Isotope data from amino acids indicate Darwin’s ground sloth was not an herbivore

Abstract

Fossil sloths are regarded as obligate herbivores for reasons including peculiarities of their craniodental morphology and that all living sloths feed exclusively on plants. We challenge this view based on isotopic analyses of nitrogen of specific amino acids, which show that Darwin’s ground sloth Mylodon darwinii was an opportunistic omnivore. This direct evidence of omnivory in an ancient sloth requires reevaluation of the ecological structure of South American Cenozoic mammalian communities, as sloths represented a major component of these ecosystems across the past 34 Myr. Furthermore, by analyzing modern mammals with known diets, we provide a basis for reliable interpretation of nitrogen isotopes of amino acids of fossils. We argue that a widely used equation to determine trophic position is unnecessary, and that the relative isotopic values of the amino acids glutamate and phenylalanine alone permit reliable reconstructions of trophic positions of extant and extinct mammals.

Xenarthrans of the collection of Santiago Roth from the Pampean Region of Argentina (Pleistocene), in Zurich, Switzerland

“…These floral changes likely affected strong faunal changes especially for herbivores. Despite a few species with omnivorous diets, such as euphractines (Carlini et al, 2016 ; Superina & Abba, 2014 ), or occasional meat consumption in some sloths, such as Darwin's ground sloth (Tejada et al, 2021 ), xenarthrans were predominantly herbivorous (e.g., Bargo & Vizcaíno, 2008 ; Gaudin & Croft, 2015 ; Toledo et al, 2015 ; Vizcaíno & Loughry, 2008 ). The high herbivore dominance of xenarthrans from this period thus makes them good indicators of paleoenvironmental change as already highlighted by many studies (e.g., Soibelzon & Tonni, 2009 ; Soibelzon, 2019 ; Tonni et al, 1999a , 1999b ).…”

Pliocene Pre-GABI herbivorous mammals from Espinar, Peruvian Andean PlateauCitation for this article: Salas-Gismondi, R., Ochoa, D., Gamarra, J., Pujos, F., Foster, D. A., & Tejada, J. V. (2023) Pliocene pre-GABI herbivorous mammals from Espinar, Peruvian Andean Plateau. Journal of Vertebrate Paleontology . https://doi.org/10.1080/02724634.2023.2237079

“…The predominance of xenarthrans in the Espinar fauna might be seen as a drawback for environmental inferences because their peculiar feeding anatomy lacks ecological analogs in modern ecosystems (Anaya & MacFadden, 1995; Bargo et al, 2006). For example, nitrogen isotopic analyses of amino acids on the Pleistocene ground sloth Mylodon darwini revealed that this species fed on items of animal origin in addition to plants (Tejada et al, 2021), a feeding behavior that would have been hard, if not impossible to infer, solely from morphology. In lack of geochemical data, however, muzzle reconstructions from craniodental morphology provide a widely used proxy to infer dietary preferences within the herbivorous guild of xenarthrans (Bargo et al, 2006).…”

Pig management in the Neolithic Near East and East Asia clarified with isotope analyses of bulk collagen and amino acids

“…Because this approach is based on a difference in the trophic isotope discrimination of two common amino acids (glutamic acid and phenylalanine), the trophic position (TP) of an organism can be estimated with more precision without the prey's values as a baseline (Ohkouchi et al, 2017; Figure 1). The difference in the δ 15 N Glu and δ 15 N Phe values ( 15 N Glu−Phe ) of terrestrial vascular plants has been reported to show the constant values with a minor variation (Ramirez et al, 2021), and the 15 N Glu−Phe value for animals increases with each step up the food chain from primary producers (Tejada et al, 2021). Therefore, an individual's TP and animal protein consumption rate can be estimated from 15 N Glu−Phe .…”