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Body odor is present in animals including humans, and its intensity can be influenced by many factors (behavioral patterns, survival strategies). Body odor has a strong genetic basis both in animals and humans, but it can be also strongly influenced by various diseases and psychological conditions.
In humans, the formation of body odors is mainly caused by skin glands excretions and bacterial activity. Between the different types of skin glands, the human body odor is primarily the result of the apocrine sweat glands, which secrete the majority of chemical compounds needed for the skin flora to metabolize it into odorant substances. This happens mostly in the axillary (armpit) region, although the gland can also be found in the areola, anogenital region, and around the navel. In humans the armpit regions seem more important than the genital region for body odor which may be related to human bipedalism. The genital and armpit regions also contain springy hairs which help diffuse body odors.
Body odor is influenced by the actions of the skin flora, including members of Corynebacterium, which manufacture enzymes called lipases that break down the lipids in sweat to create smaller molecules like butyric acid. These smaller molecules smell, and give body odor its characteristic aroma. Propionic acid (propanoic acid) is present in many sweat samples. This acid is a breakdown product of some amino acids by propionibacteria, which thrive in the ducts of adolescent and adult sebaceous glands. Because propionic acid is chemically similar to acetic acid with similar characteristics including odor, body odors may be identified as having a vinegar-like smell by certain people. Isovaleric acid (3-methyl butanoic acid) is the other source of body odor as a result of actions of the bacteria Staphylococcus epidermidis, which is also present in several strong cheese types.
In many animals body odor plays an important survival function. Strong body odor can be a warning signal for predators to stay away, or it can be also a signal that the prey animal is unpalatable. For example, some animals species, who feign death to survive (like possums), in this state produce a strong body odor to deceive a predator that the prey animal has been dead for a long time and is already in the advanced stage of decomposing. Some animals with strong body odor are rarely attacked by most of the predators, although they can be still killed and eaten by the birds of prey, who have very poor sense of smell.
Body odor is an important feature of animal morphology. It plays a different role in different animal species. For example, in some predator species, who use stalking as a means of hunt (for example big and small cats), the absence of body odor is highly important, and they spend plenty of time and energy to keep their body free of odor. For other predators, who use long running after the prey as a means of hunting (dogs, wolves), the absence of body odor is not so important, on the contrary, can play a role of warning signal. In most of the animals body odor intensifies in the moment of stress and danger.
Sebaceous and apocrine glands become active at puberty. This, as well as many apocrine glands being close to the sex organs, points to a role related to mating. Compared to other primates, humans have extensive axillary hair and have many odor producing sources, in particular many apocrine glands. In women, the sense of olfaction is strongest around the time of ovulation, significantly stronger than during other phases of the menstrual cycle and also stronger than the sense in males.
Humans can detect individuals that are blood-related kin (mothers and children but not husbands and wives) from olfaction. Mothers can identify by body odor their biological children but not their stepchildren. Preadolescent children can olfactorily detect their full siblings but not half-siblings or step siblings and this might explain incest avoidance and the Westermarck effect. Babies can recognize their mothers by smell and mothers and other relatives can identify a baby by smell.
Humans have few olfactory receptor cells compared to dogs and few functional olfactory receptor genes compared to rats. This is in part due a reduction of the size of the snout in order to achieve depth perception as well as other changes related to bipedalism. However, it has been argued that humans may have larger brain areas associated with olfactory perception compared to other species.
Joseph Jordania has suggested that human body odor played a role of aposematic olfactory (warning display signal) in early hominids, particularly at night, when hominids were sleeping in the open savannah. As many predators approach their prey against the wind, the strong body odor would inform predators about the identity and the size of the hominid group. He explained the presence of patches of hair in human armpits and genitals by the evolutionary need to produce stronger body odor. According to his suggestion, body odor lost its survival value after humans developed more advanced means of safety (controlled fire, shelters, effective weapons), and therefore started disappearing, gradually becoming an unwanted legacy of our evolutionary past.
Studies have suggested that people might be using odor cues associated with the immune system to select mates. Using a brain imaging technique, Swedish researchers have shown that homosexual and straight males' brains respond differently to two odors that may be involved in sexual arousal, and that homosexual men respond in the same way as straight women, though it could not be determined whether this was cause or effect. The study was expanded to include lesbian women; the results were consistent with previous findings meaning that lesbian women were not as responsive to male identified odors, while their response to female cues was similar to straight males. According to the researchers, this research suggests a possible role for human pheromones in the biological basis of sexual orientation.
Several different odorous substances released by the body cause behavioral effects as described in the pheromone article.
Body odor is largely influenced by major histocompatibility complex (MHC) molecules. These are genetically determined and play an important role in immunity of the organism. The vomeronasal organ contains cells sensitive to MHC molecules in a genotype-specific way.
Experiments on animals and volunteers have shown that potential sexual partners tend to be perceived more attractive if their MHC composition is substantially different. Married couples are more different regarding MHC genes than would be expected by chance. This behavior pattern promotes variability of the immune system of individuals in the population, thus making the population more robust against new diseases. Another reason may be to prevent inbreeding.
East Asians (Koreans, Chinese, and Japanese) have fewer apocrine sweat glands compared to people of other descent, and the lack of these glands make East Asians less prone to body odor. The reduction in body odor and sweating may be due to adaptation to colder climates by their ancient Northeast Asian ancestors. Axillary odor is known to be determined by the ABCC11 gene that also codes the type of earwax one has. Most of the population secrete "wet" earwax, however, East Asians are genetically predisposed for the allele that codes the "dry" type earwax, associated with a reduction in axillary odor. The non-functional ABCC11 allele—predominant amongst East Asians by 80–95%—affects apocrine sweat glands by reducing production and secretion of odorant compounds commonly found in the perspiration of other ancestral groups. This is due to a 538G>A SNP in the ABCC11 gene, which causes a loss in body odor in people who are specifically homozygous for it.
Body odor may be reduced or prevented or even aggravated by using deodorants, antiperspirants, disinfectants, underarm liners, triclosan, special soaps or foams with antiseptic plant extracts such as ribwort and liquorice, chlorophyllin ointments and sprays topically, and chlorophyllin supplements internally. Although body odor is commonly associated with hygiene practices, its presentation can be affected by changes in diet as well as the other factors discussed above.
Medical conditions 
Osmidrosis or bromhidrosis is defined by a foul odor due to a water-rich environment that supports bacteria, which is caused by an abnormal increase in perspiration (hyperhidrosis). This can be particularly strong when it happens in the axillary region (underarms). In this case, the condition may be referred to a axillary osmidrosis.
See also 
- Butyric acid
- Drug resistance
- Foot odor
- Halitosis (bad breath)
- Olfactory fatigue
- Sweat gland
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- Causes of body odor
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