Sunday 6 May 2012

What Nitrogen tells us about the diet of Mammoths.

Nitrogen has two stable isotopes, Nitrogen-14 (¹⁴N) and Nitrogen-15 (¹⁵N), which have different atomic weights, but identical chemical properties, and can be incorporated into identical compounds by organisms. Nitrogen fixing organisms (diazotrophs) tend to fix Nitrogen isotopes in the ratio found in the atmosphere, but each time the Nitrogen is passed from one organism to another, some of it is lost, with the lighter ¹⁴N being lost more rapidly than the heavier ¹⁵N. This makes it possible for scientists to estimate where an animal sits in the food chain from the proportion of ¹⁵N (δ¹⁵N) in compounds it produces; for example herbivores have a high δ¹⁵N compared to plants, and carnivores have a higher δ¹⁵N than herbivores.

Collagen from the bones of Woolly Mammoths (Mammuthus primigenius) typically has δ¹⁵N values about 3‰ higher than similar collagen from other herbivores from the same environment (tundra grasslands) such as Rhinoceroses (Coelodonta antiquus) and Horses (Equus sp.). Such a high comparative proportion of ¹⁵N would usually be taken as an indication of being a stage higher on the food chain, but Mammoths were clearly not well adapted to predating either Horses or Rhinoceroses, and even if this was not immediately obvious, there have been numerous studies of the stomach contents of frozen Mammoths, none of which have suggested that they were carnivorous.

A Woolly Mammoth, not likely to have been a predatory animal. Laurie Caple.

In a paper published in the journal Archaeological and Anthropological Sciences on 14 April 2012, Margot Kuitems, Thijs van Kolfschoten and Johannes van der Plicht of the Faculty of Archaeology at Leiden University present the results of an investigation into the δ¹⁵N values of keratin in the nails and hooves of Elephants, Rhinoceroses and Horses in a number of Dutch zoos and riding schools, and discuss the conclusions drawn from these results.

In 1986 Stanley Ambrose and Michael DeNiro of the Department of Earth and Space Sciences at the University of California, Los Angeles published a paper in the journal Oecologia, detailing the results of a study into the Carbon and Nitrogen isotope ratios in collagen from the bones of large mammals from the grasslands of Kenya and Tanzania. This revealed that like Pleistocene Mammoths, East African Elephants had raised δ¹⁵N compared to other large herbivores in the same environment. This led Kuitems et al. to conclude that modern Elephants could serve as a substitute for Wooly Mammoths in an investigation into Nitrogen isotope ratios in proteins.

Elephants in the Mwaluganje Elephant Sanctuary in Kenya. Although African Elephants are not closely related to Woolly Mammoths they also show raised δ¹⁵N compared to other large herbivores in the same environment, making them a good experimental model. Lonely Planet.

Keratin from nails is not an exact analogue for collagen from bones, but was more readily available; Kuitems et al. found that bones of deceased animals from Dutch zoos were collected by the Faculty of Veterinary Science of the University of Utrecht, but than they were cleaned with aggressive cleaning agents that rendered them useless for the purpose of this study.

In the event the study found no appreciable differences between the δ¹⁵N values in keratin from the three groups of animals yielded no appreciable variation, but this in itself was not a total disaster. Elephants (and other animals) in Dutch zoos are well fed the year round; they are not subject to the periodic environmental stresses that animals living in the wild have to endure. This suggests that the Nitrogen isotope imbalance seen in the bones of Mammoths and African Elephants is the result of environmental stress, rather than something that stems automatically from Elephant metabolism or behavior.

Kuitems et al. observed that Elephants have quite short guts for their size compared to Horses or Rhinoceroses, making them less efficient at food processing. This would mean that in times of short food supply they would be more reliant on recycling Nitrogen internally, which would tend to raise their δ¹⁵N value. This would be further increased if the Elephants were to engage in coprophagy, eating their own dung, a good strategy for a herbivore with a short digestive tract. This was not observed in the (well fed) Dutch zoo Elephants, but could still be a possibility in wild East African Elephants and Pleistocene Mammoths.

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