Residents in Espoo have noticed a lot of creatures scurrying around recently.
Rattus Norvegicus – brown rats – have been spotted in apparently greater numbers especially near metro stations leading some locals to wonder if perhaps the rodents have been using the recently opened Länsimetro extension to migrate west from Helsinki.
The short answer, say experts, is no. But the longer answer is more interesting.
“Wherever there are people, there are always rats” says Jukka Kiesi, Environmental Inspector for the City of Helsinki.
“Norwegian rats, we call them ‘iso rotta’ or big rat in Finnish, are quite big. When it’s an adult it might be almost 30cm and plus the tail. They can be really big and when people call me they always say it’s extra, they say it’s the size of a cat!” he laughs.
It’s Kiesi’s job to keep tabs on Helsinki’s rat population, but it’s the responsibility of individual property owners to call exterminators if there’s any rat problem at a particular building.
There’s no specific data on how many rats there are in the capital city region, although a study is ongoing to try and get an idea of the numbers.
“Some say there are as many rats as there are people. But I think that would too much. We would see them all day and everywhere” says Kiesi.
“There might be 100,000 or 200,000. Nobody knows yet” he adds.
Metro construction brings out the rats
The Länsimetro extension opened almost two years ago, in November 2017, and linked the edge of Helsinki with eight new stations going west into Espoo along 13.5km of tunnels and track.
During the construction phase there was a marked increase in the number of rats seen along the route.
“Starting of subway sites and building in general usually get the rats moving and because of that, there will be more observations from the inhabitants” explains Minna Oja from Espoo City’s Centre for Urban Technology.
It turns out the reason for so many rat sightings is because the Rattus Norvegicus doesn’t like construction.
“Generally wherever there is construction the rats move, because they are frightened by the construction when they are in their burrows” says Tuomas Aivelo a Postdoctoral Researcher at the University of Helsinki, and one of Finland’s leading experts on urban rats.
The shockwaves from blasting metro tunnels literally drive the rats from their homes and that’s why they become a very common sight at construction areas.
Why rats don’t use metro tunnels to migrate
It turns out that rats like to stay close to home – their home, and our homes too.
“Thinking of cities where we know how the population structure of rats goes, that would be New York and obviously in New York there are lots of sightings of rats in subway stations. But it doesn’t seem to be that they migrate through the tunnels” says Aivelo.
“The population structure of rats is rather limited, rats don’t move much, and might only move within 50 metres, so if a new rat tries to get into areas where there are already rats, they will defend aggressively” he explains.
In New York there’s been no data to suggest that subway tunnels make the dispersal of rats any easier, with distinct populations uptown and downtown.
Tuomas Aivelo thinks it will be there same here in Finland.
“Subway tunnels really aren’t the places rats want to go. There’s long distances between the stations, and there’s no food down there” he says.
In Helsinki, as in New York, rats are more likely to thrive in the sewage tunnels underneath the city where there’s plenty of food.
So why are more rats being spotted?
It seems that local residents are seeing more rats around the new metro stations, rather than inside the stations, on underground platforms, or running along the tracks.
That’s likely because more human activity brings more trash to the metro stations. More trash cans, more dropped litter, more scraps of food for the local rats to feast on.
“It would be exceptional to see rats indoors, you’re more likely to see them around subway stations” says Aivelo.