How can a creature's physiology involuntary react to whether a child is being raised or not?

Question

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

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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?

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Additional information:

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.

Answer 1

Pheromones.

Simply put, the child gives off the Pheromones. Spending time near the child, such as via raising, would cause the parents to undergo these sorts of changes. Once the child matures, they no longer give off the Pheromones so the parents can go back to mating.

This does have some weird implications, like housing and the like. A mature child still living with parents would be affected by the pheromones, too, perhaps leading children to leave quickly. It would also be possible to "Skip" the raising by having a nanny or other surrogate take the place of one or more of the parents, just like the practice of a royal wet nurse. This could lead to the skipping parent(s) being able to mate more often (As long as they have sufficient surrogates). At the same time, the surrogates wouldn't be able to mate at all, as once they're done with one kid, they've got another one dropped on them.

Answer 2

The easiest is to copy actual human physiology.

Human females normally (it's not a hard rule, but it "generally" works this way) don't undergo menstrual cycle (i.e.: no ovulation) while breast feeding. Other species exhibit the same behaviors (e.g.: cats).

Make fathers also breast-feed (possibly a different kind of "milk") and the child to require both of them to reach maturity and you have all triggers you need.

Answer 3

While I think the 'Pheromones' answer is a really good one, it basically assumes that you're aliens have an (almost) human biology - which is not a bad thing in itself, but you can also choose differently.

I have two other idea's that might work for you (but don't necessarily match all of your 'additional info')

• Seasonal breeding
  This is actually very common on earth and humans are one of the exceptions and not the norm. If your aliens are only able to mate during a specific season (and one 'year' lasts about as long as raising a child) then obviously the parents can not get another child, as long as they have one (but also non-parents can't get children in that time). Many species on earth do this, because it's actually a lot less complicated if all children are born at similar times.

• Alien breeding
  On earth a male usually gives is sperm (or pollen, ...) to a female and then a child grows, so the male never knows if he was actually successful and can (and often will) seek out more females. Your aliens can work differently: a more complicated and longer mating process could start hormonal reactions in both parents, that signal the growth of a child and disable fertility for next X months in both parents.
  You can go with all sorts of details here: from joining together in a cocoon for a few months to mating regularly over a long period to sustain or channel the birth of a child (i.e. an embryo is not formed by one interaction of male and female, but multiple ones)