Evolution breeds the super rat

Enemies — they’re all around us, or that’s how it sometimes feels. Not burglars or terrorists, but non-human enemies in the natural world. The media describes some of these as “super—” but in what way are they super and where do they come from?

In this week’s science lecture, Professor Rob Smith, Dean of Applied Science at Huddersfield University, investigates the world of the super rat.

Modern technology insulates most people in western society from what were for millennia the brutal facts of a hard life and an early death.

Disease and food shortage kept human populations in check for thousands of years.

The death toll due to disease fell when it was realised that bacteria were responsible for many plagues, rather than divine retribution.

Basic hygiene, immunisation and public health measures such as providing clean water were the first effective defences.

Chemical warfare added greatly to the armoury in the mid 20th century.

Antibiotics had an enormous impact in reducing mortality caused by bacterial infections. DDT saved millions of lives by killing mosquitoes and other insects that carry disease.

Anticoagulant rat poisons did the same job by allowing effective control of rats for the first time.

But nature always fights back. We now have super rats and superbugs.

These are new strains of natural enemies, described as Super because they have evolved to be resistant to our chemical weapons.

Our natural enemies have moved ahead in the evolutionary arms race.

In response, we could try to rely less on drugs and pesticides but mostly we rely on scientists to develop new chemicals. These solutions are short term — natural enemies sooner or later evolve defences against our chemicals.

How do our enemies keep up in the evolutionary arms race? DNA is the genetic material that codes for how most organisms develop and function.

This genetic code is copied accurately from one generation to the next but changes in the DNA code called mutations sometimes happen.

Mutations are mostly harmful but a few are useful. The sort of change that protects rats against poisons is very useful to rats, if not to us.

Rats with mutations that protect against poison will live while other rats die and the protective, resistance mutation will be passed on to the next generation.

This is how super rats evolved — through a process of Darwinian natural selection of favourable mutations in genes, which are passed on to young rats and inevitably spread through a population.

The first anticoagulant rat poison in Britain was warfarin. An anticoagulant stops blood clotting so that poisoned rats gradually lose blood and die.

Resistance to warfarin and similar chemicals soon appeared and became widespread in the 1960s, making it harder to control rats.

Chemists then developed more powerful chemicals that worked well for about 20 years until new resistance mutations evolved.

We have now run out of choices for controlling rats in parts of the UK.

In some places (mostly in the south), the modern super rat is out of control; legal rat poisons do not work and only inefficient methods (traps, guns and dogs) are available to control rats.

Super rats also appeared in East Yorkshire and caused one pig farmer to close his business though luckily, they do not seem to have spread.

There are newer chemicals that can control super rats, but these cannot legally be used outside buildings in Britain because of fears that they might accidentally kill other animals.

The best thing we can do to prevent rat numbers building up is to make life harder for them by clearing up rubbish and food and using rat poison only when it is really necessary.

Removing rubbish helps because rats do not like crossing open spaces where predators can catch them.

Rats cannot live and breed without food so waste food should only be left out in rigid bins.

Unfortunately, many people irresponsibly help rats by leaving out both piles of rubbish and waste food such as half-eaten pizzas.

Much of the problem of super rats is our own fault, the rest is down to evolution.