Demythologizing Arctotherium, the Biggest Bear Ever

Quite a few years back, so long ago that I can’t really remember much more than the fact that I once visited it, my parents took me to Space Farms Zoo and Museum. Tucked away in northern New Jersey, the roadside attraction is not so much a zoo or a museum as a throwback to the curiosity cabinets of Renaissance Europe – a ramshackle collection of odd natural and historical artifacts in varying states of preservation. According to those who have been there more recently than I have, the first two cents the museum’s proprietor earned are displayed not far from the Native American skulls, and the Space Farms website boasts everything from a “primitive tool barn” to a “one-of-a-kind complete miniature circus” and “more than 100,00 other unique items.” If Space Farms ever had one claim to fame, though, it was Goliath.

An Alaskan grizzly bear, Goliath lived at Space Farms between 1967 and 1991. He was enormous. Said to measure twelve feet long and weigh a short ton, he has often been heralded as the largest bear ever held in captivity. Sadly, however, Goliath lived in disturbingly inadequate conditions, and, the pet cemetery section of Roadside America reports, by the end of his life Goliath’s rheumatism was so bad that he couldn’t even drag himself across his concrete floor to get a drink of water. Surprising no one, the managers of Space Farms gave Goliath a second life. A mount made from Goliath’s pelt stands in front of the museum’s fireplace, and his skull rests in a glass case nearby.

Goliath was undoubtedly one of the biggest bears ever seen alive. Assuming his oft-repeated stats are correct, he fell in the upper size range for Alaskan grizzlies. But he was not the biggest bear of all time. That title, as far as we presently know, goes to Arctotherium angustidens.

The first of five Arctotherium species, A. angustidens was a descendant of earlier bears that traveled to South America when the Panamanian land bridge opened a connection to North America about three million years ago. Scientists have known about this species for quite some time – it was first described in 1880 – but the bones of an elderly male specimen just described by Leopoldo Soibelzon and Blaine Schubert are gigantic. In a photo included in a National Geographic News report, Soibelzon holds up the humerus – upper arm bone – of the bear next to that of an elephant, and the two are nearly equal in size.

The newly-described bones were discovered in 1935 during construction of the San Juan de Dios hospital in La Plata City, Buenos Aires Province, Argentina. They date to about one million years ago, and include both the right and left arms. (Part of a shoulder blade was found, as well as a few hand bones, but these fossils have gone missing.) This was probably a very old, cranky male bear. The degree of fusion between the heads and shafts of the arm bones indicate that this was an elderly individual, both humeri show old injuries along one of the bony crests where the massive arm muscles would have attached, and the left radius – one of the lower arm bones – shows signs of infection.

But just how big was this bear? There is no doubt that it was quite large – the limb bones of other bears look puny by comparison – but, without a complete skeleton, Soibelzon and Schubert had to use a bit of arithmetic to estimate the bear’s mass.

Depending on the estimation method used, the Arctotherium from La Plata City weighed anywhere between 2,162 and 4,500 pounds, although the authors admit that the upper limit is a bit too high. Instead, they propose that male Arctotherium angustidens maxed out around 3,500 pounds, and a skeletal reconstruction included in the paper is just shy of eleven feet tall. This is about as tall as the largest grizzly and polar bears can stand, but Arctotherium angustidens was a much more robust animal.

Based upon the new mass estimate, the prehistoric South American bear was at least a thousand pounds heavier than even the largest known modern bears, and nine times as heavy as its closest living relative, the spectacled bear. (The mass of the other giant bears of the Pleistocene – the European cave bear and the deep-snouted bear Arctodus simus of North America – were not directly estimated in the study, but the fact that they had smaller arm bone measurements indicates that they would have received lower mass estimates through the methods used.) Indeed, Arctotherium angustidens was not the direct ancestor of today’s giant grizzly and polar bears. This bear, which Soibelzon and Schubert state was “probably the most powerful terrestrial carnivoran of the late Cenozoic”, was actually the biggest and most carnivorous member of a group that became increasingly smaller and more herbivorous over time.

