Pitfalls in comparing modern hair and fossil bone collagen C and N isotopic data to reconstruct ancient diets: a case study with cave bears (Ursus spelaeus)

Coupled social and ecological change drove the historical extinction of the California grizzly bear (Ursus arctos californicus)

Large carnivores (order Carnivora) are among the world's most threatened mammals due to a confluence of ecological and social forces that have unfolded over centuries. Combining specimens from natural history collections with documents from archival records, we reconstructed the factors surrounding the extinction of the California grizzly bear (Ursus arctos californicus), a once-abundant brown bear subspecies last seen in 1924. Historical documents portrayed California grizzlies as massive hypercarnivores that endangered public safety. Yet, morphological measurements on skulls and teeth generate smaller body size estimates in alignment with extant North American grizzly populations (approx. 200 kg). Stable isotope analysis (δ13C, δ15N) of pelts and bones (n = 57) revealed that grizzlies derived less than 10% of their nutrition from terrestrial animal sources and were therefore largely herbivorous for millennia prior to the first European arrival in this region in 1542. Later colonial land uses, beginning in 1769 with the Mission era, led grizzlies to moderately increase animal protein consumption (up to 26% of diet), but grizzlies still consumed far less livestock than otherwise claimed by contemporary accounts. We show how human activities can provoke short-term behavioural shifts, such as heightened levels of carnivory, that in turn can lead to exaggerated predation narratives and incentivize persecution, triggering rapid loss of an otherwise widespread and ecologically flexible animal.

High δ13C values in red squirrels Sciurus vulgaris explained by a reliance on conifer seeds

Eurasian red squirrels Sciurus vulgaris have unusually high δ¹³C values compared to other forest rodents, which is seemingly related to the consumption of ¹³C-enriched conifer seeds. To test this hypothesis, we analyzed the hair of wild and zoo-kept red squirrels, seeds of pine and spruce, and feathers of specialized and opportunistic consumers of conifer seeds, crossbills Loxia spp., and woodpeckers Dendrocopos major. Data on the isotopic composition (δ¹³C and δ¹⁵N values) of hair or feathers of other species of forest rodents and birds were obtained from published studies. The range of mean δ¹³C values of hair of wild forest rodents (19 species) exceeded 10 ‰. All squirrel species had high ¹³C content, S. vulgaris having maximum δ¹³C values. In contrast, S. vulgaris kept in captivity had an isotopic composition similar to other captive rodents. The feathers of crossbills were enriched in ¹³C compared to other forest birds (15 species), while seeds of coniferous trees often had higher δ¹³C values compared to seeds of other woody plants. Distinctiveness of the isotopic composition of mammals and birds feeding on the seeds of coniferous trees suggests that this resource can be traced through the entire forest food web.

Carbon-isotope composition of artiodactyl tooth enamel and its implications for paleodiets

The stable carbon-isotope composition of mammalian tooth enamel is a powerful tool for reconstructing paleodiet and paleoenvironment. Its application in the fossil record relies on a thorough understanding of the isotopic composition of mammalian diets in modern ecosystems. We compiled and evaluated a global dataset of the carbon-isotope values of artiodactyl tooth enamel, supplemented by new samples, for 79 extant species. After correcting for differences in atmospheric carbon-isotope composition, body mass, and digestive physiology, we compared the inferred carbon-isotope values of ingested forage (δ13Cdiet) among seven feeding categories. The artiodactyl herbivore dietary spectrum is expressed through a wide range of δ13Cdiet values, with the most depleted mean value in frugivores and the most enriched in obligate grazers. In general, grazing species have a broader range of isotope values than browsing species, suggesting a wider dietary niche breadth. Notably, variable grazers exhibit a bimodal distribution of δ13Cdiet values, with North American and Asian taxa consuming C3 diets and African taxa consuming C4 diets, reflecting the amount of C4 vegetation in the environment. Variation in δ13Cdiet values also occurs among terrestrial ecoregions and artiodactyl clades. Grassland ecoregions differ significantly from forest ecoregions. We detected a low but significant phylogenetic signal in the mean δ13Cdiet values of extant species, with some of the oldest ruminant lineages having maintained C3 feeding and pure C4 diets being restricted to two bovid clades. Determining variation in δ13Cdiet values in different feeding categories and lineages will help refine paleoecological and paleoenvironmental reconstructions from the rich fossil record of artiodactyls.

