Cane Toads Deterred by Cold Feet

Climatic Conditions Limit Spread of Aggressive Bufo Marinus

Sep 12, 2008

A toad 'sprint' run at different temperatures has demonstrated that climatic conditions may be the best barrier to cane toads' aggressive spread across Australia.

Dr Michael Kearney, from the Department of Zoology at the University of Melbourne, and collaborators from other Australia and US universities, have shown that the toads get cold feet – literally!

Since their introduction to Australia in the 1930s, cane toads have been steadily advancing across northern Australia and have already invaded Brisbane and Darwin and Kakadu National Park and are heading south towards Sydney.

A devastating pest, an accurate prediction of their final range and rate of movement is essential in limiting their spread.

Dr Kearney, Dr Ben Phillips from Sydney University’s School of Biological Sciences and Dr Chris Tracy from the School of Science at the Northern Territory’s Charles Darwin University organised a series of sprint races for cane toads – a mini Olympics to see how the reptiles could perform in differing climatic conditions.

“We ran 44 toads from four different places in Northern Australia, from Townsville inland to near the invasion front,” said Dr Kearney.

Using toads collected from four populations across the invasion front, they ran two-metre sprints for the reptiles at a range of different temperatures to see what would slow toads down the most.

“We found that cane toads can barely hop once they get below about 15 degrees Celsius,” he said.

Computer Model Predicts Toads Movement Patterns

After the races the researchers roped in the skills and computer modelling developed by Dr Warren Porter at the University of Wisconsin, Madison, USA, to predict how cold toads would get at different times of the year across Australia.

‘This modelling was used to predict the microclimatic conditions toads and their babies would experience in nature,” Dr Kearney explained.

“In the case of the adult toads, we predicted what that would mean for their body temperatures, and then we used our data from the sprint study to predict how fast and far they could move.”

Dr Kearney said their study is unique in being based on an understanding of the capabilities of the toad itself, whereas many other studies are based on correlations between climate and the places where the toads are living now.

“The modelling also predicted the duration, depth and temperature of ponds in which the toads breed and we used that to see how it affected the potential for the eggs and tadpoles to survive and develop,” he said.

“We put this all together into a final prediction which showed how many months per year a toad could breed in different parts of Australia.”

The study, Modelling species distributions without using species distributions: the cane toad in Australia under current and future climates, was published in Ecography.

Mediterranean Climate a No-no for Bufo

Fortunately for people living in southern Australia, the cane toad is not likely to arrive, or survive. These places have what's termed a ‘Mediterranean climate’ with cold wet winters and warm dry summers.

“These are perfect conditions for growing wine, but you are unlikely to meet a toad at a winery” said Dr Kearney.

“In many of these places the air temperature at night – the active period for toads - is often above 15 degrees Celsius, but this only happens during summer, and evaporation in the dry summer air cools them down too much.”

Toads Clue to Climate Change Effects

Dr Phillips, who co-wrote the paper, was originally a herpetologist, but has switched his focus from snakes to cane toads.

“My research nowadays mostly focuses on the toads themselves and how the process of invasion has changed them (evolutionarily speaking) from relatively sedentary, but healthy, homebodies with modest reproductive tendencies into restless, unhealthy uber-breeders," he told me.

“I think it is all very interesting because the process driving these changes should happen in any species shifting its range, and we know that lots of species are already shifting their range because of climate change.”

He added it was ironic to be using “one environmental disaster (toads) to inform us about the effects of a completely different one (climate change)”.

The copyright of the article Cane Toads Deterred by Cold Feet in Reptiles & Amphibians is owned by Sue Cartledge. Permission to republish Cane Toads Deterred by Cold Feet in print or online must be granted by the author in writing.