1Contents
-
Introduction
1.1. Smell
1.2. Pheromones
1.3. Pheromone detection
1.4. Pheromone production
1.5. Pheromone effects on animal reproductive behaviours
1.6. Pheromones and human reproductive behaviours
1.7. Pheromones and the battle of the sexes -
Conclusion
References -
Introduction
The importance of pheromones in intra-species communication
has long been known in insects. A classical example
is bombykol, the sexual attractant of the butterfly Bombyx
mori. Bombykol is produced by the female butterflies in
odour glands of the abdomen. Male butterflies detect the
pheromone with sensory cells, located in the antennae and
can find the females by the gradient of her odour. As little as
one molecule of bombykol is enough to stimulate the
receptor cells and facilitate the orientation reaction. Several
studies suggest that pheromones play an important role also
in mammalian social behaviour and thus in humans as well.
The present article reviews the current evidence how
pheromones influence human life and interactions and
discusses the consequences for human sexual attraction and
mate-choice.
1.1. Smell
According toKohl et al. [1] the sense of smell has largely
been underestimated in reproductive behaviours and it has
long been assumed that humans are ‘microsmatic’ (poor
smellers) and rely essentially on visual and verbal cues
when assessing potential mates. Certainly visual stimuli
play a key role in the perceptions of others within a
sociosexual context, especially at a distance, but when
individuals get closer and personal intimacy is increased, it
is likely that smell also plays a key role a variety of
sociosexual behaviours. Recent studies have indeed
suggested that olfaction (conscious and unconscious)
can play a significant role in human reproductive biology.
Zajonc’s [2] ‘affective primacy’ hypothesis states that both
positive and negative affect can be evoked with minimal
stimulus input and only minor cognitive involvement.
Olfactory signals induce emotional responses even if an
olfactory stimulus is not consciously perceived: this is due
to the fact that olfactory receptors not only send projections
to the neocortex for conscious processing (e.g. the nature of
a particular aroma) but also to the limbic system for
emotional processing (e.g. memories and affect associated
with a particular smell).
1.2. Pheromones
The term ‘pheromone’ was introduced by Karlson and
Luscher [3] and it derives from the Greek words ‘pherein’
(to carry) and ‘hormon’ (to excite). Pheromones are referred
to as ‘ecto-hormones’ as they are chemical messengers that
are emitted into the environment from the body where they
can then activate specific physiological or behavioural
responses in other individuals of the same species.
According to McClintock [4] pheromones can be divided
into two classes. Firstly, ‘signal pheromones’ produce shortterm
behavioural changes and seem to act as attractants and
repellents. Secondly, ‘primer pheromones’ produce longerlasting
changes in behaviour via their activation of the
hypothalamic–pituitary–adrenal (HPA) axis [4]. In particular,
it is assumed that primer pheromones trigger the
secretion of GnRH from the hypothalamus, which in turn
triggers the release of gonadotropins (LH, FSH) from the
pituitary gland. These gonadotropins influence gonadal
hormone secretion, e.g. follicle maturation in the ovaries in
females, testosterone and sperm production in males. In
support, in various species the short-term exposures of
females to males have been associated with a corresponding
rise in testosterone [5]. Four specific functions of
pheromones have been determined: opposite-sex attractants,
same-sex repellents, mother–infant bonding attractants and
menstrual cycle modulators [6]. It is the first category that
this review will focus upon though may draw upon evidence
from the other categories wherever relevant.
1.3. Pheromone detection
In most mammals, a specialised region of the olfactory
system called the vomeronasal organ (VNO), also referred to
as ‘Jacobson’s organ’ is responsible for pheromone
detection. The principal evidence that the VNO plays a
role in mammalian pheromone detection comes from lesion
studies where removal of the VNO produces reliable
impairments in reproductive behaviours [7]. The VNO is
located above the hard palate on both sides of the nasal
septum and it is lined with receptor cells whose axons
project to the accessory olfactory bulb, which sends its
projects to the hypothalamic nuclei [8]. Pheromones can
thus potentially influence sexual and reproductive behaviours
and endocrine function via the HPA axis [9]. There
has been some scepticism concerning the ability of humans
to detect and respond to pheromones due to the facts that
VNO appears to vestigial in some primates, and the
accessory olfactory bulb is not discernable in humans [9].
