Feral Cities (31 page)

But while London has opted to do no more to keep them at bay, Paul Doumer's dream of a rat-free city lives on elsewhere. Copenhagen has talked about eradicating rats by 2015 and New York has been experimenting with ways to sterilize its rats. So far just three large cities claim to be rat-free. The first two are Calgary and Edmonton in Canada, and both owe their lack of rats to the swift action taken in the 1950s to keep brown rats from entering the entire province of Alberta. The third is the Hungarian capital Budapest. It faced a tougher challenge: eliminating rats that had already conquered the city.

In 1970 Budapest was a city under siege, a place where Boelter's claim of one rat for each of its two million people actually held true. A third of properties were infested, including the whole of downtown, and each year the damage caused by rats was costing the city the equivalent of $40 million in today's money. In response the city declared all-out war on the rats and brought in Bábolna Bio, the pest control division of a state-controlled stud and chicken farm, to eliminate them.

What followed was a blitzkrieg. For almost two years Bábolna's extermination squads combed the streets, homes, factories, and sewers for rats. No location was left unchecked. The Communist government had required all citizens and businesses to open their doors to the pest controllers, and everywhere the exterminators went they laid down rat poison. By the end of the crackdown in December 1972, more than one and a half thousand tons of rodenticide had been spread over the city, and hundreds of thousands of rats were dead.

Since then Bábolna Bio, which is now privately owned, has been monitoring every site rats could use to recolonize the city, from railroad stations and markets to rivers and sewers, and acting fast
to stamp out any rats that do show up. As a result Budapest has been near enough rat-free for nearly half a century. Today just one in every thousand Budapest properties gets a rat infestation every year. In comparison, three in every hundred homes and five in every hundred commercial properties in Britain have rats.

While Budapest proves that eliminating rats is possible, it is not an easy task, says Bábolna Bio's managing director, Daniel Bajomi. “Theoretically the same results could be achieved in New York or Paris too,” he says, but it needs strong political will, a lot of money, and “a political system that ensures access to every premises.”

And even then rats still cling on in small numbers. “Rats still live in the sewer system in minimal numbers,” says Daniel.

Rats are not the only things living beneath cities. There are fungi in the sewer, for starters. “It's dark and dank, so it's the ideal environment for growing mushrooms and you do get some odd growths growing down there off the walls in a few places,” says Rob. “We've got toads and frogs too, and some horrible little black flies that always manage to get in your eyes and your mouth whenever you have a deep breath.”

Another resident of subterranean London is the mosquito
Culex pipiens molestus,
which first gained attention during the Second World War when it began feasting on people sheltering from German bombs in London Underground tunnels.

But
Culex pipiens molestus
is more than just another bloodsucking fly. It's actually a special underground form of the northern house mosquito,
Culex pipiens,
that can be found across the temperate zone of the Northern Hemisphere, from the northern half of the United States to Europe and onward, through Central Asia, to China and Japan.

Above ground the northern house mosquito behaves much like we expect mosquitoes to. The females need to feed on blood to develop their eggs, and while they much prefer to suck the blood of
birds, if that's not an option us or another mammal will suffice, and they have no qualms about entering homes to find that vital meal. In fall, the females that are still alive enter diapause—a state of suspended animation that helps them survive until they reawaken in the spring.

But although it is the same species, the
molestus
form behaves very differently and is limited to dark, underground worlds like the London Underground, other metro systems, or the sewers. Unlike its brethren on the surface,
molestus
doesn't do diapause. Instead it is active year-round, cocooned by the largely constant temperatures of the urban underground, which is insulated from the winter cold and summer heat by the surrounding soil.

The
molestus
form also doesn't need blood. It can lay its first raft of eggs without feeding on an animal, a feat it can even manage when sugar is in short supply, which is often the case in the dark, flower-free underground. Instead it turns to blood when it hopes to lay more eggs and, since birds are rarely seen underground,
molestus
prefers mammal blood and makes rats, mice, and the occasional metro passenger its target.

Most of the time, the above- and below-ground forms of the northern house mosquito live separate lives. One roams the surface world, biting birds and the occasional person. The other buzzes around in the dark, laying eggs in stagnant pools of underground water and catching opportunistic blood meals from passing rats. So separate are their lives that the mosquitoes of the London Underground are genetically distinct enough from the mosquitoes living above ground to suggest the subterranean population originally entered the tunnels as far back as the late 1800s, when the deep Bakerloo and Central lines were being constructed.

The genetic differences don't stop at the surface. The mosquitoes may have colonized both the east-west Central line and the north-south Bakerloo line, but the only place these lines meet is at Oxford Circus station, and even then they are separated by a maze of escalators, stairs, and underground passageways. As such there's
little opportunity for the mosquitoes living on different Underground lines to meet, and over time that seems to have caused their populations to become distinct through a process known as genetic drift.

