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PHEROMONES

HUMAN PHEROMONES
Androstenol and androstenone are components of sweat that are probably derived from androgen hormones. There is five times more androstenone in male axillary sweat than in female sweat. Perhaps as a result, there is a sexual dimorphism in the microbes which inhabit the axilla (Cornebacteria dominate in males as opposed to Micrococcaceae in females). Concentrations of androstenol in male urine is three times higher than in females. Females typically perceive androstenol as attractive and it may increase arousal while they tend to be repelled by the odor of androstenone. Given this, it is not known why males produce high quantities of androstenone (Hohl, 2001; Thornhill, 1999). Women rate olfactory cues as more important than visual cues in mate selection and arousal. Women's greatest sensitivity to androstenol is during ovulation (Thornhill, 1999).


The odor of vaginal copulins (short chain fatty acids) in monkeys is a stimulant for sex and human females synthesize the same copulins. The perception of components of female axillary sweat allow females to allow their menstrual cycles to come in sync with each other. Males can react differently to axillary sweat produced at different stages of the menstrual cycle (Hohl, 2001; Gangestad, 1998).Males in many mammal species (including humans) increase testosterone production in response to the presence of females (Hohl, 2001). Exposure to male pheromones affect females in a number of ways. This exposure can alter LH levels, mood, increase number of social contacts with men, and can cause irregular menstrual cycles to become more regular (Hohl, 2001).

The substances thought to represent human pheromones include 4,16-androstadien-3-one (AND) which is derived from testosterone and is present in male sweat and estra-1,3,5(10),16-tetraen-3-ol (EST) which is similar to estrogen and present in female urine. Exposure to EST and AND increases activity in sexually dimorphic regions of the human hypothalamus in a gender specific manner. When heterosexual men smell AND or women smell EST, they utilize brain regions which process common odors. Men who smell EST increase activity in the paraventricular and dorsomedial nuclei of the hypothalamus, unlike the response in women. Women and homosexual men who smell AND increase activity in the preoptic and ventromedial nuclei of the hypothalamus, unlike the response in heterosexual men. These regions of the hypothalamus are involved in the control of sexual behavior in animals (Savic, 2005).

MHC and ODOR
MHC molecules are an essential component of the immune system's ability to distinguish "self" from "nonself". Apparently, these variable "self" molecules can also contribute to individual odor, although the mechanism through which this occurs is not known. There is some evidence that MHC differences are a factor in determining the reaction to the sweat of other people. Surveys have shown that odor is a more important determinant for human females than males (Herz, 2002).
Mice preferentially mate with individuals dissimilar MHC genes. Mice can be trained to distinguish between urine samples whose donors differ only in MHC loci and untrained mice have been shown to discriminate between the odors of mice which differ only in the alleles of a classical MHC gene (dm2). MHC genes are thus involved in odor (Penn, 1998). Thus, natural selection would have acted on MHC molecules in both their capacities to defend against pathogens and for their odor (Carroll, 2002).
Women prefer the odor of dissimilar MHC men and the odors of these men are more likely to remind them of partners they have had in the past. These preferences were not observed in women taking contraceptives (Wedekind, 1995). MHC-disassortative mating preferences might increase the number of individuals which are heterozygous for genes in the immune system and thus enhance the ability to combat diverse microbes (Penn, 1999). Some studies have indicated that humans (particularly women) prefer the body odor of those whose MHC complex differs from theirs (Penn, 1999; Hohl, 2001; Gangestad, 1998).

MHC and INBREEDING
MHC molecules are expressed in vomeronasal neurons and there is interest in determining whether this allows the VNO to measure degrees of relatedness of potential mates as a potential mechanism to suppress inbreeding (Hegde, 2003). Male mice preferentially mate with females with different MHC proteins and females preferentially nest with females with similar MHC proteins (Beauchamp, 2000). In mice, fetal MHC proteins produce odors that can be detected in maternal urine. Male mice can distinguish between, and react to, the odor in maternal urine, showing a preference for females whose fetuses carried MHC proteins differing from those of the male (Beauchamp, 2000). Females prefer urine of uninfected males to that of infected males (Ehman, 2001). Untrained mice can distinguish between individuals which differ in some but not all MHC genes (Carroll, 2002). Humans can also distinguish between the odors of mouse strains which differ only in their MHC (Wedekind, 1995).
In rodents, females spontaneously abort same MHC embryos more frequently and in humans, the in vitro fertilization of same-MHC embryos are more likely to fail (Penn, 1999). Studies among an inbred group of humans indicated that couples which share an MHC haplotype have longer than normal interbirth intervals and that there were fewer marriages between same-haplotype individuals than would be expected by chance (Penn, 1999).
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OTHER SIGNALS
While all males would have an advantage in producing an attractive scent, females have an advantage in detecting "honest signaling" from males to evaluate them in mate selection (Thornhill, 1999).
The deviation from perfect symmetry, termed fluctuating symmetry, has been correlated with a man's number of sexual partners, the number of extrapair partners, and even the number of their partner's orgasms (Gangestad, 1998). Fluctuating asymmetry (FA) can be influenced by mutations, inbreeding, and homozygosity. Low FA increased probability of lower physical and mental health. The bacteria which colonize skin may be related to health. (Thornhill, 1999).
Female desire changes over the course of the menstrual cycle, with the greatest number of extra-pair couplings occurring around the midpoint of the menstrual cycle. Women prefer the scent of men with a greater degree of symmetry than less symmetrical men in the midpoint of the menstrual cycle. These olfactory preferences are not observed at other points of the menstrual cycle or in women taking contraceptives (Gangestad, 1998). Ovulating women prefer the odor of more symmetrical men although this is not observed in women on contraceptives or men's preferences for women (Thornhill, 1999).
Many perfume components in use today were used by humans living five thousand years ago in areas such as China, India, and Egypt. Analyses have indicated that perfume preferences vary with an individual's MHC alleles (Milinski, 2001).