Not a lot of people know this, but out of the 3,500 mosquitoes species out there, just a few bite humans. This immediately begs the question: what’s so special about these species and their relationship with humans?

In a new study, researchers investigated the evolutionary pathways that enabled some mosquitoes to adapt to human settlements and grow a taste for our blood.

According to the new study, increased population density played a major role in mosquito adaptation to human blood. However, a dry climate was even more important.

That’s quite an important insight to have, considering mosquitoes — small and annoying as they may be — are known to spread dangerous infectious diseases. For instance, the findings suggest that increased urbanization in the coming years might increase mosquito biting and the risk of disease in tropical regions and beyond.

“We found that different African populations of the mosquitoes that spread dengue, Zika, chikungunya, and yellow fever vary widely in how attracted they are to humans. Mosquitoes living near dense cities were more willing to bite humans than mosquitoes in rural areas, but climate was even more important: in places with intense dry seasons mosquitoes showed a strong preference for humans,” Noah Rose, a researcher at Princeton University and co-author of the new study, told ZME Science.

“Mosquitoes that prefer humans showed differences in the same sets of genes, so we think this preference evolved just once about 5,000-10,000 years ago, likely as an adaptation helping them survive long dry seasons. Many cities in Africa are growing extremely quickly; our model suggests that mosquitoes in these places may evolve greater preference for human hosts, which could lead to them spreading disease more effectively.”

The Princeton researchers focused on their attention on Aedes aegypti, a highly successful mosquito species that inhabit areas in tropical, subtropical, and in some temperate climates, and the primary vector of infection for dengue, Zika, yellow fever, and Chikungunya virus.

In the grand scheme of things, Aedes aegypti is an oddity because it is one of only a handful of species from Africa that bite humans. The researchers used this to their advantage since it allowed them to empirically verify where specifically mosquitoes interact with humans the most and where they prefer to bite other animals instead.

“This study was very challenging to conduct, because we had to collect mosquitoes from a really wide range of habitats across a huge geographic range. So we were collecting mosquitoes everywhere from in the middle of a rainforest to the middle of a huge bustling city. Each of these places has unique challenges, whether it’s watching out for dangerous wild animals like lions and elephants, or getting out of a giant traffic jam on a busy city street. This was only possible because of the experience and expertise of our whole scientific team, which included people with years of experience in each of the countries and habitats in which we collected mosquitoes,” Rose said.

Using special traps, the team collected Ae. aegypti from outdoor sites in more than 27 locations across sub-Saharan Africa. In the lab, mosquitoes from each population were exposed to various animal scents (such as guinea pigs, quail, and humans) in a controlled environment in order to assess their preferences. The analysis of the recorded data suggests that mosquitoes from dense urban cities were attracted to people more than those from rural or wild areas.

But since this pattern of preference for human odors only held in extremely dense modern cities, it is highly unlikely that this was the original reason why Ae. aegypti mosquitoes evolved to bite humans.

The secondly identified pattern of mosquito preference for human blood seems more revealing and points to a more plausible avenue for genetic adaptation. Specifically, the researchers found that the insects that lived in drier, hotter regions had a strong preference for human scent when compared to other animal scents.

Somewhat counter-intuitively, the climate seems to have mattered more than having many blood bags on two legs in close quarters. Even more surprisingly, many mosquitoes living in dense cities don’t particularly prefer to bite human hosts — it is only when cities become extremely dense and are located in areas with intense dry seasons that our blood becomes very enticing to the insects.

“I think it’s because mosquitoes in habitats with intense dry seasons become particularly dependent on humans for their life cycle. Aedes aegypti larvae live in small, contained bodies of water. In the ancestral state, this was places like tree-holes or sometimes rock pools. Later, they adapted to surviving in human-associated containers like pots of stored water, or more recently buckets and tires,” Rose wrote in an email.

“In places with long, hot dry seasons, there are very few natural habitats for mosquitoes, but they may be able to survive year round by taking advantage of the habitat that humans make for them by keeping water stored near their homes. So these mosquitoes have a very close relationship with humans, which may have led them to specialize on biting humans.”

Genes concentrated in a few key regions of the mosquito genome seems to have driven this evolutionary shift in the insects’ biting preferences.

The researchers also modeled how climate change and expected urban growth might shape mosquito preferences in the near future — and it doesn’t look too good.

While climate change isn’t expected to cause important changes in dry season dynamics in sub-Saharan Africa over the next 30 years, many cities are expected to expand massively.

“Surprisingly, when we checked what our model predicts for near-term climate change in the next few decades, we didn’t see major changes in the precipitation variables that are important for mosquitoes. However, cities are growing extremely quickly, so we saw that our model predicts more human-biting in many cities across Africa due to urbanization effects. Longer term climate change could drive important behavioral shifts, but we haven’t extended our model that far — in the near term the urbanization effects seem to be more important,” Rose said.

In the future, Rose and colleagues will further investigate the interplay between climate, genetics, and urbanization in mosquitoes’ biting preferences.

“Overall, we hope this study will help people understand that all mosquitoes are not the same. Some spread disease much better than others. Even within species, there is enormous diversity. Mosquito history and human history are intertwined, and global changes driven by humans are also likely to drive further mosquito evolution,” Rose concluded.

The findings appeared in the journal Current Biology.