A framework for age estimation of marine mammal teeth using killer whales (Orcinus orca)

Abstract

Accurate age estimation is an important factor in understanding the temporal ecology, life history, and population structure of marine mammal species. The presence of annual growth layer groups (GLGs), bands of differing densities within teeth, has been documented and studied for decades as a method of age estimation. However, the methodology is still associated with complexities and uncertainties that challenge practical applications. Characteristics of GLGs can vary by species, and interpretation of GLG boundaries can vary by person. These complications are especially notable for elusive species, which are underrepresented in dentin studies. Here, we examine a dataset of ∼80 killer whale (Orcinus orca) teeth within an established framework to improve empirical understanding of GLG growth and identification. This is, to our knowledge, the most comprehensive dataset for this species and the first to reference known-age individuals. We describe characteristics of killer whale GLGs, including GLG width, presence of accessory lines, and possible distortion from pulp stones (i.e. calcified masses). We also compared our own age/GLG estimations with those of numerous alternate readers who have studied these teeth in the past, finding potentially large discrepancies (0 to 38 years). To achieve more accurate age estimations and better understand the discrepancies, we were able to examine teeth from wild, age-referenced killer whales. In addition to analysing the GLGs, we also calculated the pulp:tooth area ratio as another measure through which to understand age, as the pulp cavity gradually fills with dentin as the animal ages. We found support for an exponential relationship, and we found that killer whale ecotype is a factor for consideration, since pulp:tooth area ratios were consistently lower for residents than for transients. Our study provides a modern foundation for age estimation of killer whales, an elusive species of increasing conservation concern in some regions. We also provide a structured and empirical framework for better understanding age estimation via dentinal GLGs in any marine mammal species.