ESTIMATING FITNESS - A COMPARISON OF BODY CONDITION INDEXES

Behavioral ecologists might often benefit by the ability to directly m easure an animal's body condition as an estimate of foraging success, and ultimately fitness. Here we compare the reliability and effectiven ess of three indices of body condition that have been heavily used in the morphometrics literature. We examined the ratio index (body mass/b ody size), the slope-adjusted ratio index (based on regression slopes generated from a reference population), and the residual index (the re siduals of a regression of body mass on body size). We present the res ults of tests performed in the field and laboratory on two ecologicall y and evolutionarily divergent spider species: the vagrant wolf spider Pardosa milvina (Araneae, Lycosidae), and the colonial orb-weaver Met epeira incrassata (Araneae, Araneidae). The ratio index correlated wit h body size, which weakened the strength of conclusions that could be drawn. The slope-adjusted ratio index requires an independent and larg e data set with which to generate the expected values, and was likewis e sensitive to body size. The residual index, with appropriate transfo rmations to achieve homoscedasticity, was the most reliable index beca use it did not vary with body size, and we recommend its general use i n behavioral studies that require a condition estimate.