Cold comfort for reptiles in warmer world

Dani Cooper
ABC

Commonly known as being cold blooded and in need of some sunshine, the world's ectotherms may be struggling to keep cool in the future.

The finding raises concerns about how animals that regulate their body heat using air temperature will cope in a warmer world predicted by climate change.

Writing in today's issue of Proceedings of the National Academy of Sciences, a team of Australian and US researchers say the impact of climate change on ectotherms will depend on how global warming-induced changes in habitat alter the ability to access shade.

Another factor will be the animals' capacity to alter the seasonal timing of their activity and reproduction.

Lead author Dr Michael Kearney, of the University of Melbourne's Department of Zoology, says the results of their modelling are counter-intuitive.

"The majority of the world's animals are cold blooded," Kearney says. "So if it gets warmer you would think it might be better for them."

Keeping cool
However, Kearney says using modelling that combines spatial data on climate, geography and vegetation, with behavioural models they were able to determine whether the priority of thermoregulating ectotherms such as reptiles and insects was to keep warm or stay cool.

"What we've found is that for a large fraction of the planet's animals their main priority is to thermoregulate to stay cool," Kearney says, "and global warming is going to make keeping cool harder."

Not only will air temperatures be higher, he says, but the availability of habitat in which to shade could be altered.

For the study, with Professor Warren Porter, of the University of Wisconsin's Department of Zoology, and the University of Sydney's Professor Rick Shine, the team studied the effects of climate change on a small Australian lizard known as the heath monitor (Varanus rosenbergi).

The modelling was based on the lizard living in three different climate zones - temperate coastal, Melbourne; arid continental, Alice Springs; and tropical coastal, Darwin.

The consequences of three different behaviours - sitting in full sun, sitting in full shade and moving in and out of shade to maintain a preferred body temperature - in those three zones was then mapped.

Kearney says the big surprise of their work was to find that so-called "cold-blooded animals" were more focused on keeping cool than on warming up.

Modified behaviour
This means increased temperatures predicted under climate change scenarios would impact greatly on animal behaviour.

He says these effects could include changing the timing of activities, such as foraging and reproduction, which could have a flow-on effect to "whole ecosystems".

Kearney says warmer environments may also increase energy costs for the animal while also constraining activity time as they spend more time seeking shade.

"Effectively their rent goes up, but the time they've got to find an income goes down," Kearney says.

Under the modelling Kearney says it is clear there will be a "mismatch between required and available shade" in regions such as northern Australia and northern Africa, where there is too little shade, and temperate Australia, North America and Europe, which has too much shade.

"Human activities such as deforestation are dramatically altering the degree of shading available for thermoregulating ectotherms in tropical regions," the authors say in the paper.

"Climate change will also alter vegetation cover through processes such as increased carbon dioxide and changed fire frequency."

Kearney says the ability of a species to tolerate climate change will be whether it can modify the seasonal timing of activities such as reproduction.

"Without such lability, the feasible options to maintain population viability are greatly limited and likely will require substantial evolutionary shifts."

ABC Science