However, it has since been reported that humans do
possess a functional VNO that responds to pheromones
(even in picogram amounts) in a sex-specific manner
[10–12]. Recently, the identification of a pheromone
receptor gene expressed in human olfactory mucosa has
further strengthened the case for a functioning VNO [13].
Further evidence comes from patients with Kallmann’s
syndrome, which occurs due to the underdevelopment of
the olfactory bulb in the embryo and minimal GnRH
secretions from the hypothalamus. Individuals have
underdeveloped gonads, lack secondary sexual characteristics,
are anosmic, and preliminary research indicates that
they show no response to pheromones (personal communication
cited in [1]).
1.4. Pheromone production
The main producers of human pheromones are the
apocrine glands located in the axillae and pubic region. The
high concentration of apocrine glands found in the armpits
led to the term ‘axillary organ’, which is considered an
independent ‘organ’ of human odour production. Apocrine
glands develop in the embryo, but become functional only
with the onset of puberty. At sexual maturation, they
produce steroidal secretions derived from 16-androstenes
(androstenone and androstenol) via testosterone, and as
such, the concentrations of several 16-androstenes is
significantly higher in males [14]. Freshly produced
apocrine secretions are odourless but are transformed into
the odorous androstenone and androstenol by aerobic
coryeform bacteria [15]. In the vagina, aliphatic acids
(referred to as copulins) are secreted and their odour varies
with the menstrual cycle [16]. It is now possible to isolate
and manufacture synthetic human pheromones and such
compounds are often used in research as they are relatively
easy to make, convenient to store, and easy to apply.
1.5. Pheromone effects on animal reproductive
behaviours
Preliminary studies in the 1960s demonstrated that
exposure to boar odour elicited the mating stance in females.
Subsequent experiments showed that application of male
urine or semen to the female’s snout also produced the same
effect. Studies have appeared to demonstrate a number of
confirmed effects of pheromones in animals. Firstly the
‘Lee-Boot Effect’ [17] describes the effects of the social
environment on the female reproductive cycle. The authors
noted that when female mice were housed 4 in a cage their
oestrous cycles became synchronised and extended.
Secondly, the ‘Whitten effect’ [18] confirmed that female
mice housed together displayed an extended oestrous cycle,
but further noted that when a male was introduced the
females ovulated synchronously 3–4 days later. The
substance was found to be androgen-based pheromones
secreted in the male’s urine.
Thirdly, the ‘Bruce effect’ [19] describes the effect of
housing pregnant mice with males that were not their
original mates. Within 48 h of such pairings, significantly
more miscarriages were observed in the females. Subsequent
mating with the new male within 3–6 days then always
followed the failed pregnancy. The inclusion of castrated or
juvenile male strangers had no such effects. This appears to
be a male tactic of blocking the pregnancy by a previous
male and bringing the female quickly into oestrous. Finally
the ‘Vandenburgh effect’ [20] notes that young female rats
exposed to adult males for 20 days after weaning entered
puberty earlier than female pups not exposed to males. Male
pheromones stimulate puberty, probably by releasing LH,
which stimulates follicular growth, presumably so that they
can mate earlier. A related effect was noted in that female
mice housed alone attain puberty earlier than female mice
housed together, females can thus delay puberty in their
conspecifics, probably by suppressing LH and FSH release
from the anterior pituitary gland.
1.6. Pheromones and human reproductive behaviours
Several authors have speculated that pheromones may
influence human sociosexual behaviours (e.g. [21,22]) and
evidence for the effects of putative pheromones on human
sexual behaviours has come from several sources:
- Human correlates of animal effects
McClintock [23] reported that human female college
students demonstrated synchrony in their menstrual
cycles when housed in shared accommodation (Lee–Boot
effect). Preti et al. [24] extended this research by applying
extracts of female sweat to the upper lips of female
volunteers three times per week for 4 months. At the end
of this time the participants showed significantly greater
menstrual synchrony than volunteers in a control group.
