Queensland, Australia through a single warm, wet summer and cool, dry winter.

Typical common mist frog habitat in Wooroonooran National Park in Queensland, Australia. Photo by Besty Roznik.

The study revealed some basic information about the common mist frog’s lifestyle. “By tracking common mist frogs using harmonic direction finding, I discovered that they are relatively sedentary frogs that are restricted to the stream environment, and prefer sections of the stream with riffles, numerous rocks, and dense vegetation,” she said. To ensure their survival she suggested that “dense, native vegetation” should be maintained alongside streams.

Moreover, she found that during the summer, the frogs dwelled amongst vegetation away from the river. During winter, however, the frogs moved around less and spent more time in the water. This behavior increased their chances of contracting chytridiomycosis in winter.

The common mist frog of Australia perched on a stem. The tiny species is threatened by habitat loss, climate change, and disease. Photo by Betsy Roznik.

Roznik also found that the common mist frog occasionally enjoys sunbathing in patches of sunlight that stream through the forest canopy. This behavior is crucial to reducing their risk of succumbing to chytridiomycosis as the fungus is very particular in its thermal needs and can be killed if it’s dried out by the sun.

“[P]roviding canopy openings for populations at risk may be one beneficial management strategy for common mist frogs,” she said.

Canopy openings may be essential in combating the pathogenic fungus’s assault on other amphibian species, as well, according to Roznik. Her PhD research, which tracked three frog species, found that even in areas where chytridiomycosis was endemic the rates of infection varied greatly between the species. Species with higher body temperatures and that spent more time on land were affected less than those with lower body temperatures that dwelled in and around water, such as the mist frog.

Harmonic direction finders have a number of advantages over radiotelemetry. The tags are of course very small, weighing less than ten percent of a small frog’s mass — the recommended threshold so as not to overburden or injure the frog. They are quite simple to apply: Roznik made use of a silicon belt, securely fastened around the frog’s waist with cotton string. They don’t require batteries, prolonging potential field research. And of great importance in a field with limited funds, they are cheap and easy to produce. Roznik said one tag costs around $5, whereas a miniature radio transmitter can cost as much as $100.

A common mist frog wearing a harmonic direction finder tag. Photo by Betsy Roznik.

But the technology isn’t without its downsides. For one thing, the signal cannot pass through solid objects, which can make it difficult to find a tiny frog.

“Common mist frogs often shelter under rocks in the stream, so sometimes I was unable to locate my frogs,” Roznik said. But she struck lucky with her subject’s chilled out nature. “[T]hey are relatively sedentary, so I was usually able to find them when they changed locations,” she said.

The harmonic direction finder’s range is also limited, to under 15 meters, so it isn’t ideal for highly mobile species. Andrius Pašukonis, a biologist at the University of Vienna who has used harmonic direction finding to study the brilliant-thighed poison dart frog (Allobates femoralis), wasn’t quite so lucky with his fleet-footed subject.

A brilliant-thighed poison dart frog wears a harmonic direction finder tag on its waist. Note the tiny springtail hitching a ride on its head. Photo by Andrius Pašukonis.

“Our study area is in a primary rainforest and these little frogs will easily run through areas such as big treefalls and liana tangles, not easily accessible for large clumsy mammals like we are,” he told Mongabay. “To make the matters even trickier, frogs move the fastest in heavy rain…So at times we found ourselves in the middle of a tropical downpour climbing in a massive treefall area listening for a faint signal from a frog moving somewhere under.”

Pašukonis found in his study, which was published in Biology Letters, that brilliant-thighed poison dart frogs can learn their way home through the dense rainforest. And although he contends that this particular finding may not have any direct conservation value at the moment he told Mongabay last spring that “we can’t protect what we don’t understand.”

Roznik is of the same opinion. “Tracking amphibians is a very useful way to learn more about the habitats they use so that we can protect those habitats.”