Elucidating the responses and role of the cardiovascular system in crocodilians during diving: Fifty years on from the work of C.G. Wilber

In 1960, C.G. Wilber in a laboratory-based study described for the first time the changes in heart rate with submergence in the American alligator noting in particular the marked bradycardia that occurred during forced dives. This short review summarizes the major advances in our understanding of diving and the responses and role of the cardiovascular system of crocodilians during submergence in the 50 years since Wilber published his findings. These advances are attributable in part to the technological advances made in physiological monitoring devices and wildlife telemetry that have not only provided greater elucidation of the hemodynamics of the unique crocodilian cardiovascular system but also allowed the natural diving behaviors and heart rates in free-ranging crocodiles to be recorded. Of note, telemetric field-based studies have revealed that wild free-ranging crocodiles typically undertake only short dives, less than 20 min, yet crocodiles are also capable of dives of many hours in duration. In contrast to Wilber's study, dives recorded from free-ranging crocodiles were found to be accompanied by only a modest bradycardia, highlighting the often confounding effects associated with captive animals monitored under laboratory conditions. More recent studies have also documented the complex central flow and pressure patterns of crocodilians, including a pulmonary to systemic shunt that can be initiated by a unique intracardiac valve located in the subpulmonary conus. The role and significance of this cardiac shunt remains controversial and the focus of recent lab-based studies. We contend that elucidation of the role and significance of the cardiac shunt in crocodilians will only be achieved by monitoring telemetrically the central cardiovascular flows and pressures in non-captive animals that are undisturbed and free-ranging. This presents the challenge ahead in the next 50 years.