My experience is that hormones can be triggered by fooling the brain.
When I was breastfeeding my two daughters we had a girl staying with us.
She was in her late teens and had never been pregnant.
Curious about what it felt like to breast feed Chloe asked if we could put a baby on to her nipple just to see.
I agreed and of course baby latched on ,sucking furiously trying to get mother's milk.
This became a regular thing , in as much as I would feed and then before they were completely done I would pass them over to Chloe for her pleasure. My daughters would suckle and then gradually fall asleep , very calm and relaxing for all of us.
After less than a month Chloe started producing her own milk ,not much at first but a steadily increasing flow.
Her breasts grew plump and round and she became a fully lactating "mother".
My theory is that nature/survival of the species takes over , if a child ,any child, needs nourishment a female will be able to provide it whether she is a mum or not.
Anyway there it is , Chloe BTW loved her big bouncy boobs,lol.
Also , after I stopped feeding my little ones I continued to "express" my milk [ I liked the fact that at last I had breasts as I was totally flat before ] and doing so kept my boobs "full" and firm .
I didn't have to express much just a few mins in the shower everyday kept them "topped up".
If I had an occaison that required "big and bouncy" or a revealing outfit I would hold off a couple of days before the event and they would swell almost a cup size larger.
Even today [at 67] if you play long enough with my nipples I will get that feeling of "uhoh here comes dinner" ,it doesn't arrive of course.
Oh and if I hear a baby cry my nipples tingle .
Females are always females I guess
Nicky x