The chemistry of love

For Aubrey Smith

Valentine's Day arrives during a subtle but significant shift in the seasons. Here in Southern Ontario, though we still expect more than a month of cold, snowy weather, the days are growing longer. Daylight is increasing. This has a profound if subconscious effect on all creatures.

In a tree at the edge of the park I watch four grey squirrels, Sciurus niger. This species is usually solitary. They keep themselves cautiously distributed, with hardly any aggression, along the band of trees by the river.

However these lengthening days stimulate the hypothalamus, a small but important part of the brain. It causes the master gland of the endocrine system, the pituitary, to secrete various hormones into the blood. These chemical messengers induce important changes throughout the body. Some are more vital, like the ones that mature and develop the reproductive organs.

But over time animals have evolved secondary responses to these hormones. Nature is thrifty. Evolution finds additional uses for old body parts. Many of the secondary effects of hormones include behavioural strategies for survival. These responses often differ between species. In the female grey squirrel, those hormones flowing in January bring the onset of estrus.

Males can probably smell these chemicals. Most vertebrates have a vomeronasal organ toward the back of the cartilage that divides the nostrils, one on each side. It's a small pit containing specialized scent receptors sensitive to pheromones, chemicals that members of a species use to communicate with one another.

It isn't necessarily an obvious, stinky kind of odour. The male squirrel might not know he smells anything. It's possible the nerves are hard-wired into a part of the brain that effects his own body chemistry--and his behaviour--directly.

Until recently it was only known that the human fetus had a vomeronasal organ, but it seemed to diminish and disappear before birth. It is well-developed in many other vertebrates, like birds, mammals and even some primates, but seems to be absent in Old World apes, the group from which we evolved. Only in the past two decades have researchers found concrete evidence that we do in fact have these mysterious little pockets, and that we may be communicating unconsciously by airborne chemical stimuli. It would explain why women who live together develop synchronized menstrual cycles. Still, there is some doubt among biologists as to whether these nerves still function, and if so how. Part of the mystery involves a pharmaceutical company called Monell which claims to have isolated human pheromones, but is still conducting research and is protecting the chemicals by patent. Many of its findings haven't been disclosed to the scientific community.