Sure, Humans Might Lack a Jacobson’s Organ. We’re Still Decent at Sniffing Out Mates.

Slate’s animal blog.
Aug. 25 2014 8:07 AM

Are Humans Any Good at Pheromones?

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A man and a mouse lemur enjoy the golden hour.

Photo illustration by Lisa Larson-Walker. Photos by Pindyurin Vasily/Shutterstock and Arjan Haverkamp/Flickr Creative Commons

Excerpted from Wild Connection: What Animal Courtship and Mating Tell Us About Human Relationships by Jennifer L. Verdolin. Out now from Prometheus Books.

As if your face, your eyes, your symmetry, your hair, your waist, your teeth, and sometimes even your feet weren’t enough, there’s even more going on than meets the eye, and this other consideration may just be the ultimate deciding factor in initial mate attraction—it’s how you smell. When discussing mating systems with my students, I always tell them, “It wasn’t that you saw each other across the room; you smelled each other!” There is one man I know who I could probably smell a mile away. And I mean that in a good way. For me, his natural scent is so thoroughly intoxicating that I can barely think when I am around him. Worse still, I am like a basset hound when it comes to sensing his presence in my environment, and I am convinced it is because I smell him before I see him!

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The idea that one can become inebriated by the natural smell of another is not as strange as it may sound. Both males and females of many species succumb to the scent of desire. Just look at the delightful, pudgy, brown lemming male: He has a keen sense of smell when it comes to the ladies. It’s a rough life for lemmings. They are pretty much on the bottom of the food chain, and they only live for about a year and a half. Not one for dillydallying, this little rodent packs a lot into that short life span.

Despite living in the arctic, lemmings are active all year round. With the clock ticking down, there is no time to hibernate for these guys. Females can have several litters a year, raising anywhere from four to nine babies at a time. Females have the uncanny ability to sniff out better mates, and their noses lead them right to the dominant male. Have you ever walked into a room and said, “Boy, you can just smell the testosterone in the air?” Apparently this is what female brown lemmings are discussing as well.

Male lemmings not only have a knack for smelling females that are ready to mate but also for smelling those that haven’t already mated with another male.

When females mate with multiple males, it is harder for males to be sure of their paternity.

Male lemmings try to get around this by detecting whether a female has already mated. This, of course, implies that males leave a chemical calling card that other males can detect. From beetles to bees and lizards, females do give off a different scent if they have already mated or if they are ready to mate.

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What does all this chemical calling card stuff have to do with us? Lo and behold, we are just as sensitive to the scent of the opposite sex as the humble lemming. Humans can discriminate odors in just a single whiff, which at a minimum takes approximately 400 milliseconds. Like male beetles, bees, lizards, lemmings, and a whole suite of other species, men can discern the scent of a woman ready to become pregnant.

They find the smell of sweat from women who are close to ovulation more pleasing and even sexier.

And not just their body odor, men also prefer the voice, the complexion, and basically everything about a woman near ovulation. The thing is, men know women are ovulating because they can smell it, but they don’t know that they know!

One of my friends swears by this phenomenon. She claims that she gets a lot more attention from men right before she begins ovulation. Whether it is holding the door open for her, buying her a cup of coffee or a drink, or being asked out, like bees to honey the men flock to her, only to disappear again once she passes that magical time. Women, the same holds true for us. When we are ovulating we strongly prefer the scent of a male, but not just any male, a more symmetrical male.

Beyond the simple fact of whether one prefers certain scents, there is increasing evidence that how an individual smells, the person’s pheromone signature, if you will, may be linked to that person’s genetic health—specifically, his or her immune or disease-fighting genes. These are known as major- histocompatibility-complex, or MHC, genes. By distinguishing at a cellular level between self and other, they are involved in identifying and fighting off invading pathogens.

Mothers, fathers, and close relatives like grandparents and aunts and uncles have been shown to be able to identify the odor of a related infant compared with an unrelated one. In the case of fathers and other relatives, they can do this even if they have had no prior exposure to the baby! When we look to animals, we find similar results. Individuals seem to be able to tell the difference between relatives and nonrelatives based on smell alone. And it is largely thought that this is due to the scent one gives off based on the particular set of MHC genes you have.

While this is fascinating—and potentially a topic for another book—what does this have to do with finding and choosing a mate? Studies with lab mice reveal that, all other things being equal, individuals will choose a mouse mate that is most dissimilar in the MHC genes.

This phenomenon extends far beyond the lab. One of the cutest species I have had the pleasure of studying is the gray mouse lemur. This nocturnal primate, native to Madagascar, is small enough to fit in the palm of my hand, reminding me of a miniature Topo Gigio, an Italian television puppet character popular when I was growing up.

Looking at gray mouse lemurs in the wild reveals that their mate choices are also MHC-dependent.

The benefits of this are twofold. First, they avoid mating with relatives, and second, by combining different genes from two parents, offspring have the maximum diversity in their disease-fighting genes. This second benefit may help offspring survive better when fighting off infections and disease.

I mentioned that mothers and other close relatives can distinguish the smell of a related versus nonrelated infant, but does this extend to detecting the best genetic match based on MHC composition? Yes, indeed. Just like paper wasps, the house mouse, seabirds, primates, and countless other animals, human females have a stronger sexual interest in the odor of males who differ from them on the MHC-gene level. Even more interesting is that in already-paired couples, women were less sexually responsive to and had fewer orgasms with partners who had similar MHC compositions.

