The resting frequency of echolocation signals changes with body temperature in the hipposiderid bat Hipposideros armiger

Doppler shift (DS) compensating bats adjust in flight the second harmonic of the constant-frequency component (CF(2)) of their echolocation signals so that the frequency of the Doppler-shifted echoes returning from ahead is kept constant with high precision (0.1–0.2%) at the so-called reference frequency (f(ref)). This feedback adjustment is mediated by an audio–vocal control system that correlates with a maximal activation of the foveal resonance area in the cochlea. Stationary bats adjust the average CF(2) with similar precision at the resting frequency (f(rest)), which is slightly below the f(ref). Over a range of time periods (from minutes up to years), variations of the coupled f(ref) and f(rest) have been observed, and were attributed to age, social influences and behavioural situations in rhinolophids and hipposiderids, and to body temperature effects and flight activity in Pteronotus parnellii. We assume that, for all DS-compensating bats, a change in body temperature has a strong effect on the activation state of the foveal resonance area in the cochlea, which leads to a concomitant change in emission frequency. We tested our hypothesis in a hipposiderid bat, Hipposideros armiger, and measured how the circadian variation of body temperature at activation phases affected f(rest). With a miniature temperature logger, we recorded the skin temperature on the back of the bats simultaneously with echolocation signals produced. During warm-up from torpor, strong temperature increases were accompanied by an increase in f(rest), of up to 1.44 kHz. We discuss the implications of our results for the organization and function of the audio–vocal control systems of all DS-compensating bats.