The Scent of a Man—and a Woman’s Response

Jeffrey Mogil and his co-investigators produced an article that generated comment in several media outlets. The most interesting, perhaps, is that provided in The Economist: Sex, writhes and videotape. This lede was provided:

"Rodents feel less pain when men are around. For scientists, that is worrying"

Scientists have figured out a way to determine how much pain mice are suffering by observing their facial characteristics. When Mogil and his team injected a substance into the legs of mice to induce pain, they observed that the level of pain suffered depended on which of the observers happened to be present. The intention was to verify that the presence of an observer might affect the response of the mice. What they actually found was that the effect of the observer appeared only when males were present. In fact, what the mice were keying on was not the observer himself, but on the smell of the observer. Clothing worn by a male would induce the same effect.

It should be understood that the scent of a man did not produce a soothing effect; rather, the investigators were able to determine that what was actually happening was fear generation, and fear, via a process known as fear-induced hypoalgesia deadened the pain. From Wikipedia:

"Fear induced hypoalgesia is another example of a mechanism controlled by opioids. It is postulated that fear is a defense mechanism that has evolved over time to provide protection. In the case of hypoalgesia, a decreased response to pain would be very beneficial in a situation where an organism’s life was at stake, since feeling pain would be a hindrance rather than a help. It has been well documented that fear does cause a decrease in pain response however much like the exercise induced hypoalgesia, the exact mechanisms of action are not well understood. Studies have shown that opioids are definitely involved in the process, yet opiates alone do not completely explain the analgesic response. What the other mechanisms of action are is still unknown."

Was everyone aware that our body produces various forms of illegal substances in order to get itself through the day?

The article in The Economist summed up the situation in this colorful manner:

"Simply put, the animals were being scared painless. (A significant increase in faecal pellets suggested they were scared shitless as well.)"

And on a more serious note:

"This is an important finding. At the least, it may account for some failures to replicate results in animal experiments, a perennial problem in the field. And, because stress affects numerous bodily responses besides pain, research in many areas may be affected. Dr Mogil suggests, therefore, that the sex of those who conduct experiments needs to be a matter of record, included routinely in the methods sections of research papers."

The possibility that the results of experiments with animals may be compromised by scents produced by humans is certainly important, but should it be so surprising? We, as members of the animal kingdom, have many attributes in common with the other members. We share much of our genetic makeup and produce and use many of the same chemical substances. That natural selection within a species would produce variations in usage and response to chemicals should not be surprising. From her book Mother Nature: Maternal Instincts and How They Shape the Human Species, Sarah Blaffer Hrdy provides this perspective:

"Foragers, primates, mammals—human legacies spiral backward through time, like the coils of DNA that connect us, linking us to long-ago life-forms. Evolutionarily, humans are a ‘mixed bag.’ A line from an old nursery rhyme, "Snips, snails, and puppy-dog tails," isn’t too far off the mark—not just for boys but for everybody. A thrifty matron and inveterate recycler, Mother Nature is slow to discard leftovers. Conservative retention of useful molecules explains why the same endorphins [opioid], the natural morphine that made the pain of my children’s births bearable, are also released in an earthworm when my garden spade accidentally severs it. The innate immune system that protects my body from bacteria makes use of the same kind of proteins that perform this function in fruit flies."
　
What will be most interesting about this article we have discussed are not the precise details, but the indication that human scent can have effects on animals. If a human’s body odor can produce responses in mice, can it produce responses in other humans?

Consider this information from Wikipedia on how body odor affects animal and human activities.

Body odor is largely influenced by major histocompatibility complex (MHC) molecules. These are genetically determined and play an important role in immunity of the organism. The vomeronasal organ contains cells sensitive to MHC molecules in a genotype-specific way. Experiments on animals and volunteers have shown that potential sexual partners tend to be perceived more attractive if their MHC composition is substantially different. Married couples are more different regarding MHC genes than would be expected by chance. This behavior pattern promotes variability of the immune system of individuals in the population, thus making the population more robust against new diseases. Another reason may be to prevent inbreeding.

While we may not be as adept as some animals at consciously detecting smells, it does not mean that we don’t register and process their existence. And matches are not made in heaven, they are made by genetic mismatching—isn’t that fascinating.

"Humans have few olfactory receptor cells compared to dogs and few functional olfactory receptor genes compared to rats. This is in part due a reduction of the size of the snout in order to achieve depth perception as well as other changes related to bipedalism. However, it has been argued that humans may have larger brain areas associated with olfactory perception compared to other species."

Hyrdy follows up on this notion that humans form positive or negative responses to others due to their body odor.

"A curious reminder of how adaptive it once was for a pregnant woman to be near kin when pregnant persists in a modern woman’s sense of smell. As has been experimentally demonstrated for other mammals, ovulating women prefer the scent of males who have genetically produced immunological attributes….different from their own. Presumably this is to decrease the chances that a woman will mate with close kin. Mice manage to avoid incestuous matings this way, by sniffing the male’s urine. Women use body odors instead. They can distinguish between man’s smelly T-shirts, and rank them as either ‘attractive’ or unattractive.’ Instead of preferring alien smells, very different from their own, however, women taking birth control pills that simulate pregnancy exhibit the reverse preference: they prefer the smell of those with immune systems genetically most similar to their own….Perhaps women whose bodies have been artificially induced to simulate pregnancy subconsciously gravitate toward kin."

If so, it would be interesting to ponder what might be the effects of birth control pills on natural selection among humans.

When Mogil’s team performed their experiments on mice they substituted aromas from androstenone and androstadienone, compounds found in male sweat and urine, and observed the same result as with actual males. Yes, just as do their mice brethren, men leave their markers in their urine.

Wikipedia provides this information about androstenone:

"Androstenone….is a steroid found in both male and female sweat and urine. It is also found in boar's saliva, and in celery cytoplasm. Androstenone was the first mammalian pheromone to be identified. It is found in high concentrations in the saliva of male pigs, and, when sniffed by a female pig that is in heat, results in the female assuming the mating stance. Androstenone is the active ingredient in 'Boarmate', a commercial product made by DuPont sold to pig farmers to test sows for timing of artificial insemination."
　
It seems likely that some will be amused by the association of the scent of men with that of pigs.

And then there is this:

"To animals, the smell of androstenone can act as a social sign of dominance, or it can be a way of attracting a mate. This smell, to some animals, has a huge impact on behavioral patterns in the specimen."

Different scents mean different things to different species. Perhaps Mogil and company didn’t so much discover anything very new, but they did package their results well.

And this about androstadienone:

"Androstadienone….is a chemical compound that has been described as having strong pheromone-like activities in humans. It is a metabolite of the sex hormone testosterone: however, androstadienone does not exhibit any known androgenic or anabolic effects. Though it has been reported to significantly affect the mood of heterosexual women and homosexual men, it does not alter behavior overtly, although it may have more subtle effects on attention. Androstadienone is commonly sold in male fragrances, it is purported, to increase sexual attraction."

So, heterosexual women and homosexual men respond similarly to male scent. Can you still believe that homosexuality is a lifestyle choice?

We often find it amusing to observe other animals as they go through their sniffing rituals. Perhaps we do many of the same things—we just do them subconsciously.