Assessment of digital image analyses for use in wildlife research

Due to the decreased cost and increased availability of digital camera technology, digital image analyses are rapidly replacing traditional caliper-based approaches to wildlife morphometry. The substitution of image-based measurement methods over those utilizing calipers is based on the assumption that both methods are comparable with respect to accuracy, precision, time required and inter-observer variation, yet this hypothesis has not been explicitly tested. In this article, we evaluate these aspects of each method using three life stages of the marbled salamander Ambystoma opacum. We found that digital image-based measurements were significantly more accurate, faster to obtain, and resulted in reduced inter-observer measurement variation relative to caliper measurements, yet the former was associated with reduced precision among repeated measurements relative to the latter. Based on our observations, we recommend that digital image-based measurements represent a useful alternative to caliper measurements and are particularly useful for organisms that pose risks to investigators, normally maintain non-linear body orientations, or cannot easily be removed from their habitat.