I haven't yet had a chance to look for the original source of this intriguing report, but here is what London's "Daily Telegraph" is saying:

Neanderthals may have died out because their bodies overheated as the Earth grew warmer, according to new research.

Analysis of DNA obtained from Neanderthal remains has revealed key differences from modern humans that suggest their bodies produced excess heat.

While in the cold climate of an ice age this would have provided the species with an advantage, as the earth warmed they would have been less able to cope. Ultimately this would have caused their extinction around 24,000 years ago. ...
Here's the key point:

Professor Patrick Chinnery, a neurogeneticist at Newcastle University ... said ... "Differences in these mitochondrial DNA sequences might explain why modern humans were able to survive while Neanderthals were not ..."
Neanderthals could have died out because their bodies overheated - Telegraph