Genetic drift is about randomness and sampling error. Flip a coin a thousand times and you should end up with an even split where the result is half heads, half tails. But toss a coin ten times and there's a good chance that you would get more heads than tails because there is more chance of a disproportionate result when you have a small sample.

A similar principle applies to the genetics of small, isolated populations, like the London Underground mosquitoes. So when Bakerloo line mosquitoes breed, some genes, quite randomly, become overrepresented from one generation to the next. Over time this random effect makes them less and less like the population on the Central line, where genetic drift may have led to different genes being passed down.

As a result the isolated mosquito populations of the London Underground become less and less like each other as well as distinct from those on the surface. But when the above- and below-ground forms of the northern house mosquito meet, the results can be fatal.

One place this happened was in the villages west of Thessaloniki in northern Greece, where open sewers and cesspits allowed the usually isolated forms to interbreed. The fusion of underground and overground mosquitoes created hybrids with a heightened taste for the blood of both birds and mammals, as well as the ability to survive winter by entering diapause.

In August 2010 these hybrids caused the first outbreak of the West Nile virus in Greece, as the mosquitoes transferred the disease from migratory birds to people. It became one of the largest-ever outbreaks of the potentially deadly virus in Europe with more than 250 people infected and 35 dead.

It was not an isolated event. There's plenty of evidence suggesting that these hybrids caused the 1999 West Nile virus outbreak in New York that signaled the arrival of the disease in the Americas. Whereas in Europe the above- and below-ground forms rarely meet, in the United States, for reasons unknown, more than four out of ten northern house mosquitoes are hybrids. So when the virus reached New York, via bird or an infected person who caught it abroad, the mosquitoes were already well placed to spread it from bird to bird, bird to human, human to bird, and human to human.

Since then West Nile virus has spread across the United States, aided by migratory birds, hybrid northern house mosquitoes, and—in the South—their close relative
Culex quinquefasciatus
, the southern house mosquito.

Intriguingly, the
molestus
form that enabled these hybrids seems to be a recent development in evolutionary terms, emerging just ten thousand years ago. No one is sure why it came to be, but there are two theories. One theory is that a population of northern house mosquitoes got isolated during a glaciation. The other theory is that it developed in response to living alongside people, evolving to make the most of us and our settlements.

This latter theory is a startling idea, one that suggests cities may do more than just change the behavior of animals. Could it be that our cities are rewiring the very DNA of animals?

Mark Stoeckle's got mail: a small white envelope with his address at Rockefeller University scrawled on in black ink. He cuts it open and we look inside to find the squashed remains of a cockroach.

Mark gives it an approving nod. It's an American cockroach, and that's exactly the kind of roach he wants in his mail. For Mark, a geneticist at the New York City university, this is a regular occurrence. “They dribble in. Some days we might get none. Other days we get several.”

The mail is just the start. People give him cockroaches wherever he goes. “I actually went to a meeting last night and someone gave me a plastic bag of cockroaches, and they were apologizing, saying, ‘I tried to keep them alive.'”

Dead is preferred, Mark replied.

Mark's been collecting them too, scooping them off the Manhattan streets as he walks to and from his West Side home. “I carry a little plastic bag to collect them in,” he says. “The morning is the best time to find them because, I think, the birds eat them and the
ones you see out on the street in the day are dead while the ones at night are roaming around.

“I also went to some building supers in the neighborhood. The supers know where the cockroaches are and they were helpful. Some say we don't have any in our building, but that is sort of impossible for New York—if the building is connected to the sewer system, somewhere there are probably cockroaches.”

What happens now? I ask, looking at the dead roach in the envelope. “I stick it in the freezer,” he replies.

The freezer at Mark's laboratory is full of cockroaches, each stored in individual plastic vials with their origin recorded on them. Reassuringly, his roach collection is not a strange hobby but part of a
CSI
-style investigation into the genetics of New York City's most unwanted roommates.

It all began with sushi. “We've been doing projects with high school students for several years and, when my daughter was in high school, I had been working on DNA barcoding, which is a simple way of identifying species by their DNA,” Mark explains. “My daughter had the idea that we could test sushi and so we did.

“We found a quarter of the fish in sushi was mislabeled, and the results got in the
New York Times
and on television. That gave us the idea that there are really interesting questions you could do with students where you could make real discoveries.”

One of those high school projects tested the DNA of household bugs. It was supposed to be routine work, identifying everyday creepy crawlies, but one of the cockroaches didn't match the DNA records. Thinking that they may have found a new type of cockroach living in New York, they gathered more specimens and asked some experts to take a closer look. It wasn't a new species.

“They said that they are American cockroaches, but they were genetically distinct from what was in the database,” says Mark.

That was enough to get Mark thinking about a new study, one that probed the genetic history of the Big Apple's cockroaches. To do this work he would need plenty of roaches, so rather than
collecting them all himself, he asked the public to send in the bugs.