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Description

Britain, ~65,000 years ago

We are at the transition from interstadial to stadial, the beginning of MIS 4, or the end of MIS 5a. The climate has gotten colder, and ice caps have expanded. The Island of Britain, for 20,000 years an insular no man's land, is once again re-colonised by man, and the other beasts of the European mainland.

A band of these men (Homo neanderthalensis) have just taken down a reindeer (Rangifer tarandus), one of the few large game they can find on the island - much else is rare in these lands. Though this is a virgin land for them, their ancestors once stalked this lands millennia before the, when it was a warm haven, teaming with game, where hippo would be found wallowing in the water and great forest elephants would tower above all, but those times have gone, and this is a cold steppe, inhabited by little but bison and reindeer, but will soon become absorbed into the realm of the mammoth.

As they begin to prepare the kill, they are stopped by one of the locals. A great tyrant brown bear (Ursus arctos tyrannus) - one which dwarfs any that roams the men's homelands, has been attracted by the smell of a free meal. Usually a brown bear from their lands, would be cautious to approach a band of predators like humans, but this bear has never seen anything like them before, and knows no reason to fear them. Stealing prey from other hunters is no unknown task for his kind. The tyrant brown bear has been king of this land for many millennia, it has no fear of men.

And it is hungry.

Some more stuff on the British MIS 5a.

Ursus "maritimus tyrannus" is pretty likely not to be a valid subspecies, but I have thought on the idea of naming Ursus arctos tyrannus as a valid subspecies instead, since they do have to be a phylogenetically distinct form of bear to the already-named mainland brown bears of late Pleistocene Europe, known as steppe brown bears - Ursus arctos priscus.
These British bears were isolated for a considerable amount of time, from about 87.22-66.8 thousand years ago, based on the dating of this period, spanning roughly 20,000 years, until the glaciers expanded and the mainland fauna were able to return to the British Isles, causing a prompt shrinking + ecological subjugation of both the brown bears and wolves:
onlinelibrary.wiley.com/doi/ab…
onlinelibrary.wiley.com/doi/ab…

By comparison, the Kodiak bears, alongside those of the other Alaskan islands, have only been isolated for about 10,000 years, and have also grown to huge sizes.
research.amnh.org/~rfr/paetkau…
Given that they are ostensibly recognized as their own subspecies (U. a. middendorffi), I supposed this may lend credence to the approach of separating the mainland Ursus arctos priscus (or whatever we may call these bears) from the larger insular offshoot Ursus arctos tyrannus, but I won't make any strict judgements regarding that, since the taxonomy of these British bears below the level of species is not really discussed in the literature.

But there is a possible scenario - of there being British tyrant brown bears while the mainland has steppe brown bears, who eventually migrated to the British Isles come the landbridge, and genetically swamped the offshoot tyrant brown bear population.

I managed to get hold of a PhD thesis which attempted to calculate the body mass of British Pleistocene fauna

drive.google.com/file/d/1IH9He…
No idea what happened to the author, can't find anything else on her but hey.

It gives a mass estimate of an average body mass of 510 kg for the brown bears of Banwell Bone Cave (identifed as MIS 4 in the thesis but later this assemblage was reassigned to representing the earlier MIS 5a), which represent the type locality for the faunal assemblage that the Kew Bridge site (the type locality for the tyrant polar bear), among various other sites belongs to - the Banwell Bone Cave MAZ.
Post cranial remains were used, and apparently according to her:

"Mass estimates from some of the specimens approach 1000 kg, but estimates cover a very large range and imply the occurrence of unusually high sexual dimorphism in this animal. A single humerus specimen from Wretton is also large in size and produces a mass estimate of 440 kg."

(Wretton had a total of 6 studied postcranial bones which yielded an average of 442kg)

