TIL some reptiles can also generate their own heat

That might be one of the reasons why warm and cold-blooded terms have been deprecated, as there is always an exception out there in nature

Salvator merianae – the Black and white tegu, also called the Argentine giant tegu – is able to keep warm by raising its own body temperature during the reproductive season even while it is in a dark burrow. This had never been observed before in reptiles

::: spoiler spoiler

[...] the group collected biopsies from the skeletal muscles of ten lizards during summer (February) and spring (September-October). Small tissue samples taken from a front foot and back foot were submitted to biochemical analysis and calorimetry.

[...] both males and females produced far more mitochondria (cellular energy organelles) [during mating season].

In addition, a protein present in mitochondria called ANT, known to be involved in a biochemical process that generates heat in birds, was more abundant and more active in the lizards during the reproductive season, when females prepare for egg laying and nest building while males seek territory and increase their gonads to get ready for mating.

UCP, another protein involved in the process of heat generation, but in mammals, was not found to be active

[...]

Until the 2016 discovery, the only reptiles known to be able to warm themselves were two species of python, large Asian snakes that can reach 5 m in length. This endothermic behavior occurs mainly while they are incubating eggs. It consists of shivering, a technique also used by some mammals and birds. S. merianae raises its own temperature without shivering, however.

Temperature regulation may be associated with the production of sex hormones, which peaks in the mating season for both males (testosterone) and females (estradiol and progesterone). Other hormones that are abundant in this phase, such as thyroid hormones, are involved in energy expenditure and mobilization of energy reserves.

“Thyroid hormones are known to trigger a rise in mitochondria. Increased production of these hormones during the mating season may be associated with abundance of mitochondria and hence increased activity of the protein ANT, which helps produce heat and is known in birds,” [...]

[...] mechanisms involved in endothermy may have appeared in vertebrates even earlier than has been estimated. Their presence in a large lizard (without shivering) and a large snake (with shivering), albeit only at a certain time of year, could be a sign that the phenomenon is more common in reptiles than scientists think.

“Both animals have large bodies, in which internally produced heat tends to take longer to dissipate. It’s possible that other large reptiles may also be warmer than the outside temperature at certain times. This is the first description in a lizard, and one that inhabits a subtropical region, of a cellular heat-generating mechanism that closely resembles what happens in birds and mammals”

Paywalled DOI :( :::