Nose Job: Loss of Smell Is Not a Trivial Effect of COVID

One of the much-reported effects of SARS-COV-2, including the effects of so-called ‘Long COVID’, is a change in smell perception. The effects of this, if they are truly long-term, might be more profound than people realise. Sense of smell is not just some amusing spice added to life, it’s a reminder that humans are just another critter, and live by the same rules.

Smell matters to sex in other mammals, and we are no exception. As Jennifer Anniston memorably put it, “The best smell in the world is that man that you love.” True as that is, the effect of partner smell goes beyond mere pleasure, and anyone with even a passing knowledge of how evolution works knows that nature doesn’t hand out free lunches. When comparing men and women, the average density of the bit of the brain that processes smell is hugely denser in women. The brain is expensive real estate. What is going on?

Honest Signals

The great biologist, Amotz Zahavi, died a couple of years ago. I meant to write something at the time, but life gets in the way. Better late than never, I hope. One of Zahavi’s key insights was that if you see anything in nature that is baroque, larger than it appears to need to be, seemingly unnecessarily complex, or just plain weird, the likely answer is Sex. Specifically, sexual selection. A well-known example is the magnificent peacock’s train but humans have other signals.

When I was just starting my Ph.D. I was incredibly lucky to be able to present my first year of work to Zahavi, who happened to be visiting London. At the time, my working hypothesis, still the dominant one in the field, was that female orgasm was a by-product of male orgasm. I’ve repeatedly said why I now don’t think that this is true—roughly that anatomy, behavior, and physiology all argue against it. A by-product can’t be more complex than the thing that it’s a by-product of and female orgasm is more complex than male. But it was Amotz Zahavi who put the idea in my head. After my talk, he took me aside and said, “Don’t tell anyone, but we call something a “by-product when we can’t think of anything to test.”

Once you start thinking of things to test, then patterns start to emerge. You have to be careful here because humans are notorious for seeing patterns where there are none. That’s why we look for surprising predictions, the ability to replicate the studies (especially by people who don’t agree with the hypothesis and all the other accouterments of scientific method), and do peer review (which often amounts to trying to fit a new piece of the jigsaw into what has already been completed).

Picky Picky Picky

When we first started studying female orgasm one of the first things that we did was ask women what scientists should be asking them about its nature. The surprising answer came back, “Ask about partner smell.” “Really?” we asked. “Really,” they replied. Whenever I give talks about our work and mention the smell finding, the women in the audience nod and say,e “Of course.” And “Well, duh!” Meanwhile, the men look astonished (the smarter ones take notes).

In our research, we found that partner smell predicted vigorous, deeply felt, and energetic orgasmic responses better than any other partner characteristic. Better than dominance. Better than masculinity. Even better than partner skill and sensitivity. Why might this be?

Recent research has revealed that women’s olfactory bulbs—the part that is sensitive to smell is fully 40 percent larger in women’s brains than men’s. Imagine finding that women’s eyes were 40 percent larger than men’s or they had tongues with 40 percent more taste buds. Nature doesn’t give free lunches, not even to those with taste buds to spare. What is all this extra hardware for? Recall the principle I started with. We think the likelihood is that it’s about sex. We have known for a while now that smell changes, apparently caused by oral contraception, can lead to changes in perceived partner attractiveness, and that helps answer one question—is it likely that humans exude pheromones? The likely answer is ‘no,’ but this doesn’t mean that smell doesn’t matter, —it means that targeted smell probably matters.

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There’s a lot of talk about pheromones; but, strictly speaking, pheromones advertise general sexual receptivity, like in mayflies that only have hours of breeding time. But humans aren't mayflies. We are choosy—both sexes—but especially women, the costs of any reproductive mistake fall most heavily on women.

What does smell advertise? Well, we are only beginning to understand this, but one theory with growing (but far from universal) empirical support is that our smell advertises our major histocompatibility complex. This is the part of our genome that advertises our immune systems.

We talk grandly of “the immune system” as if it was an organ with a simple function—like heart or lungs. But, in reality, the immune system is an incredibly complex set of interlocking defences that draw on lots of disparate bodily resources. Scientists devote whole careers to studying sections of it—so I’m not going to do much more than scratch the surface of its importance here.

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One crucial balance is between being immune to those pathogens that are local but, at the same time, not indulging in inbreeding (which has problems of its own). What this amounts to in practice, is wanting a smell that’s a bit like daddy’s, but definitely not daddy. And what that means, is that the amount by which we differ on six key markers of the MHC, predicts how sexy a woman will find a man’s smell.

This has been tested in various ways but probably the most entertaining is the smelly T-shirt paradigm. In this, the males supply some appropriate DNA (via saliva—don’t be filthy) and also wear a T-shirt at night for a few days (no deodorants or perfumes allowed). Said T-shirts are placed in sealed containers and then women come along, break the seals, and rate the attractiveness of the smells. It works the other way around too—with female smells and male smells—as demonstrated here:

Work is still ongoing but seems to point to the idea that it is immune system compatibility rather than just random mixing (technically called “heterozygosity”) that we find attractive. Although Wedekind and Füri (1997) did not find women to be more sensitive than men in their particular study, others have. For instance, the work of Allen, Cobey, Havlíček, and Roberts, (2016) not only found that women were more sensitive to odour than men, but that this sensitivity was more susceptible to disruption by perfume.

The corollary is also true—women are more likely to be disgusted by all sorts of stimuli—including smell—than men are (Fleischmann, 2016). Disgust is protection against potentially dangerous interactions and we ignore it at our peril. There is thus a real chance that we may be getting in our own way by not allowing ourselves to be led by the nose.

If all of this is being disrupted by the effects of COVID, then the effects on relationships could be severe. I'm not sure how useful this sort of information is to sex therapists, relationship counsellors, and similar but maybe forewarned is forearmed?