How bears evolved herbivory...twice!


Skeleton of a brown bear (Ursus arctos) standing up, at the Zoological Museum of Naples. 

Bears (Ursidae) are an ecologically diverse and widely distributed group of carnivorans: originating in the Late Oligocene-Early Miocene, they would go on to colonize almost every continent, so that bears can be found in Eurasia, North and South America, and until recently, Africa (Galdies 2022; Luna-Aranguré & Vázquez-Domínguez 2024).

Bears have a long history in Europe, here we have on the left an Auvergne bear (Ursus minimus) from the Pliocene, and on the right an Etruscan bear (Ursus etruscus) from the Early Pleistocene, both from Italy and both displayed in the Museo Paleontologico di Montevarchi. They've both been posited as possible ancestors of cave and brown bears, with U. etruscus probably being the best candidate (see Galdies 2022), and U. minimus has also been regarded as the possible common ancestor of all extant members of Ursus (see Charters et al. 2024). U. etruscus is also among the bear species that used to roam Africa.

They range from species with a broad distribution and ecological plasticity, like the brown bear (Ursus arctos) and the American black bear (Ursus americanus), to specialists like the mainly myrmecophagous sloth bear (Melursus ursinus) and the polar bear (Ursus maritimus), specialized to the extreme cold and dry environments of the Arctic, to the giant panda (Ailuropoda melanoleuca), famously specialized to a diet of bamboo (Luna-Aranguré & Vázquez-Domínguez 2024).

A taxidermy Eurasian brown bear (Ursus arctos arctos) displayed at the Museo delle Scienze (MUSE), Trento. Brown bears were historically present throughout the Italian Alps, but through the years their numbers have kept diminishing (mainly because of human persecution), until in the late '90s only an estimated 3-4 individuals remained. Since then, bears have been reintroduced in the region from Slovenia, while a number of occasional natural returns from Austria and Slovenia also occurred. There are now around 100 individuals of the subspecies in Italy. You can read about all this here (in Italian), see also Groff & Mustoni (2018), also in Italian.

The black bear displayed at the Zoological Museum of Naples (a recent addition to the collection). Specifically, this is labelled as a Vancouver Island black bear (U. a. vancouveri), a subspecies sometimes recognized on the basis of a distinctly large skull (see Kubiak et al. 2022).

Despite their diet mainly consisting on ants and termites, sloth bears (here a taxidermy specimen at the Turin Museum of Natural History) are famously nasty, highly aggressive beasts standing their ground against tigers, and attack more people than any other bear (Sharp et al. 2024). Its myrmecophagous diet may actually have something to do with this, and I may return on this topic in the future.

Great size variation in polar bears, exemplified here on one hand by the large specimen displayed at the Turin Museum of Natural History, and on the other by the surprisingly small specimen displayed at the Zoological Museum of Naples (compare it with the arctic fox Vulpes lagopus it is displayed with). Polar bears are the largest modern species of bear, so seeing such a smallish specimen may be surprising. Note how furry the paws are (the paw pads are smaller than those of brown bears) for heat retention, as well as the short and curved claws, which apparently don't have a great role in providing grip on snow (Orndorf et al. 2022).

Still, most bears exhibit an omnivorous diet, and this creates a problem: while many other carnivoran lineages evolved omnivorous diets, including mustelids, mephitids and procyonids, most of them seem constrained to small to medium sizes, rarely exceeding 20 kg, probably because of the high energetic requirements of large size and inefficiency of the carnivoran digestive system in processing plant matter. 
It seems that large size in carnivorans is actually associated with hypercarnivory instead (Jiangzuo and Flynn 2020).

Compare the dentition of a brown bear to that of a grey wolf (Canis lupus), display at Museo delle Scienze (MUSE).

Bears are the only exception to this, being carnivoran omnivores that regularly surpass 100 kg. The ability to hibernate may have helped in saving energy, but this can't be the sole explanation, as hibernation is also practiced by badgers and skunks. Instead, Jiangzuo and Flynn (2020) proposed that bears, particularly those of the clade Ursinae (here used in its broader meaning, encompassing all modern bear species except the giant panda, which belongs to Ailuropodinae) developed a unique and more efficient antero-posterior masticatory pattern, accompanied by anteroposterior elongation of the posterior molars.

The same brown bear skeleton from the top of the article, from a different view: note the molars.

This, together with the ability to hibernate, would have increased the ability of ursines to process plant matter, allowing larger sizes (Jiangzuo and Flynn 2020).

