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A little known sense

· ​Interview with Enrico Alleva ·

The sense of smell is a little known sense which has also been little explored from the scientific point of view too. To understand this better we interviewed Enrico Alleva. This scientist, an ethologist with a long and remarkable scientific career in important Italian and international institutions, was a student and collaborator of Nobel prize-winners such as Rita Levi Montalcini and Daniel Bovet. Since 2004 he has directed the Department of Behavioural Neurosciences at the Istituto superiore di sanità. A Corresponding Member of the Accademia nazionale dei lincei [Lincean Academy] and President of the Federazione italiana della scienze della natura e dell’ambiente [Italian federation of natural and environmental sciences], he is also a Member of the Scientific Council of the Italian Encyclopaedia Institute.

René Magritte, “The Beautiful Relations” (1967)

How important is the sense of smell for human beings?

Human beings do not have the same sharp sense of smell as other animal species, some of which reach sensory peaks unimaginable to humans: insects that can see ultraviolet light, dolphins and bats that perceive ultrasonic signals and, with regard to the sense of smell, the male silk worm which can detect the scent of the female at a distance of kilometres, thanks to its feathered, multi-directional antennae. The perception of smell is an indispensable instrument for survival for all animals, since not only can it provide information on the availability of food resources but it can also guide social interactions and mediate the establishment of parental relations. In humans the sense of smell is generally considered the last in the hierarchy of the senses, even though it is suspected that the olfactory molecules produce many more effects than those known, especially in the field of interpersonal communication (what is known as the “chemistry” of relationships. The sense of smell certainly plays a primary role in the affective and emotional sphere, since the olfactory system is directly linked with both the hippocampus (the cerebral structure that manages memory) and the amygdala and limbic system, those parts of the brain that govern the emotions. In this way, through association with past experiences and memories, the olfactory experience is laden with deep meanings according to one’s history and culture. In humans the perception of smell follows two principal routes: the classical one, which passes through the cerebral cortex and is a conscious route, and the accessory one, also called reptilian, which passes through the vomeronasal organ. This organ has the function of detecting pheromones (chemical compounds that are emitted by individuals and serve as a signal for organisms of the same species) and generates olfactory information which does not reach the level of consciousness but, at least according to some reductionist views, has to do with relational life and determines important preferences, such as those involved in mating. Experimental studies, carried out mainly on mice, show that females have greater sensitivity to pheromones and choose their mating partners on the basis of certain olfactory complements. Indeed there are basic metabolic typologies which are responsible for the immune complement system, for immuno-incompatibility, for metabolism and for odours, which is why smell has the purpose of orienting the choice of partner to an individual with a different immune complement. This would be an advantage for the offspring which would thus acquire resistance to a greater number of illnesses based on immunity. An organizational similarity exists between the immune system and the olfactory system, since our history of olfactory feats depends on the smells we have come across, just as our capacity for responding to infections depends on the germs we have encountered. This is important because the plasticity of the olfactory system, namely the capacity for strengthening and refining the sense of smell, derives from exposure to a multiplicity of olfactory stimuli. The scientist Linda Buck, awarded a Nobel Prize for medicine and physiology in 2004 together with Richard Axel, was a great scholar of the olfactory bulb and found that mammals can detect and memorize more than 10,000 smells and that chemical substances which are similar from the molecular point of view can generate olfactory perceptions that differ widely.

What role do pheromones play for human beings?

Studies on the presence and possible role of chemosensory perception in determining choices and forms of behaviour are difficult, since the social and human relational system is highly complex and the components of culture and acquired knowledge undoubtedly play a primary role. In the past great headway was made in the study of human pheromones but still today there is no definitive evidence on the part pheromones play in humans, even though certain experiments have suggested that women, who are more sensitive, might be orientated to choosing their mating partner on the basis of information transmitted to them by male pheromones. According to this view – as I repeat, reductionist – men transmit to women (but the contrary may not be true) information on the state of their health and on the degree of their genetic closeness with a mechanism similar to that described above for mice. I bitterly disputed a study published in the review Trends in Ecology and Evolution which sought to demonstrate that when they ovulate women are attracted by the odour of a high-ranking male, as if there were a biological evaluation of smells aimed at successful reproduction. Quite apart from the difficulty of defining a high-ranking male, it seems to me to be risky to make such reductionist statements.

Giuseppe Arcimboldo, “Spring” (1563)

You seem to understand that there might be a gender difference with regard to the sense of smell.

The sense of smell certainly has a natural history of gender differences and it is the male who usually has a more highly developed sense of smell, since it is up to him to go in search of the female, an activity for which it is fundamentally necessary to follow the traces of a scent, what ethologists call “a cue”. This behaviour, vividly defined in the saying “man is a hunter”, stems from a Darwinian rule according to which men are responsible for all the potentially more dangerous activities, such as moving around. In fact, for the preservation of the species a high number of females must be maintained and it is therefore males who run the risk of being eliminated. In humans, however, it is women who show a more acute sense of smell. It is not clear whether this is due to biological reasons linked to the natural history of our species, or, given the plasticity of the olfactory system, to personal history, that is, to exposure to a greater number of olfactory molecules, for example in the preparation of food. A proof of female sensitivity to olfactory stimuli is the synchronization of menstruation in communities of women (in convents or in work environments), a phenomenon, dependent on the sense of smell, which has been amply described and demonstrated. However in humans the differences in the sense of smell are mainly linked to age. The standard of this sense is already established in the uterus. In fact between the fifth and the eleventh week in the uterus the first receptors of smell appear and the nerves and olfactory bulbs begin to be formed, while the twelfth week sees the appearance of the gustatory papillae, the receptors of taste, a sense that is closely linked to the sense of smell. The idea of the foetus as an absolutely amorphous being, which until birth is thus a blank slate is profoundly wrong. For the entire prenatal period hearing and smell are very important in the hierarchy of the senses. The foetus knows, for example, its mother’s voice, and receives gustatory and olfactory stimuli that come from the mother’s diet and enter into the amniotic fluid. Newborn babies are thus born with a gustatory memory which enables them to orientate themselves in their new environment with greater assurance and, since they have a tendency to a basic phenomenon called neophobia, they suckle their mother’s milk because they recognize its smell. The ethologist Irenäus Eibl-Eibesfeldt, a student of Konrad Lorenz, has suggested that olfactory memory lies at the root of the way in which the newborn creature finds the mother’s nipple, a typical, automatic response linked to recognition of the mother and of the mother’s milk. Smell is very important in the early months of life; later its role diminishes, both because it is used less than other sensory stimuli and because of the establishment of greater complexity in the intellectual functions which are based in the cerebral cortex and become crucial in learning behaviour.

Jan Brueghel the Elder, “Flowers in a Basket and a Vase” (1615)

To conclude, the olfactory system is very complex and many of its implications in our lives escape us.

We must not of course forget that strong cultural differences exist which are linked to the perception of smells, as I have already mentioned; many behavioural rituals, such as ablutions or the use of incense, have to do with this sense. In this regard it is also important to remember that the sense of smell very easily creates habituation; a too familiar smell is no longer noticed, in the sense that we are no longer aware of its presence, and this can also have a cultural content.

Mariella Balduzzi

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