PS 15-190 - Fatness matters: Blood biochemical markers for understanding body condition in bighorn sheep.

Abstract: In wild animals with access to variable food resources, the significance of building and maintaining body fat is critical for growth and reproduction, and therefore population maintenance. Desert bighorn (Ovis canadensis nelsoni) and Sierra bighorn sheep (Ovis canadensis sierrae) inhabit environments where prolonged periods of limited forage availability are common, thus requiring them to maintain adequate fat reserves to meet their metabolic demands. Ultrasound measurement of subcutaneous fat and body condition score are used to calculate ingesta-free body fat, and is the preferred method for monitoring body condition in wild bighorn populations. However, this only measures the amount of body fat on the animal, rather than the animal’s current energy balance; it requires specialized equipment and a trained observer. Biochemical blood tests are an economical option for evaluating animal health from blood samples, and measures may provide information on the metabolic condition of the animal, in addition to the current body fat. Our aims in this study were (i) to determine which blood biochemical indicators are indicative of subcutaneous fat measured by ultrasound; and (ii) to identify metabolic markers that are not associated with current body fat that instead may point to the animal’s current energy balance. We collected data from bighorn sheep in the Mojave National Preserve, Death Valley National Park, and the Sierra Nevada mountains across three years – 2020–2022. We investigated the relationship between biochemical measurements and ingesta-free body fat using generalized linear mixed models. Subcutaneous fat thickness was measured at the thickest point on the rump and rump body condition was scored to calculate ingesta-free body fat according to the methods described by Stephenson et al. 2020. Blood was collected at the same time point. Blood samples were tested by Oregon State University Veterinary Diagnostic Laboratory using biochemical parameters suggestive of animal health. This methodology utilizes a simple, cost-effective model to identify the physiological predictors associated with a contemporary measurement of body condition, and adds information on the animal’s current energy balance. Determining biochemical indicators of poor body condition enhances our understanding of the overall health of bighorn sheep, and therefore broader population dynamics.