Left to choose her own mate, a female guppy will opt for a brightly coloured orange fellow – unless she notices that the other girls are going for a slightly less orange male, in which case she will too. She will even be prepared to dump a previously selected bright orange mate for a duller one.

Lee Alan Dugatkin, the US biologist to whom we owe this knowledge, has observed that guppy love is like the human variety. A failure at high-school dating, he noticed early on that the boys who got the dates seemed to get more.