If you desire the sixth sense or sex, just have a sniff

Lyall Watson has set out his conclusions on the nature of smells and their detection in this useful book. He considers that there is no modern classification of odours that can displace Linnaeus's seven scent classes of 1752.

Most people accept that there is nothing more memorable than a smell, but are unable to describe what they smell for lack of a descriptive vocabulary. Few realise that we detect odours by two distinct paths. Receptor cells in the nose respond to smelly substances by signalling to olfactory bulbs in the brain via the olfactory nerve. Jacobson's organ, discovered in 1811 but long neglected, consists of two tiny pits beside the nasal septum that contain different receptor cells. These cells transmit signals to accessory olfactory bulbs in the brain via a different nerve. Surprisingly, Jacobson's cells respond to some molecules without scent, giving us an apparent sixth sense. The trigger molecules include most pheromones, which are important initiators or terminators of some aspects of animal behaviour.

These findings are so recent that few organisms have been investigated in detail, but Jacobson's organs have been found in some reptiles and most mammals, although not in fish or in birds. They are best developed in land mammals and poorly developed in aquatic species. From an evolutionary point of view, Watson thinks it likely that smell pre-dates sight and hearing in primitive fish and that Jacobson's organs may have evolved in Triassic reptiles. The organ is involved in the habit of some reptiles of sniffing the air with extended tongues.

The Jacobson's organs of mammals vary in size and are sensitive to scents of urine, faeces, sloughed skin and glandular secretions. They are involved in the scent-marking of populations and territories, in social recognition, and in identifying the opposite sex when ready to mate. With their organs excised, voles are unable to mate or to distinguish the sex of another vole. Despite the importance of scented plants in nature, this book gives few examples of pheromone mimics such as the attraction of pigs by truffles, or the flavonoids of subterranean clover, which act as a contraceptive in sheep.

Although the Jacobson's organs of humans are small compared with those of tracker dogs, we produce odours from our armpits, genitalia and excreta. Some of these contain pheromones, few of which have been isolated. Production and detection of pheromones are involved in the induction of oestrus in females by males, the coordination of the menstrual cycle in all-female groups, and the bond between mothers and newborn babies.

Meanwhile civilised humans spend $5 billion a year on perfumes, depilatories and deodorants to disguise their personal scents. No aphrodisiac pheromone has been found; it is likely that there is no universal aphrodisiac, but that each individual may be turned on by a different mixture of several unknown pheromones. An interesting observation is that myrrh and frankincense used in churches contain phytosterols, which mimic human hormones.

This book is written in a style that is easy to read and understand. The author marshals his facts in the manner of Darwin, and he frequently refers to the importance of scents in literature, such as the madeleine of Proust. Half the scientific references are post-1980. The illustrations, of woodcuts from a 17th-century herbal of Clusius, are decorative but not instructive. There are no anatomical drawings of Jacobson's organs or the animals that use them.

This book provides an up-to-date summary of the nature of smell, suitable for biologists, animal behaviour scientists and human psychologists. It would be an asset for sixth-form and university libraries. It is less useful for botanists or steroid chemists, but does suggest fascinating areas for future research.

Humphry J. M. Bowen is emeritus reader in chemistry, University of Reading.