How far away can an insect as tiny as a female mosquito hear the males in a mating swarm?

Lionel Feugère1,2 – l.feugere@gre.ac.uk
Gabriella Gibson2 – g.gibson@gre.ac.uk
Olivier Roux1 – olivier.roux@ird.fr
1MIVEGEC, IRD, CNRS,
Université de Montpellier,
Montpellier, France.
2Natural Resources Institute,
University of Greenwich,
Chatham, Kent ME4 4TB, UK.

Popular version of paper 5aAB4
Presented Friday morning during the session “Understanding Animal Song” (8:00 AM – 9:30 AM), May 17, 2019

177th ASA Meeting, Louisville, KY

Why do mosquitoes make that annoying sound just when we are ready for a peaceful sleep? Why do they risk their lives by ‘singing’ so loudly? Scientists recently discovered that mosquito wing-flapping creates tones that are very important for mating; the flight tones help both males and females locate a mate in the dark.

Mosquitoes hear with their two hairy antennae, which vibrate when stimulated by the sound-wave created by another mosquito flying nearby. Their extremely sensitive hearing organ at the base of the antennae transforms the vibrations into an electrical signal to the brain, similar to how a joystick responds to our hand movements. Mosquitoes have the most sensitive hearing of all insects, however, this hearing mechanism is optimal only at short distances. Consequently, scientists have assumed that mosquitoes use sound for only very short-range communication.

Theoretically, however, a mosquito can hear a sound at any distance, provided it is loud enough. In practice, a single mosquito will struggle to hear another mosquito more than a few centimeters away because the flight tone is not loud enough. However, in the field mosquitoes are exposed to much louder flight tones. For example, males of the malaria mosquito, Anopheles coluzzii, can gather by the thousands in station-keeping flight (‘mating swarms’) for at least 20 minutes at dusk, waiting for females to arrive. We wondered if a female mosquito could hear the sound of a male swarm from far away if the swarm is large enough.

To investigate this hypothesis, we started a laboratory population of An. gambiae from field-caught mosquitoes and observed their behaviour under field-like environmental conditions in a sound-proof room. Phase 1: we reproduced the visual cues and dusk lighting conditions that trigger swarming behaviour, and released males in groups of tens to hundreds of males and recorded their flight-sounds (listen to SOUND 1). 



Phase 2: we released one female at a time and played-back the recordings of different sizes of male swarms over a range of distances to determine how far away a female can detect males. If a female hears a flying male or males, she alters her own flight tone to let the male(s) know she is there.

Our results show that a female cannot hear a small swarm until she comes within tens of centimeter of the swarm. However, for larger, louder swarms, females consistently responded to male flight tones. The larger the number of males in the swarm, the further away the females responded; females detected a swarm of ~1,500 males at a distance of ~0.75 m, and they detected a swarm of ~6,000 males at a distance of ~1.5 m.

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