Horseshoe bats (Rhinolophus) use echolocation calls with a prominent part whose frequency is constant over time (CF) and matches the ‘acoustic fovea’ of the bats' hearing system. The present study on European Rhinolophus species investigates whether this CF component contains reliable information on species, sex, age class or quality (size and body condition) of the caller and could therefore have a communicative value. The resting frequencies (RF) were measured from stationary, handheld bats for the species Rhinolophus blasii, R. euryale, R. mehelyi and R. hipposideros (the latter with very low sample size) in Bulgaria, where they occur in sympatry. We recorded calls directly onto a laptop computer and used a specially designed analysis algorithm to achieve high and accurate frequency resolution. After silent periods, individuals ‘tuned in’ to their RFs always from lower frequencies, corroborating the recent finding that frequencies below RF might be used for auditory feedback control as well. Rhinolophus blasii could reliably be separated from its congeners by RF, while R. mehelyi overlapped strongly with both R. euryale and R. hipposideros. Only R. blasii showed sex and age differences in RF, albeit the overlap was large. Adult female R. blasii had higher RFs than both adult males and juvenile females. In R. blasii, RF was positively correlated with forearm length, body mass and body condition index; in R. mehelyi with body mass and body condition index. However, there was no correlation between RF and these body size parameters within a sex or age class for any of the species, suggesting that RF is not a reliable honest signal for intraspecific communication that would indicate the quality of a potential mate or competitor.
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