Within minutes, mosquitoes, attracted to either the human or the nonhuman scent, would pick a tube and enter it. Later, the tubes were removed to count the mosquitoes and figure out how many preferred Dr. Rose.
The resulting data revealed that mosquitoes that originally came from very dense areas — more than 5,000 people per square mile — liked humans more. (They also had more ancestry from the human-preferring subspecies.) A bigger factor, however, was climate. Specifically, mosquitoes that came from places that had a rainy season followed by a long, hot, dry season greatly preferred humans.
Why? The scientists proposed an explanation that Brian Lazzaro, a professor of entomology at Cornell University who was not involved with the study, called “pretty convincing.” Mosquitoes flourish during the rainy season, but then must find a way to survive the dry season. Standing water, critical for mosquitoes to breed, is hard to come by in extremely arid environments. But it can be found around humans, who store water to live, and so mosquito populations from arid regions evolved to take advantage of the situation.
Dr. Lazzaro also praised the team for sequencing the mosquitoes. That procedure revealed that the human-loving mosquitoes were genetically distinct from the animal-loving ones, and found that the preference for humans developed at one location and then spread across Africa. “They really see a single origin of these human-feeding mosquitoes,” he said. “That is a little surprising to me,” he added, because there plausibly could have been multiple instances of genetic adaptation.
The Current Biology paper focused on evolutionary history, but its findings might have implications for public health. The results, combined with climate and population data from the United Nations, suggest that there will be more human-biting mosquitoes in sub-Saharan Africa by 2050, caused mostly by urbanization.
“I think it’s counterintuitive, because people know the climate is changing rapidly, so that should be the driving force,” Dr. McBride said. “But the features of the climate that we found to be important for this mosquito aren’t predicted to change in strong and clear ways that would affect the mosquito.”
Urbanization, in contrast, is occurring very quickly. “You could easily imagine that having an effect on disease transmission in big cities,” Dr. McBride said.