Abstract

The Sonoyta mud turtle (Kinosternon sonoriense longifemorale) is an endangered subspecies that inhabits a small section of the Sonoyta River in Sonora, Mexico, and Quitobaquito Springs in Arizona, USA. To assess the status of this subspecies in the Sonoyta River basin, we surveyed all historical localities for extant populations of turtles. At sites where turtles were detected, we conducted quantitative trapping and visual encounter surveys to facilitate capture-mark-recapture techniques and population size estimation. Sonoyta mud turtles were absent from 2 historical sites, but we report 1 previously undocumented locality. Demographic analyses revealed sex-based differences in survival probability, with males exhibiting a lower survival rate than females, contributing to the consistent female-biased sex ratios and reduced male abundance. Abundance estimates for the Papalote reach were comparable to or exceeded historical values, while populations at most other localities appeared to be in decline. Entry probabilities were low, suggesting limited recruitment or immigration into the active population, consistent with the hydrological isolation of the Papalote reach. We documented sexual size dimorphism, with adult females exhibiting significantly larger body mass and straight carapace length than males. Although Sonoyta mud turtles persist in the Sonoyta River, their long-term viability is uncertain because of projected declines in surface water, and their survival appears dependent on continued human management.