Behavioural analysis of swarming mosquitoes reveals higher hearing sensitivity than previously measured with electrophysiology methods

Abstract

Background: Male and female mosquitoes of many species mate in swarms consisting of males and uninseminated females; males form station-keeping swarms and mating chases ensue as soon as a male detects the flight tones of a female. The auditory organs of mosquitoes have been reported to be the most sensitive among arthropods, enabling them to track their flying mate by the sound of the female's flight tones during mating chases. Previous studies on hearing thresholds have mainly used electrophysiological methods, which unfortunately limit the natural behaviour of the mosquitoes. This study explores male auditory behavioural sensitivity to female flight tones. Methods: We developed a behavioural method for measuring hearing sensitivity in swarming male mosquitoes. Free-flying male Anopheles coluzzii mosquitoes were released in a large arena (~2 m high x 2 m x 1 m) with a visually conspicuous object on the ground that stimulates swarming behaviour consisting of elliptical flight pattern over the object. Males were exposed to a range of natural and synthetic played-back sounds of female flight emanating from a speaker located at the same height as the swarm centre and 0.9 m away from it. We monitored the responses of males to female sound by recording the flight-tone and flight-dynamic responses of males. Calibrated sound-levels were measured at the swarm centre; the changing distances between the speaker and the flying males as well as other measurement uncertainties were taken into account to estimate the sound levels that males were effectively exposed to. Results: We show that the mean male behavioural threshold of particle-velocity hearing lies between 13-20 dB SVL (95%-CI). A conservative estimate of 20 dB SVL (i.e., less than a particle velocity of 0.5e-6 m/s) is already 12 to 26 dB less than most of the published electrophysiological measurements on Johnston's organ sensilla. Our results confirm, under laboratory conditions, the high behavioural hearing-sensitivity of swarming mosquitoes recently found with swarming Aedes communis in the field, and suggest a hearing threshold at least two times lower than that of published results (26 dB SVL or 1e-6 m/s particle-velocity). In addition, we established that 1) the first harmonic of Anopheles coluzzii female flight sound is sufficient for males to detect her presence, 2) males respond with a greater amplitude to single-female sounds than to group-female sounds and 3) males respond to the playback of single-female sound as much as to pure-sound at a constant frequency and amplitude.