The cave bear (Ursus spelaeus) is a prime example of very large ursine specialized to a diet of low-quality plant material, and to deal with the long Ice Age winters with long hibernation times: the characteristic "dome" on the forehead housed very enlarged sphenoidal sinuses, which is where enzymes that regulate hibernation are segregated. It also constrained the skull to process food solely with the posterior teeth (see Pérez-Ramos et al. 2020). 
This is a very beautiful display at the Natural History Museum Vienna, showing the skeleton of an adult cave bear together with two cubs of the same species, the animal almost contained by the outlines of the cave painting placed in the background, representing the same animal, almost as if in an anatomical illustration.

Interestingly, despite being among the most herbivorous carnivorans, giant pandas didn't develop the same specializations to herbivory, having a more restricted antero-posterior jaw mobility. Instead, they evolved a different set of adaptations, including a large and very robust skull with a high sagittal crest for the insertion of powerful jaw muscles, transversally widened molars, complex dental cusps and ridges, and a very low metabolic rate. Paradoxically, these adaptations may be less efficient compared to those evolved by ursines, though, as the giant panda, along with other herbivorous ailuropodines, never reached the same maximum sizes of ursines (Jiangzuo and Flynn 2020). 

Giant panda skeleton displayed at the Natural History Museum, London. Notice the very wide skull and the tall sagittal crest (compare it with the skull of a brown bear on the right in the photo).

On the other hand, you may be surprised about the past existence of giant, hypercarnivorous ailuropodines with cursorial adaptations, the Agriotheriini, which, from the early Late Miocene to the Pleistocene, spanned throughout North America, Eurasia, and Africa (Jiangzuo et al. 2023).
You heard that right: giant hypercarnivorous running pandas in Africa.

References 
  • Charters, D., Brown, R. P., Abrams, G., Di Modica, K., Pirson, S., De Groote, I., … Meloro, C. (2024). Mandibular ecomorphology in the genus Ursus (Ursidae, Carnivora): relevance for the palaeoecological adaptations of cave bears (U. spelaeus) from Scladina cave. Historical Biology, 1–15. https://doi.org/10.1080/08912963.2024.2377703
  • Galdies J., « The bears of the European steppe: a review », Quaternaire, vol.33/1 | 2022, 47-62.
  • Groff C. & Mustoni A., 2018 - Orso bruno. In: Deflorian M. C., Caldonazzi M., Zanghellini S. & Pedrini P. (a cura di), Atlante dei Mammiferi della provincia di Trento. Monografie del Museo delle Scienze, 6: 236-239.
  • Jiangzuo, Q., and J. J. Flynn. 2020. The earliest ursine bear demostrates the origin of plant-dominated omnivory in Carnivora. Iscience 23:1–21.
  • Jiangzuo, Q., J. J. Flynn, Wang, S., Hou, S., Deng, T., "New Fossil Giant Panda Relatives (Ailuropodinae, Ursidae): A Basal Lineage of Gigantic Mio-Pliocene Cursorial Carnivores," American Museum Novitates, 2023(3996), 1-71, (14 March 2023)
  • Kubiak C, Grimes V, Van Biesen G, Keddie G, Buckley M, Macdonald R, et al. Dietary niche separation of three Late Pleistocene bear species from Vancouver Island, on the Pacific Northwest Coast of North America. J Quaternary Science. 2022; 38: 8–20. https://doi.org/10.1002/jqs.3451
  • Luna-Aranguré, C., & Vázquez-Domínguez, E. (2024). Bears into the Niche-Space: Phylogeography and Phyloclimatic Model of the Family Ursidae. Diversity, 16(4), 223. https://doi.org/10.3390/d16040223
  • Orndorf N, Garner AM, Dhinojwala A. 2022 Polar bear paw pad surface roughness and its relevance to contact mechanics on snow. J. R. Soc. Interface 19: 20220466. https://doi.org/10.1098/rsif.2022.0466
  • Pérez-Ramos A, Tseng ZJ, Grandal-D’Anglade A, Rabeder G, Pastor FJ, Figueirido B. 2020. Biomechanical simulations reveal a trade-off between adaptation to glacial climate and dietary niche versatility in European cave bears. Sci Adv. 6(14):eaay9462. doi: 10.1126/sciadv.aay9462.
  • Sharp, T.R., D.L. Garshelis and W. Larson. A most aggressive bear: safari videos document sloth bear defense against tiger predation. Ecology and Evolution. 14, e11524. https://doi.org/10.1002/ece3.11524

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