Within trophic level shifts in collagen-carbonate stable carbon isotope spacing are propagated by diet and digestive physiology in large mammal herbivores
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Abstract
Stable carbon isotope analyses of vertebrate hard tissues such as bones, teeth, and
tusks provide information about animal diets in ecological, archeological, and paleon tological contexts. There is debate about how carbon isotope compositions of collagen
and apatite carbonate differ in terms of their relationship to diet, and to each other.
We evaluated relationships between δ13Ccollagen and δ13Ccarbonate among free-ranging
southern African mammals to test predictions about the influences of dietary and
physiological differences between species. Whereas the slopes of δ13Ccollagen–
δ13Ccarbonate relationships among carnivores are ≤1, herbivore δ13Ccollagen increases
with increasing dietary δ13C at a slower rate than does δ13Ccarbonate, resulting in regres sion slopes >1. This outcome is consistent with predictions that herbivore δ13Ccollagen
is biased against low protein diet components (
13C-enriched C4 grasses in these envi ronments), and δ13Ccarbonate is 13C-enriched due to release of 13C-depleted methane as
a by-product of microbial fermentation in the digestive tract. As methane emission is
constrained by plant secondary metabolites in browse, the latter effect becomes more
pronounced with higher levels of C4 grass in the diet. Increases in δ13Ccarbonate are also
larger in ruminants than nonruminants. Accordingly, we show that Δ13Ccollagen-carbonate
spacing is not constant within herbivores, but increases by up to 5 ‰ across species
with different diets and physiologies. Such large variation, often assumed to be negli gible within trophic levels, clearly cannot be ignored in carbon isotope-based diet
reconstructions.