Behavioural analysis of swarming mosquitoes reveals high hearing sensitivity in Anopheles coluzzii

Mosquitoes of many species mate in station-keeping swarms. Mating chases ensue as soon as a male detects the flight tones of a female with his auditory organs. Previous studies of hearing thresholds have mainly used electrophysiological methods that prevent the mosquito from flying naturally. The main aim of this study was to quantify behaviourally the sound level threshold at which males can hear females. Free-flying male Anopheles coluzzii were released in a large arena (∼2 m high×2 m×1 m) with a conspicuous object on the ground that stimulated swarming behaviour. Males were exposed to a range of natural and synthetic played-back sounds of female flight. We monitored the responses of males and their distance to the speaker by recording changes in their wingbeat frequency and angular speed. We show that the mean male behavioural threshold of particle velocity hearing lies between 13 and 20 dB sound particle velocity level (SVL) (95% confidence interval). A conservative estimate of 20 dB SVL (i.e. <0.5 µm s-1 particle velocity) is 12-26 dB lower than most of the published electrophysiological measurements from the Johnston's organ. In addition, we suggest that (1) the first harmonic of a female's flight sound is sufficient for males to detect her presence, (2) males respond with a greater amplitude to single-female sounds than to the sound of a group of females and (3) the response of males to the playback of the flight sound of a live female is the same as to that of a recorded sound of constant frequency and amplitude.