Crocodiles as dinosaurs: Behavioural thermoregulation in very large ectotherms leads to high and stable body temperatures

Empirical field data describing daily and seasonal cycles in body temperatu re (T-b) of free-ranging Crocodylus porosrus (32-1010 kg) can be predicted by a mathematical analysis. The analysis provides a mechanistic explanation for the decreased amplitude of daily cycles in T-b and the increase in 'av erage' T-b with increasing mass. Assessments of 'average' daily T-b were ma de by dividing the integral of the difference between measured values of T- b and minimum operative temperature by the period of integration, to yield a thermal index expressing relative 'warmth' of crocodiles. The average dai ly T-b of a 1010 kg crocodile was 3.7 degrees C warmer than that of a 42 kg individual in summer and 1.9 degrees C warmer than that of a 32 kg individ ual in winter. The success of this mathematical approach confirms that croc odiles are simple ectotherms and that there is unlikely to be a significant contribution to their thermal biology from physiological mechanisms. Behav iour, however, is very important even in large individuals. Crocodiles in t he field typically move daily between land and water in cycles that vary se asonally. We predicted T-b for the reverse of these behavioural cycles, whi ch more than doubled seasonal fluctuations in T-b compared with the observe d fluctuations. We were also able to predict the T-b of very large, dinosau r-sized crocodiles in a similar climate to that at our study site. A 10000 kg 'crocodile', for example, would be expected to have a T-b of 31 degrees C in winter, varying by less than 0.1 degrees C during a day when operative temperatures varied by nearly 20 degrees C, from 20 to 38 degrees C. The s tudy confirms that, in low latitudes at least, large dinosaurs must have ha d an essentially high and stable value of T-b, without any need for endothe rmy. Also, access to shade or water must have been crucial for the survival of large dinosaurs at low latitudes. Furthermore, the finding of increasin g 'average' T-b as ectotherms grow larger may have implications for the met abolic rates of very large reptiles, because the Q(10) effect could counter act the downscaling of metabolic rate with mass, an effect that seems not t o have been recognised previously.