HUMANS SPEND an awful lot of time and money trying to smell different – over $2 billion a year on antiperspirants and deodorants in the U.S. alone. It’s hard to imagine what that dollar amount might be once you add cologne and perfumes to the annual tally. On a more practical note, our keen smell has served us well, allowing for an olfactory thumbs up or down on newly encountered foods (especially scavenged meat) throughout our evolution and the presumed parsing of mates along the way.
It has long been known that the funk wafting from people and foods was a consequence of bacteria – more specifically, the byproducts of bacteria. Fascinating new research among spotted hyenas suggests that just enough difference in odor-producing bacterial communities exists between clans of hyenas that group alliance, and the identification of friend or foe is possible based on smell alone. This finding reveals that scent, in addition to the more tactile senses of vision and vocals, has played an important evolutionary role in the social organization of this – and doubtless – many other mammals. You are whom you smell like.
Spotted hyenas can be found throughout sub-Saharan Africa, living in large, social groups (clans) of up to 50 or more individuals, with adults fiercely protecting clan territory and members. Researchers from Michigan State University studied four clans and their respective territories in the Masai Mara National Reserve in Kenya. After carefully determining the territorial boundaries of the four clans, four lactating females from each clan were anesthetized by a projectile delivered from a darting rifle. Once sedated, the anal scent pouch was sampled and bacteria sequenced using genomic techniques.
When marking territory, a hyena simply straddles a grass stalk and drags its rear-located scent pouch over the selected grass, leaving a lipid-rich secretion called paste. According to researchers, the paste is quite pungent and is detectable by the human nose for days after being deposited (presumably weeks, then, for the more attuned hyena).
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It is well known that the pungent smell is the byproduct of bacterial fermentation in the hyena scent gland. But the surprise was how the composition of the bacteria identified from the scent glands in the sampled females from each group clustered. That is, while all 16 females (four from each clan) harbored many of the same bacterial species, the clans could be separated based on structure and abundance of specific bacteria. Importantly, females within the same clan were more similar to one another than to females from other clans.
And because the clans harbored slightly different bacteria, these bacterial communities produced slightly different byproducts (short chain fatty acids) through fermentation of lipids and other substrates in the nutrient-rich scent pouch, which in turn yielded slightly different odors left behind on the blades of grass. Since all of the clans studied lived more or less in the same geographical area, the bacterial composition of each clan’s scent pouches could not be attributed to dietary differences. Though the researchers in the study do not directly suggest so, the clan-specific bacterial communities were likely handed down by birth through vertical transmission. Since hyena from the same clan will repeatedly mark the same area with multiple members, it is also highly likely that this mechanism of cross-infection between clan members contributes the conserved clan-specific microbial communities as well. In either case, the clan-specific odor – and thus bacterial –profile is conserved over evolutionary time.
It’s not much of a stretch to see how group-specific odor profiles among social animals such as hyenas could be applied to humans – which may have become more complicated when we became bipedal and thus could no longer, or at least easily, sniff one another’s butts. Without a doubt, while ones odor signaled to potential mates or social groups for the presence of disease (i.e., you smell rotten), this selection process likely enhanced mating preferences within one’s own tribe pushing populations to become more sexually isolated and thus sowing the emergence of ethnic differences we experience today.
While linguistics and the rapid loss of the worlds diverse languages get a lot of press, from a purely biological perspective our true differences may lie in what’s left of our stench – which may or may not cut across the more obvious ethnic differences. Clearly our homogenizing western diet and hyper-hygienic lifestyle has significantly blurred what was left of the mega and sub-clans we all once belonged. The final nail in the sniffable coffin has to be the meteoric rise in caesarean births, which do not allow for the vertical transmission and seeding of clan-specific microbial communities.
With all things microbial, there are consequences to tinkering with nature. It remains to be seen how the scrubbing and masking of our remaining microbial odors with Old Spice and Chanel No. 5 will play out. But one thing is for sure, as we transition from a purely biological species to a hybrid with synthetic attributes, a new definition of “self” will have to follow.