Human chemical communication is universal. Body odour signatures are a critical element.
"Similar to other animals, humans can extract signals from body odours that allow us to identify biological age, avoid sick people, pick a suitable partner and distinguish kin from non-kin," explained neuroscientist Johan Lundstrom at the Monell Chemical Senses Center in Philadelphia.
According to Tristan Wyatt at the University of Oxford, throughout the animal kingdom, odours mediate more interactions than any other signal.
"Substances secreted on the outside by an individual, and received by a second individual of the same species... release a specific reaction," Wyatt confirmed.
Olfactory (smell-sensing) capabilities are often very sophisticated. In humans, odours betray age; every generation has a distinguishable microfloral bouquet that differs from that of other age groups.
Several studies show that people can detect age of other individuals solely by sniffing odours they emit. Such odours differ in pungency, tang, zestfulness and other properties. They are like individual fingerprints that reveal a great deal of personal information that might otherwise remain unknowable.
According to Lundstrom, senior citizens have a distinctive microfloral signature that sets them apart.
"There is an 'old person's smell,' " confirmed Jane Mohler at the Arizona Center for Aging.
Scientists describe it as being "not unpleasant -- earthy, stale."
Every generation has its own specific and unique odiferousness.
"That's not a huge surprise since everything changes with age," Lundstrom reported.
According to Dustin Penn at Kondrad Lorenz Institute of Ethology and Claus Wedekind at the University of Oxford, human odours derive from "highly polymorphic genes of the major histocompatibility complex (MHC)".
"MHC genes encode cell-surface glycoproteins that bind with short peptides," they explained. "Protein carriers (lipocalins) transport odourants...(which) are made volatile by bacteria."
People can detect odourants via the vomernasal organ.
Sometimes the responses to odours are conscious. Sometimes they are subconscious. For instance, research shows that females subconsciously prefer the odours emitted by physically-symmetrical males.
Not only can people detect others' ages through cognitive odour analysis, but they also rank odours on the basis of relative offensiveness.
"The worst-smellers are middle-aged men," Lundstrom says.
Middle-aged women, meanwhile, have the "most pleasant odours," researchers agree.
"The odour of middle-aged women is the most beautiful of all," Lundstrom confirmed.
Senior males rank high in the scale of odour pleasantness, just above young women. At the other end of the odour spectrum come young males, ranking slightly better than middle-aged men.
Just as there is a distinctive old person's odour, characteristic odours attributed to all other age groups are readily distinguishable to other people.
According to Penn, a person's specific MHC genotype makeup significantly shapes his/her odour assessments.
"(Male) odours, as judged by a female, are assessed as more pleasant if the male's MHC genotype differs from hers," he reported.
"Body odour preferences are mostly influenced by the degree of similarity or dissimilarity of the MHC," he added, pointing out that people with similar MHC genotypes are more likely to find each other's odours unpleasant.
Researchers agree the degree of sophistication in odour detection and analysis in humans, and other animals, is extraordinary, and possibly linked with testosterone levels.
In many animals, chemicals called "pheromones" released by individuals are known to indicate danger, food trails and other important types of information, Wyatt pointed out.
Robert Alison is a zoologist and freelance writer based in Victoria, B.C.