Convergence of reference frequencies by multiple CF–FM bats (Rhinolophus ferrumequinum nippon) during paired flights evaluated with onboard microphones
The constant frequency component of the second harmonic (CF2) of echolocation sounds in Rhinolophus ferrumequinum nippon were measured using onboard telemetry microphones while the bats exhibited Doppler-shift compensation during flights with conspecifics. (1) The CF2 frequency of pulses emitted by individual bats at rest (Frest) showed a long-term gradual decline by 0.22 kHz on average over a period of 3 months. The mean neighboring Frest (interindividual differences in Frest between neighboring bats when the bats were arranged in ascending order according to Frest) ranged from 0.08 to 0.11 kHz among 18 bats in a laboratory colony. (2) The standard deviation of observed echo CF2 (reference frequency) for bats during paired flights ranged from 50 to 90 Hz, which was not significantly different from that during single flights. This finding suggests that during paired flights, bats exhibit Doppler-shift compensation with the same accuracy as when they fly alone. (3) In 60 % (n = 29) of the cases, the difference in the reference frequency between two bats during paired flights significantly decreased compared to when the bats flew alone. However, only 15 % of the cases (n = 7) showed a significant increase during paired flights. The difference in frequency between two bats did not increase even when the reference frequencies of the individuals were not statistically different during single flights.