Cutler et al. [25] also showed that the application of male
axillary secretions to the upper lips of female volunteers
also had a regulatory effect on the menstrual cycle
(Whitten effect). Ellis and Garber [26] showed that girls
in stepfather-present homes experienced faster puberty
than girls in single-mother homes, the younger the
daughter when the new male arrived on the scene then the
earlier her pubertal maturation (Vandenburgh effect). - Laboratory studies
In an early report, Kirk-Smith et al. [27] asked 12 male
and female undergraduates to rate photographs of people,
animals and buildings using 159-point bipolar scales (e.g.
unattractive–attractive), while wearing surgical masks
either impregnated with androstenol or left undoctored.
Mood ratings were also completed. In the presence of
androstenol, male and female stimuli were also rated as
being ‘warmer’ and ‘more friendly’. Van Toller et al. [28]
showed that skin conductance in volunteers exposed to
androstenone was higher than that of non-exposed
volunteers thereby providing evidence as to the
physiological effects of pheromone exposure. However,
Benton and Wastell [29] had groups of females read
either a neutral or a sexually arousing passage whilst
exposed to either androstenol or a placebo substance.
While sexual arousal was higher in the ‘arousal’
condition, the authors found no evidence that exposure
to androstenol had influenced sexual feelings.
Filsinger et al. [30] asked males and females to rate
vignettes of a fictional target male and female using
semantic differentials, and also to provide a selfassessment
of mood. The test materials had been sealed
into plastic bags, which were either impregnated with
androstenol, androstenone, a synthetic musk control, and
a no-odour control. Females exposed to androstenone
produced lower sexual attractiveness ratings of the target
male, while males exposed to androstenol perceived the
male targets to be more sexually attractive.
The interpretation from such studies is further
complicated by two factors. Firstly, female olfactory
sensitivity is moderated by the menstrual cycle with
smell sensitivity peaking at ovulation [31]. Benton [32]
reported that androstenol application influenced ratings
of subjective mood at ovulation, and Grammer [21] found
that females rated androstenone differently at various
phases of their menstrual cycle. Secondly, the use of oral
contraception may affect smell sensitivity and gonadal
hormone levels thereby possibly disrupting pheromone
detection. Use of the contraceptive pill does indeed
appear to influence female perception of androstenone
[21].
More recently Thorne et al. [33] employed a repeatedmeasures,
double blind, balanced crossover design to
assess the possible influence of menstrual cycle phase
and contraceptive pill use. Sixteen pill and non-pill users
were tested during both menses and mid-cycle in both
pheromone-present and pheromone-absent conditions.
During each session (four in all) the volunteers rated male
vignette characters, and photographs of male faces, on
various aspects of attractiveness. Pheromone exposure
resulted in significantly higher attractiveness ratings of
vignette characters, and faces. Use of the contraceptive
pill or menstrual cycle phase had equivocal effects on
some vignette items but neither had any influence on
female ratings of male facial attractiveness.
Not all laboratory studies have found positive results
however (e.g. [34]), and some authors are sceptical that
higher primate reproductive behaviours are significantly
influenced by pheromones [35]. Thus, while the current
scientific opinion regarding the existence of human
pheromones remains positive, opinion remains divided as
to whether such substances do in fact influence human
sociosexual behaviours. This is probably due to the fact
that while a wealth of laboratory-based studies has been
conducted, very different methodologies mean that
comparisons between studies are difficult. Furthermore,
methodologically solid double blind, placebo-controlled,
crossover studies are few and far between, the Thorne et
al. [33] study being an exception. However, that study
was laboratory based and simply required participants to
rate the attractiveness of hypothetical opposite-sex
characters based on written descriptions and photographs.
The ecological validity of such laboratory-based
studies is therefore questionable. - Real-life studies
While laboratory studies are able to exert more control
over the varying factors involved, of potential greater
relevance are studies assessing the effects of pheromones
in real-life situations. Early studies were, however, not
promising. For example, Morris and Udry [36] prepared
aliphatic acid smears, formulated to mimic concentrations
shown to be effective in enhancing monkey
reproductive behaviour. The solution was smeared on
the chests of 62 married women on eight randomly
assigned nights through three menstrual cycles. Volunteers
did not report any increase in sexual intercourse on
these test nights. However, Cowley and Brooksbank [37]
asked males and females to wear a necklace either
containing an opposite-sex pheromone or a control
substance while they slept. The next day, they found that
women who had worn the male pheromones in their
necklace reported significantly more interactions with
males than the control group.
