Bearzilla’s Diet

WordPress has a feature that lets me see how many daily views my blog articles get.  For several years my article entitled “Bearzilla: the Biggest Bear Ever” is almost always the single highest viewed article of the day. https://markgelbart.wordpress.com/2012/12/10/bearzilla-the-biggest-bear-in-history/  The subject of that popular blog entry is Arctotherium angustidens, an extinct South American species of bear that reached estimated weights of 3500 pounds–the largest size of any bear known to science.  In that blog entry I also discuss the largest specimens of extant species of bears and include a photo I ripped off from google images of a 2100 pound polar bear.  I suspect that photo is what draws so many views.  I came across a fairly recent research paper in the Journal of Paleontology about A. angustidens with enough information for me to write an addendum to my original article.

Illustration showing the early Pleistocene giant short-faced bear that lived in South America. It later evolved into a smaller more herbivorous species.

Scientists studied the pathology, morphology, chemical signatures, and biomechanics of A. angustidens bones to determine what this species ate.  Missing, broken, and worn teeth were common.  The evidence of these dental problems suggests the bears were damaging their teeth when they clumsily gnawed on bones.  Some bear teeth even had bone splinters lodged in them.  An individual young female bear had a tooth infection caused by a bone splinter in its tooth, and this was the probable cause of death.  These giant bears had large teeth cheek similar to the extant giant panda (Ailuropoda melanoleuca), but pandas don’t exhibit tooth damage on their diet of bamboo.  The frequent occurrence of tooth damage in Arctotherium can only be explained by a diet high in bone consumption.

An analysis of stable isotope ratios in Arctotherium bones does suggest this species included lots of meat in its diet, but it also ate plant material.  The scientists conclude Arctotherium was an omnivore.

When large bears first colonized South America they competed with just a few large carnivores such as saber-tooth cats.  There was an abundance of large slow-moving prey the bears could wrestle down.  Eventually, more species of predators colonized the continent, and some prey species evolved into faster runners.  Other prey species–the large ground sloths for example–may have evolved into stronger adversaries as well.  Bears that consumed more plant material had a better chance of surviving than those that competed with predators or failed to obtain prey.  This may be why Arctotherium’s descendants  evolved to eat more plants than meat.

Reference:

Soibelizon, Leopoldo; et. al.

“South American Giant Short-Faced Bear (Arctotherium angustidens) Diet: Evidence from Pathology, Morphology, Stable Isotopes, and Biomechanics”

Journal of Paleontology 88 (6) 2014

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Bearzilla: The Biggest Bear in History

With a massive presence and a terrifying roar Arctotherium angustidens rampaged over South America about 2 million years ago, chasing frightened predators away from their kills and gorging itself on the scavenged meat.  These giant bears reached a weight of 3500 pounds, so to sustain this bulk, they needed to eat approximately 50 pounds of meat per day.  They occasionally captured wounded or sick prey animals, and they likely consumed some vegetable matter, but their primary survival strategy was “kleptoscavenging.”  Like their distant North American cousins, the giant short-faced bear (Arctodus simus), Arctotheriums trotted long distances, covering grand territories, and with their keen sense of smell they could detect the scent of blood from miles away.  They followed their nose to the source and used their great size to intimidate big cats, wolves, and terror birds from the meat those successful carnivores had worked so hard to procure.

Size comparison between Arctotherium angustidens and Homo sapiens.  This was probably the biggest species of bear in earth’s history.

The bones of an old male Arctotherium were discovered in 1935 during the construction of an hospital in La Plata, Argentina.  The skeleton was not described in the scientific literature until 2010.  The scientists re-examing the specimen determined the individual suffered and recovered from several injuries, likely incurred from battling other male bears.  The old bear also had arthritis.  From the bones they estimated a mass of 3500 pounds.

By about 800,000 BP Arctotherium angistidens no longer occurs in the fossil record.  Some scientists suggest it became extinct due to competition from other predators, but I disagree with this notion.  Instead, I think they evolved into a more herbivorous bear because of environmental changes.  A large “kleptoscavenger,” like Arctotherium, can only survive in an environment with abundant prey animals and predators.  Grasslands can support this kind of biomass, but forests don’t.  I propose that cyclical climatic conditions caused South American grasslands to convert to woodlands.  Arctotherium angistidens was forced to eat more vegetable matter in this woodland environment and eventually evolved into different species, culminating in Arctotherium wingei.  The latter species was as herbivorous as its distant cousin, the still extant spectacled bear (Tremarctos ornatus) and was adapted to live in a similar forested environment, until it went extinct at the end of the Pleistocene.

Bears, cats, and wolves invaded South America when the Central American landbridge rose above sea level 3 million years ago.  The South American versions of bears and saber-tooths initially grew larger than their North American cousins, perhaps because prey animals were larger and slower here.

Arctotherium was in the same family as the North American short-faced bears which included the giant short-faced bear (Arctodus simus), the lesser short-faced bear (Arctodus pristinus), and the Florida spectacled bear (Tremarctos floridanus).  The lesser short-faced bear was probably ancestral to the giant short-faced bear.  The giant short-faced bear was mostly carnivorous but ate some plant matter, while the spectacled bear was mostly herbivorous but ate some animal matter.  A. pristinus was more of an omnivore than the other 2 species.  Fossils of the giant short-faced bear and the spectacled bear have been found at late Pleistocene sites in southeastern North America, but the latter is far more abundant.

Photo from a 1960 issue of Sports Illustrated of the hunter standing next to the taxidermic mount of the biggest wild polar bear ever recorded.  He shot and killed it in Alaska. It weighed 2210 pounds.  This was about the average size of the extinct giant short-faced bear.

Scrape marks on the side of Riverbluff Cave, Missouri.  They are 15 feet high and were made by the giant short-faced bear.  The bear stood 12 feet tall on its back legs and must have reached up with its arms over its head to reach that height.

Size comparison between the giant short-faced bear and a Yellowstone grizzly.

As the top above photo shows, the largest wild polar bear ever recorded was shot in Alaska in 1960.  It weighed 2210 pounds.  This was only slightly larger than an average male giant short-faced bear.  I’m sure a few individuals of the latter species exceeeded that size.  They reached that great size from a combination of genetics and a high protein diet.  Scientists estimated  the giant short-faced bear required 35 pounds of meat everyday.  In environments with fewer prey animals, they may not have reached a particularly large size.  Grizzly bears (Ursus arctos) are the exact same species as the Kodiak bear.  The former average about 500 pounds, yet the latter reach weights of 1500 pounds.  Kodiak brown bears enjoy a high protein diet rich in salmon, while grizzly bears are forced to survive on roots, berries, rodents, and only the occasional scavenged kill.  A Yellowstone grizzly cub, if moved to Kodiak Island, would probably grow much larger on a diet of all the salmon it could eat than it would if it stayed in Yellowstone.  The largest brown bear in captivity grew to 2100 pounds–a mass equal to the average weight of a giant short-faced bear.  This bear had a more favorable diet in its zoo environment than it would have in the wild.

See also: https://markgelbart.wordpress.com/2011/03/10/pleistocene-bears-of-southeastern-north-america/

Reference:

Soibelzan, L.H.; B.W. Schubert

“The Largest Known Bear, Arctotherium angustidens, from the early Pleistocene Pampan Region of Argentian with a Discussion of Size and Diet in Bears”

Journal of Paleontology 85 (1) 69-75 2011