Ancient DNA reveals differences in behaviour and sociality between brown bears and extinct cave bears

Abstract

Ancient DNA studies have revolutionized the study of extinct species and populations, providing insights on phylogeny, phylogeography, admixture and demographic history. However, inferences on behaviour and sociality have been far less frequent. Here, we investigate the complete mitochondrial genomes of extinct Late Pleistocene cave bears and middle Holocene brown bears that each inhabited multiple geographically proximate caves in northern Spain. In cave bears, we find that, although most caves were occupied simultaneously, each cave almost exclusively contains a unique lineage of closely related haplotypes. This remarkable pattern suggests extreme fidelity to their birth site in cave bears, best described as homing behaviour, and that cave bears formed stable maternal social groups at least for hibernation. In contrast, brown bears do not show any strong association of mitochondrial lineage and cave, suggesting that these two closely related species differed in aspects of their behaviour and sociality. This difference is likely to have contributed to cave bear extinction, which occurred at a time in which competition for caves between bears and humans was likely intense and the ability to rapidly colonize new hibernation sites would have been crucial for the survival of a species so dependent on caves for hibernation as cave bears. Our study demonstrates the potential of ancient DNA to uncover patterns of behaviour and sociality in ancient species and populations, even those that went extinct many tens of thousands of years ago.

Morphometry of upper cheek teeth of cave bears (Carnivora, Ursidae)

“…; Fortes et al. ), we can assume that different versions of the structures of P4 and M2 are fixed in different populations. Against this background, the diversity of M1 becomes more noticeable and in several cases comparable with premolar and second molar diversity.…”

“…Another indirect confirmation of differences in the genetic and epigenetic regulation of the teeth parameters is associated with an increase in relative orderliness for P4 and M2 at the population level. Reasoning by analogy and taking into account the relatively sedentary life style of cave bears ( Abelov a 2006;Frischauf et al 2015;Fortes et al 2016), we can assume that different versions of the structures of P4 and M2 are fixed in different populations. Against this background, the diversity of M1 becomes more noticeable and in several cases comparable with premolar and second molar diversity.…”

“…; Robu ), and/or by philopatria that suggests limited movement of bears during their lifetime that imposes constraints on the gene flow (Ábelová ; Fortes et al. ).…”

“…It has been also found that sites of the same age are geographically grouped with regards to cave bear dental measurements. This phenomenon may be explained by regional peculiarities in the bear diet (Taboda et al 2001;Baryshnikov et al 2003Baryshnikov et al , 2004Bocherens et al 2014;Robu et al 2018) Rabeder et al 2008Rabeder et al , 2010Rabeder et al , 2011To skan & Bona 2012;Krajcarz et al 2016;Robu 2016), and/or by philopatria that suggests limited movement of bears during their lifetime that imposes constraints on the gene flow ( Abelov a 2006;Fortes et al 2016). Most studies of cave bear tooth morphology and morphometry, as well as those of their skull and postcranial elements, are based on geographically restricted material.…”

Optimized DNA sampling of ancient bones using Computed Tomography scans

“…The outermost layer of long bones is usually removed prior to sampling for ancient DNA in an effort to remove modern DNA contamination (Damgaard et al., ; Fortes et al., ; Hansen et al., ; Rohland & Hofreiter, ). While this idea is intuitively plausible and observed in the majority of samples analysed here, we found that for some bones, this is not the case.…”