Category Archives: Olfaction

Sex and Evolution?

I like Michael Stoddart’s books in general–he has some great contributions to make and is one of a few popular scientists promoting olfaction. While reading his most recent book, I have been a bit frustrated by the simplistic view of human evolution and behavior. A recent Guardian piece by him “Smell evolution and the sex brain: Why we’re monogamous and use perfume” captures the source of my frustration. As a biological anthropologist, I find it hard to read the following sentence:

To keep male and female together to provide protection for babies, a suite of anatomical and physiological features evolved to promote the constant availability of sex throughout the year – irrespective of the monthly ovulation cycle.

The argument is that a suite of traits evolved (e.g., reduced sexual dimorphism, hidden estrus) to render human females receptive to sex at any time and this has led to monogamy–meanwhile male receptivity to sex is used as an explanation for purported male promiscuity. Huh! The constant male bias in science is at the heart of taxonomy–our class is called mammal because male scientists felt the key trait of mammals was the use of mammary glands to feed offspring.

Increasing the diversity of voices in academia has allowed us, slowly, to move away from teleological explanations for human behavior based on western society. In biological and evolutionary anthropology, human reproduction is a hot topic and more complicated. Stoddart does qualify his statement a bit:

Yet Homo Sapiens is the only species among the 5,500 kinds of mammal to maintain monogamous family relationships – or at least serially so – and to live in densely populated areas. This combination is extremely rare in nature.

Marriage, as an institution is barely thousands of years old (our species is 200,000 years old) and the concept of marrying for love younger still. Divorce is higher today partly because there are fewer economic and political structures keeping people together–religion is what is left and that doesn’t appear strong enough for most people–divorce was central in Henry the VIII’s split with the Pope. Most cultures are polygamous even if most end up practicing monogamy (mainly due to financial and/or political constraints–not enough money or power to gain more spouses). Perhaps the clearest statement we can make on pair-bonding is that humans can, and often do, come together in a pair-bond for a period of time with a goal of child rearing but this shared interest isn’t immediately linked to sexual monogamy–they are separate issues. The period of shared interest (if it occurs) enables the child to reach a point where the ‘village’ can take on some of the burden through formal and informal education. But, even western society regularly abandons its children–part of the year, I live next door to a youth shelter and drop-in center so I see it daily.

I suppose most humans are humanists–Jon Marks is a biological anthropologist who has written many books on the subject from an evolutionary perspective with a goal to distinguish us from all the other primates. I am not a humanist even if I do appreciate what we have accomplished as a species (there’s a lot to be ashamed of too…). I think there is an inherent fallacy in not recognizing that we are animals and that we cheat and lie and love and, yes, react to odors just as animals do. We may be enculturated to curb instincts but the instincts that we are enculturated to curb and how we do so vary cross-culturally. The goal to overcome our instincts with reason is a cultural one, not a biological or evolutionary one.

And, contrary to this blanket statement:

Today we have a global fragrance market equal to the GDP of a medium-sized country. But because our nose (unlike the VNO) ultimately sends all smells for rational analysis by the brain, we do not slavishly respond to sex smells in the way dogs or mice do. An alluring perfume may help a relationship, but no perfume comes with a guarantee!

odors are first processed in the areas of the emotional center of the brain where memories are also deal with–we react to odors before the frontal lobe (where reasoning attempts to modulate instinct) gets the data and formulates a response. Maybe we wear perfume because it smells good–it takes us to places we want to be or reminds of us of memories we love or smells like things we love to eat–maybe wearing perfume is about sensuality not mating. Why is so much academic work reductionist? But, perhaps that is why I am a biological anthropologist, rather than a biologist. Still, I take the point that we may not react to odors with the full behavioral response other animals might, but we react nonetheless. The closing statement of the piece is perhaps the strangest, and to an anthropologist, the most off-putting:

And so we can live in at least relative harmony with our fellows, benefitting from the long-term genetic and evolutionary advantages provided by monogamy, while participating socially in everything society has to offer.

There probably should be more biological anthropologists writing popular press books on human evolution and this gives me even more motivation to get my long overdue book Smell of Evolution out!

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Filed under anthropology, Anthropology and Evolution, Olfaction, primate social life, religion, Science, Senses, sex

The chirality of smell

Limonene_struttura

Limonene

As I prepare the first half of my Science of Smell online class, I am having fun looking for various examples of all things biomolecular, biochemical, and genetic related to olfaction. If I were a taste and flavour chemist or a molecular gastronomist, I’d probably be interested in somehow exploiting the chirality of biomolecules in food and drinks!

