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The Olfactory Lives of Primates

ISSUE:  Spring 2006

Dear Chris,

I’m sorry you couldn’t make it to Brad and Caitland’s wedding. It was pretty good. The beginning dragged—the usual, everyone standing around, sniffing each other’s breath, figuring out who was from Caitland’s family, who from Brad’s. You could see people getting confused by Jessica, Caitland’s niece, the one who’s adopted.

At one point, this very attractive second cousin of Brad’s came up and sniffed my butt. Okay, I’ll admit it, it made my day. Then these two drunk guys from Caitland’s office got into this pissing contest, completely soaked a table-cloth doing it. I don’t know why they bothered—one of the guys had no testes, and he was terrified of the other guy—majorly acrid sweat.

Totally poignant with Hugh, Caitland’s dad. She didn’t think he’d last for the wedding. You could smell the cancer from across the room. I’ve always liked the guy.

One really funny thing—Caitland’s two sisters were both ovulating (it occurred to me that Brad had done that to them, which would have been really crappy of him), and they kept sniffing Tom’s crotch. They were trying to make it seem like this big joke, but it was obvious that they were, like, totally serious. Lame.

Scientists are capable of being as childish as anyone else, and fights often erupt over what name to give to some new discovery. This happened in the middle of the last century and concerned the name for a part of the brain. Deep in the brain’s underbelly, far from that gleaming cortex doing string theory physics, is this interconnected cluster of ancient regions, like the amygdala, hippocampus, and hypothalamus. The old guard of neuroscience had long called this network the rhinencephalon—“nose brain.” This made sense. In a rat, there’s this gigantic mass of brain cells that detects odors, and it funnels all that olfactory information, by way of cable-like “projections,” into that amygdala/hippocampus/hypothalamus network. Meanwhile, a bunch of young Turk neuroscientists ignored this rhinencephalon label. They were analyzing what the amygdala, hippocampus, etc., actually did. For example, what behavioral changes would occur if one of those regions was damaged? And they were finding some very interesting effects—changes in sexual or aggressive or maternal behavior. These folks concluded that this network was about emotion (and for reasons I’ve never figured out, they called this network the “limbic” part of the brain).

Rhinencephalon or limbic system? Smell or emotion? Coke or Pepsi? After factional violence that left thousands dead, a kumbaya-esque solution became obvious. For your typical mammal, there was no conflict—emotions equal odors. Sexual, aggressive, or maternal behavior never occurs outside the context of olfaction.

Olfaction is a unique sensory modality for a mammal. To appreciate this, we have to consider how neurons—the main type of brain cell—communicate with each other, often in long sequences of cells. Neuroscientists play a game of counting how many neurons it takes to go from Brain Region A to Region B. If B is only one neuronal connection away from A, A is likely to have a big influence on B’s function. But if A has to send its message snaking through a line of ten neurons to reach B, A’s not going to be of much consequence to B.

For sensory systems, how many steps does it take to get from the eye, ear, or patch of skin to that emotional limbic system? Roughly ten steps. Take vision. First, there’s a layer of neurons in the cortex that breaks the visual scene into dots, then a next layer turning the dots into lines, then collections of lines, on and on. Finally, an Ice Age later, by a neuron’s temporal standard, visual information trickles to the limbic system, and you activate an emotional response appropriate to seeing the face of someone intent on, say, seducing you or ethnically cleansing you. All of the sensory systems, that is, except olfaction. How many steps from smelling something to the limbic system? Just one.

The result is an intertwining of the limbic and the rhinencephalonocentric worldviews. Most mammalian species give off odors—pheromones—containing all sorts of information relevant to emotion in the recipient. An animal’s pheromones communicate its genetic pedigree, gender, approximate age, if it is ovulating, if it has testes, if it is sick, happy, or terrified. Just consider—a dog sure doesn’t want an opponent to know that he’s scared. But he can’t will his anal scent glands not to exude the stress hormone–laden pheromones that broadcast his terror. But he can sure try to put a lid on those scent glands—by tucking his tail between his legs.

All of that is the type of information that can change emotions in the recipient, and in most mammals, more readily than can information from other sensory systems. Olfaction can also shape bodily responses that accompany particular emotions. Get the right pheromonal information looping to the hypothalamus, and you nudge it toward increasing your heart rate. There are certain pheromones released by rodents that will change the likelihood of ovulation, others that will change when puberty occurs. Even certain pheromones that make a pregnant female miscarry.

And olfaction has easier access to memory than the other senses. One of the key structures related to memory formation and retrieval is the hippocampus (ponder the significanceof the fact that a key portal to memory sits in a brain network most relevant to emotion). And, unlike those meandering projections from the other sensory systems, olfaction fast-tracks its information to the hippocampus, with fewer intervening steps. As a result, a rat can learn an olfactory task faster than we can learn some visual ones—trashing that business about us sitting at the top of a pyramid of species complexity.

