Smell, evolution and the sex brain: why we're monogamous and use perfume
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Version 0 of 1. The sense of smell plays a vital part in sex throughout the animal kingdom. It brings males and females together and hastens the ripening of eggs and sperm. In mammals, the so-called “secondary olfactory system” is responsible for stimulating the sex brain. This system works not through the nose, but through a structure called the vomeronasal organ (VNO), a chemical sensory organ lying above the hard palate in the roof of the mouth. Related: I lost my sense of smell – it was hard not to panic | Julie Bindel Stallions and rams can be seen drawing back their lips to open the VNO when in the vicinity of a female on heat. Female mammals produce special smells to let potential mates know they’re ovulating. The smell of a bitch on heat attracts male dogs from miles around, who gather to pay court amid much snarling and fighting. In addition to smell advertisements, many of our primate relatives provide visual signals of ovulation in which the ano-genital region becomes inflated into brightly coloured fleshy cushions. Yet Homo Sapiens is the only species among the 5,500 kinds of mammal to maintain monogamous family relationships – or at least serially so – and to live in densely populated areas. This combination is extremely rare in nature. We know our ancestors were monogamous because examination of their fossilised bones shows no greater difference in body size between the sexes than occurs in ourselves today. Males of polygamous animals like gorillas and lions are larger than females, and sometimes very much larger. To keep male and female together to provide protection for babies, a suite of anatomical and physiological features evolved to promote the constant availability of sex throughout the year – irrespective of the monthly ovulation cycle. As a device for keeping a male and female interested in one another it was a winner, but during a critical phase in our evolution it proved to be an Achilles’ heel. During the Miocene epoch (20-5m years ago) the world’s climate became distinctly drier, particularly in the tropics. Forest degeneration forced our ancestors onto the developing savannah in search of food. A vast new food resource was becoming available in the form of herds of large grazing mammals, but to primates lacking large canine teeth or slashing claws they were as good as inaccessible. Our ancestors learned that by hunting in a team, instead of singly, large mammals could be killed to provide sufficient food for all. This lifestyle required that several families lived communally so that there were always enough hunters at hand. These new arrangements threatened the bond between male and female because every male in the community knew when every female ovulated, raising the real risk that a female might find herself carrying the young of a male other than her mate. In evolution there is no genetic advantage to a male nurturing another male’s genetic legacy; infanticide is widely practiced by polygamous primates so males can be sure they are providing only for their own offspring. Evolutionary pressure to maintain monogamy and the family bond increased, and it first resulted in the disappearance of visual advertisements of ovulation. Smells advertising ovulation disappeared when a gene mutation that interrupts the flow of information from the VNO to the sex brain – nicknamed “Adam” by its discoverers – appeared about 16m years ago. This was at about the time our early ancestors were starting to live communally on the open plains. Related: Why can’t English speakers say what they smell? With no visual advertisements, and the hobbling of the secondary olfactory system, our ancestors’ ovulation was concealed. Over time, the VNO regressed to a bony residue remaining in us today much like the remnants of our ancestors’ tails. Concealed ovulation allowed our early ancestors to benefit from the new food source through cooperative hunting, while maintaining the genetic integrity of their families. “Adam” didn’t affect the brain’s neural pathways that were originally used to trigger sex. We can stimulate them with sensual perfumes made from – among other things – the sexual signals of musk deer, beavers and civets. Today we have a global fragrance market equal to the GDP of a medium-sized country. But because our nose (unlike the VNO) ultimately sends all smells for rational analysis by the brain, we do not slavishly respond to sex smells in the way dogs or mice do. An alluring perfume may help a relationship, but no perfume comes with a guarantee! And so we can live in at least relative harmony with our fellows, benefitting from the long-term genetic and evolutionary advantages provided by monogamy, while participating socially in everything society has to offer. |