Realistic heat pulses protect frogs from disease under simulated rainforest frog thermal regimes

Sasha E. Greenspan, Deborah S. Bower, Rebecca J. Webb, Elizabeth A. Roznik, Lisa A. Stevenson, Lee Berger, Gerry Marantelli, David A. Pike, Lin Schwarzkopf, Ross A. Alford

The body temperatures of frogs and other ectotherms fluctuate with ambient conditions. Globally, frogs are threatened by a fungal disease called chytridiomycosis. Batrachochytrium dendrobatidis (Bd), the fungus that causes chytridiomycosis, is sensitive to warm temperatures; growth and reproduction of the fungus cease between 26-28°C. As a result, frog populations in sunny habitats rarely decline from the disease. In rainforest habitats, where chytridiomycosis has taken a heavy toll on amphibian populations, frog body temperatures may reach 26-29°C for a few hours per day. We performed a controlled laboratory experiment to answer the question: “What are the effects of exposing a model frog host (Litoria spenceri) to 26°C or 29°C for a few hours per day?

Infections developed more slowly in frogs exposed to daily 4-hour heat spikes of 26°C or 29°C than in frogs in constant temperature treatments without heat spikes (control). Frogs that experienced daily heat spikes were also less likely to exceed Bd infection intensities at which illness and death become likely. Most frogs that experienced the hotter daily heat spikes (29°C) even cleared their infections after approximately nine weeks. When we grew Bd in nutrient media, independent of frog hosts, it grew more slowly when exposed to heat spikes than in constant-temperature control treatments, suggesting that mild heat spikes have direct negative effects on the physiological processes of Bd.

Our results suggest that even in habitats where average temperatures may be suitable for fungal growth and reproduction, infection risk or the outcome of existing infections may be heavily influenced by short but frequent exposures to temperatures that only slightly exceed the thermal limits of Bd. Our findings provide support for management interventions that promote warm microenvironments for hosts, such as small-scale removal of branches overhanging critical habitat or provision of artificial heat sources.

Image provided by authors.

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