I have noticed that my own body odor varies depending on the location on the body. The specific odor of a specific part, such as underarms, generally doesn’t vary much over time if BO is present. For example, it is unlikely I could mistake my own underarm odor for my own foot odor, but underarm odor on March 1st would be almost identical in smell to underarm odor of Sepember 9th. What is likely causing the difference in smell?
I can think of the following reasonable hypotheses:
Different bacteria. Once upon a time, after I hadn’t taken a shower in a little while and had some odor, I rubbed one part of my body and rubbed it into another in an attempt to see if the second part would start to smell like the first due to bacterial transfer, but it didn’t happen.
Different constitutents of sweat, e.g. if the chemical constituents of underarm sweat is significantly different (e.g. in terms of what chemicals are there, or their relative concentrations) from foot sweat.
Different clothing compositions (for example, chemicals commonly found in shoes but not in shirts could contribute to foot odor)
Mostly 2. We have different kinds of sweat glands on different parts of the body. The smellier apocrine glands are mostly in the armpits and groin, while the merocrine (or eccrine glands) are more widely distributed but most concentrated on the palms and the soles of the feet. There are also sebaceous glands next to the hairs.
The secretions of the apocrine glands become particularly smelly when acted upon by bacteria, and there may be different kinds of bacteria on different parts of the body. But the basic cause for the difference in smell is differences in the kind of secretion.
Your underarms and groin do secrete a different type of sweat (that is, sweat with different things in it). The hair there is at least partly to help localize the scent and disperse it into the air (rather than letting it drip or rub all over the body). And since the scent is associated with sexual maturity, you don’t get the smelly sweat or the hair until puberty. Bacteria are a contributor to the final smell.
Feet are more in the realm of bacteria and environmental conditions. Your feet wouldn’t smell if you went barefoot. Shoes and socks trap body heat and sweat to create a warm, moist environment that is perfect for bacterial growth. You don’t wear any other clothing that is so tight and waterproof or you’d probably get it elsewhere. Actually, I have encountered a few baseball gloves that smelled a lot like feet.
Another reason is that different parts of your body are covered differently. Feet & crotch, for example, almost never see the light of day. Less air circulation -> more moisture -> bacterial growth, which then act upon things as the wise **Colibri **already specified.
It’s no coincidence that the stinkier parts of your body are also the hairiest, waxing and razors aside. There was a time in our species’ existence when scent cues were still important, and hair aids in producing our unique specific odors.