Greater Japanese horseshoe bats (Rhinolophus nippon) gradually converge their echolocation call frequency to colony members.

In bats using CF-FM echolocation calls, CF component frequencies vary among colonies, across geographic regions, and between sexes. In this study, we investigated frequency changes of the second harmonic of the CF component (CF2), which is the dominant harmonic in the multi-harmonic echolocation calls, that occur following the mixed housing of resident and wild-caught populations of greater Japanese horseshoe bats (Rhinolophus nippon). By focusing on individual-level frequency adjustments in the context of colony mixing, we tracked CF2 frequencies in a total of 101 individuals across 15 capture events over a one-month period. CF2 frequencies initially differed between the resident and wild-caught populations. These intergroup frequency differences gradually decreased after mixing, accompanied by a reduction in overall variability within the colony. This convergence was achieved asymmetrically, such that bats with lower CF2 frequencies adjusted their frequencies upward, while bats with higher CF2 frequencies individuals showed little or no change. No systematic changes were observed when initial frequencies did not differ between two groups. Echolocation in group contexts would be expected to suffer from severe acoustic interference caused by overlapping calls from conspecifics; however, CF bats instead tend to converge on a shared CF frequency. Because this convergence was primarily driven by individuals using lower frequencies, it may be interpreted as a strategy to allow individuals to share a "silent spectral window" above the CF2 frequency, which is critical for Doppler-based detection. In other words, for CF bats, in which the use of Doppler information is fundamentally important, frequency convergence may function as a strategy for avoiding acoustic interference.