Body mass predicts isotope enrichment in herbivorous mammals

Abstract

Carbon isotopic signatures recorded in vertebrate tissues derive from ingested food and thus reflect ecologies and ecosystems. For almost two decades, most carbon isotope-based ecological interpretations of extant and extinct herbivorous mammals have used a single diet–bioapatite enrichment value (14‰). Assuming this single value applies to all herbivorous mammals, from tiny monkeys to giant elephants, it overlooks potential effects of distinct physiological and metabolic processes on carbon fractionation. By analysing a never before assessed herbivorous group spanning a broad range of body masses—sloths—we discovered considerable variation in diet–bioapatite δ13C enrichment among mammals. Statistical tests (ordinary least squares, quantile, robust regressions, Akaike information criterion model tests) document independence from phylogeny, and a previously unrecognized strong and significant correlation of δ13C enrichment with body mass for all mammalian herbivores. A single-factor body mass model outperforms all other single-factor or more complex combinatorial models evaluated, including for physiological variables (metabolic rate and body temperature proxies), and indicates that body mass alone predicts δ13C enrichment. These analyses, spanning more than 5 orders of magnitude of body sizes, yield a size-dependent prediction of isotopic enrichment across Mammalia and for distinct digestive physiologies, permitting reconstruction of foregut versus hindgut fermentation for fossils and refined mean annual palaeoprecipitation estimates based on δ13C of mammalian bioapatite.

Isotopic equifinality and rethinking the diet of Australopithecus anamensis

“…I combine published ungulate and primate δ 13 C enamel values from the Omo‐Turkana Basin during A. anamensis' times (4.2–3.9 Ma) and convert those values to δ 13 C diet values based on the body mass‐dependent ε * enamel‐diet values of Tejada‐Lara et al () [Equation (5)]. Since foregut or hindgut fermenting metabolism of fossil taxa is unknown, I use Tejada‐Lara et al's () equation for all organisms combined: ε * = 2.4 + 0.034 (BM), where BM equals body mass. Faith, Rowan, Du, and Koch () recently classified all Omo‐Turkana fossil ungulates utilized here by mammal size class.…”

“…Based on published body size estimates of A. anamensis , I examine if changing the ε * enamel‐diet value from Cerling et al's () +14‰ based on large ruminant and non‐ruminant mammals to a body‐size dependent ε * enamel‐diet value after methods of Tejada‐Lara et al () significantly alters the estimation of A. anamensis' %C 4 diet. I also explore if different vegetation structures may be responsible for potential isotopic equifinality between A. anamensis and “savanna” chimps.…”

“…Recently, Tejada‐Lara et al () determined the ε * enamel‐diet value of mammalian herbivores is predicted by body mass: ε * = 2.4 + 0.034 (BM), where BM equals body mass ( R 2 = 0.62, p = .0002). This study revealed that herbivore ε * enamel‐diet values range between +9 and +16‰ and are also influenced by gut physiology.…”

“…Pliocene hominins are reconstructed with body masses of ∼30–50 kg (McHenry & Coffing, ) and are thought to have been hindgut fermenters with similar gut proportions as chimpanzees (Aiello & Wheeler, ; Milton, ; Leonard, Robertson, Snodgrass, & Kuzawa, ). The application of Tejada‐Lara et al's () equation positions A. anamensis' and other Pliocene hominins' ε * enamel‐diet values between +12 and +13‰.…”

Integrative isotopic Paleoecology (δ13C, δ18O) of a Late Pleistocene vertebrate community from Sergipe, NE Brazil

“…Tejada-Lara et al (2018) suggested that body mass influences physiological carbon enrichment (□* diet-bioapatite ) in mammals, and provided equations to determine these values of enrichment. Carbon isotopic data presented here are from mammals (extinct and extant) with body mass varying from 38 to 6,300 kg (Table 1), □* diet-bioapatite varied between 12.47 to 14.84 ‰, we used four values: +12 ‰ for taxa weighting less than 75 kg; +13 ‰ for taxa weighting between 75 kg to 600 kg; +14 ‰ for taxa weighting between 600 kg to 3,500 kg; and, finally, +15 ‰ for taxa weighting more than 3,500 kg (Table S3).…”