The quiet of my evening wildlife watching was suddenly broken by a thick Boston accent. “Oh my gawd! Look! It’s a grizz! That’s the last animal I needed to see! It’s a grizz!”
He was right. Lumbering across the valley was a big dark shape that could only be a bear. It was not very close, being little more than a dot moving along the distant treeline, but through the zoom lens of my camera it was just possible to make out the hump that distinguishes black bears from grizzly bears. It was the closest I would get to Yellowstone’s largest predator during my visit to the national park (at least that I know of), but in the not-too-distant past an even larger cousin of the grizzly roamed much of North America.
Three visions of the short-faced bear: Arctodus as a predator, a scavenger, and an herbivore. By Oscar San-Isidro, from Figueirido et al., 2010
Arctodus simus, the short-faced bear, was part of the recently-lost Pleistocene megafauna which disappeared from this continent around 11,000 years ago. The first human inhabitants of this continent undoubtedly encountered it from time to time, and the prospect of meeting a bear which stood five feet at the shoulder is a chilling one. It was the epitome of the big, bad bear, but just how bad was it? As argued by paleontologists Borja Figueirido, Juan Perez-Claros, Vanessa Torregrosa, Alberto Martin-Serra, and Paul Palmqvist, the popular image of the short-faced bear as a hypercarnivorous superpredator may have obscured the reality of the beast.
There are a few things that “everyone knows” about Arctodus: that it was short-faced, had long limbs, and was more carnivorous than living grizzly bears. Its long limbs, especially, have been taken as evidence that it ran down its unfortunate prey, though it would not have been above running dire wolves or saber-toothed cats off their kills, either. From time to time some researchers have proposed that Arctodus was an omnivore that scavenged more than it hunted, and one study even hypothesized that it largely relied on plant food, but the vision of the bear as a highly-predaceous carnivore has remained the most popular.
A comparison of the skull of Arctodus (top), a black bear (2nd from top), a grizzly bear (third from top), and a polar bear (bottom). From Figueirido et al., 2010.
When Figueirido’s team re-examined the anatomy of Arctodus and compared it to a variety of other carnivores (a total of 411 individuals from 57 species), however, they found that the short-faced bear had much more in common with living omnivorous bears than had been previously appreciated. Even though its short jaws and long limbs had previously been used to suggest that Arctodus was similar in habit to feline predators, it turned out that it was not that different from living bears in the proportions of its face and limbs. While Arctodus had a deeper snout, for example, the length of its face was not significantly shorter than what is seen among its living cousins. It was not really a “short-faced” bear at all.
Three restorations of Arctodus, revealing the skeletal, muscular, and external anatomy. By Oscar San-Isidro, from Figueirido et al., 2010
The details of the arms and legs of Arctodus are a little more complicated. Its limbs were not as long as would be expected if it were a high-speed-pursuit predator, but, for its size, Arctodus had longer and more slender limbs than would be predicted according to the relationship between limb length and size seen in other bears. Maybe it had a little more speed than modern grizzlies (which can run faster than you can), but it did not have the anatomy for cheetah-like sprinting, either. Furthermore, the authors suggest that the image of Arctodus as long-legged may be a sort of optical illusion caused by its short back. The skeletal proportions of the bear are different from those of cats and dogs, and so it looks a little more leggy than it really is. (This is true of skeletons of extant bears, as well. Most people are familiar with stuffed mounts of bears from showrooms and museums, but few have seen a skeleton of a bear, which in my opinion also look a deceptively long-legged.)
But what about the isotopic studies which have supported the idea that Arctodus preferred meat? Isotopes of elements such as oxygen, carbon, and nitrogen have been used to determine what sort of food animals ate and what environments they lived in, and studies of isotopes in Arctodus remains have suggested that it was primarily a carnivore. As the authors of the new study note, however, the bears on which these studies have been carried out have been especially large individuals from Alaska. Perhaps they, like living Kodiak bears, ate more flesh than their smaller counterparts that lived further south in the continent. If this is correct then the habits inferred on the basis of the bears from Alaska might represent only those populations, or perhaps only those particular individuals, and cannot be used to create a blanket statement about how all Arctodus behaved. In general, Arctodus appears to have been similar to extant omnivorous bears in habit (and perhaps regional variability), leading the authors to conclude:
In short, we suggest that A. simus may be best envisaged as a colossal omnivore whose diet probably included varying amounts of meat according to food availability. Of course, we do not wish to imply that A. simus did not prey occasionally on bison, deer, or ground sloths, nor that it did not scavenge the carcasses left over by the hypercarnivores such as saber-tooth cats (Smilodon fatalis and Homotherium serum), giant lion (Panthera atrox), and dire wolf (Canis dirus). We simply affirm that A. simus did so in a similar manner as some North American populations of brown bears (e.g., Alaska and Yukon) currently do so.
Rather than representing mutually-exclusive alternatives, then, the restorations of Arctodus by Oscar San-Isidro envision different aspects of its feeding habits. Unlike the big cats of its time it was a generalist feeder that could subsist on a variety of food sources and exploit the leftovers of other predators. Perhaps some individuals or populations were more predatory than others, distinctions that will require further study to determine, but even if it was not a rapacious carnivore I still find Arctodus every bit as fascinating.
Figueirido, Borja, P√©rez-Claros, Juan A., Torregrosa, Vanessa, Mart√≠n-Serra, Alberto and Palmqvist,, & Paul (2010). Demythologizing Arctodus simus, the ‘Short-Faced’ long-legged and predaceous bear that never was Journal of Verterbrate Paleontology, 30 (1), 262-275 : 10.1080/02724630903416027