It’s happened to all of us – a slightly flimsy top, a cold day, and all of a sudden there they are – like two bullets standing to attention.
Man or woman, none of us are immune to it, but why exactly do our nipples go hard when the temperatures drop?
Well, the answer to this important question lies in the wrinkles in your areola – the dark area of skin surrounding your nipples reports Bustle.
When the nipples are flaccid, the areola is both larger and more wrinkly – giving the nipple as a whole a large surface area.
These wrinkles allow the surface to expand and contract, becoming flat when the nipples are erect – effectively lowering their overall surface area.
But why is this process stimulated by the cold? Well there’s no cut and dry answer to this, but according to Redditor /u/penguinluvinman:
The ultimate function of this reflex is to decrease the amount of surface area on the nipple that is exposed to the cold.
This decreases the amount of body heat that is lost to the cold and ultimately the amount of energy it takes to sustain homeostasis (maintaining normal body temperature).
Like goosebumps – it’s simply an evolutionary method for controlling the loss of body heat.
Female nipples also obviously become erect to enable babies to latch on for breastfeeding, but why do they get hard when we’re sexually aroused?
This one seems to be down to hormones – specifically oxytocin.
Oxytocin is a ‘stress’ hormone often associated with labour, childbirth, lactation and orgasm which – rather than the ‘flight’ response is linked to ‘tend and befriend’ behaviours.
Quite simply, oxytocin causes the muscles underneath/around your nipples to contract, making them hard.
So, it turns out those little guys actually have quite a lot going on behind the scenes.