Hatchling crocodiles maintain a plateau of thermal independence for activity, but at what cost?

Crocodilians show a broad plateau of thermal independence for sustained activity. It has been hypothesized that this reflects a performance breadth necessary for carrying out ecologically important behaviors across a range of ambient temperatures. Here, we swam Saltwater Crocodiles (Crocodylus porosus) in a thermally controlled flume at 23, 28, and 33°C and recorded oxygen consumption (VO2) before and after swimming activity. Ambient temperature altered spontaneous VO2 in a positively linear manner, but there was no significant difference in the distance the crocodiles would swim voluntarily. Excess postexercise oxygen consumption (EPOC) increased 10-fold between swimming trials at 28 and 33°C, and the anaerobic debt took 3 times longer to clear at the higher temperature. The results show that, although C. porosus demonstrated a broad thermal breadth for swimming performance, a higher degree of anaerobic metabolism was required to sustain activity at the upper limits of the thermal plateau. Why crocodiles should choose to sustain an anerobic debt rather than reduce their swimming activity when exposed to high experimental temperatures is perplexing, but the study findings provide a physiological rationale for some of the diel and seasonal activity patterns observed in wild crocodilians.