Mosquitoes evolved to suck human blood when they couldn’t find water

Mosquitoes evolved to bite humans if they lived in places with intense dry seasons, according to a study of African mosquitoes. The insects need water to breed and may have latched onto people because we store it in large quantities.

Many mosquitoes bite a wide range of animals, but some have specialised in biting humans and nobody knew why until now. Aedes aegypti mosquitoes often specialise in humans, bringing diseases like Zika, dengue and yellow fever with them. But some African populations of the species have a wider diet.

“No one had actually gone through and systematically characterised behavioural variation in Africa,” says Noah Rose at Princeton University in New Jersey. To do this, he and his colleagues captured A. aegypti eggs from 27 sites in sub-Saharan Africa and raised them in a lab.

They then put the mosquitoes in a chamber where they could catch a whiff of either a human or an animal – a guinea pig or a quail – to see which they would move towards to attempt to bite. A huge range of preferences was found.

The researchers then built a model to determine which factors affected the mosquitoes’ preferences. Those living in areas where the dry season was long and intense were much more likely to prefer humans. There was also a smaller effect of urbanisation: 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 rear their young. But humans often create sources of standing water, whether by storing rainwater in barrels or by irrigating crop fields. Mosquitoes that lived thousands of years ago may have been drawn to these places and thus evolved to bite humans.

The story may well be different for Anopheles mosquitoes, which spread malaria, says Rose. These mosquitoes are only distantly related to A. aegypti and have a different life cycle. “The adults can go into a state called aestivation, where they dry out through the dry season,” says Rose.

The model does suggest that more populations of A. aegypti will start to prefer humans between now and 2050. Africa is becoming urbanised and this is expected to have a strong impact on the mosquitoes’ evolution. Surprisingly, climate change may not make a big difference to this over the next three decades, because it isn’t predicted to drastically alter Africa’s dry seasons.