Testing Photogrammetry in Assessing Health of Houston Toads (Bufo [=Anaxyrus] houstonensis)

The Houston toad (Bufo [=Anaxyrus] houstonensis) is an endangered amphibian species that occupies a small range in Texas, USA. Despite recent increases in juvenile detections, obtaining data is limited by a narrow temporal window of juvenile emergence. This necessitates the rapid collection of ecological data. Because of this, we seek to test the quality of image-based measurements as an alternative to assessing the body condition of Houston toads. We used caliper- and image-based measurements of wild-caught adult toads and captive-bred juveniles, while recording handling time for each method with the juveniles. We compared scaled mass indices (SMI) and residuals from ordinary least squares regressions (OLS) between methods and life stages. Handling time of juvenile toads was significantly lower (p < 0.0001) for the image-based trial than the caliper-based trial. While SMI values violated key assumptions for a valid Condition index (CI), OLS condition index values did not. OLS condition values from the image-based trial were also not statistically significantly different to those from the caliper-based trial. These observations suggest that our image-based measurement technique is a valuable alternative to gaining morphometric data, and that applying this data to an OLS residual index is a more appropriate approach to monitoring individual- and population-level health in Houston toads.