New perspectives for estimating body condition from mass/length data: the scaled mass index as an alternative method

TLDR

Body condition is linked to health and fitness, and ecologists typically estimate it using OLS residuals of mass versus length, though recent validations have been questioned. The authors propose the scaled mass index as a new condition index with methodological, biological, and conceptual benefits. They develop the scaled mass index using scaling principles and compare it empirically and theoretically to OLS residuals. Reanalysis across seven species and 20 analyses shows the scaled mass index outperforms OLS residuals as a predictor of energy reserves, making it a useful tool for ecologists.

Abstract

Body condition is assumed to influence an animal's health and fitness. Various non‐destructive methods based on body mass and a measure of body length have been used as condition indices (CIs), but the dominant method amongst ecologists is currently the calculation of residuals from an ordinary least squares (OLS) regression of body mass against length. Recent studies of energy reserves in small mammals and starlings claimed to validate this method, although we argue that they did not include the most appropriate tests since they compared the CI with the absolute size of energy reserves. We present a novel CI (the ‘scaled mass index’) based on the central principle of scaling, with important methodological, biological and conceptual advantages. Through a reanalysis of data from small mammals, starlings and snakes, we show that the scaled mass index is a better indicator of the relative size of energy reserves and other body components than OLS residuals, performing better in all seven species and in 19 out of 20 analyses. We also present an empirical and theoretical comparison of the scaled mass index and OLS residuals as CIs. We argue that the scaled mass index is a useful new tool for ecologists.