Expert Commentary

Fungus makes tree frogs sing louder, could be linked to global extinctions

A study published in Nature looks at whether a fungus that makes tree frogs sing louder may be changing mating behavior to spread a deadly disease.

Frogs have been disappearing worldwide at unprecedented rates, according to several studies, prompting concerns from ecologists and environmentalists who worry about mass extinctions. Scientists say the frog pandemic represents the greatest mass extinction of land vertebrates since the dinosaurs. Although no single cause is to blame for the decline—which has been linked to habitat destruction caused by logging and clear-cutting of forests, acid rain, pollution and disease, among other things—one factor is a fungal disease that some scientists say has already killed 200 species of frogs and poses a threat to up to one-third of the world’s frogs, salamanders and other amphibians.

The fungus batrachochytrium dendrobatidis (Bd) causes a condition known as chytridiomycosis that kills amphibians by destroying their skin, disrupting their immune systems and causing heart failure. Scientists first became aware of it in the 1990s, when numerous species of frogs in Australia and Central and South America were dying off in record numbers.

Frogs are important to the ecology, according to a wide body of research, because of their status as “indicator species”: a frog’s health can point to problems in its habitat. If the air, water or food in a frog’s habitat is polluted, a frog will exhibit telltale symptoms. Land management agencies are trying to fashion policies to protect them. According to the U.S. Fish and Wildlife Service, “amphibians are in desperate need of conservation strategies in order to prevent any chances of extinction.” The agency has a proposal that seeks to reduce the spread of chytrid fungus by reducing the intercontinental trade of amphibians for use as pets and food under the Lacey Act.

A new study, “Fungus Makes Tree Frogs Sing,” published March 2016 in Nature, says that Bd may be spreading by making the mating calls of infected males more attractive to females. Researchers recorded the mating calls of 59 male Japanese frogs in rice paddies in South Korea between 10 p.m. and midnight throughout their breeding season, from early June to mid-August 2011. They recorded 10 calls from each frog on each of several nights then tested them for fungus.

Study findings include:

  • Infected frogs tended to sing more rapidly and longer than non-infected frogs. Since females generally prefer males that sing longer and faster, infected males will probably have more success mating.
  • More robust mating calls could be a sign that the fungus is manipulating the frogs’ reproductive behavior—longer calls attract more frogs, potentially spreading the disease. Or the frogs may be acting on a biological imperative to mate because of a shortened lifespan.
  • Amphibians have robust immune systems and are able to adapt in order to tolerate infection rather than eliminate it. But this ability has a drawback: Infections can kick start physiological and behavioral responses in their hosts that help them recover from infection, but also manipulate these responses to facilitate their spread. For example, Bd may cause its hosts to suppress symptoms of illness while at the same time boosting their efforts to mate and reproduce, thus passing on the disease to new hosts (i.e., female frogs who mate with infected males).

Such studies are important because they will help land management agencies fashion policy to protect frogs and try to stave off extinctions. Study authors note that long-term field studies of Bd’s effects on amphibians in all their life stages are needed to fully understand the threat; this study only looked at only one frog species and only one life stage. Currently, it’s difficult for scientists to gage how many frog deaths can be attributed to the fungus because of frogs’ unique ability to adapt to disease: a frog may harbor Bd but show no clinical symptoms and yet still infect other frogs. Study author Bruce Waldman, a behavioral ecologist at Seoul National University, notes in the journal Science, that the research up till now “shows that Bd continues to be an enigma.”

Recent research: A 2013 study in the journal EcoHealth, “Amphibian Pathogens in Southeast Asian Frog Trade,” looks at how the global trade of amphibians for food or as pets spreads disease, particularly Bd.

Keywords: tree frogs, frog extinction, ecology, batrachochytrium dendrobatidis (Bd)


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