Cornell and Binghamton University researchers report for the first time that mosquitoes can hear over distances much greater than anyone suspected.
The findings were published Feb. 7 in the journal Current Biology.
Until now, it was believed that organisms required eardrums for long-range hearing, and that the feathery antennae with fine hairs that mosquitoes and some insects use to hear only worked at close distances of several centimeters (a few inches).
A series of experiments has now provided neurophysiological and behavioral evidence that Aedes aegypti mosquitoes – which transmit such diseases as yellow fever, Dengue, Zika, West Nile and Chikungunya viruses – can hear specific frequencies as far away as 10 meters (32 feet) or more.
These frequencies overlapped well with the frequencies of female mosquitoes in flight as well as human speech.
“It’s been known for quite a long time that male mosquitoes are drawn to the sound of the female’s beating wings,” said Ron Hoy, the D & D Joslovitz Merksamer Professor of Neurobiology and Behavior and the paper’s senior author. Gil Menda, a postdoctoral researcher in Hoy’s lab, is the paper’s first author.
Hoy noted that since mosquitoes mate in mid-air, the sound of the female’s wings buzzing sets the males in motion. Previous experiments to prove that males are drawn to the sounds of females in flight were done at close range, which reinforced the idea that they only hear at close range – up to 30 centimeters (approximately 1 foot).
Past research by Menda and Hoy to prove hearing in jumping spiders gave them the methods and skills needed to tap the auditory nerves of mosquitoes, and record the electrical potential of the excited nerves.
Initial tests in the lab revealed the mosquitoes’ auditory nerves picked up sounds from across a room. To prove this principle, the researchers set up an experiment in Barton Hall, a field house with a 30 meter (100-foot) ceiling that would reduce echoes. Menda fitted mosquitoes with an electrode in their brains and made neurophysiological recordings of the auditory nerve being stimulated by pure-tones emitted from a loudspeaker 10 meters away.
“They’re hearing at distances that normally require ear drums, but these are hairs,” said Hoy. Ear drums work by picking up pressure from sound waves, while tiny hairs sense sound from air particles vibrating at certain frequencies.
They then moved the nerve physiology equipment to a super-quiet anechoic room run by collaborator Ron Miles, professor of mechanical engineering at Binghamton University. “It’s the quietest room in the Northeast and possibly in the country,” Hoy said.
“We found the sweet spot of frequency that the mosquitoes are sensitive to was between 150 to 500 hertz,” Menda said. Menda played back the tones of females’ wings beating, which occurs at a frequency of about 400 hertz. In behavioral experiments, when these 400-hertz tones were played from as far as 3 meters away – the length of the room – male mosquitoes in a mesh cage all instantly took to flight. The behavioral reaction was proved in individuals, to make sure they weren’t taking flight as part of a group response.
The mosquitoes’ frequency range for hearing also overlapped with human speech. “The most energetic frequencies of an average human vowel is in the range of 150 to 900 hertz,” Hoy said, so “they should be able to hear” people speaking.
Also, using the anechoic room, “we showed the sensitivity of the male mosquito was so low that when I played a tone, it was hard for me to hear it, but I can see the mosquito can hear it,” Menda said. They recorded excited auditory nerves at 30 decibels. Human speech is typically spoken at 45 to 70 decibels, also within the mosquito’s sweet spot.
While the study provides both neurophysiological and behavioral evidence that male mosquitoes hear sounds from far field, it offers no proof that they use it to home in on people. The insects are known to pick up sensory cues such as carbon dioxide, odors and warmth to locate people. But the results do show an intriguing correlation, Hoy said.
Though the study does not suggest viable new avenues for mosquito control, it does open the door for developing highly sensitive directional microphones and hearing aids that use fine hairs that sense the speed of air particles as they are jostled by passing soundwaves.
Laura Harrington, Cornell professor of entomology, and Paul Shamble Ph.D. ’15, a John Harvard Distinguished Science Fellow at Harvard University, are co-authors of the paper, which was funded by the National Institutes of Health and a Gates Challenges Explorations Grant.