RATS are four times the pest we thought they were.

Ken Aplin at the Smithsonian Institution in Washington DC looked at the genes of black rats (Rattus rattus) from 32 countries to work out their evolutionary history. He found that the rats fell into six distinct groups that diverged from one another about a million years ago, before modern humans evolved.

Four of these groups include rats that now live off humans, by eating our rubbish, for example. That means they became pests on at least four separate occasions, Aplin says (PLoS One, DOI: 10.1371/journal.pone.0026357).

Rats from different lineages may respond differently to rodenticides, says Aplin.