However estimates from the Windy Knoll site (which is also identified as MIS 4 here but in the later literature is also reassinged to MIS 5a) gives much lower estimates of 330kg, even though the sample size of studied remains is similar to that of Banwell Bone Cave, which suggests either geographic variation or a female-heavy sex bias at the site.
Sexual dimorphism wasn't taken into account to give two different means due to the added difficulty in sexing bones. I suspect they would represent a single brown bear population. But creating a mean from the existing sites gives an overall average of about 414 ± 187kg - the error bars are high, again sexual dimorphism, I imagine this figure would be polarized considerably taking into account sexual dimorphism, since modern brown boars are approximately 1.8 times larger on average than sows.
Assuming the specimens used for the British MIS 5a bears were a 1:1 sex ratio (unlikely but let's assume) I got average weights of 532kg for boars and 296 kg for sows assuming modern brown bear dimorphism. AFAIK modern Kodiaks weigh on average about 390kg for males and 210kg for sows, and polar bears 500kg and 227kg for boars and sows respectively. Dunno how reliable the weight estimates are, but the methodology seems pretty sound and only postcrania were used, not teeth, and certainly the sample size of postcrania was pretty huge, like about 396 bones, and large samples are also used for other sites/ages, so I'd wager that at least the body masses relative to bears from other time periods in the British Isles holds true.
In any case, the Banwell Bone Cave MAZ (=MIS 5a) brown bears were the largest brown bears that are known to have inhabited the British Isles, larger than bears from other ages.
I

nterestingly, the later MIS 3 brown bears in the UK are estimated to average 342 ± 85.6kg. Assuming equal contribution of male: female bones (lets assume.......) and assuming modern brown bear dimorphism, I get 438kg and 244kg for boars and sows respectively in this population, which is still a tad bigger than modern Kodiaks. Interestingly, this includes the site of Sandford Hill with 341 ±66.9kg average weight. Marciszak (2017) spoke of the Sandford Hill brown bear remains as displaying a very large size similar to the very large Polish remains we talked about a few months ago, and also a morphology consistent with an assignment to Ursus arctos priscus. While large, Pappa (2016) actually found that the MIS 3 brown bears of the UK were fairly herbivorous (incl. Sandford Hill). Not suggesting that they were necessarily not steppe brown bears, but I suspect this might be due to the absence of the cave bear in the British Isles during the late Pleistocene opening up a niche for a herbivorous bear, which would have reduced competition from the carnivore guild (which at the time was the usual mammoth steppe guild of cave lions, hyenas and wolves).

But yeah, given that the British steppe brown bear remains of this time are of similar size to those of central Europe, I'd say they'd be a good proxy for how big the steppe brown bear was in general, and that would mean that while huge, they were still smaller than the short-lived, ecologically liberated tyrant brown bears.

Yeah I think I dun fucked up on the scale, proportions etc. of the reindeer, and also the sense of distance/size relative to said sense of distance of the bear.

Description

Britain, ~65,000 years ago

We are at the transition from interstadial to stadial, the beginning of MIS 4, or the end of MIS 5a. The climate has gotten colder, and ice caps have expanded. The Island of Britain, for 20,000 years an insular no man's land, is once again re-colonised by man, and the other beasts of the European mainland.

A band of these men (Homo neanderthalensis) have just taken down a reindeer (Rangifer tarandus), one of the few large game they can find on the island - much else is rare in these lands. Though this is a virgin land for them, their ancestors once stalked this lands millennia before the, when it was a warm haven, teaming with game, where hippo would be found wallowing in the water and great forest elephants would tower above all, but those times have gone, and this is a cold steppe, inhabited by little but bison and reindeer, but will soon become absorbed into the realm of the mammoth.

As they begin to prepare the kill, they are stopped by one of the locals. A great tyrant brown bear (Ursus arctos tyrannus) - one which dwarfs any that roams the men's homelands, has been attracted by the smell of a free meal. Usually a brown bear from their lands, would be cautious to approach a band of predators like humans, but this bear has never seen anything like them before, and knows no reason to fear them. Stealing prey from other hunters is no unknown task for his kind. The tyrant brown bear has been king of this land for many millennia, it has no fear of men.

And it is hungry.

Some more stuff on the British MIS 5a.

Ursus "maritimus tyrannus" is pretty likely not to be a valid subspecies, but I have thought on the idea of naming Ursus arctos tyrannus as a valid subspecies instead, since they do have to be a phylogenetically distinct form of bear to the already-named mainland brown bears of late Pleistocene Europe, known as steppe brown bears - Ursus arctos priscus.
These British bears were isolated for a considerable amount of time, from about 87.22-66.8 thousand years ago, based on the dating of this period, spanning roughly 20,000 years, until the glaciers expanded and the mainland fauna were able to return to the British Isles, causing a prompt shrinking + ecological subjugation of both the brown bears and wolves:
onlinelibrary.wiley.com/doi/ab…
onlinelibrary.wiley.com/doi/ab…

By comparison, the Kodiak bears, alongside those of the other Alaskan islands, have only been isolated for about 10,000 years, and have also grown to huge sizes.
research.amnh.org/~rfr/paetkau…
Given that they are ostensibly recognized as their own subspecies (U. a. middendorffi), I supposed this may lend credence to the approach of separating the mainland Ursus arctos priscus (or whatever we may call these bears) from the larger insular offshoot Ursus arctos tyrannus, but I won't make any strict judgements regarding that, since the taxonomy of these British bears below the level of species is not really discussed in the literature.

