Tracing the Source of the Elephant And Hippopotamus Ivory from ...

4‰ lower than those of marine plants, and terrestrial plants have a wide spread of δ 15 N
values. For terrestrial foodchains in general, there is a 2-3‰ trophic level shift higher
from plants to herbivores, as well as a 2-3‰ trophic level shift from herbivores to
carnivores, which is due to the preferential excretion of 14 N, leaving behind 15 N in the
animal. Marine ecosystems also have trophic levels shifts, but more of them, so that
“δ 15 N values in marine organisms and those in other aquatic settings (e.g., rivers and
lakes) are higher than in terrestrial ones (up to 20‰)” (Larsen 1997: 283).
The δ 15 N values are furthermore influenced by climate, so that “cool forest soils
have low δ 15 N values, owing to higher nitrogen fixation and mineralization rates, and hot
savannah or desert soils have higher δ 15 N values. Other contexts producing generally
high δ 15 N soils include areas with a history of evaporation (e.g., saline soils) and those
enriched in organic matter” (Larsen 1997: 283). Indeed, the nitrogen isotope ratios
( 15 N/ 14 N) in bone collagen of African mammals has been shown to be related to rainfall
or water stress, and these values are caused by differential fractionation of nitrogen
during fixation or absorption in plant groups (Tykot and Staller 2002; van der Merwe et
al. 1990: 744). The δ 15 N values in bone collagen of elephants and hippopotami is higher
in more arid habitats, which is due to the higher 15 N levels of the local vegetation, as well
as the effect of drought stress on the protein metabolism of the animals, which is
expected in areas with an annual rainfall of