Evolution of Marine Mammal Foraging Behavior Based on Ca and C Isotope Ratios in Bioapatite

University Of Minnesota-Twin Cities, Minneapolis MN

Investigators

Abstract

Though the trophic structure of marine ecosystems is known to have increased in complexity over time, methods of quantifying this change are lacking. The proposed research will expand upon promising preliminary work using Ca isotope ratios as paleoecological indicators and develop this isotopic system as a proxy for the trophic level of extinct marine mammals. By combining Ca isotope analyses with C isotope ratios, which vary with foraging habitat in marine ecosystems, this project will provide a new means to examine the structure of ancient marine vertebrate ecosystems. Since the trophic structure of foodwebs has a significant influence on the pathways by which nutrients and energy move within and between ecosystems, analysis of the Ca isotope composition of biogenic hard parts may aid in elucidating how nutrient cycling and energy flow have fluctuated over time. Furthermore, by expanding current knowledge of how ancient ecosystems functioned and how they differed from today s ecosystems, a44Ca measurements can help improve interpretations of how climatic and environmental perturbations have impacted or were impacted by the Earth s biosphere. Ultimately, this project will provide new insight into the evolution of whales, whose from a terrestrial ancestor represents one of the most remarkable transitions in the history of life on Earth. Although the ultimate objective of this research is to understand the role of temporal changes in foraging behavior in the evolution of Cetacea (the clade that includes living whales, porpoises, and dolphins), the influence on Ca isotope ratios of factors other than trophic level must be examined first in modern species before Ca isotope ratios can be applied to extinct species. Samples of marine mammals in four, distantly related groups (sirenians, mustellids, cetaceans, and pinnipeds) will be analyzed as a means of addressing the impact of phylogenetic history on a44Ca values; the degree of similarity of a44Ca values for species at the same trophic level will verify whether a44Ca values reflect trophic level differences regardless of evolutionary history. The C isotope composition of each sample will be analyzed as a proxy for foraging habitat preference; a stronger correlation between a44Ca with trophic level rather than a13C values will indicate that trophic level has a stronger influence on a44Ca values than the foraging habits of a species. Variation in a44Ca within a single individual will be explored by analyzing the Ca isotope composition of multiple skeletal elements from a single individual from each of the four modern groups of marine mammals sampled. Once the factors influencing a44Ca values in modern marine mammals have been constrained, the Ca and C isotope composition of Eocene and Oligocene cetacean and sirenian bioapatite will be analyzed to assess the potential of this proxy for paleoecological studies. The diversification of cetacean feeding strategies in the Oligocene should coincide with a large change in a44Ca and a13C values of cetacean bioapatite, providing a clear signal of whether original a44Ca values can be preserved and interpreted from the fossil record. Fossil bones of sirenians will also be analyzed as a control for potential changes in seawater a44Ca values, since the trophic level of this group has remained constant over time and any changes in the a44Ca values of this group will reflect changes in seawater a44Ca values. Broader impacts. The broader impacts of this project are threefold. First, the grant will further the training and development of a recent Ph.D. who will be supported as a postdoc. Under the supervision of the PIs, this researcher will have primary responsibility for refining the methodologies, acquiring and analyzing the samples, and interpreting and disseminating the results. Second, the research will provide a new and potentially powerful paleoecological tool to the broader community that can be applied to other ancient marine ecosystems and, after further development, may also have applications in terrestrial ecosystems. Finally, the results will complement our increasingly detailed knowledge of whale evolution. Once the proposed research is completed, the ecological and morphological changes associated with the evolution of whales will be one of the best documented cases of macroevolutionary change in the vertebrate fossil record. Given the historical place of whale origins in anti-evolution polemics, this research will surely have lasting impact in undergraduate and K-12 class rooms.