Mouth gaping as an effective thermoregulatory device in alligators

Abstract

GAPING is a characteristic activity of several species of crocodilians in which the animal lies ashore with its mouth held open for prolonged periods^1–4. It has stirred the interest of humans for millennia: gaping is depicted in Egyptian papyri and was mentioned in the writings of Herodotus and Pliny. Today interest continues in gaping and other aspects of behaviour in these large reptiles, the last of the subclass Archosauria, the group of reptiles that included the dinosaurs^1,5. Gaping is a complex habit that involves variable degrees of mouth closure and sometimes includes rhythmic gular movements. Behavioural observations have suggested that gaping is a thermoregulatory response^3,4 but until now physiological studies have been inconclusive. Initial reports⁶ stated that gaping was effective in cooling young Nile crocodiles (Crocodylus niloticus) but a recent experiment on Caiman crocodylus indicated that this behaviour had no thermoregulatory significance⁷. Here we present evidence from controlled laboratory and field experiments that gaping is an effective mechanism for reducing head temperatures of alligators (Alligator mississippiensis). Gaping reduced the rate of heat gain by the heads of small and large alligators when these animals were exposed to high light radiation loads at moderate air temperatures. The magnitude of this effect increased with size. Gaping had little effect on body temperature and did not reduce the final equilibrium temperature of the head.