The pole-warming effects of ongoing climate change and the position of polar bears (Ursus maritimus) as top predators of the Arctic ecosystem meet at a precarious nexus, making this large mammal a modern day “canary in the mine.” Petherick et al. (2021) take a pulse of polar bear diet by examining key signatures in the microscopic attritional patterns on the surface of polar bear dental enamel. Through the novel application of a well-developed tool, they provide a four-dimensional perspective on how Arctic climate change has affected polar bear food choices through time.

The bear family, Ursidae, includes eight living species found mostly in the northern continents and South America. Like other large-bodied members of Carnivora, the mammal group that includes most large terrestrial top predators and their relatives, bears have borne the brunt of the effects of ecosystem shifts set in motion by prehistoric and ongoing climate change and habitat destruction. Local extinctions or extirpations have already been evidenced in bears such as the grizzly (Ursus arctos) in California and giant pandas (Ailuropoda melanoleuca) in southeast Asia, plus several prehistoric ursids that were among the casualties of end-Pleistocene megafaunal extinctions (the short-faced bear Arctodus and cave bears in the Ursus spelaeus lineage) across the Holarctic.

It is with this backdrop that polar bears have come into focus as a vulnerable ursid in an equally vulnerable environment. Previous research have shown that polar bears are under attack from multiple threats to their population and ecological viability, from genetic bottleneck (Miller et al., 2012) to a masticatory system specialized for the soft tissue-based diet they have adapted to consuming only in the last 600,000 years or so (Slater et al., 2010). Thus, data that can be used to monitor shifts in polar bear food choices are important for understanding ongoing changes in their ecology as Arctic sea ice continues to melt.

Challenges associated with monitoring polar bear diet are several. Conventional tagging/tracking has proven to be difficult in the Arctic environment (Durner et al., 2009). Ad hoc observations of polar bear kill or feeding sites offer a firsthand but partial view of dietary changes (Brook & Richardson, 2002). On top of these challenges, longitudinal changes in polar bear diet are not easily deciphered, because most historical collections of polar bear specimens comprise processed and prepared skeletons and skins that are not amenable to all types of analyses or are too rare to warrant destructive analysis. A nondestructive methodology that directly measures dietary shifts in both modern and historical polar bear specimens is desired to advance our understanding of the ongoing effects of climate change on polar bear populations.

One such tool is dental microwear texture analysis (DMTA), used by Petherick et al. (2021) to quantify dietary signatures on polar bear dental enamel to test hypotheses about their dietary shifts through time. Dental enamel microwear methodology has its roots in mammalian herbivore dietary analysis (Walker et al., 1978). In conventional dental microwear analysis, 2D images of tooth surfaces generated by scanning electron microscopy (SEM) or specimens examined under stereomicroscopes represent main data sources; the analysis involves counting scratches and pits and summarizing the numbers and ratios of those features on the spectrum of grazers to browsers. Conventional microwear analysis is relatively simple to conduct on museum specimens of varying conditions, but the method requires quantification of interobserver error to make data comparable across studies and researchers.

DMTA provides two major advances over conventional microwear analysis. First, interobserver error is minimized by sampling entire sections of the enamel surface using confocal microscopy and computed scale-sensitive indices of surface topography rather than subjective counts of scratches and pits that vary in appearance at different magnifications. Second, confocal microscopic images could be captured in three dimensions, providing not only planar textural properties, but also measurements of topographic relief and heterogeneity. Textural measurements such as surface roughness (Asfc), uniformity of surface feature directions (epLsar), and relative size of attritional features (tfv) replace counts of large and small scratches and pits. These indices summarize physical signatures of the masticatory environment (including both ingested food and nonfood environmental particulates as well as tooth–tooth contact) as recorded on the dental enamel surface.

The application of DMTA to polar bear research is timely and important for both practical and heuristic reasons. Polar bears are both the largest living mammalian predators that spend the majority of their time on land and one of few terrestrial predators that preferentially feed on other carnivores (DeMaster & Stirling, 1981). Their unique niche makes them important model organisms for understanding tertiary consumer interactions within food webs, but it also means that their ecosystem services cannot be readily replaced by other predators. The polar bear lineage is also a relict lineage in a sense, considering their origin during the Pleistocene glacial–interglacial intervals alongside other large or very large bears of the northern continents (e.g., short-faced bears, cave bears, grizzly bears), but being the only species of that Ice Age group that is not extinct or extirpated from most of their historical geographic range. Lastly, in macroevolutionary terms, specialist carnivore lineages can exhibit reduced evolvability during periods of abrupt environmental change (termed the “macroevolutionary ratchet”; Van Valkenburgh, 2007), thus polar bears may be predisposed to exacerbated consequences of rapid, anthropogenic environmental perturbations given their specialized dietary niche.

Putting these polar bear characteristics into context, findings reported by Petherick et al. (2021) are consistent with a scenario where hard-food consumption by Alaskan polar bears remained low throughout historical times despite fluctuations in Arctic temperatures from 1000 years ago to the end of the 20th century. The lack of a significant hard-food signal in DMTA data in pre-21st century polar bear dental enamel suggests a history of feeding ecology that operated within the bounds of polar bear skull functional morphology (Slater et al., 2010). This makes it all the more alarming when the authors also report a statistically significant increase in hard-food signal from DMTA data in 21st century polar bear specimens. These results are reminiscent of what was thought to be similarly tough times for Late Pleistocene predators. One hypothesis postulates that the masticatory system of late Pleistocene dire wolves was mismatched for the magnitude of end-Pleistocene environmental deterioration and decreased prey availability compounded with increased competition, based on elevated tooth breakage frequency (Van Valkenburgh & Hertel, 1993). However, DMTA signatures in dire wolves do not show increases in hard-food signal relative to modern gray wolves in a bulk, time-averaged sample (DeSantis et al., 2015). It would be interesting to see if additional lines of evidence such as gross-level tooth breakage patterns are consistent with DMTA data in polar bears as their diet shifts landward to harder and tougher foods.

Continued deterioration of Arctic sea ice and therefore prime polar bear and prey habitat push polar bear populations into an uncertain future. The extent to which polar bear populations undergo selection for their masticatory ability to incorporate more terrestrial (i.e., harder) food sources will be pitted in a race against increased niche overlap between polar and grizzly bear habitats as the former shifts their home ranges inland toward those of the latter. Will polar bear populations have to rely on hybridization with grizzly bears as a means of adaptation to a changing environment, will they successfully shift their hunting strategy to reduce competition with their sister species, or will they become one more casualty in the ongoing extinction event that is accelerating in this century? No matter the outcome, the future of polar bear populations is inextricably tied to how the broader momentum toward a “tipping point” for Earth's biota is managed here and now.