So olfaction rules. Certainly in a rat, which devotes an outlandish 48% of its brain to olfactory processing. Rhinencephalon indeed. Same olfactory obsession in dogs, where you take the dog out for a quick walk in a drenching thunderstorm, and he has twenty spots in the neighborhood to sniff in order to catch up on gossip. There are even dogs who are professional noses—those bloodhounds trained to smell out explosives and drugs and anti-Dubya writing in suitcases. Olfaction is even a big deal for nonhuman primates. Much less of the brain is devoted to olfaction than in a rodent, with an emphasis instead on vision—for example, the evolution of sophisticated stereoscopic color vision for spotting a smidgen of a color signaling the right kind of ripe fruit, amid a riot of rain forest foliage. But that emphasis on vision only counts for so much. Troop of baboons, someone gave birth during the night, new mom is now on the ground, holding the baboonito, and everyone comes over to check it out, to ask the same question that we all ask—boy or girl? And they don’t spread the kid’s legs to look. They spread them to sniff.

So what about humans? We’re more olfactory than most would guess. After all, we spend fortunes on perfumes and Dainty Floral Country Garden sprays to use in the bathroom after we’ve really cut one. We do the olfaction/hormone business familiar to other mammals. For example, there is that tendency of women roommates to synchronize their menstrual cycles, and this is accomplished through pheromones. Or that women can smell (but not be consciously aware) if some guy has his testes. Or that men can smell (again, not consciously) whether someone is ovulating. (High-tech science: get some women volunteers at known points in their cycles to rub their undeodranted armpits with a cotton swab, stick it in a mason jar, and let guys sniff the insides of various jars. And not only do guys tend to distinguish the smell of the ovulatory cotton swab from the others, they think it smells better.) And that olfaction/memory linking works in humans: this accounts for that Proustian moment where an unexpected odor wafts by and, suddenly, you’re transported back to kindergarten with this wild intensity, back amid the primary-colored plastic chairs and the Elmer’s glue. Olfaction does things to our human heads that the other sensory modalities can only dream of.

But despite those amazing human olfactory abilities, we still have a very atrophied olfactory system, compared to other mammals. We can’t remotely distinguish odors as well as most other species. We devote only 1% of our brain mass to olfaction. But here’s the measure that really amazes me.

A truly unsettling finding came a few decades back that chimps and humans share 98% of their DNA. 98%. This is boggling. So this naturally leads to the question—what’s the 2% difference about? The answer came recently. A few years back, that mammoth public works venture, the Human Genome Project, decoded the human genome, producing a billions-long sequence of letters that comprises all our DNA. And then, a few months back, the chimpanzee genome was revealed. And at last, we get to see what that 2% difference is.

And it turns out to be weird. Not a thing having to do with our brains working so differently than theirs—no genetic explanation for literature, art, megastates, termite sticks, the bunny hop (which, increasingly, strikes me as logical). Our “me-ness” is not all that anchored in our genes. There were some genetic differences concerning body hair. Others about immune function—chimps handle malaria better than we do, we handle tuberculosis better than they do. Some about reproduction, making it unlikely that there’s some human/chimp hybrid out there. But the biggest difference concerned olfaction. When an odor—a molecule that has floated off, becomes airborne, from sweat, from a mound of cinnamon, or a rotten egg, or pollinating flower or exhaust pipe—reaches your nose, it binds to a subset of literally thousands of different olfactory receptors which, when activated in particular patterns, send a Guess what I just smelled message to your brain. Those receptors are coded for by genes, and it turns out that half, half of the genetic differences between chimps and humans concern olfactory receptors. They’ve got them, and we’ve functionally disabled ours into what are called pseudogenes. As we split off from our last common ancestor a few million years back—an ape already with an atrophied olfactory system—the most common genetic shift that would ultimately differentiate us from chimps was that we decayed into having a lousy sense of smell.

And what are the consequences? We have become disproportionately specialized in our other senses. We can be aroused by an erotic picture. We can be moved to tears by music. We can read this essay in Braille.

But even our nonolfactory senses are not so hot. Raptors see better than we do. Nocturnal animals hear better. Our senses aren’t great. But our thinking about our senses is amazing. You can decide whether some curry tastes the same as the one you made last July. You can remember the sensation of someone’s hand moving down your body. Or you can sit alone in a loft with a stack of empty sheets of music paper and imagine what the symphony will sound like. We have been freed from the concrete here and now of sensation—as we have been from the here and now of emotion, of thought, of everything. And while that may make for less interesting wedding parties, it sure is central to what makes us human.


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