Two studies which have often been cited as the
strongest evidence yet provided for the influence of
pheromones on human sociosexual behaviour are those
of Cutler et al. [38] and McCoy and Pitino [39]. Both
studies employed double blind, placebo-controlled
methods and focussed upon the effects of synthetic
pheromones on self-reported sociosexual behaviours in
young men [38] and women [39]. In the first study [38] 38
male volunteers recorded the occurrence of six sociosexual
behaviours (petting/affection/kissing; formal
dates; informal dates; sleeping next to a partner; sexual
intercourse; and masturbation) over a 2-week ‘baseline’
period. Over the next 6 weeks the volunteers kept the
same records while daily applying a male pheromone or a
control substance added to their usual aftershave lotion.
The authors reported that a significantly higher proportion
of pheromone users compared to placebo users
showed an increase from baseline in ‘sexual intercourse’
and ‘sleeping next to a romantic partner’. In general 58%
of the pheromone group compared to 19% of the placebo
group showed increases in two or more behaviours
compared to baseline; 41% of the pheromone group
compared to 9.5% of the placebo group showed increases
in three or more behaviours compared to baseline.
In the second study [39] 36 female volunteers recorded
the occurrence of the same six socio-sexual behaviours and
an additional behaviour ‘male approaches’ over a 2-week
‘baseline’ period. Over the next 6 weeks they then either
applied a synthetic female pheromone or a control
substance added to their usual perfume on a daily basis.
While the groups did not differ in their sociosexual
behaviours at baseline, a significantly higher proportion of
the pheromone group showed increases in the following
behaviours: ‘sexual intercourse’, ‘sleeping next to a
partner’, ‘formal dates’ and ‘petting/affection/kissing’.
However, as pheromone exposure can shift the timing of
ovulation, the authors recalculated the data to only include
the first experimental cycle. After these recalculations the
pheromone group only significantly differed from the
placebo group in ‘sexual intercourse’ and ‘formal dating’.
In terms of percentages, three or more sociosexual
behaviours increased over baseline in 74% of pheromone
users but only 23% of placebo users. As there was no
increase in self-reported masturbation the authors argued
that the changes did not reflect changes in sexual
motivation, but that the pheromones had ‘‘positive sexual
attractant effects. . .’’ (p. 374).
The results of these studies appear to provide
impressive evidence for the effects of synthetic pheromones
on sexual attractiveness. However, there are a
number of methodological problems with the studies,
which make the findings less emphatic. Firstly, the
studies did not control for the attractiveness of the
volunteers nor make allowance for this when allocating
the conditions. If for example the pheromone groups had
contained slightly more attractive individuals than the
control groups, then subsequent positive effects attributed
to pheromones may be misleading. Secondly, all the
data were of the self-report kind (prone to error and
subjective bias especially as ‘backfilling’ was allowed in
the second study) and as such no objective record of the
putative effects of pheromone versus placebo were
obtained. Thirdly the groups differed widely in terms of
their dating status with some being married, some in
long-term relationships and others being single. Those in
relationships would have certainly recorded more of
certain sociosexual behaviours than the single volunteers,
it would have been better if the entire subject pool were
single males seeking more dating/sex opportunities.
Fourthly, the baseline period of 2 weeks is difficult to
equate with a testing period of 6 weeks even though
average differences from baseline were analysed. How
can we be sure that the social behaviour of the volunteers
changed not as a result of pheromone exposure but by
other factors during the experimental period, e.g. going
on holiday, celebrating at an office party? While the
actual behaviours were recorded, the context within
which those behaviours occurred was not controlled for.
The evidence from these two studies thus indicates
that certain sociosexual behaviours are increased in
males and females who wear pheromones, compared to
baseline. However, the studies do not convincingly show
that the pheromone and placebo groups were well
matched; that the baseline and experimental conditions
were matched in terms of various temporal and
behavioural factors; that objective changes in sociosexual
behaviours did occur; and that the pheromones served as
a ‘sexual attractants’ rather than say a mood enhancer,
confidence builder, etc. - Genetic signalling
Various ‘good genes’ theories of sexual selection have
emphasised the importance of immunocompetence
[40,41] in that females can obtain good genes for their
offspring by mating with males whose genes are
complementary to their own. A possible mechanism
by which this can be achieved is via body odour. The
major histocompatibility complex (MHC) is a large
chromosomal region containing closely linked polymorphic
genes that play a role in immunological self/
non-self recognition; this genetic information is relayed
by androgen-based pheromones [42]. Numerous studies
in rodents have now established that MHC genotype is
involved in odour production, and such odours are used in
individual discrimination [43]. House mice learn the
MHC identity of their family during development and
avoid mating with individuals carrying familial MHC
genes; they do so through the use of odour cues from
urine (e.g. [44,45]). Is there any evidence that humans
possess these abilities?