Despite newspaper restorations of this bear rearing up on its back legs and snarling, Arctotherium angustidens was not a hypercarnivorous monster that fed exclusively on giant sloths and other large mammals. True, Arctotherium angustidens was big enough to take down large prey and run the sabercat Smilodon off a meal – just as Yellowstone’s grizzly bears steal kills from wolves today – but it probably had a mixed diet and included some salad with its steak. As found in a 2010 study by Soibelzon and Borja Figueirido, the skull and jaws of A. angustidens wer most similar to those of omnivorous species like the sun bear, the Asian black bear, and the North American “short-faced” bear Arctodus simus, another prehistoric species thought to have been extremely predatory but recently reconstructed as an omnivore. Broken teeth hint that Arctotherium angustidens often chewed on bones, supporting the idea that it ate animal matter, but overall it probably had a more cosmopolitan diet like that of modern grizzly bears. As a predator, it was an opportunist that did not exclusively rely on meat to survive.

The dietary flexibility of the giant South American bear makes its disappearance around 800,000 years ago all the more perplexing. Based upon its size and early arrival in South America, paleontologists used to think that Arctotherium angustidens was a very carnivorous bear that only had to compete with Smilodon for large prey. There were other predators around – such as jaguars, the fox-like Theriodictis, and the wolf Canis gezi – but these were in an entirely different weight class. It was only later, around the time that the giant bear disappeared, that other heavyweight predators evolved on the continent, and so it has been assumed that increased competition for prey did in A. angustidens. Yet, if the new analysis by Soibelzon and Figueirido is correct, then A. angustidens had a more flexible diet, and the idea that it was simply outgunned by other predators becomes difficult to uphold.

Nevertheless, we do know is that the largest of the Arctotherium species ate more meat than those the species that followed it. The ranges of each species in time are still being worked out, but Arctotherium angustidens was succeeded by A.vetustum, A. bonariense, A. tarijense, and A. wingei. (Frustratingly, these species don’t have common names, so you’ll have to bear with me here.) The skulls and jaws of each of these species were more like those of the spectacled bear, a living representative of the same bear sub-group which is almost entirely herbivorous, and these bears are thought to have included more plants into their diet while still scavenging meat when opportunities to do so arose. Among the last of these bears, Arctotherium wingei may have even been as herbivorous as the living spectacled bear, but it is difficult to be sure since the only known skull is from a young individual and lacks the informative lower jaw.

Together, the decrease in size and the apparent diet shift towards more plant food led Soibelzon and Figueirido to suggest that increasing pressure from other predators pushed Arctotherium towards a more herbivorous diet during the past 800,000 years, but we don’t know for sure. There is much that remains unknown about the natural history of this almost entirely extinct group of bears. Today, only the spectacled bear carries on the legacy of the family which once included Arctotherium, Arctodus, and their kin – a modest relative of some of the biggest bears of all time.

[Many thanks to Cameron McCormick, who sent me the description of the new giant bear specimen and blogged about it himself here.]

Top Image: Goliath during his later years, in the fall of 1985. From Flickr user The Rapscallion.

References:

FIGUEIRIDO, B., & SOIBELZON, L. (2009). Inferring palaeoecology in extinct tremarctine bears (Carnivora, Ursidae) using geometric morphometrics Lethaia, 43 (2), 209-222 DOI: 10.1111/j.1502-3931.2009.00184.x

Soibelzon, L., Pomi, L., Tonni, E., Rodriguez, S., & Dondas, A. (2009). First report of a South American short-faced bears’ den (Arctotherium angustidens): palaeobiological and palaeoecological implications Alcheringa: An Australasian Journal of Palaeontology, 33 (3), 211-222 DOI: 10.1080/03115510902844418

Soibelzon, L., & Schubert, B. (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 DOI: 10.1666/10-037.1