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I remember one, sweltering day in Malaysia, I was climbing the very long staircase to a Buddha statue in a cave, when I suddenly felt enveloped in an overpowering smell of sweat. My first thought was that I must have forgot to wash, possibly for several weeks, but then I realised that I was in the middle of a troupe of monkeys.

And now I've come to wonder what the function is of this very characteristic smell. There seems to be connection to sexual maturity, since children don't start producing the smell until puberty, and the overwhelming base of evidence I collected that day, seems to suggest that this may be universal among primates.

Has any research been done into the function smell of sweat, socially or otherwise?

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    $\begingroup$ Why do you think it has a function? $\endgroup$
    – user438383
    Jul 31, 2023 at 11:05
  • $\begingroup$ It might not have a function. But maybe it's there as an indication that one should clean themselves to prevent disease. We similarly developed smelling dead bodies and excrement as foul to stay away from them because they can cause disease despite other animals like blowflies being attracted to these smells. $\endgroup$ Aug 2, 2023 at 2:08
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    $\begingroup$ Sweat does not smell much on its own, its the bacteria that eat the sweat that smell. $\endgroup$
    – John
    Aug 2, 2023 at 2:44
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    $\begingroup$ @user438383 Well, mostly because it only really starts at puberty. The fact that other primates also smell strongly suggests to me that this an adaptive trait that goes several million years back; and although we in modern society find the smell rather offensive, it is not actually smell you connect with decay. I believe more in the idea that it signals adulthood and perhaps that you are ready to fight for your territory or group, which may explain why we now try to mask the smell with deodorant, since we live rather stressful lives in dense cities. $\endgroup$
    – j4nd3r53n
    Aug 2, 2023 at 7:46
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    $\begingroup$ @j4nd3r53n thats fine, it's just always good to question whether a trait is really adaptive has a function or whether it's just a random byproduct of something else. Often things can appear to be adaptive when they are really just nothing :) $\endgroup$
    – user438383
    Aug 2, 2023 at 7:59

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It's a tricky query to answer, so here's some prelim info and someone can be more specific from there:

Apocrine glands are a type of sweat gland found in certain mammals, including humans. These glands are responsible for producing a thick, odorous sweat that is involved in various physiological and social functions. Here are some key characteristics and functions of apocrine glands:

Location:

Apocrine glands are typically found in specific areas of the body, such as the armpits (axillae), genital region, and around the nipples in humans. In other mammals, they may be located in different regions depending on the species.

Odorous Secretion:

Unlike eccrine glands, which produce a watery and odorless sweat, apocrine glands secrete a thicker sweat that contains proteins, lipids, and other compounds. When this sweat comes into contact with bacteria on the skin's surface, it can lead to body odor.

Activation:

Apocrine glands are activated during emotional arousal, stress, and sexual activity. The sympathetic nervous system controls their secretion, and hormonal factors can also influence their activity.

Scent Communication:

One of the primary functions of apocrine glands in mammals is scent communication. The odorous sweat they produce contains unique chemical compounds that can convey information about an individual's identity, reproductive status, and social position to other members of the species. This is particularly important in social species where scent marking and recognition play a crucial role in group dynamics and hierarchies.

Sexual Signaling:

In some species, apocrine gland secretions may play a role in sexual attraction and mate selection. The distinct scents emitted by individuals can serve as cues for potential mates to assess each other's fitness and genetic compatibility.

Developmental Timing:

Apocrine glands are not fully functional until puberty in humans. Their activation and development are influenced by hormonal changes during adolescence.

Associated with Hair Follicles:

Apocrine glands are associated with hair follicles, and their ducts open into the hair follicle canal. This is in contrast to eccrine glands, which release sweat directly onto the skin's surface.

Here's a ref from nature stating that human primates may use sweat pheromones to communicate: https://www.nature.com/articles/npre.2008.2561.1.pdf

Primates, especially those living in social groups, rely heavily on communication to maintain social bonds, establish hierarchies, and coordinate group activities. Scent plays a crucial role in primate communication, and sweat scent could have evolved as a means for individuals to convey information about their identity, reproductive status, and emotional state to other members of their group.

mate selection in primates can be influenced by olfactory cues. Scent signals emitted through sweat may allow potential mates to assess each other's genetic compatibility, overall health, and reproductive fitness.

the scent-marking behavior using secretions from sweat glands may serve as a way to establish territorial boundaries and prevent conflicts with neighboring groups.