Heavy reliance on plants for Romanian cave bears evidenced by amino acid nitrogen isotope analysis

Heavy reliance on plants is rare in Carnivora and mostly limited to relatively small species in subtropical settings. The feeding behaviors of extinct cave bears living during Pleistocene cold periods at middle latitudes have been intensely studied using various approaches including isotopic analyses of fossil collagen. In contrast to cave bears from all other regions in Europe, some individuals from Romania show exceptionally high δ15N values that might be indicative of meat consumption. Herbivory on plants with high δ15N values cannot be ruled out based on this method, however. Here we apply an approach using the δ15N values of individual amino acids from collagen that offsets the baseline δ15N variation among environments. The analysis yielded strong signals of reliance on plants for Romanian cave bears based on the δ15N values of glutamate and phenylalanine. These results could suggest that the high variability in bulk collagen δ15N values observed among cave bears in Romania reflects niche partitioning but in a general trophic context of herbivory.

Stable isotopes of H, C and N in mice bone collagen as a reflection of isotopically controlled food and water intake. ISOTOPES IN ENVIRONMENTAL AND HEALTH STUDIES 2019;55:129-149. [PMID: 30793970 DOI: 10.1080/10256016.2019.1580279]

²H/¹H ratios in animal biomass reflect isotopic input from food and water. A 10-week controlled laboratory study raised 48 mice divided in two generations (8 mothers Mus musculus and their offspring). The mice were divided into four groups based on the combination of ²H, ¹³C, ¹⁵N-enriched and non-enriched food and water. Glycine, the most common amino acid in bone collagen, carried the ²H, ¹³C, ¹⁵N-isotopic spike in food. ANOVA data analysis indicated that hydrogen in food accounted for ∼81 % of the hydrogen isotope inventory in collagen whereas drinking water hydrogen contributed ∼17 %. Air humidity contributed an unspecified amount. Additionally, we monitored ¹³C and ¹⁵N-enrichment in bone collagen and found strong linear correlations with the ²H-enrichment. The experiments with food and water indicate two biosynthetic pathways, namely (i) de novo creation of non-essential amino acids using hydrogen from water, and (ii) the integration of essential and non-essential amino acids from food. The lower rate of isotope uptake in mothers' collagen relative to their offspring indicates incomplete bone collagen turnover after ten weeks. The variance of hydrogen stable isotope ratios within the same cohort may limit its usefulness as a single sample proxy for archaeological or palaeoenvironmental research.

Microwear and isotopic analyses on cave bear remains from Toll Cave reveal both short-term and long-term dietary habits

Dietary habits of the extinct Ursus spelaeus have always been a controversial topic in paleontological studies. In this work, we investigate carbon and nitrogen values in the bone collagen and dental microwear of U. spelaeus specimens recovered in Level 4 from Toll Cave (Moià, Catalonia, NE Iberian Peninsula). These remains have been dated to > 49,000 ¹⁴C BP. The ability of both proxies to provide data on the diet of U. spelaeus at different times in the life-history (isotopes: average diet of life; microwear: last days/weeks before death), allows us to generate high-resolution and complementary data. Our results show lower values (δ¹³C & δ¹⁵N) in cave bears than in strict herbivores (i.e. Cervus elaphus) recovered from the same level of Toll Cave. On the other hand, 12 lower molars (m1) were analysed through low-magnification microwear technique. The cave bears from Toll Cave show a microwear pattern like that of extant bears with omnivorous and carnivorous diets. These data are discussed in the framework of all available data in Europe and add new information about the plasticity of the dietary habits of this species at the southern latitudes of Europe during Late Pleistocene periods.

Evolutionary history and palaeoecology of brown bear in North-East Siberia re-examined using ancient DNA and stable isotopes from skeletal remains

Over 60% of the modern distribution range of brown bears falls within Russia, yet palaeoecological data from the region remain scarce. Complete modern Russian brown bear mitogenomes are abundant in the published literature, yet examples of their ancient counterparts are absent. Similarly, there is only limited stable isotopic data of prehistoric brown bears from the region. We used ancient DNA and stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopes retrieved from five Pleistocene Yakutian brown bears (one Middle Pleistocene and four Late Pleistocene), to elucidate the evolutionary history and palaeoecology of the species in the region. We were able to reconstruct the complete mitogenome of one of the Late Pleistocene specimens, but we were unable to assign it to any of the previously published brown bear mitogenome clades. A subsequent analysis of published mtDNA control region sequences, which included sequences of extinct clades from other geographic regions, assigned the ancient Yakutian bear to the extinct clade 3c; a clade previously identified from Late Quaternary specimens from Eastern Beringia and Northern Spain. Our analyses of stable isotopes showed relatively high δ¹⁵N values in the Pleistocene Yakutian brown bears, suggesting a more carnivorous diet than contemporary brown bears from Eastern Beringia.

Applications of stable isotope analysis in mammalian ecology

In this editorial, we provide a brief introduction and summarize the 10 research articles included in this Special Issue on Applications of stable isotope analysis in mammalian ecology. The first three articles report correction and discrimination factors that can be used to more accurately estimate the diets of extinct and extant mammals using stable isotope analysis. The remaining seven applied research articles use stable isotope analysis to address a variety of wildlife conservation and management questions from the oceans to the mountains.