Updated Range of Immensity for Arctotherium: New Record for Largest Known Bear
South America is currently home to a single species of ursid, the highly elusive and uniquely patterned spectacled bear ( Tremarctos ornatus ). This species is slightly diminutive in size compared to other bears, and is known for being fairly docile and highly vegetarian (one study found that the only 7% of spectacled bear scat sampled contained animal remains, and even those were just beetles and ants; Troya et al. 2004). These tractable traits, however, belie a fearsome family tree.
The spectacled bear is actually the last surviving member of the group known as the short-faced bears—the tremarctines of the genera Arctodus and Arctotherium_. The most famous of these is Arctodus simuscontent&task=view&id=102&Itemid=60, known for having been found in great numbers in the La Brea Tar Pits. These bears were seriously formidable, attaining sizes that would have made modern grizzlies and polar bears look paltry.
During the Great American Biotic Interchange in the late Pliocene, ancestral tremarctines made their way across the newly minted isthmus that finally connected North and South America. Subsequent to this shuffle of species, many extinctions and radiations occurred, as species adapted to novel predators, prey, and habitats. Here is where we saw the tremarctines develop into the now infamous crew of car-sized carnivores, the last of which (excluding our spectacled bear) went extinct in the early Holocene (Soibelzon et al. 2005). The largest of the large in this group was Arctotherium angustidens, a South American behemoth of legendary proportions, currently known as the largest bear species ever to have lived.
Apparently A. angustidens could be even more immense than we were previously aware. A study recently published in the Journal of Paleontology (Soibelzon and Schubert 2011), describes the biggest bear fossil yet to be unearthed, an enormous individual—apparently a middle-aged male—discovered in Argentina.
Soibelzon and Schubert calculated that this individual had a mass ranging from anywhere between 983 and 2,042 kg (2,167-4,502 lb), depending on which formula is used to make the estimate. So, even using the most conservative formula, we had a bear that literally weighed a ton. For the sake of comparison, grizzlies ( Ursus arctos ) only tend to range up to 600 kg and polar bears ( Ursus maritimus ) up to 800 kg, although the record mass reported for a polar bear is ~1002 kg, on the low end of the estimates for this A. angustidens.
If bears used to attain such gargantuan sizes, and occasionally still do for some outlier individuals, why are the tremarctines now represented solely by the runty spectacled bears, which usually only range between 60 and 175 kg—an order of magnitude lower than some of their extinct relatives?
That phenomenon—the disappearance of the most massive species of a clade and persistence/radiation of smaller species—isn’t solely confined to Arctotherium. It appears that although the Pleistocene saw the development of massive ursids on several continents: Arctotherium in South America, Arctodus in North America, and the European Ursus spelaeus, none seemed to have made it out of the epoch alive. Arctotherium, however, did seem to burn out earlier than the other megabears, disappearing in the middle of the Pleistocene rather than making it through to the early Holocene like some of the other tremarctines.
Soibelzon and Schubert suggest that the large size of these species constrained their available dietary niche, and environmental conditions around the time of the Pleistocene-Holocene boundary favored smaller, more flexible species that were more general in their dietary habits and didn’t have quite the same enormous energy demand. Previous studies have supported the idea that increasing size sometimes brings on obligatory hypercarnivory, which is often the expressway to extinction. Van Valkenburgh et al. (2004), in a study of ancient North American carnivores, showed that at least two major clades exhibited an association between large body size, dietary specialization, and shorter species duration.
If large size is so hard to maintain, then why did it develop in the first place? The answer may have to do with the Great American Biotic Interchange, mentioned above. When the two American landmasses were connected, and animals were free to venture forth to new continents, they encountered ecological communities that were very different from what their adaptations had prepared them for back on their continent of origin. When the bears ambled south across the isthmus, they found a community full of large herbivores but with a very meager roster of carnivores with which to compete. At first, there were only two major predators in the area, A. angustidens and Smilodon, the infamous machairodont saber-tooth cat. A huge, widely available resource base loosened constraints on body size, and all of the open niche space allowed for diversification of the taxon.
Thus, the reign of A. angustidens was spectacular yet brief. As the process of adaptive radiation allowed carnivores to diversify, a more crowded scene began to emerge, resulting in more competition for resources. For example, Finarelli (2007) demonstrated that intraguild dynamics appear to be a factor in taxonomic diversification and the development hypercarnivory within three subfamilies of basal canids. In addition, environmental changes, such as the glacial cycles that punctuated the Pliestocene, could drastically change resource availability before carnivores had the time to adapt to new foraging conditions.
To offer an analogy, consider the difference in the practicality of driving an H3 hummer and a Prius. When oil is abundant and cheap, high demand for fuel can be accommodated. But when competition and/or scarcity, brought on by higher demand from the rest of your neighbors and possibly a change in environmental circumstances, the advantages of being more energy efficient and flexible will skyrocket. Likewise, the hugest of the huge bears stalked South America for a time during the Pleistocene, but did not make it to the home stretch of the epoch with its smaller and more generalist congeners.
The discovery of this extra-massive A. angustidens individual is important on multiple levels. It is extremely fascinating to have evidence demonstrating that bears once grew to proportions to which we were previously unaware. In addition, it brings up some interesting factors to consider when thinking about the effects of dietary specialization and intraguild competition in the face of declining resource availability.
LEOPOLDO H. SOIBELZON AND BLAINE W. SCHUBERT (2011). THE LARGEST KNOWN BEAR, ARCTOTHERIUM ANGUSTIDENS, FROM THE EARLY PLEISTOCENE PAMPEAN REGION OF ARGENTINA: WITH A DISCUSSION OF SIZE AND DIET TRENDS IN BEARS Journal of Paleontology, 85 (1), 69-75 : 10.1666/10-037.1
Finarelli, J. 2007. Mechanisms behind active trends in body size evolution of the Canidae (Carnivora: Mammalia). American Naturalist 170: 876-885.
Soibelzon, L. H., E. P Tonni, and M. Bond. 2005. The fossil record of South American short-faced bears (Ursidae, Tremarctinae). Journal of South American Earth Sciences 20: 105-113.
Troya, V., F. Cuesta, and M. Paralvo. 2004. Food habits of Andean bears in the Oyachachi River Basin. Ursus 15: 57-60.
Van Valkenburgh, B., X. Wang, and J. Damuth. 2004. Cope’s rule, hypercarnivory, and extinction in North American canids. Science 306: 101-104.
Spectacled bear image credit
Arctotherium illustration: Soibelzon and Schubert (2011)