Most female mammals schedule their sex life into just the couple of days in their cycle when they’re fertile. So why do humans do it whenever the mood strikes? (Well, in theory, at least.)

To find out, Dessa explores stinky t-shirt tests and all sorts of things we’d better not mention here. Parental discretion is strongly advised. 

Deeply Human is a BBC World Service and American Public Media coproduction with iHeartMedia.