Guest essay by Eric Worrall
A study by University of Queensland Professor Craig Franklin claims that crocodiles may be endangered by global warming.
According to the Press Release;
Professor Craig Franklin of the UQ School of Biological Sciences said saltwater crocodiles exposed to long-term elevated water temperature spent less time submerged once water temperature exceeded 31.5 degrees Celsius.
“We thought that crocodiles – like many animals – would adjust to temperature changes so life continues,” he said.
“However, we were surprised to find they had little capacity to compensate for water temperature changes and seemed to be hard-wired to operate at certain temperatures.
“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.”
The abstract of the study;
Diving in a warming world: the thermal sensitivity and plasticity of diving performance in juvenile estuarine crocodiles (Crocodylus porous)
Essie M. Rodgers, Jonathon J. Schwartz and Craig E. Franklin*
Air-breathing, diving ectotherms are a crucial component of the biodiversity and functioning of aquatic ecosystems, but these organisms may be particularly vulnerable to the effects of climate change on submergence times. Ectothermic dive capacity is thermally sensitive, with dive durations significantly reduced by acute increases in water temperature; it is unclear whether diving performance can acclimate/acclimatize in response to long-term exposure to elevated water temperatures. We assessed the thermal sensitivity and plasticity of ‘fright-dive’ capacity in juvenile estuarine crocodiles (Crocodylus porosus; n = 11). Crocodiles were exposed to one of three long-term thermal treatments, designed to emulate water temperatures under differing climate change scenarios (i.e. current summer, 28°C; ‘moderate’ climate warming, 31.5°C; ‘high’ climate warming, 35°C). Dive trials were conducted in a temperature-controlled tank across a range of water temperatures. Dive durations were independent of thermal acclimation treatment, indicating a lack of thermal acclimation response. Acute increases in water temperature resulted in significantly shorter dive durations, with mean submergence times effectively halving with every 3.5°C increase in water temperature (Q10 0.17, P < 0.001). Maximal dive performances, however, were found to be thermally insensitive across the temperature range of 28–35°C. These results suggest that C. porosus have a limited or non-existent capacity to thermally acclimate sustained ‘fright-dive’ performance. If the findings here are applicable to other air-breathing, diving ectotherms, the functional capacity of these organisms will probably be compromised under climate warming.
To their credit Professor Frankly et al put some fairly strong caveats in the body of the study itself, for example;
The deleterious effects of climate change on ectotherms may, however, be counteracted by compensatory responses. Thermal stress can be buffered by both behavioural and physiological strategies. Pockets of thermally favourable habitat can be sought out or shuttled between to maintain body temperature within a preferred thermal range (Seebacher, 2005; Seebacher and Franklin, 2005). Alternatively, long-term changes in thermal regimens (e.g. seasonal shifts in temperatures) can induce physiological changes, where the thermal effects on biochemical processes are blunted (Johnston and Dunn, 1987).
I think I’ll go with the caveats on this one. Crocodiles are a survival of a much warmer world – for example, crocodiles were far more prevalent in the Cretaceous Period, which lasted from 145 to 66 million years ago, was on average 4C hotter than today, and had an average CO2 level of 1700ppm, over 4x higher than today. The largest crocodile which ever lived, twice as big as the largest modern species, enjoyed a widespread habitat during this warm period.
In addition, many species of crocodile, such as Australia’s infamous salty, can swim 100s of miles in a month – so they would have no difficulty migrating to more comfortable habitats, if their current habitat ever becomes unsuitable for any reason.