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Bears of the Pleistocene
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Here's something better I found:
http://www.zin.ru/labs/theriology/staff/..._2002b.pdf It puts a specimen of I. punjabiensis at 500 kg.
Arctodus simus and Arctotherium angustidens
The North and South Ameican short faced bears, as more commonly referred to. Undoubtedly, these two species are at the pinnacle of the size pyramid of mammal carnivores. The North American form initially held the title of the largest bear, until the South American form came to light, specifically by Soilbenzon and Schubert in 2011. They packaged several different equations from difference sources and came up with estimates, and proposed a largest mean mass of 1,591 kg. A few months later, Figeorido et al. (2011) was published, and they proposed a value of 957 kg. In my opinion, these two values aren't on a level playing field for comparison. The South American form was estimated from many equations, some which look to be discussing a universal scaling factor rather than a genus specific. The North American form was from one equation set, focused solely on the bear genus. So, for a better comparison, we could apply the Figuerido et al. equation to the largest South American form specimens. The equation to use is: log(mass) = 2.77 * log(humerus length) - 4.68 One fossil of interest is a 620 mm humerus, equation a mass of 1135 kg. Fossil two, humerus length 615 mm, equation to a mass of 1110 kg. This puts the south form at about 200 kg heavier than the north form. A glaring area for improvement would be comparing bone diameters. The humerus diameters cannot be compared at the moment, as Soilbenzon and Shubert (2011) took diameters at the midshaft rather than at 55% length of the bone, rendering Figuerido et al.'s humeral least shaft width incompatible. The case doesn't close here, as somewhere online there's a mention of an even longer North American short faced bear humerus. Once I find it I'll post some more. Regarding equations. The Figuerido equation has one flaw, which could be major. The weights used in the regression are not of the individuals whose bone measurements were used, but rather literature values. In that way, Christiansen (1999) had a better database. Soilbenzon and Schubert also passed of the 2 metric ton weight of the South American form as an overestimate, which it may have been. But in itself, it isn't really reasonable to pass of an estimate based on objective mathematics just because one cannot fathom it, as has been done a bit too much. For all we know, the Figuerido estimates could be underestimates as the literature weights were less than the actual weights of the specimens where bone measurements were derived from. The South American short faced bear's official report has been attached. 
For a long time now, I have been searching for evidence concerning the size range of Pleistocene brown bears; especially those of Ice Age North America. My theory is that with the abundance of mega-fauna ( so much available meat ) that those grizzlies might well have been in a size catagory with coastal brown bears. But so far I have come up empy-handed. What do the fossil records show?
Me too, i am also interested about the brown bear fossil record, but not much info came out of water.
But i also heard they were a bit larger than the modern brown bear which should be deemed as normal as the Pleistocene animals generally faced less human pressure than their modern counterparts.
What's exactly the size of the coastal brown bears? I've read conflicting measurements. Let's say 400-750 kg for males, so 225-750 kg including both genders.
Have you read the attached documents? They don't look to be of bears any larger than modern specimens. The situation is more complex than just prey diversity and availability. Keep in mind that, even with more prey species, there were also more predator species, so tighter niches. Looking at North America, we have at the least (weight class in paranthesis): Smildon fatalis (100-280 kg), American lion (150-400 kg), Short faced bear (300-1000 kg), Dire wolf (50-80 kg, althought possible pack hunter, increasing prey size), and Homotherium (100-250 kg). Now where does that leave the brown bear? The brown bear could likely be omnivorous to better cope with such crowded predatory conditions. The 225-750 kg range essentially overlaps with all these aforementioned species. The high end of the range is overlaps greatly with the short faced bear. This is ecological unfavorable, thus fossil records showing not so large brown bears make sense. 
tigerluver, why do you say the dire wolf was a "possible" pack hunter? I would have thought it was quite obvious that they were.
I agree with you Richard, I just tend to state the behavior of extinct fauna with some uncertainty as more often than not, someone may find a caveat in the conventional understanding.
Tigerluver, this fossil evidence would suggest that the grizzly bears of the Pleistocene were no larger than inland brown bears of today. However, it takes more than this small amount of evidence to draw any real conclusions. There must certainly be more than this handfull of grizzly fossils.
