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 Franics 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.