The issue of which plant types elephants (or other herbivores) actually consume and how these are important nutritionally to growth processes in the animal can be traced through an elegant analytical method. Plants vary in their isotopic composition of carbon depending on the photosynthetic pathway of fixing carbon dioxide. The ratio of the two stable isotopes of carbon (the common 12C and the rarer 13C) differs widely between the so-called C4 plants (mainly the tropical grasses and sedges) and C3 plants (most dicot trees, shrubs, and herbs, as well as bamboos and temperate grasses). The C/ C ratio, expressed conventionally as S1JC per mil, has a range of about -30 to -26 per mil in the C3 plants and -14 to -11 per mil in the C4 plants.
The carbon isotope ratio in organic tissues of an animal could be expected to reflect the isotopic values of its diet. Typically, these analyses are carried out on a relatively inert protein, such as collagen in animal bone. A correction has to be incorporated for a shift in the isotopic ratio from the dietary source, through discrimination along metabolic pathways, to the final protein product (for collagen, this correction factor of about +5 per mil is known both through experimental work and through empirical studies of diet in pure C4 and pure C3 plant feeders). Similarly, the stable nitrogen isotope (15N/14N) ratio may reflect environmental conditions (e.g., rainfall and water stress) or even dietary components (e.g., leguminous plants), but much less work has been done with this technique.
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