Climate Change Is Freeing Ancient Infections From Their Icy Prisons
- Thread starter B00Mer
- Start date
Scientific Reports
Nature Publishing Group
Solving the woolly mammoth conundrum: amino acid 15N-enrichment suggests a distinct forage or habitat
Rachel Schwartz-Narbonne, Fred J. Longstaffe, [...], and Grant Zazula
Additional article information
Abstract
Understanding woolly mammoth ecology is key to understanding Pleistocene community dynamics and evaluating the roles of human hunting and climate change in late Quaternary megafaunal extinctions. Previous isotopic studies of mammoths’ diet and physiology have been hampered by the ‘mammoth conundrum’: woolly mammoths have anomalously high collagen δ15N values, which are more similar to coeval carnivores than herbivores, and which could imply a distinct diet and (or) habitat, or a physiological adaptation. We analyzed individual amino acids from collagen of adult woolly mammoths and coeval species, and discovered greater 15N enrichment in source amino acids of woolly mammoths than in most other herbivores or carnivores. Woolly mammoths consumed an isotopically distinct food source, reflective of extreme aridity, dung fertilization, and (or) plant selection. This dietary signal suggests that woolly mammoths occupied a distinct habitat or forage niche relative to other Pleistocene herbivores.
Woolly mammoths (Mammuthus primigenius) were keystone herbivores in the Pleistocene mammoth steppe1,2. This megacontinental biome was inhabited by a now-extinct community of mammals, dominated by woolly mammoth, horse and bison. The mammoth steppe reached from north-western Canada, across the exposed Bering Isthmus, to Western Europe3. The ecological role of woolly mammoths within this ecosystem has been a subject of vigorous investigation3,4,5. Reconstructions of woolly mammoth behaviour and physiology have been largely based on morphology4. Isotopic studies of bulk tissues have provided independent tests of morphology-based hypotheses, as well as suggesting new ones3,6,7,8,9. Compound-specific isotopic studies can provide a further level of understanding of ecosystem functioning within the mammoth steppe.
Bulk collagen nitrogen isotopic compositions (δ15NBulk) are commonly used in ecological studies to reveal the diet and trophic level of a species, as these values typically reflect the isotopic compositions of the plants at the base of the food web plus a 2–5‰ increase with each trophic level10. As a result, the δ15NBulk values of mammoth-steppe herbivore collagen are commonly ~+6‰ where the values of carnivores (~+9‰) are higher, reflective of this trophic enrichment3. The carnivore- rather than herbivore-like δ15NBulk values of woolly mammoth collagen (~+8‰3) are seemingly problematic and therefore require examination. The various hypotheses to explain this phenomenon (unique diet, niche feeding in a special habitat or distinct metabolic processes3,6,7,8,9,11,12,13,14) have different implications for our understanding of the now-vanished mammoth steppe ecosystem, woolly mammoth ecology, and related factors that contributed to extirpation of the woolly mammoth in this region.
Woolly mammoths may have consumed plants with higher δ15N values, such as graminoids and herbs rather than woody vegetation7,12,13, as suggested by the morphology of their enamel plates4. However, an herbaceous diet alone is not sufficient to fully explain the woolly mammoth’s high δ15N values; some further form of habitat or plant selection is also required15. While modern Arctic graminoids and forbs from some sites have a δ15N range of –0.3 to +10‰, the average value of these species ranges from ~+1 to ~+4‰16, and still other studies have reported maximum δ15N values for modern sedges of +2‰17 and for modern herbs of +5.3‰18. The majority of these plants, therefore, are not sufficiently enriched in 15N to explain the woolly mammoth δ15NBulk values. Plants growing in drier habitats, however, have higher δ15N values than plants from a more mesic environment7, and woolly mammoths may have eaten plants experiencing water-stress3,8. Water stress can also cause 13C-enrichment of plants19. This enrichment, however, is unlikely to be directly observable in woolly mammoth collagen, because its carbon isotopic composition is likely dominated by the low δ13C values of fat reserves used to survive the winter7.
Several other factors could also have contributed to high δ15N values for woolly mammoth collagen. Woolly mammoths that had small ranges, or repeatedly travelled the same routes, could have deposited significant quantities of faeces in those areas12, causing 15N-enrichment in plants arising from this dung fertilization20. Partially decayed plant material can also have higher δ15N values than the original living plant21. Woolly mammoths may have removed snow and ice cover by trampling and (or) with their tusks5, allowing them to forage on winter-killed plants generally not utilized by other large herbivores that did not share the mammoth ecological niche. It has also been proposed that woolly mammoths had distinct metabolic processes, such as increased levels of nitrogen recycling associated with winter starvation22,23 or poor quality food with low protein levels3,7,8,9,11,13,14,24, or that woolly mammoths engaged in coprophagy12.
The nitrogen isotopic compositions of the individual amino acids in collagen, as opposed to bulk collagen, enable discrimination between 15N-enrichment occurring at the base of the food chain prior to consumption (source amino acids) versus that associated with metabolic processes (trophic amino acids) (Fig. 1). Phenylalanine (Phe) and glutamate (Glu) have been identified as characteristic of source and trophic amino acids, respectively25,26. The δ15NPhe value reflects the isotopic composition of those amino acids in plants at the base of the food web, while the Δ15NGlu-Phe spacing (δ15NGlu – δ15NPhe) serves as a proxy for metabolic enrichment of 15N in the consumer’s body25
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4460640/