Stinky matters

Just ’cause you can’t smell as finely as a dog, or don’t use the perceptions as subtly, doesn’t mean you can’t smell well, or don’t use odor as signaling/response. You totally do both.

There’s a terms issue to stomp into first, regarding pheromones. These are very loosely lumped into the olfactory category (“smell”), but also technically not. The term refers to a specific type of heavy chemical observed in a number of vertebrates, sensed by the vomeronasal organ and processed via receptors, pathways, and brain sectors which are very different from those of the olfactory system. It’s short-range and fast-acting, and varies in effect or content; in some cases it has a direct, rather drastic impact on the recipient’s physiology and behavior. It’s mostly been studied in domestic and captive animals; good examples include cats, snakes, elephants, and horses. Reception is often actively facilitated via “uptake” facial movements, snakes’ famous tongue sensing, for example; stallions’ version of this, called the “Flehmen response,” looks hilarious to a human. (In chatting about this post with my wife, a veterinarian, she instantly imitated the horse-face before I even mentioned it.)

Technically, humans and a number of other mammals are exempt from any talk of pheromones for the simple reason that we, along with the other African primates, (i) don’t emit such chemicals and (ii) lack a working vomeronasal organ. However, the term was immediately picked up in pop culture as any smell, and (predictably) fixated on sexual arousal. If my students over the years are any indication, the on-the-street understanding is that people give off pheromones and turn others on. I’ve noted that many texts have bowed to the inevitable and define pheromone extremely broadly, often as far as “chemical signaling,” therefore, pretty much any smell that has any behavioral effect, and similarly, to define smell as any chemical perception, but I remain cranky about it. Also cranky about the Omni magazine level of insistence that our little remnant bit of vomeronasal organ “must” be doing something, along the lines of The X-Files and the pineal gland.

Odor is a different and almost infinitely broader phenomenon. The chemicals in question are much smaller and thus much longer-range, and the sensory apparatus picks up thousands of them from hundreds of sources per rather small units of time. Following processing at the olfactory bulbs, the input goes into the same complex hopper of cerebral integration that sight, touch, and hearing do (I’d include taste in that, but taste is mainly composed of smelling odors anyway), and therefore 1:1 correspondence across individuals between a given odor and a given response is not expected.

A certain amount of odor comes from external sources – what’s settled onto you and your clothes through the course of the day. The primary contributor, though, is bacterial emissions. Wherever they’re most numerous on your body surface, that’s the part that smells – hairy crevices the most. They breed fast, so with more time and more moisture, the more smelly. Don’t make an icky face, though! That particular reaction is a dosage thing. The lovely familiar attractive “scent” of that person is merely less of the rank, off-putting stench they emit later in the day.

It’s not even the tiniest bit controversial among biologists that humans utilize smell as a major sense, especially about one another, all mixed in with the other senses, as a contributing factor to identity, hygiene, preferences of all sorts, and hundreds of consequential decisions. It’s subject, however, to the usual ‘curse’ (actually merely a feature) of functioning physiology: that the precise effect is profoundly influenced by what all the other influences happen to be doing, as well as what they and scent have done in the past for this individual. Therefore although trends are often very consistent (“real”), the effect in any given individual for a given instance is highly contingent (i.e. low-ish correlation), and you’re not going to find the “the one chemical” that prompts a given response, as you might indeed find for a pheromone in a given species.

For example, decades ago, the research branch of International Flavors and Fragrances (which is freaking huge) thought they’d found the One Stank to Rule Them All, based on rhesus macaque females’ apparent preferences for sexual partners, and quickly opened a patent thereto. However, it turned out that an enclosure size issue was generating significant effects and confounding the initial interpretation of the results. Sadly I didn’t find the exact patent on-line, but here’s a 2014 patent application which rivals any federal grant application in its research explanation and justification.

By contrast, Claus Wedekind and co-authors’ famous preference studies among humans (MHC-dependent mate preferences in humans, 1995) pointed rather nicely to (note, I do not use the absurd word “proved”) a connection between smell-based preference and kinship. Or rather, an inverse connection, meaning that being related to someone diminished the possibility of attractiveness. Theory supports this effect as a means, among other things, of avoiding near-kin mating and maximizing the immunological diversity of one’s offspring.

This raised the important topic of how one’s smell is related to one’s major histocompatibility complex (MHC), the mammalian “extra” immune system which is very highly targeted, very reactive (i.e. changes with experience), and very individualized. Remember how I said your stench body odor personal fragrance is generated by the bacteria on you? Well, which bacteria are on you is a very specific subset of all those who could be: they’re the ones your MHC doesn’t target. A given individual mammal’s MHC is very, very limited relative to the incredible range of this feature, and as I have said before, it is literally, and by far, the most genetically individualized thing about you. Therefore the odor you characteristically give off is very much “you,” and it’s also very identifiable as kin and not-kin by those around you.

A moment, please, to cope with the incorrect and inevitable use of the word “pheromone” in connection with this research in all media releases or summaries, and indeed, anything to do with human smell and sexual attraction. Nnnnghhh! Wrong! On the internet! Must breathe … also, please note the desire or misconception to find “the one thing” that makes “the one response” happen. All research on attractiveness, even when it’s not confounded by experimental design like the IF&F one, finds high-impact but also highly-contingent variables – and lots of them.

Hi Wayne!

The best work on MHC and partner preference comes from Wayne Potts at the University of Utah, using mice. MHC features are among the very few non-pathological phenotypes which are genuinely subject to single-gene effects, including knock-in knock-out technology. Briefly, which by definition does violence to the thoroughness of the studies, the results are confirmatory. Mice’ sexual partner preferences clearly include MHC heterogeneity as a consistent contributing factor, and it’s perceived via smell. This is the main paper to read: The evolution of mating preferences and major histocompatibility complex genes (1999), by Dustin Penn and Wayne. You can get hit between the eyes regarding the number of supportive and further-on publications about this via the link to Wayne’s lab’s webpage below.

The same old bias or desire or misconception has dogged this work all along, putting Wayne and his co-workers through an unnecessarily difficult acceptance and publication process. Reviewers who should know better always wanted to see one emitted protein, synthesized by the mouse and not from a bacterium, that made a mouse want this mouse “now, now, now,” in an completely stereotyped fashion that’s at least possible for actual pheromone work, but not consistent with what we know about odors.

That’d conclude my intended point, but no discussion of human odor is complete without reflecting on the former German Democratic Republic (“East Germany”) security organization, Ministerium für Staatssicherheit (MfS, or Stasi), surely the leading case study of an espionage/security organization that had crawled into its own end orifice and curled up there. Among its multitude of offenses, they’d bring in some citizen and grill him or her for a while on, basically, nothing, seeking maximum stress before letting them go with a warning. Then they’d remove the gauze that had been inserted into the specially-constructed chair, now soaked with sweat, carefully label it by name and citizen ID number, and pop it into a sealed jar for later. “Later” meaning, if ever necessary, when they’d be pursuing this person with dogs.

Overall, human scent (and more broadly any airborne chemical sensing) offers a bizarre interplay to study among data-driven research and social application, with the latter ranging among commercial scent products, national security and citizen-identification, and indeed, the media discourse which standardizes widespread (mis-)understanding. Historians of science, get on it – this is what you need to be looking at.

Links: Potts research lab at U of Utah, and a cool paper by two principals in the topic, MHC genes, body odours, and odour preferences (Wedekind and Penn, 2000)

Next: A tale I will thee tell


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