From New Scientist:
First warm-blooded lizards switch on mystery heat source at will
The first known warm-blooded lizard, the tegu, can heat itself to as much as 10 ̊C above its surroundings – making it unique among reptiles.
But bizarrely, it only switches on its heating system at certain times of the year.
…
Even when the scientists removed access to sunshine or food for a few days, the lizards still warmed up before dawn. But how do they do it?
Last year another group reported the first known warm-blooded fish – the opah – which generates heat by the muscular flapping of its fins.
What became of all those theories about how warm-bloodedness evolved in mammals and birds, but not in reptiles and fish? What really drives these trends?
But Tom Kemp, emeritus research fellow at the University of Oxford, says that warm-bloodedness is such a complex phenomenon that it is too simplistic to think that any one feature or adaptive purpose is alone responsible for its evolution. More.
Hey, a start in the right direction!
And from… New Scientist?
See also: Dino blood cells revive “warm-blooded” controversy
Furry, feathery, and finny animals speak their minds
and
Evolution: What the fossils told us in their own words
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This enthusiast reports the tegu as intelligent: