This project will exam the biochemical composition and physical structure of marine mammal (pinniped) teeth in an effort to reconstruct seal diets and environments throughout their lives, from in uteuro to juvenile to adult stages. The Hawaiian monk seal (Monachus schaundslandi), along with their genetic cousin the Mediterranean monk seal, is one of the most endangered species on the planet, largely due to a combination of environmental and anthropogenic influences. The analysis of both organic and inorganic components of the animals' tooth complexes, in coordination with age assessment, will allow us to ascertain critical information regarding changes to their habitats and the effects of such changes on the animals. Canine teeth from 124 deceased Hawaiian monk seals, encompassing the past four decades, will be sectioned and growth layers counted to ascertain the age of the animals. Pulverized samples from specific growth layers of the teeth will be analyzed for stable isotope ratios of oxygen and carbon (δ18O, δ13C, respectively). The isotope ratios provide a record of dietary and environmental change and are reflected in the growth of a tooth as the animal ages. These data provide a critical link in piecing together our knowledge of the central North Pacific Ocean, and how seemingly unknown, environment fluctuations can be reconstructed through time to help explain direct and indirect effects on biological organism.