Fat reserve assessment in Pyrenean chamois using body measurements

Body condition is a measure of the energetic reserves stored in tissues and organs of animals in anticipation of periods of special energetic demands or to attenuate food shortages. For more than half a century, the kidney fat index (KFI) has been the most common invasive method for assessing body condition in ungulates. Since KFI requires animal necropsy, other non-invasive indicators based on body measurements have been suggested to assess body condition of mammals. These include the residuals from an ordinary least squares regression of body mass and linear measure of size (OLSr), the scores from a Principal Components Analysis on body measurements (PCAsc), and the scaled mass index (SMI). These indices, however, are often difficult to interpret and little effort has been made to confirm whether they are related to direct measures of fat reserves. We used the Bland-Altman method and linear models applied to biometric data to explore whether these three biometric indices can be used to assess body condition in 94 adult male and female Pyrenean chamois (Rupicapra pyrenaica pyrenaica). Animals were hunter-harvested in the Eastern Pyrenees (Spain) during two contrasting periods of food availability. We found that OLS residuals from the regression between body weight and hind foot length were the best proxy for fat reserves in both periods of the year. This simple, low cost and non-invasive biometric indicator can be used for monitoring body condition of chamois populations and probably in other ungulate species with similar life strategies.