Prick up your ears
To begin listening attentively.
This derives from the ability of dogs, horses etc. to lift their ears to hear more clearly. The allusion was known by 1626, when Francis Bacon used it in his Essays - On Fame. The allusion to erectness in this phrase, and the similar 'cock an ear' are the source of the slang terms 'prick' and 'cock' for penis.
Prick Up Your Ears was also the title of a film and stage play based on the life of the English playwright Joe Orton, but had an entirely different meaning there, as befitted Orton's scandalous reputation.