Chirality refers to the non-symmetrical nature of some molecules. Non-symmetrical molecules are like our right and left hands: they appear the same in reverse but you cannot superimpose the image of one over the other in the mirror nor can a left-handed glove fit a right hand. In the pharmaceutical industry, chirality is very important because the enantiomer of a biomolecule that produces a positive outcome (like reducing morning sickness using thalidomide or hyperactivity using Ritalin) may cause a harmful effect (birth defects) or no effect. The handedness is determined by the stereocenter of a molecular. Those with ‘right’ handed stereocenters are ‘R’ or + enantiomers and those with ‘left’ handed stereocenters are ‘S’ or – enantiomers (the S comes from sinister, Latin for left).

Our olfactory receptors are clever things: they can tell the difference between the right and left hands! The two most common examples are of carvone and limonene.  The R-carvone/(-) carvone is recognized as mint and its enantiomer as carraway. R-limonene/(+) limonene smells ‘orange’ and its enantiomer is lemon (see image to the right). So next time you smell oranges and lemons at the same time, recognize the power of your nose to be ambidextrous by distinguishing between the two biomolecules!

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Filed under Food, Olfaction, Science, Senses, smell of the week

Is odorant diversity driving olfactory receptor genetic variation?

Olfactory receptor genes have more variation than most gene families in the human genome. The only family with greater diversity is the major histocompatibility complex (MHC). Both families also exhibit high heterozygosity. Due to its association with disease, the MHC is well-studied. The explanation for the maintenance of MHC diversity is pathogen-driven selection–either through heterozygote advantage or frequency-dependent selection (see here for a review); a small number of papers (here’s two: and 2) have also argued for divergent allele advantage. A diversity of  pathogens will result in a diversity of MHC genes over time; as a species develops resistance to a disease, an evolutionary respones occurs in the disease-causing agent. The common analogy is the evolutionary arms race, also called the Red Queen Hypothesis.

If we apply that same model to olfaction in light of a few recent findings. there might be something worth pursuing. We know we can smell millions of odors. Such a diversity of odorants in the environment that vary from region to region may result in incredible diversity in the human olfactory receptor subgenome–especially if we look at it from the perspective of divergent allele advantage.

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Inhale for health

This research is a bit old (October 2013) but recently caught my eye:

Research out of Japan shows that walking in the woods also may play a role in fighting cancer. Plants emit a chemical called phytoncides that protects them from rotting and insects. When people breathe it in, there is an increase in the level of “natural killer” cells, which are part of a person’s immune response to cancer.

“When we walk in a forest or park, our levels of white blood cells increase and it also lowers our pulse rate, blood pressure and level of the stress hormone cortisol,” Michelfelder said.

There is rare evidence of cancer (osteocarcanoma) in prehistoric hunter-gatherer populations (see here for a nice public science summary) and mummies (another public science review is found here). This may be because we can’t detect it and accurately determine its frequency. Modern techniques like CT scanning make inroads into non-invasive paleopathology data gathering but skeletons have a limited capacity to reveal diseases of the past. This is partly because the lesions (like most pathologies) often don’t reach the bones, take too long to reach the bones before death, or are nonspecific.

The rarity of evidence for cancer may also be because it simply wasn’t there. Most cancers occur at the end of of or after reproductive years; the shorter human life span ‘in the wild’ would likely lead to fewer cases of cancer experienced by our prehistoric relatives and not impact net reproductive success (meaning any cancer-causing genes would persist in human populations). Persistent cancer-causing genes interact with the modern environment and longer life span to reach modern cancer frequencies. I wonder if lifestyles that take one into the woods for significant periods of time (e.g., prehistoric hunter-gatherers, modern populations leading ‘traditional’ lifestyles) reduce cancer incidence?

I am reminded of something I heard when I was a kid about the actor Dirk Benedict (from Battlestar Galactica–the original Starbuck!–and the A-Team) having had overcome prostate cancer by disappearing into the woods and the wilds of the country and eating a macrobiotic diet. I looked up his story to see if I remembered correctly. I had mostly:

When I learned I had a tumor—I refused to be tested for malignancy—I weighed 180 pounds. When I came out of the mountains of New Hampshire six weeks later, I weighed 155. I went to stay in a friend’s cabin because I didn’t want any distractions, any temptations, anybody calling up to say, “Let’s go have a bagel.” Well, all hell broke loose. Some days I felt on top of the world, and other days I couldn’t get out of bed. Sometimes I couldn’t walk up the stairs, and sometimes I’d ride, run and chop wood for 24 hours.

I never did go into a hospital. Instead, I packed up my duffel bag and became a vagabond, traveling to Montana, Maine, California, New York City, Wisconsin, hitchhiking across the country once and driving across twice.

Did the woods help? Maybe! The sense of smell–yet another benefit!

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The Scent of a Man

A new study (published here) suggest that scientists unable to replicate  behavioral studies in rats and mice may be due to the presence of male researchers.

The presence of male experimenters produced a stress response in mice and rats equivalent to that caused by restraining the rodents for 15 minutes in a tube or forcing them to swim for three minutes. This stress-induced reaction made mice and rats of both sexes less sensitive to pain.

