December 25, 2015, by Tim Radford
Saltwater crocodiles, like this one in Australia’s Northern Territory, may soon find themselves in hot water.
Image: By Matt via Wikimedia Commons
Rising temperatures in northern Australia could force its predatory crocodiles to seek refuge in cooler waters further south.
LONDON, 25 December, 2015 – Global warming poses a challenge to some of Australia’s scariest citizens – the saltwater or estuarine crocodiles of the far north. These cold-blooded killers could find tomorrow’s world just too hot for comfort.
Crocodiles, like other amphibians and reptiles, are ectotherms: cold-blooded creatures that become sluggish in cold weather and must warm up in the sunshine before they can hunt and eat.
But new research by Essie Rodgers, a PhD student at the University of Queensland and colleagues, published in the journal Conservation Physiology, suggests that these top predators could face a world in which it becomes too warm to take to the water to kill.
The news is not entirely a surprise: other research has established that lizards, for example, have a narrow range of heat tolerance.
But Rodgers and her colleagues put juvenile crocodiles to the test. They submerged them in water at temperatures likely to occur under different climate change scenarios.
Right now, summer water temperatures in northern Queensland’s rivers reach 28°C. Under a moderate rise, these could get to 31.5°C. If humans go on burning ever more fossil fuels and releasing greenhouse gases that warm the atmosphere even more, then the waters could reach 35°C.
The scientists found, to their surprise, that the animals spent less time submerged as temperatures rose. “Acute increases in water temperatures resulted in significantly shorter crocodile dives,” Rodgers said. “Their submergence times halved with every 3.5°C increase in water temperatures.”
The lethal temperature for crocodiles was in the high 30s to low 40s: air temperatures could easily get higher in the region, which is why crocodiles need to spend so much time submerged: up to 11 hours a day.
Craig Franklin, of the University of Queensland, professor of zoology and a co-author, said he and his colleagues had thought that the predators would be able to adjust to changing temperatures. But that didn’t seem to be the case.
On the move
“We are not sure what this means, but it’s likely that if the water is too hot, crocodiles might move to cooler regions, or will seek refuge in deep, cool water pockets to defend their dive times,” he said.
The study is unfinished. Dive times are only part of the crocodile’s survival equipment. The next step, the researchers say, is to see what rising water temperatures will do for the swimming capacity, bite force and aerobic scope of the air-breathing, submarine hunter Crocodylus porosus.
As a class of animals, the crocodiles survived when the dinosaurs perished: they have faced a lot of climate change in the last 200 million years, and individual saltwater crocodiles can live for as long as 70 years.
But, the scientists warn, the Australian estuarine crocodiles are unlikely to acclimatise to what they call “the negative consequences of elevated temperatures on dive capacity.” So to survive, the animals may have to become climate migrants, and move south. – Climate News Network
Tim Radford, a founding editor of Climate News Network, worked for The Guardian for 32 years, for most of that time as science editor. He has been covering climate change since 1988.