Amazing reptile fact 1. Reptiles are cold blooded. Or are they?

Wikipedia

So reptiles are cold blooded. But not always. The Tegu lizard, Salvator merianae, from Argentina warms itself in the sun during spring and summer. At night they retreat to their burrows. In winer (April to September) they hibernate. At the end of hibernation in September the males emerge from their burrows ready to mate.
Their testosterone is high. The muscle in their face, which allows them to bite, swells ready for a fight.  Meanwhile the females are in the borrows, gathering nesting material and laying eggs. She guards them too - a kind of maternal care you see in birds and mammals.
This time of year, the tegus can heat themselves up and keep hotter longer, than would be expected just from basking. Birds and mammals do this all the time, so they don't need to bask in the heat. They make their own warmth.
So why is a reptile, cold-blooded, doing this?  Does this show evolution at work? Is this what was happening when the first mammals or birds emerged from the reptiles?

Read more at Tattersall et al., Seasonal reproductive endothermy in tegu lizards, Science Advances, 2016, 2. 1-7. Found at http://advances.sciencemag.orghttp://advances.sciencemag.org