Nitrogen and carbon isotopic composition of bone collagen from marine and terrestrial animals

The stable nitrogen and carbon isotope ratios of bone collagen prepared from more than 100 animals representing 66 species of birds, fish, and mammals are presented. The b15N values of bone collagen from animals that fed exclusively in the marine environment are, on average, 9% more positive than those from animals that fed exclusively in the terrestrial environment; ranges for the two groups overlap by less than I Wo. Bone collagen 6”N values also serve to separate marine fish from the small number of freshwater fish we analyzed. The bone collagen b”N values of birds and fish that spent part of their life cycles feeding in the marine environment and part in the freshwater environment are intermediate between those of animals that fed exclusively in one or the other system. Further, animals that fed at successive trophic levels in the marine and terrestrial environment are separated, on average, by a 3% difference in the 6”N values of their bone collagen. Specifically, carnivorous and herbivorous terrestrial animals have mean b15N values for bone collagen of +8.0 and +5.3%0, respectively. Among marine animals, those that fed on fish have a mean b15N value for bone collagen of + 16.X& whereas those that fed on invertebrates have a mean 615N value of +13.3%0. These results support previous suggestions of a 3%~ enrichment in 615N values at each successively higher trophic level. In contrast to the results for d”N values, the ranges of bone collagen 6°C values from marine and terrestrial feeders overlap to a great extent. Additionally, bone collagen 6°C values do not reflect the trophic levels at which the animals fed. These results indicate that bone collagen 6”N values will be useful in determining relative dependence on marine and terrestrial food sources and in investigating trophic level relationships among different animal species within an ecosystem. This approach should be applicable to animals represented by prehistoric or fossilized bone in which collagen is przserved.