Weighing our measures: approach-appropriate modeling of body composition in juvenile Steller sea lions (Eumetopias jubatus)

Abstract

While many approaches to modeling body condition exist, ranging from arbitrary morphometric indices to sophisticated cone modeling, few approaches have attempted to develop a standardized, simplified method for determining total body fat and protein in otariids. Our goal was to develop a method for predicting the body condition of juvenile Steller sea lions (Eumetopias jubatus (Schreber, 1776)) using simple morphometrics such as measurements of girth, length, mass, and blubber depth. We compared a candidate set of models to determine which metrics best predicted total body water (TBW) measures obtained by deuterium isotope dilution. Furthermore, we used AICc (Akaike’s information criterion corrected for small sample size) model selection methods and cross-validation to choose and validate the best suite of predictors. TBW was best predicted by a model that included mass, standard length, axial girth with the addition of blubber depths on the lateral side of the neck and dorsal surface of the hip. The results presented here show that blubber depth is an important addition to modeling body composition and may improve upon nonlethal, population-level estimates of nonisotopically derived values of TBW in juvenile Steller sea lions. Additionally, our models present a model development framework for other research efforts for use in determining body condition in otariids.