Perhaps this is why some men are constantly obsessed with whether or not a woman has an orgasm? As if that weren’t bad enough, closely MHC-matched couples also engaged in a higher number of extrapair copulations. Translation: more cheating.

Similarity of MHC composition may also explain why some couples have difficulty getting pregnant, and it may even explain the frequency of spontaneous abortions.

With nature guiding the way and with such severe consequences, how do we ever end up mismatched?

One argument for how we end up mismatched is that we don’t have the capacity to detect MHC composition using our olfactory ability, especially since we lack the Jacobson’s organ, which is found in the nasal cavity of many animals. This organ is first in line when it comes to olfactory sense and processing. Next time you see your cat smell something and hold its mouth open with upper lips curled and teeth exposed in what is called the flehmen response, you can bet something tweaked its Jacobson’s organ.

Although you may never have heard of it, scientists have been hotly debating whether you have a Jacobson’s organ, or vomeronasal organ. This mysterious and contentious organ is the secondary sensory organ of the accessory olfactory system with specialized neurons that process chemical cues, separate from those associated with primary olfactory processing center. Interestingly, fish lack this accessory organ, suggesting that perhaps, life on land may have been the impetus for the evolution of the Jacobson’s organ. Where, if you have one, would you find this special structure? Depending on the species, it can be located at the base of the septum or in the roof of the mouth.

So who has it and who doesn’t? As usual, except for fish, there is no simple answer to this. Since we are primates, let’s just investigate what’s going on with this group. For the families that include bush babies and lemurs, also known as the strepsirrhine primates, we find a fully functional, anatomically complete vomeronasal organ. In the catarrhine primates, like macaques, it is generally absent, or reduced, with some indeterminate function.

When we start looking at the group that includes tarsiers, monkeys, apes, and us, things get a little messier. Perhaps ironically, this group, the haplorhine primates, literally translates in Greek to mean “simple-nosed” primates. Some species have it, some species don’t, some have it but it doesn’t work, and more importantly, throughout the order, its size is greatly reduced. For instance, both night monkeys and spider monkeys have a Jacobson’s organ, but it is not functional in the spider monkey.

When it comes to humans, it is clear that developing embryos have a Jacobsen’s organ which then seems to disappear. In adults a depression, or pit, consistent with the Jacobson’s organ is present at least on one side of the nasal cavity about 2 centimeters up into the nostrils, but it looks more like a remnant structure. So is this pit, or pits if you are one of a few that have it bilaterally, a functional Jacobson’s organ, or is it just a leftover of our evolutionary past?

There are a few things to consider. First, the epithelium tissue lining these depressions in humans is not well-developed. Second, there is a lack of sensory neurons that even if the tissue were functional could connect it to the brain, sending along whatever chemical information was being perceived. Third, and perhaps more importantly, almost all of the genes involved in protein expression of a functional vomeronasal organ are pseudogenes in humans dating back as far as 23 million years ago— around the time we went our separate ways from Old World monkeys. What are pseudogenes? They are dysfunctional copies or relatives of functional genes. Put simply, they don’t work.

But why all the fuss? What does this little organ actually do? Some have suggested that its purpose is solely for the detection of pheromones, or hormones involved in a variety of social functions, from recognizing individuals to mate selection. The implication then being that we, and other species that lack a functional Jacobson’s organ, cannot detect, process, or even respond to pheromones. That is quite a leap. Indeed, we have other genes, separate from those linked to this touchy organ that are involved in detecting pheromones. Not surprisingly, they are linked to the main olfactory system. MRI studies have shown that molecules involved in discriminating between “self” and “other” odors, called peptide ligands, activate not just the vomeronasal organ in animals but also parts of the brain. Though humans lack the organ in our noses, we certainly have brains, and these same molecules, when we smell them, light up the same area in our brains, too. This then means that regardless of which camp you fall into, the “we do” or the “we don’t,” the scales are tipped in favor of our ability to detect and respond to pheromones. So sniff away because even if we don’t have a functional Jacobson’s organ, we have noses with lots of neural connections to our marvelous brains with which to process information in the very functional main olfactory system we do have.

However, this brings us back to the question then—why do we end up mismatched?

It might have something to do with birth control pills. The irony, or perhaps tragedy, of hormonal birth control is that it interferes with how a woman’s nose knows. When women take birth control pills, some studies suggest that this natural ability to discriminate between similarity and differences in MHC composition may be disrupted. The research isn’t entirely clear, but this could cause women to be more sexually attracted to the odor of males with MHC genes more similar to themselves. Not the best match.

I was discussing this with my friend Stacey, who exclaimed, “That must be why I couldn’t stand the smell of my ex-husband!” She went on to explain that when she met her first husband she had been taking birth control pills. Several years into their marriage, after she discontinued the pill, not only was she unable to get pregnant, but she no longer cared for the smell of her husband.

My advice: sniff a potential mate. I personally like the neck. Good for smelling babies and good for smelling men. If you are not on birth control and he (the man, not the baby) passes the sniff test, then that is just one more step toward finding a potentially good mate.

Excerpted from Wild Connection: What Animal Courtship and Mating Tell Us About Human Relationships by Jennifer L. Verdolin Out now from Prometheus Books. 

Wild Connection: What Animal Courtship and Mating Tell Us about Human Relationships

Dr. Jennifer L. Verdolin, an expert in animal behavior, is currently a research scientist affiliated with the National Evolutionary Synthesis Center at Duke University in Durham, North Carolina.

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