Elemental concentrations in mammalian tissues: Novel indicators of ecology and biology of mammals

The diets of marine mammals are complex. Methods of quantifying the relative contributions of different prey items to their diets are complicated by 1) the marine environment, which makes extended observation difficult; 2) differences in digestibility and temporal variability of prey, which cause stomach content or fecal analyses to provide only partial information on a species' diet; and 3) a lack of tight correlation between morphology and diet, which means morphological adaptations often cannot supply information on exact prey type. Biochemical techniques like stable isotope analysis supply information on trophic level and foraging preferences of marine mammals; however, these techniques are limited in that they cannot always supply information on food type, and can only be used over a small spatial area, as variation in baseline isotopic values with location makes direct comparison of consumers from different locations difficult.;In this study, I assessed Sr/Ca and Ba/Ca ratios of marine mammal bone as global biochemical proxies for diet type. Combining new and existing Ba, Ca, and Sr data for environmental and biological specimens from terrestrial and marine sources, I determined that variations in Sr/Ca and Ba/Ca ratios in marine samples are appreciably different from their terrestrial counterparts. Although trophic discrimination controls Sr/Ca and Ba/Ca ratios of terrestrial consumers, diet type is the dominant control on Sr/Ca and Ba/Ca ratios of marine mammals. Through a series of analyses of skeletal material from extant species, I demonstrated that ontogeny, environment (i.e., location, depth, freshwater influences), and biology (i.e., diet, physiology) can influence Sr/Ca and Ba/Ca ratios in mammalian bioapatite. Yet, the impact of each is correctible and can be useful in providing additional information into the life history and ecology of an individual or taxon. These results support the idea that Sr/Ca and Ba/Ca ratios are global proxies for diet type in marine systems that complement diet information provided by other biochemical proxies (i.e., stable isotope analysis). With additional research, Sr/Ca and Ba/Ca ratios applied to fossil samples could supply important paleoecological information on the importance of diet type in the evolution of marine mammals.