It will take more than this to completely convence me that a grizzly, with a nose estimated to be seven times more acute than that of a bloodhound, would ignore the smell of a mammoth carcass and be satisfied with eating acorns and digging for ground squirrels.
The Pleistocene brown bear came out from such tightly competitive environment, so i can't imagine that they were no stronger than today when facing against so many fearsome rivals.
In most paleontoligical cases, the sample size from which conclusions are drawn from are normally very small. I'd stray away from calling such conclusions are invalid when there are no better options.
Sure, bears may could get bigger, a few fossils doesn't tell enough, I agree. Although, I would like you to understand that I am thinking under Hutchinson, Grinnel, Elton, etc. niche principle. You're mammoth carcass idea. Ignoring the fact of resource depletion, that carcass will potentially attract all the aforementioned competitor predators. The American lion, a pack of dire wolves, Smilodon, and certainly Arctodus are massive hurdles blocking access to the carcass (this is interference competition). There's not as much available as one would like to think. Moreover, often one predator is capping the size of the other. There are a lot of Pleistocene predators that can size cap the brown bears of that age. The reality of the situation is still much more complex than this. Maybe some other factor not discussed here allowed the bears to grow the modern coastal sizes. Maybe time will change the status quo.
Just one last thought to leave here. Could you imagine a mother grizzly in this environment defending her cubs? -A big job.
Male bear also had to do its job. Otherwise, the mother and the cubs would be an easy meal for other large predators.
Here's a whole chunk of new bear reading.
Some more fossils.  *This image is copyright of its original author  *This image is copyright of its original author The records are hard to analyze. A lot of the fossil material references in the attached paper I can't find, probably because there from late last century, and not digitized yet. The Harington et al. (2014) specimen looks to be around 240 kg. The skull fragment doesn't seem to add up to a large anima on the other hand, but I am not sure. Then we have measurement-less quotes like these: "The complete skeleton from El Capitan Cave, as well as portions of several others, were of black bears, distinguished from the living bears on the island only by their large size. Their size seemed especially significant since they appeared to be females based on the lack of bacula and the gracile structure of their skulls. Even more significant was the discovery of even larger bear remains that we identified as brown bear. (Harington et al. 2014)" What exactly is large®/giant size? Unfortunately, for now, this is the best idea of the relative size of the Pleistocene brown bear form. It is still a real possibility than brown bears underwent a size increase after the competition died out. Also, it seems Matheus (1995) went over the mammoth carcass scenario as well in a way: "If short-faced bears were large, aggressive scavengers capable of stealing carcasses from other large carnivores, then it seems unlikely that brown bears could dominate them in direct interference competition. And while brown bears may have preferred to feed on animal carcasses, it seems more likely that they would have avoided direct confrontation with a dominant bear. Instead, brown bears would have been doing what they do best - making use of a variety of dietary resources and habitats." I admit, the giant size of some Pleistocene species really is fascinating. But that's a double-edged sword... "The ecological plasticity of brown bears and their ability to hibernate may have been the keys to their ultimate survival at the end of the Pleistocene, while Arctodus, the highly specialized forager, was not able to find a niche in Holocene ecosystems. Most likely, carcass densities on Holocene landscapes fell below levels necessary to sustain minimal viable populations of short-faced bears. Since many bears hibernate to survive poor food availability during winter, this may be an indirect indication that short-faced bears, and perhaps all New World bears, never evolved this strategy to survive seasonal dietary bottlenecks.(Matheus 1995)"
Once again, I have been proven wrong. I was seeking the truth; thank you Tigerluver for taking the time. But, this brings up so many questions. Where were did the grizzlies live? I'm sure that many chose the mountains, but the theory of their aggressive nature suggests that many lived on the open prairie. In what type of terrain have fossils been found? And, would you say that these Pleistocene grizzlies were basically in the size range of modern inland grizzlies?
I will add; I'm not too put back because of this. Arctodus simus was a real monster; much bigger even than a Kodiak with a skull that more resembled that of a tiger, complete with bone-crushing jaws. Too much even for the biggest grizzly or any other predator of his time and location. I will add: along with the grizzly there lived giant wolves, giant jaguars, scimitar cats, saber-toothed cats, giant lions, and giant bears. Among these apex predators, only the grizzly survived the end of the Ice Age.
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 *This image is copyright of its original author Read the attached paper for more information. |
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