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

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.