Let’s imagine ourselves in a farm of yesteryear, where they milk the cows by hand. Look carefully at what they do. The person milking the cow gently pulls and squeezes each teat. A few seconds later there is a squirt of milk into the bucket. It’s the same in lactating women. The baby doesn’t suck the milk out of the breast, but the stimulation of his/her mouth causes a similar spurt of milk. There are women who claim to be able to squirt their milk across the room. The ejection of milk into the hungry baby’s mouth is an essential part of maternal care.
Milk ejection comes about because of a reflex, set up by the baby’s mouth sucking on the nipple. The nerves from the breast carry this stimulus to the brain, where it activates a group of very special nerve cells. These cells make oxytocin, which is then released, via the pituitary gland, into the blood, and travels to the breast. The breast has special muscle cells that respond to oxytocin by contracting, thus expelling the milk. It all happens rather fast.
Oxytocin also has a well-known role in parturition. The womb becomes very sensitive to oxytocin late in pregnancy, and blocking its action prevents or delays delivery. It’s released during childbirth. It is equally well-known that infusions of oxytocin, or compounds that mimic its action, can accelerate or precipitate birth, and are widely used clinically for this purpose. So oxytocin has a coordinated role in childbirth and nourishment of the baby.
That's all oxytocin was supposed to do, until about 20 years ago.
Now join me in a small field. It’s an experimental one, and there are two stakes in the ground. One has a small pile of hay tied to it; the other has a lamb, about a month old, who is bleating in the way lambs do when they are separated from their mothers. In one corner is a ewe: she has never had a lamb, and is not pregnant. She does have a tiny pump that can inject a small amount of hormone into her brain. The scientists inject saline, which the ewe does not notice. They release the ewe. The ewe ignores the lamb and makes straight for the hay. Now they repeat the experiment, this time injecting a small amount of oxytocin. Now she ignores the hay but makes for the lamb, and tries to mother it.
So oxytocin is more than a milk-ejection or birth-promoting hormone: it also activates maternal behavior and bonding with the young. It may also encourage mothers to eat more. This simple chemical (it’s a small peptide) thus coordinates several of the essential elements of successfully raising offspring. It all fits. Interestingly, oxytocin levels in female rats correlate with how assiduous they are as mothers, and how vigorously they defend their young. Oxytocin sets up motherhood. In women, oxytocin is released during vaginal delivery. If this plays a part in the inset of maternal feelings towards the newborn baby, then one might expect this to be less after Caesarian section: but this is not the case. ‘Bad’ mothering has not been laid at the feet of deficits in oxytocin or the way the brain responds to it. There is, so far, not too much direct evidence that oxytocin is at the center of human maternal behavior, but considerable suspicion that it might be.
For a while, the story stopped there. But what about males? They have oxytocin in their blood as well (it goes up after orgasm), but its function, if any, remained mysterious. The males of different species have very distinct patterns of mating: some pair for a season, or for life, others are more promiscuous. It was reported that the monogamous prairie vole had more oxytocin in its brain that the more promiscuous mountain vole, and that the latter could be made less promiscuous by being given oxytocin. But the story has got more complicated: it seems that the distribution of the receptors for oxytocin in the brain, the molecules that detect and act on its presence, distinguish monogamous from polygamous species, and may be more important. But even in monogamous species (for example, marmosets) interaction between partners is increased by adding oxytocin to their brains: they cuddle more often!
Many animals, including, of course, humans, live in social groups. Social species do seem to have different levels of oxytocin and its receptors than more solitary species. It’s not simply more or less, but differences in their distribution in parts of the brain. Oxytocin, as well as promoting sociability within a group, can also accentuate hostility to members of other groups. It may also enhance the ability to recognize faces, an essential social attribute. So it has complex, but powerful, actions on social organization. Whilst we can’t yet measure oxytocin in the living human brain very well, giving it in a manner than enables it to reach the brain increases trust in others and the ability to infer another’s emotional state (empathy). But generalizations are dangerous: oxytocin given to subjects with borderline personality disorder actually decreased trust: individual characteristics in this context, as in many others, are important.
Oxytocin does seem to play an important role in bonding. This includes sexual pairings, but also mother-infant interactions, as well as more general sociability and acquiring social skills. Attempts to relate oxytocin to deficits in these qualities – for example, autism – have not been very successful so far, though investigations continue. And there is another aspect of bonding: it’s not random. A mother bonds with her own baby, less with others. Simply altering oxytocin can’t explain that, or that a male will bond with one female but not another (and vice versa). Oxytocin plays a fascinating role in some of the fundamental behaviors of many species, including humans. Whilst it may contribute to the essential biological functions of sexual selection or maternal behavior, it’s not a complete explanation. You may be encouraged to fall in love by your oxytocin, but whom you choose depends on something else.
Social and sexual bonding is so important, but so variable between species and individuals, that it's unlikely that a single factor is responsible. Other important behaviors, such as eating, drinking or sex, have multiple controls, just as has your car's braking system. So oxytocin (and its receptors), powerful and intriguing as it is, will not act alone. But it's a striking example of the way that ancient and fundamental behaviors are regulated by relatively simple molecules, and how we humans inherit these mechanisms from our evolutionary past.