Tegu lizards are cold-blooded, but it turns out they really crank up the heat when it's time for some lizard love — a surprising finding that could help scientists get one step closer to solving a mystery of evolution.

The half-metre long lizards, native to large swathes of South America, spend most of the day basking and foraging in the sunshine before retiring to burrows at night, where they quickly cool to the surrounding temperature.

They are, after all, ectotherms — creatures that depend on heat sources from the environment to regulate their internal temperature and, with a few notable exceptions, lack specialized tissues to generate heat.

But a new study published in the journal Science Advances this week has shown that during the mating season tegus significantly increase their internal body temperature and maintain it well above that of the surrounding environment. 

Over the course of the breeding season from September to November, the average internal temperature increase in the tegus monitored was between 4 C and 5 C, but some lizards warmed as much as 10 C. 

Tegu lizard Tattersall

A thermal image of tegu lizards in their burrow at night. Their body temperatures remained elevated above the ambient temperature throughout the mating season. (Glenn Tattersall)

It's a "reasonably unique" phenomenon among reptiles, says Glenn Tattersall, an animal physiologist at Brock University in St. Catharines, Ont., and lead author of the paper. A select few larger reptiles such as sea turtles, komodo dragons and some crocodiles are known to be able to elevate their internal temperature in certain circumstances.

But it was a surprise to see it occurring in tegus.

According to Tattersall, the results reinforce the contested theory that the evolution of endothermy, or "warm-bloodedness," is linked to sexual reproduction.

Being warm-blooded is very energetically expensive, and its evolutionary origin in birds and mammals — which share no common warm-blooded ancestor — is hotly debated. 

Interestingly, tegus are a similar size to some of the first known mammals. Uncovering how and why these cold-blooded reptiles elevate their body temperature during the mating season could partly illuminate the evolutionary dynamics of early warm-blooded animals. 

As for the how, Tattersall says it is likely a combination of conserving more heat absorbed during daylight hours and an increase in the lizards' metabolic rate.

"They are raising their metabolism. We don't know in what tissues, but it appears to be globally throughout most of the body," he told CBC's Quirks & Quarks in an interview set to air Saturday at noon.

Tegu lizard

Until now, it was believed that tegu lizards were consistently cold-blooded year-round. (Glenn Tattersall)

"It's probably a two-to-three times rise in their resting metabolism."

In females, higher internal temperatures might be related to enhanced egg production or embryo development. In males, it could be linked to the development of secondary sexual characteristics, such as jaw muscles that grow bigger during the breeding season.