Detecting between-individual differences in hind-foot length in populations of wild mammals

Hind-foot length is a widely used index of skeletal size in population ecology. The accuracy of hind-foot measurements, however, has not been estimated. We quantifiedmeasurement error in adult hind-foot length in yellow-belliedmarmots (Marmota flaviventris (Audubon and Bachman, 1841)), mountain goats (Oreamnos americanus (de Blainville, 1816)), and bighorn sheep (Ovis canadensis Shaw, 1804) from long-term capture–recapture studies. Fitting a linear mixed effect model for each species separately, we found that hind-foot length was significantly repeatable in the three species, but repeatability was low, ranging from 0.30 to 0.47. Measurement error explained 53%–66% of the variance in foot length. Differences of 6, 13, and 27 mm would be indistinguishable frommeasurement error for marmots, goats, and sheep, respectively. At least 4–6 measures per individual were needed to detect variation in foot length between individuals of a population using a mixed effect model. Researchers should strive to limit measurement errors because inaccurate measures may obscure important biological patterns.