The cave bear (Ursus spelaeus), an extinct species that roamed Europe during the Pleistocene epoch, has long fascinated paleontologists. Predominantly found in caves, as reflected in both its common and scientific names, these massive creatures were contemporaries of early humans, sharing their habitat until their extinction around 25,000 years ago. A key area of research focuses on reconstructing the cave bear's diet, aiming to understand the factors that contributed to its demise. This article delves into the dietary habits of the cave bear, synthesizing evidence from isotopic analyses, dental microwear studies, and morphological data to provide a comprehensive overview of its feeding ecology.
Cave Bear Taxonomy and Morphology
The cave bear was first described in 1774 by Johann Friedrich Esper, who initially mistook its remains for those of a polar bear. It wasn't until two decades later that Johann Christian Rosenmüller formally classified the species. Cave bear remains exhibit considerable variability in size and morphology, leading to the recognition of several subspecies or closely related species, including Ursus rossicus, Ursus ingressus, and Ursus kudarensis. These "cave bears" sensu lato display significant genetic divergence, suggesting a complex evolutionary history.
Both cave bears and brown bears descended from the Etruscan bear (Ursus etruscus) that lived from about 5.3 million years ago to 100,000 years ago. The last common ancestor of cave bears and brown bears lived between 1.2-1.4 Mya. Middle Pleistocene remains of the cave bear lineage are assigned to Ursus deningeri (Deninger's bear).
Cave bears were massive animals, comparable in size to the largest modern bears, reaching lengths of up to 2 meters. Males weighed between 350 and 600 kg, while females were smaller, weighing 225 to 250 kg. A common misconception is that most cave bear skeletons in museums are male.
Dental Adaptations and Dietary Inferences
Cave bear dentition provides crucial clues about their dietary preferences. Unlike other bears, cave bears of the last Ice Age lacked two or three premolars; to compensate, the last molar is very elongated, with supplementary cusps. The three anterior premolars were gradually reduced, then disappeared, possibly in response to a largely vegetarian diet. In a fourth of the skulls found in the Conturines, the third premolar is still present, while more derived specimens elsewhere lack it. The last remaining premolar became conjugated with the true molars, enlarging the crown and granting it more cusps and cutting borders. This phenomenon, called molarization, improved the mastication capacities of the molars, facilitating the processing of tough vegetation.
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Cave bear teeth were very large and show greater wear than most modern bear species, suggesting a diet of tough materials. However, tubers and other gritty food, which cause distinctive tooth wear in modern brown bears, do not appear to have constituted a major part of cave bears' diets on the basis of dental microwear analysis. Seed fruits are documented to have been consumed by cave bears.
Isotopic Evidence: A Predominantly Herbivorous Diet
Isotopic analysis of cave bear bones has revolutionized our understanding of their diet. Studies focusing on the isotope composition of collagen, a protein found in bones and teeth, reveal that cave bears primarily consumed plants. Collagen is an essential organic component of the connective tissue in bones, teeth, cartilage, tendons, ligaments and the skin. The examination of the isotope composition of individual amino acids in the collagen shows that the bears lived on a strictly vegan diet.
Scientists from Japan, Canada, Belgium and Germany conducted isotope studies on the collagen from the bears' bones.
The low values of δ15N are purportedly linked to a predominantly vegetarian diet. Conversely, Richards et al. and Robu et al. in Peştera cu Oase (Romania) show values of δ15N of the U. spelaeus that place it at the same level as contemporary carnivores, suggesting an omnivorous diet for this species. It is worth mentioning that decreasing δ15N values can result not only from reduced consumption of animal protein in the diet but also from variations in soil δ15N values due to climatic conditions linked with vegetation cover or by a higher amount of nitrogen-fixing plants in the animal’s diet. According to Fernández-Mosquera et al., δ15N values in nitrogen-fixing plants are lower than in plants that do not fix nitrogen. In addition, an analysis of bear blood revealed that the δ13C values during hibernation decrease, while the δ15N increases. Hence, bear species have an interesting and complex metabolism (aspects of which remain poorly understood but which need to be considered when interpreting isotopic data).
Dental Microwear: Complementary Insights
Dental microwear analysis (DMA) offers a complementary perspective on cave bear diets. By examining microscopic wear patterns on tooth surfaces, scientists can infer the types of food consumed in the weeks or months leading up to an animal's death. Dental microwear from the high altitude site of Ramesch-Knochenhöhle in the Totes Gebirge indicates that some cave bears living in mountains ingested large amounts of sand as a result of feeding on alpine vegetation. However, some evidence points toward the occasional inclusion of animal protein in cave bear diets.
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A study of the feeding habits of U. spelaeus that combined stable isotopes extracted from bone collagen and dental microwear compared to a wide variety of extant species of ursids has never been published before. This work was performed on the fossil remains of cave bear from the Toll Cave (NE Spain) which is located at Mediterranean climate latitudes. Moreover, the cave bear remains and other bones from this site have been radiocarbon dated. It should also be noted that information on Mediterranean latitudes is practically non-existent, and these can be interesting when contributing data of the diet of this animal in more temperate environments.
Regional Variations and Dietary Flexibility
While the general consensus points towards a predominantly herbivorous diet for cave bears, regional variations and dietary flexibility likely existed. The rapid climatic fluctuations that took place during the late Pleistocene led to repeated changes in both the environment and the vegetal landscape. These variations generated modifications in the biogeographical distribution of mammals in Europe.
The strongest correlation was observed between the δ15N value of bulk bone collagen and the δ15N value of phenylalanine that represents δ15N of the nitrogen source in a food web. This indicates that the bulk collagen δ15N values of the Romanian cave bears reported here can be explained by the following two possibilities, that are not mutually exclusive: (i) a δ15N shift of the baseline in this local ecosystem, and/or (ii) the consumption of some specific plants with high δ15N values by these cave bear individuals. This means that the reported high δ15N values in the Romanian cave bears’ bulk collagen can be explained by the exclusive consumption of plants rather than by the trophic level effect caused by consumption of animal meat.
Extinction and Dietary Specialization
The extinction of the cave bear around 25,000 years ago remains a topic of intense debate. Compared with other megafaunal species that also became extinct during the Last Glacial Maximum, the cave bear was believed to have had a more specialized diet of high-quality plants and a relatively restricted geographical range. This was suggested as an explanation as to why it died out so much earlier than the rest. Some experts have disputed this claim, as the cave bear had survived multiple climate changes prior to extinction.
Some evidence indicates that the cave bear used only caves for hibernation and was not inclined to use other locations, such as thickets, for this purpose, in contrast to the more versatile brown bear. This specialized hibernation behavior would have caused a high winter mortality rate for cave bears that failed to find available caves. Therefore, as human populations slowly increased, the cave bear faced a shrinking pool of suitable caves, and slowly faded away to extinction, as both Neanderthals and anatomically modern humans sought out caves as living quarters, depriving the cave bear of vital habitat. This hypothesis is being researched as of 2010. In addition to environmental change, human hunting has also been implicated in the ultimate extinction of the cave bear.
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