But there is a possible scenario - of there being British tyrant brown bears while the mainland has steppe brown bears, who eventually migrated to the British Isles come the landbridge, and genetically swamped the offshoot tyrant brown bear population.

I managed to get hold of a PhD thesis which attempted to calculate the body mass of British Pleistocene fauna

drive.google.com/file/d/1IH9He…
No idea what happened to the author, can't find anything else on her but hey.

It gives a mass estimate of an average body mass of 510 kg for the brown bears of Banwell Bone Cave (identifed as MIS 4 in the thesis but later this assemblage was reassigned to representing the earlier MIS 5a), which represent the type locality for the faunal assemblage that the Kew Bridge site (the type locality for the tyrant polar bear), among various other sites belongs to - the Banwell Bone Cave MAZ.
Post cranial remains were used, and apparently according to her:

"Mass estimates from some of the specimens approach 1000 kg, but estimates cover a very large range and imply the occurrence of unusually high sexual dimorphism in this animal. A single humerus specimen from Wretton is also large in size and produces a mass estimate of 440 kg."

(Wretton had a total of 6 studied postcranial bones which yielded an average of 442kg)

However estimates from the Windy Knoll site (which is also identified as MIS 4 here but in the later literature is also reassinged to MIS 5a) gives much lower estimates of 330kg, even though the sample size of studied remains is similar to that of Banwell Bone Cave, which suggests either geographic variation or a female-heavy sex bias at the site.
Sexual dimorphism wasn't taken into account to give two different means due to the added difficulty in sexing bones. I suspect they would represent a single brown bear population. But creating a mean from the existing sites gives an overall average of about 414 ± 187kg - the error bars are high, again sexual dimorphism, I imagine this figure would be polarized considerably taking into account sexual dimorphism, since modern brown boars are approximately 1.8 times larger on average than sows.
Assuming the specimens used for the British MIS 5a bears were a 1:1 sex ratio (unlikely but let's assume) I got average weights of 532kg for boars and 296 kg for sows assuming modern brown bear dimorphism. AFAIK modern Kodiaks weigh on average about 390kg for males and 210kg for sows, and polar bears 500kg and 227kg for boars and sows respectively. Dunno how reliable the weight estimates are, but the methodology seems pretty sound and only postcrania were used, not teeth, and certainly the sample size of postcrania was pretty huge, like about 396 bones, and large samples are also used for other sites/ages, so I'd wager that at least the body masses relative to bears from other time periods in the British Isles holds true.
In any case, the Banwell Bone Cave MAZ (=MIS 5a) brown bears were the largest brown bears that are known to have inhabited the British Isles, larger than bears from other ages.
I

nterestingly, the later MIS 3 brown bears in the UK are estimated to average 342 ± 85.6kg. Assuming equal contribution of male: female bones (lets assume.......) and assuming modern brown bear dimorphism, I get 438kg and 244kg for boars and sows respectively in this population, which is still a tad bigger than modern Kodiaks. Interestingly, this includes the site of Sandford Hill with 341 ±66.9kg average weight. Marciszak (2017) spoke of the Sandford Hill brown bear remains as displaying a very large size similar to the very large Polish remains we talked about a few months ago, and also a morphology consistent with an assignment to Ursus arctos priscus. While large, Pappa (2016) actually found that the MIS 3 brown bears of the UK were fairly herbivorous (incl. Sandford Hill). Not suggesting that they were necessarily not steppe brown bears, but I suspect this might be due to the absence of the cave bear in the British Isles during the late Pleistocene opening up a niche for a herbivorous bear, which would have reduced competition from the carnivore guild (which at the time was the usual mammoth steppe guild of cave lions, hyenas and wolves).

But yeah, given that the British steppe brown bear remains of this time are of similar size to those of central Europe, I'd say they'd be a good proxy for how big the steppe brown bear was in general, and that would mean that while huge, they were still smaller than the short-lived, ecologically liberated tyrant brown bears.

Yeah I think I dun fucked up on the scale, proportions etc. of the reindeer, and also the sense of distance/size relative to said sense of distance of the bear.