Modern vertebrate endotherms (birds and mammals) have high rates of metabolism, elevated body temperatures, usually tight control over body temperature, and a reasonable scope for further increases in metabolism through locomotor activity. Vertebrate ectotherms, on the other hand, rely on behavioural thermoregulation and cardiovascular adjustments to facilitate warming, and generally lack specific biochemical and cellular mechanisms for sustained, elevated metabolism. Nevertheless, the ancestral condition to endothermy is thought to resemble that of many extant reptiles, which raises the question of the origins and selection pressures relevant to the transitional state.
Numerous hypotheses have emerged to explain the multiple origins of endothermy in vertebrates, including thermoregulatory, locomotory, and reproductive activity as possible drivers for these sustained and elevated metabolic rates. In this context, all possible heat generation and heat conservation capacities need to be considered.
We recently demonstrated evidence for facultative endothermy in an extant lepidosaur, the tegu lizard. The following graph shows the measured heart rates of tegu lizards living in their underground burrow throughout the year, showing a phase that looks like ectothermy (passive temperature dependence throughout the hibernation period), followed by periods where heart rate (and thus, metabolic heat production) rise 2,3 or up to 5 times greater than expected for their measured body temperature. This is a facultative form of endothermy, appearing in the reproductive period:
Since lepidosaurs are a sister group to the archosaurs, understanding how a novel form of endothermy evolved will open up opportunities to test the compatibility or incompatibility of the various endothermy hypotheses, with the potential to elucidate and resolve long contentious ideas in evolutionary physiology. In short, heating up to mate is the take home message from the tegu measurements, although we have much left to do to understand how they do this and its link to the evolution of endothermy in general.
In thermal image format, here is what the data above can be summarized as:
Tattersall G.J., Leite C.A., Sanders C.E., Cadena V., Andrade D.V., Abe A.S., Milsom W.K. 2016 Seasonal reproductive endothermy in tegu lizards. Sci Adv 2, e1500951. (doi:10.1126/sciadv.1500951).
Tattersall G.J. 2016 Reptile thermogenesis and the origins of endothermy. Zoology (Jena). (doi:10.1016/j.zool.2016.03.001). Link here for free access until Dec 6, 2016