Autonomic control of heart rate exhibits diurnal shifts in a crocodilian

Over the diurnal cycle most reptiles show large changes in internal body temperature and heart rate (fH). The raising of fH, as the surface of a heliothermic reptile warms, increases cardiac output and facilitates in optimising the preferred daily body temperature (PDBT). In mammals, the fine tuning of cardiac output by the autonomic system can be observed through distinct oscillatory patterns in fH. This study examined 'Caiman latirostris' (n = 6) to determine if similar oscillations in fH were present, and to assess if they exhibited a diurnal component associated with daily shifts in body temperature. A surgically implanted miniature datalogger recorded every heart beat and the dorsal surface temperature (Tds) of animals free-ranging in a semi-natural habitat. All 'C. latirostris' exhibited rapid warming of Tds between 0700 and 1200. This was correlated with a rapid increase in fH, accompanied by erratic beat-to-beat oscillations in instantaneous fH. As Tds cooled, fH decreased and the short-term oscillations were abolished, resulting in a much slower rate of change in instantaneous fH. The two distinct fH rhythms may serve to optimize the PDBT over the diurnal cycle.