I had no idea U. arctos is found in Europe. One wonders which way that migration across the Bering land bridge went.
ChatGPT:
How big is the geographic difference?
At the broadest level, adult brown bears range from relatively small bears in resource-poor populations to enormous coastal bears. The biggest populations occur where food is exceptionally abundant, especially along productive salmon rivers and coasts.
A useful rough picture for adult body mass is:
Population/environment Adult males Adult females
Small Eurasian / northern or interior populations ~100–200 kg ~60–130 kg
Typical North American interior grizzlies ~180–300 kg ~100–180 kg
Large Alaskan/Yukon grizzlies ~250–400+ kg ~150–250+ kg
Exceptional coastal Alaska/Kodiak bears 400–600+ kg possible 200–350+ kg possible
These are broad ranges rather than standardized population averages; age, season, sex, reproductive status, and whether the animal has recently fed heavily can shift an individual substantially.
The really striking part: Alaska alone
The geographic variation isn’t just a difference between Europe and North America. Brown bears differ markedly from one Alaskan region to another. A study of 11,651 Alaska brown bears found region- and sex-specific growth patterns, with large differences in asymptotic size among regions. The researchers associated much of this variation with habitat quality, particularly salmon availability.
D
DOI
+1
Another Alaskan study comparing four ecosystems found that bears living where salmon were abundant were generally larger. Among adults within populations, lean mass itself varied enormously: approximately twofold among females and three- to fourfold among males.
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Wiley Online Library
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So two adult male brown bears from the same general part of Alaska can differ dramatically, let alone bears from opposite ends of the species’ range.
Why does this happen?
Food is probably the biggest driver. Brown bears are extraordinarily adaptable omnivores. Where they have access to huge seasonal pulses of high-energy food—salmon, spawning fish, carrion, berries, nuts, etc.—they can accumulate enormous fat and muscle reserves.
Conversely, bears in colder, less productive or food-limited environments tend to be smaller. Studies of Alberta grizzlies, for example, found that regional productivity and environmental conditions during both early life and adulthood affected body size.
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PubMed Central (PMC)
Climate and latitude matter, but latitude alone doesn’t predict brown-bear size particularly well. European populations illustrate this nicely: researchers comparing 1,771 bears from northern and southern Europe tested several competing explanations for geographic differences in mass rather than simply assuming that northern bears must be larger.
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PubMed
And then there are the giants
The famous Kodiak brown bears and the enormous coastal bears of Alaska and British Columbia aren’t a different species. They’re Ursus arctos populations living in environments where food availability permits exceptionally large body sizes.
This produces a somewhat counterintuitive situation:
A “grizzly bear” can be a ~150-kg adult male in one population and a 400+ kg adult male in another, while both are Ursus arctos.
And individual variation within a population can itself be huge. In fact, the scientific literature emphasizes that brown-bear body size is highly plastic and that ecological conditions can produce substantial differences even among neighboring populations.
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Wiley Online Library
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One other interesting consequence is that males become disproportionately larger in the biggest populations. The Alaska study found that sexual size dimorphism increased with asymptotic body size, meaning that exceptionally food-rich environments don’t simply make both sexes larger by exactly the same amount.
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DOI
The fossil record appears to be pretty clear. From Europe into Siberia, then across Beringia (now separated by the Bering straights, then after the last glaciation, down into Canada then the lower 48. Ursus Arctos is a latecomer to North America south of Canada. Probably sometime about 13K to 10K years ago. People were already here.
Ursus Arctos probably helped with the mass extinction of mega fauna.