Behavioural postures and the rate of body temperature change in wild freshwater crocodiles, Crocodylus johnstoni

I recorded body temperature and behaviour of eight Croconylus johnstoni in the wild over a 2-yr period in order to quantify the effect of posturing on body temperature and to provide a mechanistic explanation of how behaviour affects body temperature. Behaviour was categorised according to the propo rtion of a crocodile's surface area exposed from the water (0% exposed [=di ving] to 100% exposed [=basking]). Crocodiles did not simply shuttle betwee n the extremes of 100% exposed and diving but showed an array of intermedia te postures. Rates of body temperature change were negative for exposures l ess than 40% and positive for 60%-100% exposed. This was due to the differe nce between operative temperature and body temperature, which was negative during diving but increased with the percentage of exposure, up to 25 degre es-30 degrees C during basking. For any particular posture, the rate of bod y temperature change decreased with increasing mass. Thermal time constants were shortest during diving and longest during basking. A heat-transfer eq uation predicted the rate of body temperature change well, except that it u nderestimated the rate of body temperature change during 80% and 100% expos ed. Exposing only a small part of their body when in water (20%) slowed hea t loss considerably, allowing crocodiles to spend more time in the water wh ile maintaining body temperature within their preferred body temperature ra nge.