The horny dilemma of the woolly rhino: isotope analysis hints at the ancient diet

The link between isotope ratios and diet

The diet of ancient humans and animals can be reconstructed by carbon and nitrogen isotope analysis of preserved tissues, such as bone, hair and hooves. For instance, the ¹²C/¹³C isotope ratio gives an indication of whether the subject consumed C₃ or C₄ plants.

The C₃ plants fix carbon from atmospheric carbon dioxide by reaction with ribulose biphosphate to produce phosphoglycerate, whereas the C₄ plants produce oxaloacetate in the first step of photosynthesis. C₃ plants flourish in cool, wet, cloudy climates and make up the majority of plants, including wheat, barley, potatoes, rice and sugar beet. C₄ plants tend to inhabit hot dry environments and include maize, Bermuda grass and many types of desert plant.

The two carbon fixation pathways produce different isotope ratios for carbon and nitrogen, so revealing the type of diet. The delta¹³C values for C₃ and C₄ plants, determined with reference to an established standard, are clearly separated, falling in the ranges -22 to -30 permil (parts per thousand) and -10 to -14 permil, respectively. So knowledge of these figures helps to reveal how ancient creatures lived.

Woolly rhino horn structure contains environmental information

The woolly rhinoceros is a poorly studied species due to the dearth of well-preserved soft tissue specimens. The animals displayed a small frontal horn accompanied by a large nasal horn and these represent some of the main body parts that have been preserved by permafrost.

One nasal horn housed in the Ice Age Museum in Moscow, from an animal which lived in the late Pleistocene period, is poorly preserved, with both ends missing and the remaining surfaces eroded. However, its appearance led two Russian scientists to believe that it was sufficiently intact to yield useful isotopic information.

Alexei Tiunov from the Institute of Ecology and Evolution RAS, Moscow, and Irina Kirillova from the museum, were interested in the characteristic banding pattern on the horn. Surface erosion had exposed alternate bands of light-coloured convex zones and dark, hollow zones along the length of the horn.

Earlier reports had tentatively linked this structure to the change in seasons, while the band widths were similar to the annual growth increments in the horns of the modern rhino, with the number of bands equating to the age of the beast.

Poorly preserved horn still yields consistent isotope data

The researchers took clippings from each zone along the horn and subjected them to carbon and nitrogen isotope analysis using a continuous-flow mass spectrometer. The total carbon and nitrogen contents were also determined, using elemental analysis.

The ratios of total carbon/nitrogen were very similar to those of modern keratin tissues and did not vary between the dark and light zones of the horn. In addition, there were only slight variations in the delta¹³C and the delta¹⁵N values from the outer and inner layers. These are good indications that chemical changes in the horn were minimal and there was little contamination of the keratin from which the horn was constructed.

Both delta¹³C and the delta¹⁵N oscillated along the length of the horn in an approximate sinusoidal pattern which correlated with the banding pattern. The higher values were associated with the darker zones and the lower values with the lighter areas.

By comparing the isotope variations with those reported for mammoth hair, and for tissues from living species such as horse hair, moose hooves and caribou antlers, the researchers tentatively equated the dark zones with summer diet, similar to that of the caribou, consisting largely of grasses. The light zones were associated with the cold season, when the rhino consumed a diet approaching that of the moose, with increased amounts of shrubs and woody vegetation.

This conclusion was supported by reports of traces of grass and twigs in the mouths of preserved woolly rhino heads and of wood in mammoth dung. The pollen analysis had identified the plants as grasses, Artemisia, Betula, ferns and club mosses.

The facial features of the woolly rhino are consistent with that of a grazing animal, like the woolly mammoth, and they might have fed on grasses and sedges as a staple part of the diet.

Tiunov and Kirillova stressed that their interpretations need to be substantiated by more studies but the data illustrate the value of isotope analysis for studying the diet of extinct species of grazing creatures.

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.