Rats, like wood pigeons, badgers and foxes, are synurbic – i.e. a species that thrives in urban habitats. Cities provide perfect conditions for rats, offering warmth and shelter and plentiful supplies of food – and cities are getting bigger. Human antipathy to rats is well founded: they breed quickly and spread disease. Until recently, unwanted rats have been controlled by the use of warfarin, a blood-thinning (anticoagulant) drug used medicinally on humans. It has been the perfect poison for rats, being odourless and colourless; rats will return to eat food laced with warfarin until a lethal dose is accumulated. But rats are developing resistance to warfarin and passing this resistance to their offspring. We do not yet have an alternative. The rat population will continue to increase as cities grow.