Warming World Too Hot for the Cold-Blooded?

Despite thriving in the balmier past, cold-blooded animals like reptiles may struggle to combat overheating as climate warms

Robert Goodier, Scientific American 27 Mar 09;

A new study warns that cold-blooded land animals like lizards and insects in the tropics may wither as the world warms.

"Cold-blooded" is the layman's term for ectotherms—animals whose body temperature is contingent on the surrounding environment, rather than internally regulated like that of warm-blooded creatures. They thrive in temperatures ranging from 68 degrees to 104 degrees Fahrenheit (20 to 40 degrees Celsius), above which they overheat. As the globe warms, researchers warn they may be forced to swelter in burrows and under bushes with little time to eat, find mates or rear young.

"Our models suggest that for many reptiles, the room to move may be pretty small," says Rick Shine, an evolutionary biologist at the University of Sydney in Australia and co-author of the study published recently in Proceedings of the National Academy of Sciences, USA. "They are likely to be in trouble with climate change," he adds, noting they will be confined to shrinking shady patches under disappearing forest canopy and in bodies of water where they can cool off.

The resultant loss of tropical ectotherms could be "catastrophic," Shine says, noting that it could tear apart entire ecosystems from food webs to pollination. The reason: ectotherms range from food sources like insects to top predators like snakes and crocodiles, the loss of which would have "major flow-on effects," Shine says.

Using painted, water-filled pipes of different diameters as stand-ins for lizards and other cold-blooded critters, Shine and colleagues calculated their surface areas and reflectivity to determine how much heat they absorb and release in different environments. They tested them in three Australian climate zones from tropical to temperate, measuring variables like wind speed and solar radiation in three shade levels: the open (no shade), 90 percent shade under a forest canopy, and variable, by shuttling the pipes back and forth between the two extremes to approximate the way ectotherms regulate their body temperature.

The researchers tested their model's predictions against real animals until they were satisfied it worked. Then, to give the model its global scope, they mixed the numbers with worldwide climate data and notched up the average temperature by 5.4 degrees F (3 degrees C). (The Intergovernmental Panel on Climate Change, or IPCC, predicts a global temperature increase of 2 to 11.5 degrees F, or 1.1 to 6.4 degrees C, by the end of the century.) The researchers then ran the numbers through a heat-exchange model developed by co-author Warren Porter, a zoologist at the University of Wisconsin–Madison.

"It's not just the air-temperature increase—three degrees [Celsius] in the shade is not that much. What does kill you is the sun," Porter says. "Not only do you deal with a huge amount of radiation but your water requirements skyrocket."

Shade may be the key to cold-blooded survival in the tropics, as animals will need roomy, contiguous swathes of leaves to shelter them from the sun. The double prongs of a warming climate and the loss of rainforests to farmland and pastures, however, may confine overheating ectotherms to shrinking amounts of shade.

Raymond Huey, a zoologist at the University of Washington in Seattle who was not involved in the study, says the research is the first to examine how an animal's behavior can either buffer or exacerbate the effects of climate change on a large scale. "I don't know any other team who could pull off this project, or come close to doing it as well," he praises, adding, "Porter has no peers at this type of modeling."

All this begs the question of why the dinosaurs didn't overheat. At the close of the Triassic, as the curtain opened on the age of the dinosaurs, global average temperatures may have been in the 70s F (20s C), higher even than those projected in this model—62.4 degrees F (16.8 degrees C).

"Shade would have been extremely important for them," Porter says. They must have been tied to water and dense vegetation to keep cool, they may have migrated with changing seasons, and they must have had lower metabolisms, he speculates. Large animals cool more slowly than smaller ones in hot climates, so "The big animals could not possibly have had a high metabolic rate like we do," he says, "or they would have cooked themselves." Alternatively, some paleontologists believe the dinosaurs were endotherms – that is, they regulated their body heat internally like we do.