Smell and pick your mate

Mice are known to do it. So are some other larger mammals. But whether humans put their noses to good use when it comes to choosing their mating partner has long been an issue of contention among scientists.

Probably humans too do a bit of sniffing around in order to choose the right mating partner, albeit subconsciously, says a new study that’s being reported at the European Society for Human Genetics annual meeting at Vienna today.

One, probably, need not seek far for the source of this strange behaviour. Genetics may be playing a role here, says Brazilian researcher Maria da Graca Bicalho, who is presenting the work in Vienna.

Evolutionarily, like all other living things, humans also want healthy offspring. One way to ascertain this is to make sure that they have an efficient immune system capable of fighting disease-causing agents that are abundant in the world.

In large animals, the major histocompatibility complex (MHC) — a genetic region situated on chromosome 6 — plays an important role in deciding the efficiency of the immune system. Apart from being a large region, it is an extraordinarily diverse one too.

If the parents have diverse MHCs, the chances are that their progenies will have a more efficient immune system. Dissimilar MHCs play a significant role in reproductive success too, says Bicalho, who heads the Immunogenetics and Histocompatibility Laboratory at the University of Parana in Brazil.

Studies in the past have shown that human body odour is strongly associated with MHC.

To test if people have the tendency to pick partners with MHC different from their own, Bicalho and her associates selected 90 married couples and compared their MHC compatibility with 152 “couples” picked as control from the general population. “We had computer software to randomly put men and women from the database together, treating them as if they were a real couple,” she says.

The number of MHC dissimilarities among the real couples turned out to be far more than what could have been expected on the basis of chance. “If MHC genes did not influence mate selection, we would have expected similar results from both sets of couples,” she notes.

One way in which MHC dissimilarity manifests in an organism is through body odour. There is speculation that the shape of the face — say, symmetrical or asymmetrical — too reflects the MHC code of a person.

Body smell is found to be more acceptable, with studies in laboratory mice bearing this out. For instance, tests in the past have shown that rodents find members of the opposite sex with a different MHC code “more preferable” to those with similar MHC code. However, studies on humans, conducted indirectly through “sweaty T-shirts,” produced conflicting results.

Bicalho reasons that this could be nature’s way of limiting in-breeding (breeding among relatives of a species). When couples are MHC dissimilar, the partners are bound to be genetically different, and such a pattern of mate choice decreases the dangers of endogamy (marrying within a social group) and increases the genetic variability of the offspring. So the MHC effect could be an evolutionary strategy to make humans avoid incest and improve the efficiency of the immune system.

“Although it may be tempting to think that humans choose their partners because of their similarities, our research shows that it is the differences that make for successful reproduction,” says Bicalho.

The scientists believe that the findings will play a role in understanding conception, fertility and gestation failures better. Research elsewhere has already shown that couples with similar MHC genes had longer intervals between births, which could imply early, unperceived miscarriages.

“We are sure that cultural aspects play an important role in mate choice and do not subscribe to the theory that if a person bears a particular genetic variant it will determine his or her behaviour. All we would like to say is that we should not overlook the unconscious evolutionary aspect of partner choice,” Bicalho told KnowHow.

Does she really think that body odours will determine the mating preferences in these days of perfumes and deodorants? “People tend to believe that synthetic scents will mask our own natural odours. But it seems possible that the way people choose a perfume for themselves is also related to their MHC, corresponding to a smell that they would like to enhance, and present to the world,” says Bicalho. But, she insists, this hypothesis still lacks sufficient backing of evidence.