Even though I’m a confessed insects-arthropods fan, and although pheromones are found everywhere in the realm of the insect, I feel like trying a different angle this time. Pheromones and how the affect humans, if they do at all. If you don’t want any background and don’t like to wait, go straight for point 3.

1. Pheromones. A brief recap.

Literally meaning “bearers of impulse” in greek, pheromones are organic molecules produced by a wide range of living organisms, from plants and insects to mammals. Their role, maybe even their evolutionary turn up, can be understood when compared to hormones.

Hormones (testosterone, estrogen…) are also organic compounds that, as we are very aware of during adolescence, change our body and behavior in different ways. Pheromones are produced for the same reason, but differ in their target. They can elicit different behaviors, from sexual attraction to self defense mechanisms, in an external organism/creature.

One example we find it in how acacia trees signal an alarm when an animal feeds of their leaves. Releasing ethylene into the air they warn surrounding trees, that alerted by the air-floating molecules increase the tannin levels in their greens rendering them toxic and uneatable.

That’s why giraffes only eat of one tree of every bunch at a time, wandering away readily after looking for a different cluster.

2. Bad statistics and sweaty T-shirts. What a quagmire!

It is well known that other higher mammals, pigs for example, use pheromones to communicate. For instance, many primates also produce pheromones[1]. So at this point concluding that we may use some kind of chemical communication seems everything but foolish. But we have to tread carefully now, as “opinion” is so far from “proof” as it is itself from complete ignorance. What I mean is that we have to prove this, without letting our bias take any part in it. Specially considering that it’s not the first time society rush into the matter:

The urban Legend.  Synchronicity of menstruation in women. This effect was first recorded in rodent females sharing living space and called the Lee-Boot effect. Martha McClintock studied and concluded that the same effect was present in humans in 1971. Except that, during the following decades, several better-proposed and statistically more accurate studies proved wrong the hypothesis and criticized the original work and pointed at its flaws. But the damage was already done. Even scientists working nowadays in the field still cite the original paper as solid proof [2]. A simple Wikipedia search shows that the original results were, at least, highly controversial.

The main efforts in research, due to very apparent evident, lie in studying sex-pheromones and sexual arousal. Just imagine the consequences for perfume industry, to name one, and the value of an eventual patent. However, a short status-report for the present of this research pipeline may be: it’s not clear whether or not some putative pheromones arouse humans and how they affect our behavior in that sense. Even shorter: we don’t know.

But as I read papers and looked for background in order to write a short (ehem) post it became more and more apparent to me the amount of things we give for granted regarding this topic. Thus it became also very clear that I should clarify that, when talking about human pheromones, we are far from really proving anything.

Believe it or not, many studies approach the problem through statistics and having a (usually too) small group of people smelling sweaty T-shirts. As they do so, they look at pictures of people and rate their (sex) appeal.

I’ve talked about statistical studies and how they can lead to partially or completely biased conclusions (Ancel Keys’ Study of The Seven Countries) when I wrote about margarine, fatty acids and nutrition.

Obviously, conclusions from this kind of studies are mixed and it’s hard to take them seriously. Nevertheless, other studies take a more scientifically-founded route and seem to be throwing some light over the matter, creating easier-to-assess results.

3. Steroids and hypothalamus activation. Sexual attraction.

The smell sense (olfaction) has been very undervalued in our species and regarded as poor or underdeveloped for so many years, which is something that has been proved to be completely wrong. Our smelling abilities are as good as that of an average mammal in that regard.

 

Having said that, the part of the brain responsible in other mammals of pheromone detection through the olfaction system, the vomeronasal organ, is indeed atrophied and lack proper blood irrigation. Very clever of me stating one thing just to question it in some way in the following paragraph. Evasive movement. Random fact. Animals lifting the upper lip are not making funny faces so you can mock (you can, though). This is called the Flehmen response, a way of making air pass through the vomeronasal organ to detect pheromones.We humans, even lacking a functional vomeronasal organ and funny-Flehming faces, seem to have some neural reactions when exposed to determined compounds that are worth of study .

Steroids like androstadienone (on the right down below), which is found in axilar sweat thus giving an alleged basis to the smelling-T-shirts-experiments thing, are very promising. The other I’ll name may be called EST (oestra-1,3,5(10),16-tetraen-3-ol) and is very similar chemically speaking to estradiol, a female sex hormone.

Androstadienone, produced in a quantity twenty times higher in males’ sweat compared to female’s, elicit a reaction in the hypothalamus of female subjects when inhaled. Likewise, EST-steroid elicited a similar response in male subjects [4,5]. This is important because, first, this part of the brain shows no activation at all when the subject is presented with other odorants. And second and most important, the regions activated seemed to be related to sexual behaviors with the expected results when electrically stimulated in monkeys [4].

It has also been proved that people with anosmia (no olfaction) do not react at all to these stumuli [6]. This may look a little bit obvious, but it proves that the steroids are actually being picked by the olfactory system and are also responsible of the elicited neural reaction. Proving is the golden word, remember.

The neural path for the brain response and the extent to which these steroids may interfere with our behavior remains unclear. Therefore the topic is still very open to debate, claiming for further research of the mechanisms, overall impact, and a complete list of molecules of study.

But even if the results achieved up until now seem quite dull, don’t grieve too much. To be honest, do you really want chemicals that elicit a sexual response available for use in the human society? That leaves a huge and bloody ring for ethical debate that I will slowly skirt while waving good bye…

References

[1] Social odours, sexual arousal and pairbonding in primates. C.T. Snowdon, T.E. Ziegler, N.J. Schultz-Darken, C.F. Ferris (2006)

[2] Human pheromones and sexual attraction. K. Grammer, B.Fink, N.Neave (2005)

[3] Vomeronasal organ and human pheromones. D. trotier (2011)

[4] Smelling of Odorous Sex Hormone-like Compounds Causes Sex-Differentiated Hypothalamic Activations in Humans. I. Savic, H. Berglund, B.Gulyas, P. Roland (2001)

[5] Androstenol–a steroid derived odor activates the hypothalamus in women (2010)

[6] Pheromone signal transduction in humans: what can be learned from olfactory loss. I. Savic, E. Hedén-Blomqvist, H. Berglund(2009)