The chemical signals emitted by males of any species are detectable by other species. Since males secrete these pheromones at higher concentrations than women, the effects tend to be limited to male researchers. Rats and mice acclimatize over time to the male researchers, suggesting an ‘easing’ in period prior to experimentation or perhaps, even better, even more effort to promote women in science!

Since there is growing evidence that humans respond to pheromones, I wonder if there is a similar effect caused by male researchers on human subjects; namely, is stress induced in males and females when experiments are conducted by men? Outside the lab, does the scent of a man induce stress and reduce pain response, but in a good way? I’ll end with ‘Boys‘ by Robots in Disguise.

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Filed under anthropology, Anthropology and Evolution, Olfaction, Senses, sex

Black diamonds

The genome for the Périgord truffle was published in 2010. Considering that these special truffles go for 1000-2000 ($1300-2700) an ounce, the genome has been under-exploited by culinary scientists and molecular gastronomists…until recently when specialists in bioinformatics and proteomics got together to mine the secrets of Brillat-Savarin‘s “diamond of the kitchen”. The resulting paper, released today in the Journal of Proteome Research, seeks to unlock the “puzzling biology of the Black Périgord truffle Tuber melanosporum“.

As ectomycorrhizal fungi, truffles exist in symbiotic relationships with tree roots (black truffles like both oak and hazelnuts). Cultivation first occurred successfully in France when phylloxera destroyed viticulture in France (temporarily) and made available huge tracts of land for the creation of truffle groves. Much of that knowledge was lost in the wars that followed and the trees eventually reached the end of their truffle-producing life cycle. Attempts to start again today are rife with price control politics.

I was amazed to find that I LOVED LOVED LOVED truffles because I dislike mushrooms (something about their texture turns me off). I had truffles for the first time in a town called Stenay in Lorraine region of France. They were tender and aromatic and meaty without being meat. There wasn’t anything that I had eaten that compared to them and they filled a distinct niche that nothing else had in my vegetarian diet.

The smell. The lovely smell of good truffles–so good that Peter Mayle describes a truffle breakfast eaten with one’s head under the napkin to better capture the aroma. Amazingly, the new article reports that only nine enzymes are responsible for creating over 90 different odorants associated with the unique aroma of the black diamonds of Périgord! The authors note:

It is hardly surprising that such low numbers of enzymes were shown to be involved in the production of over 90 volatiles considering over 70% of the proteome is yet to be annotated. The potential to discover novel enzymes that could be of economic, medicinal, or other uses remains a tantalizing possibility.

Attempts have been made to capture this aroma to cheaply produce ‘truffled’ food. There is even a truffle-scented vodkaThe food industry uses two  chemicals (DMS and 2-methyl-1-butanol) predominant in the odor profile in order to produce a truffle flavour. A chemical called 2,4-dithiapentane is used to simulate the aroma in olive oils. Both DMS and 2,4-dithiapentane are derived from methanethiol, which is the key odor produced in human urine after metabolism of asparagus. The ability to detect this compound has a genetic component. I wonder if this influences one’s preference for truffles…something I am exploring in my on-going research on food preference, odors, and genes! 

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Filed under Food, Olfaction, Senses, smell of the week

Is there a connection between dietary repetition and food preferences?

A Science News Post (brought to my attention earlier in the summer by @elizabethjrowe) presents research trends in food science: the pairing of retronasal olfaction and taste reception in studying flavour and the knowledge pairing of culinary experts and scientists within a relatively new journal Flavour. I am glad that food sensation (for lack of a better word to describe the complex process of perception, taste, smell, hedonic value, and preference) is getting increasing amounts of attention! From an anthropological perspective, however, the evolutionary and cultural underpinnings of these studies is still missing from the dialogue–something I hope to rectify in the coming years!

The article leans towards the idea that repetition is the driver of food preference–and it starts in the womb. Support for this idea is presented by referencing the study on babies whose mothers ate anise and garlic during pregnancy (and therefore were not averse to the odors post-natally).  I assume the reference is to Schaal et al. 2000. That paper was great and is a start to exploring cross-cultural differences in the interaction between odor perception and food preference–but there also might be variation in olfactory receptors within the sample from the Alsace region (where anise is a common food additive but the population history of which is complex).

A taste and smell scientist is quoted as supporting the idea of repetition shaping diet: “What makes lasagna loved is that the odors have been paired to a source of calories.” Odors stimulate appetite but arguing for a causative relationship among odors, loving a food, and its caloric value is premature. We have so much yet to learn about the genetic architecture of individual and population odor profiles, which ligands bind to which receptors, odor processing, perception, and consciousness let along variation among all these things. All these known unknowns make olfaction a great place to work (and the unknown unknowns exciting things to be discovered)!

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