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Mosquitoes have evolved to bite humans if they lived in places with intense dry seasons, according to a study of African mosquitoes. Insects need water to reproduce and may have clung to humans because we store them in large quantities.
Many mosquitoes bite a wide range of animals, but some have specialized in biting humans and no one knew why until now. Aedes aegypti mosquitoes often specialize in humans, bringing with them diseases like Zika, dengue, and yellow fever. But some African populations of the species have a broader diet.
“No one had actually experienced and consistently characterized variations in behavior in Africa,” says Noah Rose of Princeton University in New Jersey. To do this, he and his colleagues captured A. aegypti eggs from 27 sites in sub-Saharan Africa and reared them in a laboratory.
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They then put the mosquitoes in a room where they could pick up the scent of a human or animal – a guinea pig or a quail – to see which they would move towards to try to bite. A wide range of preferences were found.
The researchers then built a model to determine which factors affected mosquito preferences. Those who lived in areas with a long and intense dry season were much more likely to prefer humans. Urbanization also had a smaller effect: mosquitoes in cities tended to prefer humans.
A long dry season is a problem for A. aegypti, says Rose, because these mosquitoes depend on standing water to raise their young. But humans often create sources of standing water, whether it’s by storing rainwater in barrels or irrigating fields. Mosquitoes that lived thousands of years ago may have been attracted to these places and therefore evolved to bite humans.
The story might be different for Anophele mosquitoes, which spread malaria, says Rose. These mosquitoes are only remotely linked to A. aegypti and have a different life cycle. “Adults can go into a state called aestivation, where they dry out during the dry season,” says Rose.
The model suggests that more populations of A. aegypti will start to prefer humans by 2050. Africa is urbanizing and this should have a strong impact on the evolution of mosquitoes. Surprisingly, climate change may not make a big difference in this area over the next three decades, as Africa’s dry seasons are not expected to drastically change.
Journal reference: Current biology, DOI: 10.1016 / j.cub.2020.06.092
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