Carbon Isotope Variability in the Bone Collagen of Red Kangaroos (Macropus rufus) is Age Dependent: Implications for Palaeodietary Studies

Abstract

Isotopic assessment of bone collagen is often used as an environmental tracer in both contemporary and palaeoenvironmental studies. However, variability in the isotopic composition of this tissue remains poorly understood for naturally occurring and wild populations of animals. In this study the stable carbon isotope composition of both diet and bone collagen was assessed for a population of red kangaroos (Macropus rufus). Animals sampled ranged in age from approximately 10 months to 15 years. The diet of this population, estimated from faeces collected in the field, varied from predominantly C4grasses in late summer (δ13C⋍−16·5‰) to mostly C3herbage in late winter (δ13C⋍−22·5‰), with a long-term average δ13C of between −19 and −20‰. Bone collagen was enriched in13C by 3 to 4‰ in older animals relative to pouch young. Isotopic analysis of hair, used to assess more recent diet in individuals, indicated that diet selection was similar in all animals that had been weaned. We suggest that the most likely explanation for the age-dependent relationship in the δ13C of bone collagen occurs because milk (the only source of nutrition in suckling kangaroos) is not fractionated in the same manner as plant-derived carbon during its assimilation into skeletal tissue. If this is the case, then such a relationship should be most predominant in mammals that have low birth weights (relative to the adult mother) and gain significant weight from milk. Whatever the precise mechanism(s) for the observed fractionation, bone collagen of kangaroos seems to retain an isotopic memory of the carbon laid down prior to weaning, which takes several years to be diluted and replaced with carbon derived from an “adult” herbaceous diet. These results have implications for palaeoecological research where fossil skeletal tissue is used as dietary or environmental tracers particularly if the relative age of the animals sampled is unknown.