The odor produced by sweat may play a role in avoiding predators. For example, certain primate species may produce alarm pheromones through their sweat, signaling danger to other group members.

Sweat scent in primates could be a remnant of their ancestral heritage, where scent communication was crucial for survival and reproduction.

incidentally, horses, dogs, deer, they also have strong pheromones.

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  • $\begingroup$ Thank you for the article, +1. So it seems pheromones have an effect, even in the absence of smell. But evolution wouldn't have favoured sweat that would produce this characteristic smell when broken down by skin microbes, I think, unless the smell itself was important. Perhaps it amplifies the effect of pheromones? $\endgroup$
    – j4nd3r53n
    Jul 31, 2023 at 12:28
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To elaborate slightly on the existing answer in which mention is made of 'genetic compatibility', there are many compelling studies which indicate a strong preference for dissimilarity in class 1 HLA alleles between human mating partners, and that this dissimilarity is readily detectable in the body odour of respective individuals. The link below 1 is to one such study; and the following paragraph is its abstract.

The major histocompatibility complex (MHC, called HLA in humans) is an important genetic component of the immune system. Fish, birds and mammals prefer mates with different genetic MHC code compared to their own, which they determine using olfactory cues. This preference increases the chances of high MHC variety in the offspring, leading to enhanced resilience against a variety of pathogens. Humans are also able to discriminate HLA related olfactory stimuli, however, it is debated whether this mechanism is of behavioural relevance. We show on a large sample (N = 508), with high-resolution typing of HLA class I/II, that HLA dissimilarity correlates with partnership, sexuality and enhances the desire to procreate. We conclude that HLA mediates mate behaviour in humans.

Interestingly, there are also allied studies which demonstrate that the use of oral contraceptive hormones disrupts this sensitivity, and one such example is cited by the study linked.

There is clearly a heuristic argument that this dissimilarity favours a broader range of immunological memory in any viable offspring; and this hypothesis is explored in the 'Wikipedia' articles on 'Major Histocompatibility Complex' and its subsidiaries on 'sexual selection'.

To address the comment inquiring upon a causative basis for this phenomenon in the relation between MHC or HLA type and body odour, the latter section of that 'Wikipedia' article makes the following observation and cites the relevant study.

MHC may be related to mate choice in some human populations, a theory that found support by studies by Ober and colleagues in 1997, as well as by Chaix and colleagues in 2008. However, the latter findings have been controversial. If it exists, the phenomenon might be mediated by olfaction, as MHC phenotype appears strongly involved in the strength and pleasantness of perceived odour of compounds from sweat. Fatty acid esters—such as methyl undecanoate, methyl decanoate, methyl nonanoate, methyl octanoate, and methyl hexanoate—show strong connection to MHC.2

I might add here that perhaps the most consequential determination of these sorts of studies is the finding that hormonal contraceptives such as the OC pill effectively inhibit any olfactory sensitivity to this potentially important dissimilarity in HLA type.

Consider for example the implications of less than optimal breeding matches arising under the influence of these hormones and of this manipulation of female fertility on a massive scale, the persistence of which culture in the face of such evidence -- arguably in order merely to sustain the putative bodily autonomy of women in the illusory form of a simple pill -- defies reason. Common sense alone informs us that practices of this kind invite sociological and even biological calamity.

Influence of HLA on human partnership and sexual satisfaction

Infuence of MHC on odour perception of 43 chemicals and body odour

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    $\begingroup$ Does this have any relationship to the perceptible odor of sweat? $\endgroup$
    – Bryan Krause
    Aug 4, 2023 at 2:44
  • $\begingroup$ @Bryan Krause. The particular study cited rather correlates a preference for body odour between individuals in a mating couple with the dissimilarity found in their respective class 1 HLA types, than to attribute a direct effect of MHC type on such body odour. However, there are other studies which demonstrate this effect, and I have amended the answer accordingly. to say that If it exists, the phenomenon might be mediated by olfaction, as MHC phenotype appears strongly involved in the strength and pleasantness of perceived odour of compounds from sweat.: fatty acid esters, $\endgroup$
    – jeremiah
    Aug 4, 2023 at 23:01

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