Cave bears, Ursus spelaeus, are among the most emblematic animals of the Last Glacial in Europe. Their remains occur in the thousands in cave hibernacula stretching from the Iberian Peninsula to the Ural Mountains, mostly dating to Marine Isotopic Stage 3. Because of this abundance, cave bears have long been at the forefront of new techniques, from ancient DNA analysis to stable isotope studies of diet and cementum increment analysis of population structure. Yet one fundamental question, first raised in the 1920s, remained unresolved: how did cave bear cubs develop during their first year, and how did that development compare with their closest living relative, the brown bear?

Two competing models had divided researchers. The first, called the accelerated development or Ehrenberg-Veitschegger model, held that tooth eruption after den emergence was roughly the same in both species, but that skeletal growth was faster in cave bears. The second, the delayed development or Debeljak model, proposed the opposite: skeletal growth was similar, but tooth eruption was delayed in cave bears. The crux of the disagreement boiled down to whether cubs in the third and oldest mortality group died during the active season or during their first hibernation. Only semi-articulated skeletons, in which dental, cranial and postcranial development can be assessed in the same individual, could resolve the question, and such specimens are exceedingly rare.

The new study examined seven partially preserved cub skeletons. Four come from Barová Cave, all recovered from the Komínový dóm excavation pit, where more than ninety percent of macrofaunal remains belong to cave bears. A lower canine from this pit yielded the only finite radiocarbon age for a cave bear there, 46,518 to 45,055 calibrated years before present. The three Goyet skeletons were found in layer 4 of chamber B, an assemblage interpreted as a cave bear hibernaculum largely unaffected by other taphonomic processes, with radiocarbon dates between roughly 40,087 and 41,043 calibrated years before present. The Goyet cave bears carry the spelaeus mitochondrial haplotype, while the Barová bears belong to the ingressus group, which the authors, following recent nuclear ancient DNA work, treat as a single broadly defined species.

To age the cubs, the researchers used the sequence and timing of tooth development and eruption in captive brown bears described by Dittrich in 1960, defining what they call the brown bear equivalent dental age, or BEDA. The Barová skeletons yielded BEDA estimates of approximately two months, three to four months, and eight months, while the Goyet skeletons corresponded to a neonate of a few days to one week, a cub of about 2.5 months, and a cub of roughly eight months. The oldest Goyet cub, previously estimated at about seven months, was judged slightly underestimated. Two previously published skeletons from Austrian caves, from Bärenhöhle with a BEDA of about six months and from Salzofenhöhle aged eight to twelve days, completed the comparative framework.

With these anchor points established, the team analysed hundreds of isolated juvenile long bones from both caves, including humeri, radii, ulnae, femora and tibiae, measuring maximum length and the widths of the diaphysis ends. The same pattern emerged in every bone type and at both localities: three distinct mortality groups separated by gaps. The first group represents neonates that died within days of birth, whose long bone dimensions match those of newborn brown bears remarkably closely. The second group comprises cubs with a BEDA of roughly two to three months, most probably pre-emergence cubs still nursing in the den. The third group, separated from the second by the most prominent gap in the samples, contains cubs with a BEDA of around six to eight months, sometimes extending down to six months.

The critical evidence came from comparing the Bärenhöhle cub with Barová skeleton no. 1. Clear development occurred in both cranial and postcranial elements between these two ontogenetic stages, which would not have been possible during hibernation. Growth arrest lines in bear bones form once per year, associated with the interruption of long bone growth, and studies of recent brown bears show no such lines in cubs of eight to nine months but one line in eighteen-month-old cubs, consistent with the first line forming during the first hibernation. Moreover, during hibernation new proteins must be derived from lean mass rather than fat, making large-scale tissue construction implausible. The authors therefore rejected the delayed development model and accepted the accelerated model, interpreting the brown bear equivalent dental age as the real age for the third mortality group.

The comparison also revealed how cave bear cubs achieved their larger dentition. At a BEDA of six months, cave bear cubs were only slightly larger than brown bear cubs but had more massive long bones and clearly larger skulls, with the need to accommodate larger molariform teeth solved by enlargement of the whole skull rather than just the facial part. By eight months, cave bear cubs were larger than brown bears of the same dental age, with differences far more pronounced in bone width than in length. Because cave bears were herbivores requiring larger occlusal surfaces on their cheek teeth, and because cubs must process adult diet during the feasting season to accumulate fat reserves before denning, the authors argue that having the first molars in occlusion was a minimum requirement for surviving the first hibernation.

Perhaps the most surprising result is the geographic consistency of the mortality pattern. The three mortality peaks recorded at Barová and Goyet match published profiles from Slovenia, Spain, Germany, Austria, France and the Urals, spanning different periods within Marine Isotopic Stage 3 and populations carrying different mitochondrial haplotypes. Field studies of recent brown bears and American black bears show that the most critical period for cub survival is not the first hibernation but the active season after den emergence, particularly the mating season, when sexually selected infanticide and starvation dominate. The authors therefore propose that cave bear cubs aged six to eight months mostly died during their first active period, probably as a consequence of a failed transition to adult diet during weaning, possibly linked to limited food sources. Cubs that successfully reached adult-like isotopic values during their first summer appear to have managed the first hibernation and escaped this mortality peak.

The study also raises new questions about cave bear reproductive biology. Cave bear cubs were born during hibernation, as shown by isotopic data, and were born at approximately the same size as brown bear cubs despite their much larger mothers, a pattern the authors attribute to later evolutionary increase in adult body size retaining an ancestral neonate size. If weaning occurred earlier than in brown bears, as some researchers have proposed, the biological advantage is unclear, since cave bear cubs would have remained costly to raise. One possibility is that early weaning allowed some females to breed in the same year they gave birth, producing mixed-aged litters that are rare in recent bears, although cementum evidence suggests cave bear females did not produce cubs every year. Whatever the resolution, the new skeletons from Barová and Goyet demonstrate that the first year of life, from birth through weaning to the eve of the first hibernation, was the decisive gauntlet in the life of a cave bear.

Subject of Research: Early ontogeny and first-year mortality patterns in cave bear cubs based on skeletons from Barová and Goyet caves

Article Title: Cave bear cub skeletons from Barová (Czech Republic) and Goyet (Belgium) caves contribute to our understanding of cave bear palaeobiology

Article References: Plichta, A., Wagner, J., Dubjelová, N., Roblíčková, M., Jiangzuo, Q., & Ivanov, M. (2026). Cave bear cub skeletons from Barová (Czech Republic) and Goyet (Belgium) caves contribute to our understanding of cave bear palaeobiology. Fossil Record, 29(2), 787-816. https://doi.org/10.3897/fr.29.204422

Image Credits: AI Generated

DOI: 10.3897/fr.29.204422

Keywords: cave bear, Ursus spelaeus, palaeobiology, Barová Cave, Goyet Cave, ontogeny, hibernation, mortality pattern, dental age, long bones, Pleistocene, Moravian Karst