Who knows the secrets of smell?

C&I Issue 2, 2017

Who knows the secrets of smell?


John Emsley

Smell is all to do with sex – at least for insects and mammals. They have highly developed ways of sending out sexual messages, and their would-be mates have equally sophisticated methods of detecting them. Both the broadcasting and the detecting involve chemistry and a host of various molecules.

The former are easy to identify, the latter very difficult, as we discover in this book. Pelosi has devoted his research to solving these complex problems, and here he explains it in a most interesting and readable manner –pleasingly illustrated with chemical formulae.

Molecules can convey other kinds of information apart from carrying sexual messages, such as to warn others of danger, or in the case of ants, termites, and bees, to keep the colony together. Humans are not programmed to respond to smells although we know those we find attractive and those which repel us. Unlike our other senses such as sight, sound and taste, and unlike most other animals, our sense of smell is undeveloped. We seem to have evolved with little more than the ability check whether something smells safe to eat.

Pelosi covers in detail the way molecular shape determines smell. Sometimes structural variations, such as replacing the aldehyde group with a nitrile or a nitro group, will have no effect on the bitter almond smell. On the other hand – literally – the chiral versions of the same molecule can smell very different. It’s all to do with the shape and size of the receptors in our nose.

He also covers sex pheromones such as androstenone. Nothing makes females pigs more willing to mate than detecting this molecule, and this brings us to the question of whether it might also affect humans. It certainly seems to affect dogs. Pelosi tells the story of how, in 2014, John McGlone of Texas Tech University, US, found a way to stop his dog from excessive barking – he sprayed some androstenone on its nose and it worked. Such products are now available commercially.

Even if androstenone has no pheromone effect on us, might there still be something that we emit that could prove irresistible to members of the opposite sex? So far it has evaded detection and Pelosi points out that in any case our nose lacks the ability to detect it. Not that smell is without its sexual implications.

‘Home in three days; don’t wash’ is supposed to have been Napoleon’s message to Josephine, suggestive of a pheromone link between those famous lovers. However, Pelosi doubts this was ever said, and in the final chapters of his book, he concludes that there is no such thing as a human pheromone.

Pelosi moves on to the complex problem of detection and identifying the proteins that analyse smell, quickly and easily, and with the ability to distinguish a particular one from the thousands of other volatile molecules in the air. Here we learn something of Pelosi’s personal struggle to identify the nasal protein by which androstenone was identified and acted upon. But it was not easy going.

For his research, Pelosi needed to visit a local abattoir to collect pig heads and extract the protein sensors. Not only that, but he also needed to label the trigger molecule androstenone, which he did with deuterium. This adds across a double bond and converts the molecule to androstanone, a much more volatile chemical, which has an odour of stale urine. This powerfully smelling chemical percolated throughout the university building where his team worked and it binds strongly to protein-based fibres such as wool thereby contaminating many of his fellow workers. They would go home smelling as it they had ‘peed their pants’.

Some scientists still believe that there may be something equivalent to a human pheromone. Men do give off a range of molecules of which one might just be attractive to the opposite sex, although it is generally assumed that none of them are. In any case, we don’t need to rely on it.

Some young men believe that they can apply such molecules using aftershower sprays. Lynx, one of the more popular male body sprays, originally suggested that men could gain a sexual advantage by using it, so much so that the Advertising Standards Authority said their ads were offensive and degrading to women. But then women are also encouraged to use certain perfumes for the same reason. And I am sure that the advertising of such fragrances will always hint of a particular brand’s secret ability to attract the opposite sex. But even if they don’t work in this way, at least they make us nice to be near, and that’s what counts.


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