Warming climate is making crocs hotter, with unknown effects

Crocodiles in northern Australia are altering their behaviour to try to keep cool as they experience increasingly stressful temperature extremes driven by climate change.

New research has revealed that saltwater crocodiles (Crocodylus porosus) are spending more time at or close to their critical thermal limit of 32-33°C, diving less and cooling themselves off more.

“As ectotherms crocodiles can’t regulate their own temperature like birds and mammals,” says Kaitlin Barham, a PhD candidate at The University of Queensland (UQ) and first author of the paper published in the journal Current Biology.

“So, as their environment is becoming warmer, the animals in our study are also getting hotter and needing to spend more time on cooling behaviours.

“But if their time and energy is dominated by the need to stay cool, activity necessary for hunting, keeping safe from predators or reproducing, is reduced.”

Each year with Australia Zoo, the UQ research team makes the long trek to the croc spotting site, located on the Wenlock River in Queensland’s Cape York Peninsula.

There they spend 4 weeks camping out in nature.

“Every August is croc trip month,” Barham told Cosmos. “It’s actually really fun.”

“There are different trap sites along the river and if a crop goes into one of those, then … the zoo team will restrain it, and then the UQ science team will come in and we’ll attach satellite trackers.”

They also attach internal acoustic trackers under the croc’s skin to record its body temperature. While its restrained, they use the opportunity to take measurements and perform health checks.

In total, data from 203 crocodiles revealed that, since 2008, the highest crocodile body temperatures have risen by 0.55°C.

The data matches long-term trends in local air temperatures, with the Northern Australia Climate Program reporting the region’s average temperatures are increasing 0.05-0.2°C per decade.

According to study co-author Craig Franklin, a professor at the School of the Environment at UQ, when a crocodile’s body temperature rises above 32-33°C, its diving and swimming performance is impacted.

“Hotter crocodiles don’t dive for as long, which is concerning because as ambush hunters, they need to wait underwater holding their breath for a wallaby or feral pig to come past,” he says.

Elevated body temperatures increase a crocodile’s demand for oxygen when diving. So, they must return to the surface more frequently and recover at the surface for longer.

Of the 203 crocodiles, 65% exceeded a 32°C body temperature at least once during the study period; 41% exceeded 33°C; and 22% exceeded 34°C.

This heating was associated with an increase in cooling behaviours and shorter dive durations.

“We saw that crocs were putting a bit more effort into cooling behaviours, rather than warming behaviours,” Barham explains.

“That might be as simple as they’re spending more time on the bank at night, rather than on the bank during the day.”

“We think a big reason for that is that water tends to stay a pretty consistent temperature, but the air at night gets colder, so that’s an opportunity for them to cool down,” says Barham.

“It’s possible that they’re also sitting in the shade, or there is cold water running off from cool springs in the area. Those are some other resources that they could use to keep cool.”

According to Barham, it is quite possible these kinds of behaviours could be seen across the Order Crocodilia.

But, as the study focused on a population of crocodiles in a single, remote river system, it is unclear whether increasing body temperatures effected other, longer distance behaviours, such as changing in river systems or moving to the ocean, or the animals’ interactions with humans.

“What we think is really important is that we’ve studied these crocs [in] the hottest part of their range in Australia,” says Barham.

“But it would be really interesting to see how crocs that live at the southern part of their range in Australia, where they’re a bit cooler, cope with similar heat waves.”

“Around Rockhampton is generally as south as crocodiles get in Queensland.”

The researchers suggest that, if the pressures of climate change continue to increase, saltwater crocodiles may need to actively cool more frequently or potentially move to higher latitudes (south in Australia).