It's a natural response to run and hide when you feel threatened; even young crocodiles dive for cover when another creature shows an interest in devouring them. But Craig Franklin from The University of Queensland, Australia, is concerned that startled croc youngsters may not be able to remain safely submerged for as long as they do now as the climate continues warming. When so-called ‘cold blooded’ (ectothermic) animals get warmer, they consume oxygen faster as their metabolism increases, possibly restricting how long frightened crocodiles can remain submerged when avoiding a threat. Teaming up with Essie Rodgers, Franklin measured how long scared juvenile crocodiles can remain submerged at current river water temperatures and the temperatures that crocodiles are predicted to encounter in 2100 to find out how these reptiles may fare in the future.

After adapting the young crocodiles to modern day (28°C water) or future (34°C) climate scenarios for 2 months, Rodgers startled the animals with a gentle tap on the back and timed how long they remained submerged. Impressively, the crocodiles that were adapted to current climate conditions were content to remain submerged for 18.5 min, extending to over an hour if they felt particularly harassed after performing four consecutive dives. However, the hot water crocodiles could only remain under water for 9 min after a single tap on the back and the more threatened animals only stayed down for 28 min. Rodgers also measured the animals’ heart rates and oxygen consumption to try to understand why the hot water animals’ refuge tactics were so impaired, and found that the crocodiles that had adapted to 2100's predicted warmer temperatures were unable to lower their metabolism as much as the cooler crocodiles, burning through oxygen at a faster rate, forcing them to return to the surface sooner than their cooler cousins.

‘This finding suggests predator avoidance dives may be shortened if water temperatures continue to increase in marine and freshwater habitats’, warn Rodgers and Franklin, who are concerned that crocodile youngsters will become more vulnerable to predators as they are likely to have to surface more frequently if the temperature continues rising.

E. M.
C. E.
Physiological mechanisms constraining ectotherm fright-dive performance at elevated temperatures
J. Exp. Biol.