The global explosion of rats
Rats are now found in every large city in the world. These rodents are revered by some, reviled by others; a true beauty and the beast of the animal world.
There are two species of rat in the UK: the sleek black rat and the larger, more numerous brown rat but neither is native. They arrived on our shores as stowaways from Asia, and have spread across the land.
The UK population of brown rats is peaking at 15 million… and rising. But why are rats seemingly taking over the world?
Rats: Fact and fiction
The mere mention of rats draws many negative connotations. You’ve probably heard phrases such as “dirty rat” and “drowned rat”, but do rats really deserve their bad name?
It's estimated that rodents are responsible for depleting one-fifth of the global food supply every year. And rats communicate and mark their territory by urinating pretty much everywhere they go. Some estimates put the annual financial cost of soiled or damaged goods caused by rats at over £11 billion ($19 billion US).
People think of rats as unclean but they actually spend long periods grooming and cleaning every day.
However, rats do still help transmit many diseases that affect humans, most notably blamed for the Black Death plague that swept through Europe in the 14th Century, as well as the Great Plague of London in the 17th Century. Black rats do carry fleas which can transfer harmful bacteria to humans when they bite. However, recent studies indicate that rats were not responsible for such a rapid spread of the pandemic Black Death plague.
Rats impact on our wildlife too, preying on insects, small mammals, amphibians, reptiles and more. Ground-nesting birds make particularly easy targets.
But rats, whilst not part of our native fauna, are also beneficial. They provide a source of food for our predators. Foxes, stoats and barn owls for example will readily hunt and eat rats.
And through their natural foraging techniques, rats act as seed dispersers. Their burrows also tend to aerate the soil, improving its overall condition.
These scavengers also deal with the mounting piles of waste we leave behind, doing a vital job of reducing scraps and waste.
The perfect design of rats
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Click on the labels to see how each feature makes the rat fit for city living.
The role of humans
Humans have a lot to answer to when it comes to the spread of rats. For a start, it was us that brought them to the UK. And where we go, rats will surely follow.
Welcome to synurbia
Rats, like wood pigeons, badgers and foxes to name but a few, are synurbic - species that thrive in urban habitats and will actively move into our towns and cities. And it's easy to see why. We have created the perfect conditions for rats. Cities are filled with cosy spots for rats to raise families in. Houses, sewers, derelict buildings give warmth and protection. And as our urban sprawl continues, we're making more habitat for rats.
Filling their potential
We throw away plenty of food these days - highly calorific food - which the omnivorous rats devour. The return of compost heaps, putting out bird feed and intensifying agriculture are providing more rats with enough energy to reach their upper size limits.
The dawn of the 'super rat'
But by far the greatest impact we have had on the UK's rat population is our blanket use of rodenticides, driving the expansion of poison-resistant rat populations, now dubbed as ‘super rats’.
Resistance to poison
Rats eat pretty much anything. The stomach contents of one rat revealed over 4,000 food items. And their scavenging ways meant we thought we had found a simple way to control them – laying down food laced with poison.
Anticoagulant rat poisons work after being eaten by preventing the blood from clotting, eventually resulting in death. Perhaps the most famous of the 'first generation' anticoagulant rodenticides is warfarin.
Introduced in 1948, this drug seemed ideal as it is both odourless and tasteless. It is effective when mixed with food bait, because the rodents will return to the bait and continue to feed over a period of days until a lethal dose is accumulated.
However, some rats are able to cope with the poison. They are then surviving, breeding and passing on their resistance to their offspring.
Of the surveyed counties in England, some reported that 70% of the rats are thought to show resistance to rat poison.
To combat the rise of poison-resistant rats, second generation rodenticides were released - 'super warfarins'. These had some initial successes but rat populations are now showing resistance to these too. Much like the widespread use of antibiotics in humans is helping create strains of resistant 'superbugs', the use of rodenticides is creating populations of poison-resistant 'super rats'.
Natural England says: “Using more potent (‘second-generation’) compounds may, temporarily, remove the practical problems associated with control failure but is likely to increase the selection pressure and eventually select for higher degrees of resistance.”
‘Mutant’ rats hit the headlines
The news is increasingly filled with stories of giant rats and super rats. Which of these do you think is an actual news headline?