Sweat barely smells; armpit bacteria turn it into body odour
Fresh sweat is not what you smell. Apocrine glands, which switch on at puberty, release compounds that skin bacteria digest into pungent molecules. Different microbes dominate in men and women, which is why male armpits tend to smell cheesy and female ones more like fruit or onion.
Humans have three kinds of skin gland. Eccrine sweat glands work from birth, while apocrine and sebaceous glands activate at puberty. Odour comes mostly from the apocrine type, concentrated in the armpits and also found around the nipples, navel and groin. Bacteria such as Corynebacterium make lipases that break sweat fats into smaller molecules like butyric acid, and Staphylococcus hominis produces sulphur-containing thioalcohols. Men's armpits carry more Corynebacterium jeikeium, women's more Staphylococcus haemolyticus. Key ingredients of the resulting smell include 3-methyl-3-sulfanylhexan-1-ol and the steroid androstenone. Compared with other primates, humans have unusually abundant underarm hair and apocrine glands.
Other animals use scent strategically. A porcupine's stink warns predators off, and an opossum playing dead gives off a smell suggesting it has been rotting for some time. Cats, which stalk their prey, spend a great deal of effort keeping themselves odour-free, while dogs and wolves, which chase quarry over long distances, have less need to. In most species odour grows stronger under stress.
People read one another's smell more than they realise. Mothers can pick out their biological children by scent but not stepchildren, and children before adolescence can identify full siblings but not half- or step-siblings, a finding that may help explain why people raised together rarely become attracted to each other. Newborns locate the breast partly by their mother's odour and learn to tell her breast from another woman's. Scent released during frightening situations can raise activity in the amygdala of people who smell it.
Humans have fewer olfactory receptor cells than dogs and fewer working receptor genes than rats, a loss tied to a shrinking snout and upright walking, though some researchers argue our brain areas for smell are relatively large. Diet, gut microbes, medication and genes all shape how each person smells.
Source: Body odor
