How can I bypass Bergmann's rule in cold weather populations of humans?

Question

I am working on a setting with a large native population of humans who live in a year-round freezing environment, somewhat akin to the real world Antarctic. These humans should be as biologically similar to other populations as possible, however they will be taller and thinner on average than other humans, around 6'0" to 6'4".

It is important that this is a long-term population, not a group of recent migrants. Bergmann and Allen's rules suggest that this population should have adapted to be shorter, stockier, and fattier than the tall and lean populations of warmer, arid environments. What reasons can I give to explain this unusual set of adaptations for their climate?

Answer 1

Another stronger evolutionary pressure could become more important than the baseline evolutionary pressure to be short, stocky, and fat (with those evolutionary pressures dealt with another way, such as warm clothing).

For example, suppose that the region is full of short, stocky, and fat giant killer walruses.

Image of Walrus attack per Wikipedia

The way that the locals survive them is by squeezing into narrow cracks in the ice where their fat predators can't reach them, or by ice climbing up to high shelves of ice or down into crevasses that long limbs and greater height makes it easier to reach, and being thinner makes it easier to fit into.

Generally, when defying gravity, you want to be light (which being thin and not stocky helps), but you to be able to reach handholds when climbing, so you want to have long limbs and height.

Answer 2

Relatively recent climate change

This can be explained by e.g. an ice age setting in. The onset was about say, 10,000 years ago, average sea temperature went down 12 degrees Celcius. There has been no time for any biological adaptation, on an evolutionary time scale. Also, take into account you have the advantage of humans: they are creative mammals, able to slow down natural selection by protecting themselves. Humans can also adapt to changing climate in cultural ways, like wearing thicker clothing and building isolated dwellings in settlements.

Answer 3

Coat.

Scandinavia is cold and snowy in the winter. In some places the Sun does not come up for months!

But the mighty Scandinavians do not care. They are cold resistant and taller than most ethnic groups on the planet.

How do they overcome their increases surface area? They wear a coat! Fashioned from the hide and fur of a defeated snowbeast, this cloak of fortitude will keep you warm in even the harshest of storms.

Your people are like the Scandinavians. They became large during a time of plenty. Then they either moved into the Snowlands, or the Snowlands came to them. They remained tall but became skinnier from lack of food. They resist the cold using coats.

Answer 4

Technology.

Sufficient technology can overcome evolutionary pressures. If, for instance, they have sufficient technology so that shorter, stockier bodies to retain heat isn't a biological advantage, then there's no evolutionary pressure to adapt to the environment because you adapt the environment to you instead. It doesn't have to be high tech, just sufficient tech that is available to all (or at least the majority) of the population in terms of keeping warm.

In terms of the setting, you could postulate that there is, for instance, an common, widely available animal, perhaps one that ends up being domesticated, whose hide is so ridiculously efficient at heat insulation that it beats any real-world equivalents such as caribou hide and can not only be used for clothing but fashioned into blankets that can be used to insulate dwellings. The ancestors of the current people discovered how to use it before they had to deal with extreme cold, thus when the cold came, they had tech available to deal with it.