Group leader Max Planck Institute of Geoanthropology, Germany
The discovery of Steller’s sea cow (Hydrodamalis gigas) during the mid 18th century marked the first documentation of a species of sirenian inhabiting high-latitude, kelp-dominated w aters. This habitat preference suggested that the ecology, as well as the physiology, of H. gigas was radically different from that of all other extant sirenians, which are restricted to tropical/sub-tropical seagrass meadows and freshwater ecosystems. Unfortunately, human hunting led to the extinction of H. gigas preventing further study. Observations of kelp consumption made by Wilhelm Steller and other explorers suggest that kelp was part of the animal’s diet, but identifications and descriptions of the plants ingested by these animals are open to interpretation, leaving much doubt as to whether sea cows actually were significant kelp consumers. To assess the contribution of kelp to the diet of H. gigas, we analyzed carbon isotopic composition (δ13C) of essential amino acids (EAA) in bone collagen. EAA are ideally suited as source tracers because sources that biosynthesize EAA – bacteria, fungi, vascular plants and algae – each have source diagnostic δ13C EAA patterns or fingerprints that remain largely intact during trophic transfer because animals cannot biosynthesize EAA. Within aquatic primary producers, δ13C EAA fingerprints can discriminate marine kelp from seagrasses, freshwater plants, and other macroalgae.
We sampled bone collagen from three species – H. gigas (n = 7), Trichechus manatus (Florida manatee, n = 6), and Dugong dugon (Australian dugong, n = 3) – and compared δ13C EAA patterns from these samples with those recovered from representative samples of modern kelp, other macroalgae, seagrasses, and freshwater plants. Specimens of H. gigas included historic material from the Commander Islands in the Bering Sea (~200 years old) and fossil material of late Pleistocene age from Monterey Bay, California. Using multivariate classification methods, we found that the δ13C EAA patterns of H. gigas matched those of kelp, while patterns for D. dugon mostly resembled those of seagrasses, and patterns for T. manatus were split between those resembling seagrasses and those resembling freshwater plants. Our results suggest H. gigas was an important consumer within the kelp ecosystems that flourished along the North Pacific coastlines, and that this species favored kelp species over other producers available within these ecosystems. In addition, we show that compound-specific stable isotope analysis of amino acids can be a more specific method of paleodietary reconstruction than bulk SIA when well-preserved bone collagen is available for analysis.
