Copulins are pheromones produced by females when they are in estrous or fertile. Copulins are relatively simple organic compounds that are commonly found elsewhere in nature; they send signalling clues, but traditionally have not been considered powerful aphrodisiacs like pheromones based on steroid molecules. But some recent research indicates that copulins may also have a direct action on testosterone and other steroids, and may give a smelly indication of which animals are primed to breed.

Chemicals considered copulins include:

• butanoic acid (butyric acid)
• 3-methylbutanoic acid (isovaleric acid)
• hexanoic acid (caproic acid)
• propanoic acid (propionic acid)
• 2-methylpropanoic acid (isobutyric acid)
• pentanoic acid (valeric acid)

Recent research has shown that some of these compounds increase the activity of testosterone in cells - it appears that these fatty acids binds to the androgen receptor and make the testosterone signal transmit better. Coconut oil is a rich natural source of several of these copulins, and it has been demonstrated that coconut oil increases the testosterone signal, making whatever testosterone is available more effective.

Most copulins are a bit funky. Butyric acid is a notable scent in butter - in very small doses, butyric acid is pleasant. In concentrated form, butyric acid is used as a stink bomb and can trigger gagging and nausea. Valeric and isovaleric acids (which are present in the herb valerian) have been compared to the scent of unwashed socks. Caproic and caprylic acid sare related to the word capricorn (think goats). Butanoic acid is commonly described as fruity, and is in fact found in many fruits.