Short-faced bear – Arctodus spp.

[Deleted]

Arctodus simus was without any doubt the largest land mammal carnivore of North America during the Pleistocene and we are quite confident now that the largest specimens were more than 1000kg in mass.
In term of absolute dimensions, its bones are longer and wider than almost all others mammal carnivores. And concerning the robustness of its bones, this bear was more robust than previously asserted by some authors:

The limb bones of Arctodus simus are proportionally longer than those of other bears, leading to a “gracile” appearance, however, it still overlaps with Ursus and the limb bones are stouter than in the large-bodied felids (Panthera). Well-developed humerus epicondyles and a wide range of ulna rotation support an inference that Arctodus was powerful and could subdue large prey, unlike the suggestion of Sorkin (2006) that this genus has a weak medial epicondyle and reduced development of the pronator teres muscle.

Source: New Fossil Giant Panda Relatives (Ailuropodinae, Ursidae): A Basal Lineage of Gigantic Mio-Pliocene Cursorial Carnivores.

[Methane]

Jul 3, 2023 20:10:53 GMT 5 @hibernours said:

Arctodus simus was without any doubt the largest land mammal carnivore of North America during the Pleistocene and we are quite confident now that the largest specimens were more than 1000kg in mass.
In term of absolute dimensions, its bones are longer and wider than almost all others mammal carnivores. And concerning the robustness of its bones, this bear was more robust than previously asserted by some authors:

The limb bones of Arctodus simus are proportionally longer than those of other bears, leading to a “gracile” appearance, however, it still overlaps with Ursus and the limb bones are stouter than in the large-bodied felids (Panthera). Well-developed humerus epicondyles and a wide range of ulna rotation support an inference that Arctodus was powerful and could subdue large prey, unlike the suggestion of Sorkin (2006) that this genus has a weak medial epicondyle and reduced development of the pronator teres muscle.

Source: New Fossil Giant Panda Relatives (Ailuropodinae, Ursidae): A Basal Lineage of Gigantic Mio-Pliocene Cursorial Carnivores.In

Interesting, do you know how it compare to other ursids in robusticity?

[Deleted]

In fact, when we talk about robustness we must of course specify which bones we are comparing and it would take some times to do a really rigourous study because there are a lot of bear species, extant or extinct and they have different proportions. In general, bears are very robust and the extinct forms were the largest and the most powerful prehistoric land mammal carnivores that we know from good remains. Maybe some creodonts could rivalize in size but unfortunately we don't have enough data on their body proportions. In addition, each individual inside a species can have different proportions.
For example, if we compare the robustness of the humerus, Ursus arctos has a more robust humerus than a short faced bear but the difference is not extreme, sometimes they overlap.
On the other side, if you compare the skulls of these two species, then you can see that Arctodus simus had a skull much more robust than any extant bear and any extant or extinct big cats.
It depends on the bone you compare.

[Infinity Blade]

Multiple isotope analyses on Beringian and British Columbian specimens suggest a substantial amount of terrestrial meat consumption, especially caribou, so it was indeed eating meat. However, Arctodus wasn't only eating caribou. Relatively high values in some Arctodus specimens also suggests predation on muskoxen, and even killing and eating other predators. Kleptoparasitism, scavenging, and consumption of plant matter have also been suggested (Bocherens, 2015; Mychajliw et al., 2020).

The isotopic values published in Bocherens et al. (1995a), Matheus (1995), and Fox-Dobbs et al. (2008) were clearly in the range of coeval predators. These results were interpreted as reflecting a strongly carnivorous diet, in which meat acquisition may have been mainly performed through scavenging rather than active hunting (Matheus, 2003). The isotopic results published by Matheus (1995), combined with those from Fox-Dobbs et al. (2008), including a range of potential prey species such as mammoth, bison, horse and reindeer, allowed to estimate the relative contribution of these prey species in the meat consumption of short-faced bears (Fig. 13). Using the mixing model Isosource, Fox-Dobbs et al. (2008) concluded that short-faced bears, showing relatively high δ¹³C values, were specialized on reindeer. However, when isotopic data from Matheus (1995) are included, the short-faced bear individuals with the highest δ¹⁵N values could not eat only reindeer and they had to include prey with higher δ¹⁵N values than the analysed reindeer. Recently published isotopic data for muskox in Yukon show that some individuals of this species had high δ¹³C and δ¹⁵N values (up to 7.7‰; Raghavan et al., 2014). Such ungulates could have been preyed upon by such short-faced bears to explain their high δ¹³C and δ¹⁵N values. Another possibility is the consumption of some meat from other carnivores, such as wolves or Homotherium, a possibility that would support the hypothesis of klepto-parasitism of short-faced bears upon coeval predators, chasing them from the carcasses of their prey (Matheus, 1995), while isotopic values suggest that some individuals may sometimes kill other predators and consumed them as well.

The hypothesis that giant short-faced bears were specialized predators or scavengers has been challenged, based on ecomorphological investigations of this species (Figueirido et al., 2010). The high δ¹³C and δ¹⁵N values of A. simus prove that short-faced bears were not exclusively herbivores, but this does not mean that they did not also eat plant food. As explained previously, the relationship between the proportion of meat versus plant food and δ¹⁵N values of the consumer collagen is not a linear one and bears with δ¹³C and δ¹⁵N values similar to those of carnivores may in fact have consumed up to 50% plant food (Fig. 5). Therefore an opportunistic foraging strategy including plant food up to 50% and meat of reindeer, muskox and possibly some predators, seems realistic, and consistent with the isotopic data and the conclusions of the ecomorphological studies.

The rich record of Rancho La Brea allows us to contextualize this species within its contemporaneous Late Pleistocene ecosystem (Fig. 5). The CCI individual exhibits a larger nitrogen value than all other co-occurring mammalian and avian predators, and most closely resembles the California condor (Gymnogyps californianus), which in the Late Pleistocene consumed a diet of 20–30% marine protein40. The A. simus δ¹³C value (− 17.9‰), coupled with mixing model results (Supplemental Fig. 1), indicate similar partial marine resource use for this individual, which may have sustained short-faced bears in the presence of a much denser megafaunal carnivore guild. Compared with CCI predators in the Holocene, A. simus shows more marine resource use than the island fox¹² (Urocyon littoralis, δ¹³C − 18.34 ± 1.10‰, δ¹⁵N 9.51 ± 1.95‰), but less than humans^11,51 (δ¹³C − 14.09 ± 0.76‰, δ¹⁵N 16.10 ± 1.68‰) and bald eagles (δ¹³C − 13.88 ± 1.55‰, δ¹⁵N 16.79 ± 1.84‰)⁵². Though we are limited to a single individual and more study is required to understand population-wide dietary trends, we tentatively suggest that A. simus was an opportunistic scavenger of both marine and terrestrial carcasses across a wide home range, aligning with reconstructions based on Beringian specimens¹⁸. This increased resolution into the basic natural history of A. simus, particularly its plausible use of a coastal environment, permits us to more robustly address our biogeographic hypotheses.