The protection of rare amphibians depends upon accurate information about their distributions, yet cryptic, low-density populations are easily missed during field surveys. We used sites with known populations of the U.S. federally threatened species Rana chiricahuensis (n = 34) and its congener Rana yavapaiensis (n =11) in Arizona, USA to test whether a conspecific call stimulus can improve detectability of species. We show that the use of the evoked vocal responses technique improves the detectability of initially silent anuran populations of R. chiricahuensis and R. yavapaiensis by 45% and 71%, respectively. Evoked vocal responses, however, were not strictly species specific: 19% of R. chiricahuensis and 45% of R. yavapaiensis populations also increased their vocal activity in response to heterospecific stimuli. The low costs of the material (<50 USD per set) and the short time (1 min) to play a stimulus suggest that the additional costs associated with this technique will be minor relative to existing survey protocols. This technique offers the potential to improve the detection of other frog species, particularly in sites that are difficult to search visually, provided it is carried out by surveyors who can identify species by their calls.

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