While previously European cities were inhabited by black rats (Rattus rattus), nowadays they are rare, having lost the ecological competition. Black rats are rarely seen in Finland, and usually they are limited to seaports or other transport hubs.
Brown rat is one of the largest members of the muroid superfamily. While on average brown rats are approximately 400 mm nose-to-tail and 300 grams of weight, it is not uncommon to encounter males which weigh more than half a kilogram. It lives in small family groups and it can be fiercely territorial.
Brown rats originated from Eastern Asia, likely from Northern China or Mongolia. While it was slower than black rat to spread over the globe, it did eventually displace black rats. Rats are brilliantly adapted to urban life: They live with whatever they are able to forage in cities. Their incisors can gnaw through different materials. They survive well changes in weather.
At the same time as rats conquered the outdoors, greenspaces and garbage pits in cities, they arrived inside houses. They have been kept as pets since late 18th century as they are easy to breed and take care and they are generally well-behaved and intelligent. The use of rats for laboratory experimentation started in 19th century and since then many important scientific discoveries have been made possible by lab rats' sacrifices.
Not much is known about the lives of urban rats in Helsinki. While they are regularly found all around the city, they are rarely numerous enough to cause worries in inhabitants of Helsinki. Building sites and carelessly built garbage collection sites are the most common reasons for rat sightings. Rats are also a substantial part of urban food chain as they seem to be a staple diet for some urban predators, including eagle owls, foxes and hawks.