Hello all, Please see below for details regarding our recent open-access publication, in which we empirically test three common assumptions underlying stable isotope sampling of growth layer groups in marine mammal teeth. This study is part of my larger PhD work (alongside my advisor and co-author Dr. Carolyn Kurle) on killer whale ecology using teeth; I have graduated and am pursuing postdoc/employment opportunities, so please feel free to get in touch if you might be interested in working together or discussing this work further. Bowen KR, Kurle CM. 2025. Testing Assumptions for Stable Isotope Analysis of Marine Mammal Dentin Growth Layer Groups. Rapid Communications in Mass Spectrometry. http://dx.doi.org/10.1002/rcm.9997 ABSTRACT *Rationale* Stable isotope analysis of growth layer groups (GLGs) in mammal dentin is an increasingly prevalent and noninvasive approach to study animal foraging ecology. However, empirical evidence to support assumed proper methodologies for sampling GLGs is lacking. Here, we examine the effects of intratooth and intertooth variations with respect to targeted GLGs, as well as the effects of common pretreatments (e.g., formic acid and graphite) to enhance GLG visibility, on stable isotope values (δ13C and δ15N) from dentin. *Methods* We measured the δ13C and δ15N values of killer whale (Orcinus orca) dentin. We used dentin from 37 teeth to compare stable carbon (δ13C) and nitrogen (δ15N) isotope values from multiple locations within a GLG (intratooth variation), from corresponding GLGs among teeth of an individual (intertooth variation), and from treated and untreated teeth. *Results* We observed no significant differences in the δ13C or δ15N values when sampling a single GLG from multiple locations (intratooth variation) or when comparing the same GLG across duplicate teeth of individuals (intertooth variation). One tooth in a triplicate set showed a significantly different but likely biologically inconsequential δ13C value. Lastly, formic acid and graphite highlighting to accentuate GLGs did not significantly influence measured stable isotope values. *Conclusions* We validate several previous assumptions in this field of study. First, dentin samples for stable isotope analysis can be sampled from different locations across a GLG. Second, researchers can compare stable isotope values from the same GLGs of different teeth collected from the same individual in most cases, as the δ13C and δ15N values did not vary with the sampled tooth. Third, a common protocol of formic acid and graphite treatment to enhance GLG visibility does not bias the δ13C and δ15N values from dentin. We also describe factors to consider and cautions associated with these conclusions. Best regards, Kelly Bowen -- Kelly R. Bowen (Flanders) Ph.D. Candidate - Kurle Lab Ecology, Behavior, and Evolution - Biological Sciences University of California San Diego 9500 Gilman Dr. La Jolla, CA 92093