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Evolution on an island continent: feeding ecology of Pleistocene sloths

Periodic Reporting for period 1 - FEPS (Evolution on an island continent: feeding ecology of Pleistocene sloths)

Reporting period: 2022-01-10 to 2024-01-09

• What is the problem/issue being addressed?
South America (SA) contains Earth’s highest mammalian species richness, but this richness was significantly higher just 10,000 years ago, prior to the last ice age extinction that eliminated more than 80% of mammals above 40 kg in the continent. With only two living genera restricted to tree-dwelling in rainforests, sloths once were a dominant group within SA mammalian ecosystems. Fossil sloths are generally considered plant eaters, like their modern relatives, but their extremely high diversity (in terms of body mass [4-4000 kg], geographical distribution [Patagonia to Alaska], habitats [coast, mountains, rainforest] and inferred locomotion [terrestrial, fossorial, arboreal, semiaquatic]) suggests that these were ecologically more versatile than traditionally thought. Based on preliminary results obtained by the lead researcher, there is evidence that not all fossil sloths were obligate herbivores as traditionally thought. By means of amino acid compound specific isotope analyses, the primary goal of this project is to extensively analyze the dietary composition of fossil sloths assessing evidence for carnivory and the consumption of proteins of animal origin, as suggested by our preliminary results on one species. Since xenarthrans (the mammal group including sloths, armadillos, and anteaters) represented 1/3 of overall species diversity in South American mammal communities until as recently as 10,000 years ago, a discovery that several fossil sloth species possessed ecologies differing from those traditionally attributed to them could radically alter understanding of the entire community structure of terrestrial fossil mammals on the continent through time. Indeed, because large herbivores (like many fossil sloth species) greatly impact different aspects of the ecosystem such as the vegetation structure, the soil moisture, and the amount of methane released to the atmosphere, removal of some fossil ground sloth taxa from the continental herbivore guild would change previous estimations of the net primary production required to sustain a given number of megaherbivores, as well as the type of vegetation dominating the biome floor, among other aspects. Furthermore, by matching stable isotope data with the different biological, environmental, and/or phylogenetic traits of the species under study, we expect to reach a better understanding of the processes behind isotope variation and to determine predictors for these differences that will allow the application of these techniques to fossil and modern mammals. Thus, the study will also test key assumptions inherent within isotopic trophic studies, and thus it is broadly relevant to our understanding of the isotopic signals preserved in other fossil species and ecosystems.

• Why is it important for society?
The temporal setting of this project is the Pleistocene, a geologic epoch characterized by several glacial and interglacial stages that ended in an extinction event that decimated an important part of Earth’s mammalian communities (80% of large mammals in most continents). As such, the Pleistocene extinction component of this project is being (and will be) used to address modern conservation issues because it was arguably the first human-driven extinction event. By sharing insight on the Pleistocene epoch, focusing on the fantastic megafauna native to South America (with especial emphasis on giant ground sloths) and the causes that led to extinction, it is possible to raise awareness on the continent’s prehistoric past, as a means to understanding its current issues and to strengthen the commitment towards biodiversity conservation. Furthermore, one of the obligate secondary outcomes of this project will be the discovery and characterization of essential amino acid in sloths and other mammalian taxa. Therefore, this project also will be important for zookeepers and conservation biologists, as they can implement informed feeding regimes by supplementing these amino acids in the diets of captive individuals, enhancing their growth and quality of life.

• What are the overall objectives?
The main research objectives include the rigorous testing of a controversial hypothesis of carnivory and consumption of proteins of animal origin on fossil sloth species by means of amino acid compound specific isotope analyses (AACSIA) as well as to measuring the patterning of diet-animal tissue amino acid d15N offsets in fossil sloths and modern mammals, to reliably determine feeding behaviors of extinct species using stable isotope data
- Training in laboratory techniques
- Visit of fossil collections to assess appropriateness and availability of material for isotopic analysis
- Sampling regime in museum collections in Europe and the Americas
- Collagen extraction from fossil samples in laboratory at the University of Cambridge
- Visit of laboratories in the US and Europe to learn about lab management, instrumentation use, and lab techniques
- Presentation of preliminary results in a specialized scientific meeting and colloquiums at US universities
- Establishment of external collaborations with researchers in South America (Peru and Uruguay), Europe (France, Germany, and Switzerland), and the United States.
The research has identified a critical challenge in trophic ecology (i.e. a reliable way to distinguish secondary consumers in the fossil record) and is providing a new understanding of ecological isotopic relationships, permitting exploitation of isotopic evidence to its maximum potential. As such, the project is helping to develop a novel approach and critical baseline data necessary for interpretation of vertebrate skeletal isotopic analyses. Furthermore, in studying tropical South America, the project has provided British/European researchers with opportunities to expand ecological and geographical breadth of investigations in understudied settings. More specifically, the researcher has established new collaborations with international scholars at the University of Cambridge that she plans to continue far beyond the duration of the postdoc program. In this way, the project emphasizes the importance of continued international cooperation in science at a global scale.
Julia Tejada conducting observational field data