1Anthropology, University of Illinois, Urbana-Champaign, 2New York Consortium of Evolutionary Primatology, WCS, Bronx Zoo, 3Anthropology, University of Michigan, 4Psychology, University of Michigan
Thursday All day, Clinch Concourse
Recent studies suggest that digital photogrammetry offers a promising non-invasive method to estimate stature in wild populations. In one version of this method, researchers use two parallel lasers mounted on a digital camera to provide a measurement scale within each photograph. However, if measurement landmarks are difficult to identify or if the photographed animals have different postures, the precision of this method can be undermined. Here, we used digital photogrammetry to measure shoulder-to-rump length in wild female geladas (Theropithecus gelada) in the Simien Mountains National Park, Ethiopia. First, preliminary trials on domestic dogs indicated that laser-based measurements were accurate on animals of comparable size and coat thickness. Second, in the field, we measured shoulder-to-rump length in a cross-section of photos from our adult, adolescent, and immature females. We calculated two sources of variation: (1) inter-measurer error (i.e., comparisons across people taking measurements from the same photo), and (2) inter-photo error (i.e., comparisons across multiple photos and postures taken from the same animal on the same day). The inter-measurer CV was low and comparable to previous studies. The inter-photo CV was slightly higher, with differences in stature contributing to higher CVs. These results suggest that researchers should select only the most comparable photos, as measurement error seems to be attributed more to differences between photos of the same individual, and less to differences in landmark identification by measurers. Because measurement error is particularly important for longitudinal studies on growth, it is critical to identify the appropriate interval for repeated measurements.
Data collection supported by the University of Michigan (to JCB), the National Science Foundation (BCS-0715179 to JCB), National Geographic (to JCB), the Leakey Foundation (to JCB), and the Wildlife Conservation Society (to AL, CM & JCB).