Without the benefit of an internal heating system, reptiles make the most of the sun by basking for hours, soaking up the sun's warming rays. But how do they regulate their body temperature, when they are at the sun's mercy? Most reptiles speed up heating when they move into the sun by boosting their heart rate, quickly shunting the heat from the skin to the animal's core. And when the sun goes in, the reptiles' heart rate falls rapidly, to protect it from cooling. This distinctive heart beat pattern is known as heart rate hysteresis'. So when Craig Franklin and Frank Seebacher set out to find which environmental stimuli trigger heart rate hysteresis, they needed an animal that was content to bask in various different conditions. What they hadn't banked on was that their ideal basker, would come equipped with a bad attitude and fearsome set of teeth; crocodiles are the perfect model organism for this study' explains Seebacher, because they are happy to bask both in and out of water. The team set about monitoring how the reptiles' heart rates varied when they were basking under a sun lamp, and were astounded when the animals'hearts began racing as soon as the temperature changed, far exceeding the heart rates they'd expected to record if the effect was driven by heat alone(p. 1143).

But working with fully grown adult crocodiles wasn't feasible, so the team measured the heart rate of juvenile crocodiles, because they are simply more manageable! They fitted the young animals with electrocardiogram (ECG)electrodes to measure the heart rate as the temperature changed. Seebacher explains that this wasn't straightforward either, because reptile's heartbeats tend to be weak and easily affected by stress. However, once they'd managed to get reliable ECG recordings, they began raising the temperature and waited to see how the crocodiles reacted.

Seebacher explains that they had expected to see a hysteresis pattern as the temperature rose and fell, but nothing had prepared them for the way the animal's heart rate leapt as soon as the temperature increased. And when the young crocodiles cooled, they saw another enormous decrease in the youngster's heart rate. However, within less than a minute of the temperature fluctuation,the crocodile's heart rates returned close to expected levels. Could the dramatic change in the reptile's heart rate be driven by the increase in temperature alone? When they calculated the Q10 effect, it was over 4500, simply too high for the physiological response to be driven by the effect of temperature alone on the heart's pacemaker cells.

Having found that the crocodiles' cardiac system responds so drastically to temperature fluctuations, Seebacher and Franklin are keen to figure out which systems control the heart's sudden change of pace. As Seebacher explains `the extent to which [the crocodile's] heart rate can be decoupled from the intrinsic temperature effect is astounding and quite unheard of'.

Franklin, C. E. and Seebacher, F. (
2003
). The effect of heat transfer mode on heart rate responses and hysteresis during heating and cooling in the estuarine crocodile Crocodylus porosus.
J. Exp. Biol.
206
,
1143
-1151.