Colder climate = larger prey
I was fortunate enough to spend some time studying zooarchaeology - at least enough to be introduced to the viewpoint that the climate affects the entire ecosystem, which in turn, affects humans in the region.
While I don't think discussions regarding heat retention are misguided, I think they should perhaps be expanded in their scope:
animals further from the equator tended to be considerably larger - particularly during the ice ages.
It goes without saying that it takes far more coordination (individual and social) to create the tools and strategize for a wooly mammoth mammoth hunt than many of the large birds, fish, rodents, and even herd animals that dominated the equatorial zone.
Just a thought.
Biting the hand that feeds IT
