Does body size influence thermal biology and diet of a python (Morelia spilota imbricata)?

Abstract

Current theory predicts that larger-bodied snakes not only consume larger prey (compared with smaller individuals), but may also have a different range of prey available to them due to their thermal biology. It has been argued that smaller individuals, with lower thermal inertia (i.e. faster cooling rates at nightfall when air temperature falls and basking opportunities are limited), may be thermally restricted to foraging and hunting during the day on diurnally active prey, and have reduced capacity to hunt crepuscular and nocturnal prey species. This predictive theory was investigated by way of dietary analysis, assessment of thermal biology and thermoregulation behaviour in an ambush forager, the south-west carpet python (Morelia spilota imbricata, Pythonidae). Eighty-seven scats were collected from 34 individual pythons over a 3-year radiotelemetry monitoring study. As predicted by gape size limitation, larger pythons took larger prey; however, 65% of prey items of small pythons were represented by nocturnally active, small mammals, a larger proportion than present in larger snakes. Several measures of thermal biology (absolute body temperature, thermal differential of body temperature to air temperature, maximum hourly heating and cooling rates) were not strongly affected by python body mass. Additionally, body temperature was only influenced by the behavioural choice of microhabitat selection and was not affected by python body size or position, suggesting that these behavioural choices do not allow smaller pythons to vastly increase their temporal foraging window. By coupling dietary analysis, measures of body temperature and behavioural observations of free-ranging animals, we conclude that, contrary to theoretical predictions, a small body size does not thermally restrict the temporal window for ambush foraging in M.s.imbricata. An ontogenetic or size-determined switch from ambush feeding to actively foraging on slower prey would account for the differences in prey taken by these animals. The concept of altered foraging behaviour warrants further investigation in this species.