Analysing the isotopic life history of the alpine ungulates Copra ibex and Rupicapra rupicapra rupicapra through their horns

The horn of ungulate grazers offers a valuable isotopic record of their diet and environment. However, there have been no reports of the spatio-temporal variation of the isotopic composition of horns. We investigated patterns of carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopic composition along and perpendicular to the horn axis in Capra ibex and Rupicapra rupicapra rupicapra to assess the effects of animal age, within-year (seasonal) and inter-annual variation, natural contamination and sampling position on horn isotope composition. Horns of male C. ibex (n = 23) and R. r. rupicapra (n = l) were sampled longitudinally on the front (only R. r. rupicapra) and back side and on the surface and sub-surface. The sides of the R. r. rupicapra horn did not differ in δ¹³C. In both species, the horn surface had a 0.15%o lower δ¹³C and a higher carbon-to-nitrogen (C/N) ratio than the subsurface. Washing the horn with water and organic solvents removed material that caused these differences. With age, the δ¹⁵N of C. ibex horns increased (+0.1%o year^-1), C/N ratio increased, and ¹³C discrimination relative to atmospheric CO₂ ( ¹³Δ) increased slightly (+0.03%o year^-1). Geostatis-tical analysis of one C. ibex horn revealed systematic patterns of inter-annual and seasonal ¹³C changes, but ¹⁵N changed only seasonally. The work demonstrates that isotopic signals in horns are influenced by natural contamination (δ¹³C), age effects ( ¹³δ and δ¹⁵N), and seasonal (δ¹³C and δ¹⁵N) and inter-annual variation (δ¹³C). The methods presented allow us to distinguish between these effects and thus allow the use of horns as isotopic archives of the ecology of these species and their habitat.