Peeps and chirps

One of my favorite parts of summer is sleeping with the windows wide open – and, if there’s a pond or a marshy spot nearby, drifting off to the peeps and chirps of frogs calling to one another.

The global chorus of frogs, however, is getting quieter – amphibian populations are declining around the world due to many factors, including habitat loss, climate change, and disease. The deadliest disease amphibians face is chytridiomycosis, an infection caused by a type of fungus called Batrachochytrium dendrobatidis, or Bd for short.

Bd infections can devastate amphibian populations and even drive them to extinction, though some locations and species seem to be resistant to the fungus. Scientists believe that environmental differences in temperature, altitude, and moisture influence the ability of some amphibians to survive infection, or avoid it altogether. New research reported in the journal Freshwater Biology suggests that the presence of predators might play a role as well.

Scientists exposed a group of wood frog tadpoles to ‘predator cues’ – in other words, they added the excretions of predacious beetle larvae that had been fed a diet of tadpoles to the tanks containing the experimental tadpoles, signaling to the tadpoles the presence of a predator. Another group of tadpoles were kept under similar conditions, but not exposed to predator cues. The researchers also added Bd, the fungus responsible for chytridiomycosis, to some of the tanks.

The two groups of tadpoles exposed to Bd – those that experienced predator cues and those that didn’t – had equal rates of Bd infection; about thirty percent of the tadpoles were infected with Bd. The infected tadpoles in the predator-present group, however, had less than half as many fungal spores in their bodies as the tadpoles that weren’t exposed to predator cues.

The scientists suspect that the stress-inducing predator cues primed the tadpoles’ immune systems, allowing them to lower their Bd infection loads. “This is an important result,” the researchers note, “because virulence is often associated with Bd pathogen load.” In other words, lower infection loads could translate to higher survival rates.

Amphibian decline is a complex, worldwide problem, but studies on chytridiomycosis and the fungus that causes it move us closer to a solution – and a world in which nocturnal frog choirs continue to sing.

An adult wood frog showing off inflated vocal sacs.

(Image by Dave Huth via Flickr/Creative Commons license)