Abstract
Stable carbon isotope analyses of vertebrate hard tissues such as bones, teeth, and tusks provide information about animal diets in ecological, archeological, and paleontological 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 regression slopes >1. This outcome is consistent with predictions that herbivore δ13Ccollagen is biased against low protein diet components (13C‐enriched C4 grasses in these environments), 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 negligible within trophic levels, clearly cannot be ignored in carbon isotope‐based diet reconstructions.
Highlights
Stable carbon isotope analysis is routinely used to reconstruct consumer resource use patterns in modern, archeological, and paleontological contexts (Ben-D avid & Flaherty, 2012; Cerling & Harris, 1999; Cerling et al, 2015; Crawford, McDonald, & Bearhop, 2008; Hare &Sealy, 2013)
Relationships between mammal δ13Ccarbonate and δ13Ccollagen values differ across trophic levels, in terms of the well-known differences in Δ13Ccollagen-carbonate spacing and in their slopes
Whereas slopes for carnivores approximate or include 1, and Δ13Ccollagen-carbonate spacing remains more-or-less constant across the full range of diets, slopes >1 occur in herbivores and Δ13Ccollagen-carbonate spacing increases with increases levels of C4 grass intake. This result is consistent with the expectation of a bias toward 13C-depleted C3 foods in δ13Ccollagen due to the higher protein content of C3 browse than C4 grass
Summary
Stable carbon isotope analysis is routinely used to reconstruct consumer resource use patterns in modern, archeological, and paleontological contexts Lipids in particular are 13C-depleted relative to other biochemical carbon sources (Tieszen, Boutton, Tesdahl, & Slade, 1983), and a lipid-rich diet is expected to lead to low δ13Ccarbonate values This observation has often been invoked to explain why Δ13Ccollagen-carbonate spacing in carnivores is only ~4 ‰, much lower than in herbivores, as carnivores consume a greater proportion of dietary lipids (Krueger & Sullivan, 1984; Lee-Thorp et al, 1989; O’Connell & Hedges, 2017). We treat primates as a separate set of taxa because trophic level assignations of these taxa, especially baboons (Papio ursinus), are debatable, and because they likely have different physiological, anatomical, and behavioural traits distinguishing them from the “true” herbivores (mostly ungulates) included in the dataset
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