Hot bodies protect frogs from disease

By following individual frogs in the rainforest, we reveal that hot frogs are less likely to be infected with a potentially deadly fungus.

The amphibian chytrid fungus (Batrachochytrium dendrobatidis) is responsible for the often fatal amphibian disease chytridiomycosis. This disease has now been associated with declines and extinctions in hundreds of species of amphibians worldwide, and is a serious threat to global amphibian biodiversity.

However, not all amphibian species decline from chytridiomycosis, and many amphibians that have declined from the disease in some environments coexist with it in others. Why might this be?

One such reason is temperature. In the lab, the amphibian chytrid fungus grows best between 17-25°C and infected captive frogs can be cured by simply raising the temperature. Natural populations of frogs also appear to be affected by temperature, with infected frogs more often detected in cooler months and at higher (and therefore cooler) altitudes. So there’s reason to suspect that temperature may play an important role in determining whether individual frogs are infected with the amphibian chytrid fungus in nature.

To investigate this, a small team of volunteers and I tracked over 100 frogs of three species in the rainforests of northeastern Australia. Each frog was fitted with a tiny radio-transmitter or diode attached via a tailor-made waist-belt that allowed us to find each frog every day and night for up to 16 days at a time. This was no easy feat in steep, boulder-strewn streams surrounded by dense (and often prickly!) vegetation. Once we located each frog, we recorded its body temperature (using an infrared gun or thermally sensitive radio-transmitter). This was also often rather tricky- just because the transmitter is telling you the frog is close, it doesn’t mean that it’s easy to see.Frogs that sit on rocks tend to look like rock, and frogs that sit on leaves have a tendency to look like leaves!

Although frogs were often located in the same parts of the same stream (with the same air temperature), the body temperatures of individual frogs varied considerably among and within individuals. A frog sitting under a boulder in the stream was a whole lot cooler than a frog basking in the sun ten metres up a tree or sitting on a sun-warmed rock.

The results of our intensive frog-stalking revealed that the more time individual frogs were found at temperatures above 25°C, the less likely they were to be infected by the amphibian chytrid. Frogs of all species were very unlikely to be infected if 75% or more of their body temperatures were above 25°C. This is the first demonstration that individual frog thermal histories affect the probability of amphibian chytrid infection in nature.

Our findings also point towards one possible way to reduce the impact of the disease on threatened amphibians in nature. By carefully manipulating habitats to increase the availability of warmer temperatures, we may be able to help frogs reach temperatures that allow them to reduce or eliminate infection. While at first glance, our findings may suggest that global climate change may act in favour of amphibians, it’s too complicated to predict. The impacts of global climate change, particularly at the small scales relevant to a frog, are likely to vary among species and sites. While increased air temperatures at some sites might decrease disease impact, increased cloud or canopy cover may increase it.

Reference: Rowley, J.J.L. & Alford, R.A. Hot bodies protect amphibians against chytrid infection in nature. Scientific Reports 3, 1515; DOI:10.1038/srep01515 (2013).

Research from the School of Marine and Tropical Biology at James Cook University and the Australian Museum.