Heat foils frog fungus
Chytridiomycosis, a disease caused by the amphibian chytrid fungus Batrachochytrium dendrobatidis (Bd), has caused declines and, more depressingly, extinctions in amphibian populations around the world. However, two Australian researchers have found that frogs that can maintain high body temperatures are less at risk of infection from the deadly fungus.
Stoney Creek frog
The Stoney Creek frog ( Litoria lesueuri ) spends much of its time above the optimal temperature range of the amphibian chytrid fungus (17–25°C) and has not experienced population declines due to chytridiomycosis.
Link between environmental temperature and disease prevalence
Previous research has observed that some species of frog are relatively unaffected by the fungus. Other species can be badly affected in some areas but coexist with the fungus and recover in others. This suggests a link between environmental temperature and disease prevalence at the population level. Back in the laboratory, previous research has shown that Bd grows best in culture at 17–25°C. In the lab, individual frogs could be cured of their Bd infections by elevating their body temperature – a solution which could pull some species back from the brink of extinction.
Three frog species investigated
Dr Jodi Rowley and Professor Ross Alford from James Cook University in Queensland investigated three infected Australian rainforest frog species in their native habitat
Litoria lesueuri – least affected by Bd declines.
Litoria serrata – intermediate.
Litoria nannotis – most affected by Bd declines.
The frog populations were tracked in the summer/wet and winter/dry seasons at a total of four rainforest sites,and 128 frogs’ body temperatures were recorded nocturnally and diurnally every 24 hours.
Infection declines at temperatures over 25ºC
Like all amphibians, frogs, being ectotherms, rely on the external environment for heating and cooling – they bask in the sun or sit on warm rocks to warm up and go into the shade or water to cool down. The researchers showed that, in all three species, a frog’s probability of infection declines as it spends more time above 25°C–the highest temperature at which Bd grows optimally.
Green-eyed tree frog
A green-eyed tree frog ( Litoria serrata ) fitted with a temperature-recording radio transmitter. This species disappeared from many sites during epidemics of the disease chytridiomycosis in the late 1980s and early 1990s but has subsequently recovered.
The researchers were testing the hypothesis that frogs elevated their body temperatures in response to infection (behavioural fever) since the probability of infection declined dramatically with increasing time spent at higher body temperatures. However, the results were not highly consistent with this idea.
Role of natural selection in higher body temperatures
“We cannot entirely eliminate the possibility that infection may cause individuals to regulate their body temperatures at higher levels than normal. Our results may also have a simpler cause. Individuals that choose relatively high body temperatures for other reasons, i.e. to aid in growth, digestion or reproduction, may have coincidentally decreased their probability of Bd infection. While Bd remains endemic, any tendency to maintain higher body temperatures could be reinforced by natural selection.
Endangered waterfall frog
The endangered waterfall frog ( Litoria nannotis ) disappeared from many sites during epidemics of chytridiomycosis in the late 1980s and early 1990s. They spend little time above the optimal temperature range of the amphibian chytrid fungus (17–25°C).
The researchers write that natural or artificial selection for frogs that like to maintain a higher body temperature or manipulating habitats to increase the availability of warmer temperatures could reduce susceptibility to this pathogen.
The research was published on 21 March 2013 in Nature’s Scientific Reports .
Related content
Use the article Investigating frog disease to find out more about New Zealand research into chytridiomycosis and why it kills frogs. They can also find out how scientists at the University of Otago were able to cure diseased frogs.
This article discusses the threat chytridiomycosis poses to Australian frogs. Your students may like to watch this video clip, Threats to frogs, in which Dr Phil Bishop talks about the threats facing New Zealand native frogs.
