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The chemistry of love

© Van Waffle

Neurotransmitters have only a brief effect. For squirrels, brevity doesn't create a moral dilemma. After mating, they part company.

Another kind of fix

But for many animals it is more advantageous to maintain a committed pair bond. Generally these are animals which use the strategy of raising fewer young, but investing more energy in ensuring their survival. Humans aren't the only ones that long for everlasting love. It is common in swans, geese, certain primates and various other creatures. Common loon pairs will nest together for years, and if one dies, its partner rarely takes another mate.

So what happens when the haze of infatuation has passed? The emotions and behaviour associated with pair bonding depend not on the nervous system, but once again the endocrine system, or the glands. It also uses chemicals to control body functions, but they're hormones rather than neurotransmitters. And their effects are longer-term.

That prime mover, the pituitary gland, situated near the base of the brain, produces many substances. One of the most essential in this case is oxytocin. Its primary purpose is inducing labour and the onset of milk production in mammals, but it also plays a role in the bonding behaviour between mothers and infants. And it also appears at elevated levels in attached adult couples. It is secreted along with endorphins, the body's natural feel-good drugs. The role of these chemicals isn't completely understood, but it is thought to affect behaviour, creating a sense of peace and stability.

The four-year itch

But even this effect doesn't last forever. The body's tolerance to these hormones gradually increases, so our sensitivity decreases. Research indicates that after 24 to 48 months, two people can no longer rely on oxytocin to keep them in love.

Evolution seems to have adapted us to stay together long enough to raise a child through the first few fragile years. By the time our buzz of love chemicals has worn off, the child is at least a toddler, physically developed enough to look after some of its own most basic needs, like keeping out of the way and foraging for food. Now it can be watched and protected by the extended family or community, and the roles of its parents become less important.

Natural forces suggest that three or four years is long enough for humans to bond. After that, if we choose to stay together, it's an action of mind over body. Yet many relationships last far longer and remain happy. My own parents celebrated their 50th anniversary in 2001, and their relationship is close as ever. Couples who stay together seem to rely on shared values, interests and activities to keep their enjoyment alive. Knowing a mate's fondnesses, like a bouquet of flowers or a special meal, may help keep him or her feeling appreciated and happy.