Like most reptiles, tegu lizards (Salvator merianae) were thought to be consistently ectothermic: They can’t sustain a body temperature that’s substantially higher than the ambient temperature. But researchers studying these large, 2-kilogram (4-pound) lizards discovered that tegu nighttime body temperatures are elevated by up to 10ºC during mating season. The findings, published in Science Advances this week, hint at the origins of warm-bloodedness, or endothermy. 

Thanks to heat-generating tissues, birds and mammals these days are all warm-blooded, yet our common endothermic ancestor remains an evolutionary mystery. Despite huge changes in metabolism associated with digestion, small reptiles have a body temperature that might exceed ambient temperatures by no more than 1.5ºC. But is that the case for larger ectotherms? Tegus, for example, hibernate underground, and an elevated body temperature would be a huge advantage when they come out of dormancy, regrow their gonads, engage in mating behaviors, produce eggs, and incubate in their nests. 

To investigate, a team led by Glenn Tattersall of Brock University in Canada studied tegu lizards reared from eggs within a captive colony in Rio Claro, Brazil. One group spent a year in outdoor enclosures, while another group was studied in both outdoor and indoor conditions over the course of three reproductive seasons (typically October). 

During their spring and summertime, tegus warm themselves by basking in the sun – reaching temperatures of up to 35ºC (95ºF) – and then they retreat into their burrows at night. During the fall and winter seasons, they enter dormancy: They retreat completely into the burrow to hibernate without feeding, and their body temperatures are about that of the shelter they're in. Towards the end of their six-month hibernation period, testosterone levels in males rise 20- to 30-fold, and they emerge from their burrows and engage in territorial and mate searching behaviors. Females, on the other hand, stay in their burrows, gather nesting materials, and begin laying eggs in the spring. 

The team discovered that tegu body temperatures equilibrated with that of their burrows during the night throughout most of the year – but not during the reproductive season. Rather, both males and females got warmer and warmer compared to the ambient temperature. They maintained this temperature difference for eight days without access to any heat source for basking or any insulating material.

Tegus must have an enhanced capacity to augment heat production and conservation. The process might be linked to increased metabolism during the reproductive season, but the specific source of that increased heat remains unknown. 

Endothermy may have evolved as part of a reproductive strategy to help increase the survival of offspring. After all, females invest heavily in egg production: A clutch corresponds to about 40 percent of her body mass.

^{Thermal image of a (cool) tegu taken at 6 a.m. on a night when it didn’t stay in its burrow. Glenn J. Tattersall}

^{Image in the text: Glenn Tattersall}