I'm designing an intelligent alien species in which child rearing is a very labor instensive and lengthy process (their progeny take a little more than a decade and a half to mature), so much so that:
both sexes are equally involved in it
parents do not reproduce again until their child has reached adulthood
These traits are natural and instinctual, they are not cultural affectations. They are in fact so ingrained that some biological processes of these beings are affected dramatically by whether an individual is parenting young or not.
The changes are namely:
Sex drive is diminished and reproductive systems are made completely nonfunctional while an individual is raising a child, it is impossible for them to successfully mate until the latter has properly developped
The antlers possessed only by the males of this species (this is a good place to mention these aliens are the same ones discussed in this question) shed when they are in the process of fathering kids and quickly regrow when they are not
Now the question is, since these changes are completely instinctual/involuntary, what possible means could the creature's body use to differentiate between the states of being with and without young? What about when a child dies prematurely, how can the body detect this and react appropriately?
This isn't meant to be some form of magic or telepathy, for example, if you lock a male in a room after he has impregnatated a female without him being aware of such, then his body will obviously have no way of "knowing" and so it will not change to reflect the fact.
The changes aren't instantaneous, it's not a simple chemical switch and they can take many months to accomplish.
It should be pointed out that children don't simply have this effect on everyone who spends time with them. Only biological parents.