Evaluation of a Photographic Technique for Estimating Body Size in Lizards from a Distance

—Body size is often an important factor in understanding an animal’s ecology. In squamates, snout-to-vent length (SVL) is a primary metric for quantifying body size. Measuring SVL typically requires capture and handling of animals, which may be difficult due to a species’ behavior or its presence in complex habitat. Here, we evaluated an alternative to the capture, handling, and direct measurement of lizards via the use of paired photographs of animals in situ and with a ruler positioned in place of the lizard. Paired digital images were processed using the freely available image analysis software ImageJ. We found that snout-to-hindlimb length (SHL) measured directly on an animal was highly correlated with its SVL. In turn, SHL estimated from images was highly correlated with SHL measured directly on the animals and had an average error of 19.4 ± 16.2% (1 SD) compared with measurements made on the animals themselves. Estimates of total length from images were also highly correlated with total length measurements made directly on the animals and had an average error of 14.8 ± 12.9% (1 SD). There was no correlation between image measurement error and the distance from which photographs were made. Our results identify that the use of digital photographs and image analyses can in some cases eliminate the need to capture and handle lizards to quantify body size.