Virtually all rats in the UK are the Norway rat (Rattus norvegicus) – this is also known as the Brown rat or Sewer rat.
Rattus norvegicus likes to live in sewers and when the Victorians invented the sewer and put in Britains drainage infrastructure, this species of rat found the perfect place to live, feed and breed.
Numbers have been rising year on year ever since and the UK’s former native rat species – the Black rat (Rattus rattus) – has since dwindled to a few isolated pockets around the south coast.
Sewers provide a food rich and warm environment with an ample water source – their pitch black environment also suits the nocturnal instincts of the rat and they provide a predator free existence (apart from the other rats of course).
However they are not a great place to raise kids – they have fluctuating water levels with big extremes, there is not much to make a soft nest out of and there is not much in the way of privacy from other rats.
Leave a litter of new borns unattended in a sewer system and the other rats will eat them in no time!
Rats have a very black and white view on life…
Luckily though, all drains lead to a house if you travel against the flow and the rats soon discovered that these houses had dry, warm floor cavities and roof voids.
Such areas provide ideal secluded nesting sites tucked away from the rat race in the sewers so you fill find that most of the rats occupying your building fabric are in fact nesting females.
These females will still have to return to the sewer system on a daily basis to feed as the building fabrics contain no food but they co-ordinate this daily commute with when human activity peaks (i.e. during the day).
That is why noises within walls, floors and ceilings typically occur late evening and early morning – early morning is the rats leaving the building to feed in the sewers and late evening is when they return.
As buildings evolved central heating, thermal insulation and wall cavities (i.e. 1930’s onwards), building fabric temperatures rocketed which in turn increased the survival rate of resident rat populations.
Then as the DIY boom of the 1980’s combined with the widespread use of PVC drainage, the number of drain defects ramped up which in turn increased the opportunity for rats to invade building fabrics.
Over the past 20-30 years we also have climate change to contend with – warmer winters – which is also contributing to the problem by again increasing the survival rate of rats within the sewer system.
The quantity of food within the sewers is also increasing as population density with urban areas continues to increase and appliances such as dishwashers become more mainstream.
Given that these rats will also essentially eat what we eat (i.e. what ends up in the sewers) then its surprising to see how closely aligned their lives are with ours in many ways!