By 2020, the World Health Organization (WHO) estimated that nearly half of the world's population was at risk of malaria, while around 627,000 people died from the disease.
Although a malaria vaccine may soon be available (WHO recommended one for children last year), malaria is just one of several mosquito-borne diseases. And the total number of mosquito-related infections is bound to increase as climate change expands mosquito populations.
So in order to reduce the disease burden from malaria and other mosquito-borne diseases, we need to continue to develop effective tools to control mosquito populations.
A primary goal is their copulation in the air. The mosquito mating ritual involves a man identifying and pursuing a flying female by discovering her weak flight tone.
If the male cannot hear the female properly, then the hunt fails and they do not mate. Reproduction in mosquitoes depends to a large extent on their sense of hearing.
We investigated the behavior of mosquitoes that cause malaria (the Anopheles gambiae species) to understand more about how males listen for females to secure a mate. Our results have recently been published in the journal The progress of science.
But first a little background. The hearing mechanism of mosquitoes is unique, yet poorly understood.
Both sexes' ears are almost deaf to each other's flying sounds, the frequencies of which are simply too high to be heard. To hear each other, they borrow a trick from physics.
When male and female flight tones are combined in a mosquito ear, they create lower frequency - and therefore audible - "phantom tones", called distortion products.
Distortion products are found only inside the mosquito ear and cannot be heard or recorded outside it.
A male mosquito must therefore fly to hear a flying female. And his own flight tone must be within a specific frequency range to generate audible distortion products with a given female.
We listened to the flight tones of the mozzies
We recorded flight tones (or "wing beats") of mosquitoes in incubators equipped with very sensitive microphones.
Our experiments included looking at 100 males and 100 females in separate incubators, individual mosquitoes (one male or one female, separately), as well as a mixed incubator with 50 mosquitoes of each sex.
In the incubators, we sought to mimic the conditions of their natural environment with lighting and by controlling temperature and humidity.
We were able to measure the frequency of mosquito wingbeats over several days and at different times of the day.
We found that male mosquitoes, but not females, changed their flight tones in a daily pattern. By beating their wings about 1.5 times faster than the females, the males optimize their ability to spot a single female in crowded swarms.
More than ten years ago, scientists proposed and described an acoustic interaction between men and women as "harmonious convergence". While they identified the same relationship between wingbeats that allows mosquitoes of the opposite sex to hear each other (equivalent to 1.5 male wingbeats to a female wingbeat), we found that this happens by default and does not actually require any interaction between the sexes.
In particular, we found that males flapped their wings faster at dusk than at other times of the day. This makes sense because in Anopheles gambiae mosquitoes, males fly predominantly around dusk when forming mating swarms, often at 1,000 mosquitoes or more.
These swarms are visited sporadically by a few virgin females. As you can imagine, finding a mating partner is not easy.
The males' increase in wingbeat frequency at dusk changes the frequency of the distortion products, which become more audible to the male ear than those created at other times of the day. So by adjusting their wingbeats in the swarm, they are better able to hear the females and increase their chances of finding one to mate with.
The males' flight tone adjustment is partly driven by their 24-hour clocks. Hitting faster with the wings is likely to be very energy consuming for the males, so they limit this behavior to the time of swarming.
What do our results mean?
It will be important to replicate similar experiments outside the laboratory, especially among mosquito swarms in their natural habitat. We have already started working on this in Tanzania.
Yet these results open new avenues for research into the evolutionary ecology of hearing, the unique hearing system in mosquitoes, and mosquito behavior more broadly.
They could also contribute to the mosquito control effort. As part of vector control programs, mutant males will be released into the wild to collapse local mosquito populations. Mutated male mosquitoes are genetically modified, so when they mate with a female, the offspring are not viable and will die.
Mating efficiency in this context depends to a large extent on the ability of the released males to hear the "resident" females. Our results suggest that in order to create a successful program, it may be important to assess male and female flight tone distributions along with male hearing areas before releasing the mutant mosquitoes.
This would strengthen any intervention by ensuring that the mutants' mating efficiency is optimal - essentially that they can compete with the resident male mosquitoes to identify and mate with the resident females.
Joerg T Albert, Professor of Sensory Biology and Biophysics, UCL; Alex Alampounti, Research Fellow, Biophysics, UCL, and Marcos Georgiades, PhD candidate, Neurobiology and Biophysics, UCL.
This article is republished from The Conversation under a Creative Commons license. Read the original article.