Some studies have shown that women seem to prefer
the odours of immunocompatible men. Wedekind et al.
[46] HLA-typed (Human Leukocyte Antigen is the
human MHC) 49 women and 44 men and asked the
women to rate the attractiveness of the odours of t-shirts
worn by three MHC-similar and three MHC-dissimilar
men.Women rated the odour of the MHC-dissimilar men
as ‘more pleasant’, and this odour was significantly more
likely to remind them of their own mate’s odour.
Interestingly, the preferences of women taking an oral
contraceptive were reversed—they preferred the MHCsimilar
odours. This could be due to the fact that oral
contraceptives mimic the effects of pregnancy, and
pregnant females may be attracted to MHC-similar
individuals who are likely to be close kin and potential
reproductive helpers.
In a similar study, Thornhill and Gangstad [47]
measured bilateral physical traits in males and females
and then asked the volunteers to wear the same T-shirt for
two consecutive nights. Opposite-sex participants then
rated the shirts for ‘pleasantness’, ‘sexiness’ and
‘intensity’; donor’s facial attractiveness was also
assessed by different opposite-sex volunteers. Non-pill
users in the fertile phase of their menstrual cycle gave the
T-shirts worn by symmetrical males higher ratings; this
was not seen in females using the contraceptive pill, or in
females at unfertile phases of their cycle. Female
symmetry had no influence on male ratings. The authors
proposed that the so-called ‘scent of symmetry’ is an
honest indicator of male genetic quality.
In a real-life study of actual mate choices, Ober et al.
[48] found evidence for HLA-dependent mate preferences
in a population of Hutterites (a small, genetically
isolated religious sect). They found that couples were less
likely to share MHC haplotypes than chance, and in
couples that had a similar MHC they demonstrated
unusually long inter-birth intervals (unconscious avoidance
of inbreeding?).
Milinski and Wedekind [49] HLA-typed males and
females and then asked them to smell 36 scents
commonly used in perfume/aftershave. They rated each
scent on whether they liked it or not, and whether they
would use it on themselves. The authors reported a
significant correlation between HLA and scent scoring
for themselves but not for others, showing the people
unconsciously select perfumes to enhance their own body
odours that reveal their genetic make-up.
1.7. Pheromones and the battle of the sexes
Differential parental investment theory [50] predicts that
when looking for long-term relationships females should
seek out and choose males who are ready to invest resources
in their offspring. This minimizes female investment, but
maximizes overall investment through added male assistance.
In contrast, males are expected either to attempt
copulation frequently and with as many fertile females as
possible, or to develop a long-term pair bond. This helps to
ensure that either a large number of offspring survive
without significant paternal investment, or that male parental
investment occurs primarily when another male does not
father offspring.
According to this theory, it is adaptive for females and
males to develop and use cognition in mate selection, which
takes into account biological constraints. Thus, mate
selection is a task of information processing, and evolution
would have favoured individuals who were able to quickly
and reliably process information that allowed them to make
appropriate mating decisions. Adaptive cognition could be
expected to lead to optimal decision-making under a wide
spectrum of socio-economic constraints. The existence of
ubiquitous sex specific differences in mate selection criteria
[51] attests that male and female cognition is adapted to the
biological constraints of mate selection.
Neither males nor females can perceive ovulation in
humans consciously. This is surprising in the light of the fact
that it has been shown to be associated with a number of
overt physiological and behavioural changes. One ‘unconscious’
mechanism associated with these menstrual cycle
changes might be that of olfactory perceptions.
Alexander and Noonan [52], and Symons [53] have
argued that hidden oestrous has evolved because females
need to trick males into forming a bond. Males unaware of
female’ s fertility would remain bonded to ensure
impregnation and paternity. A female providing clues to
her ovulation might risk losing male investment, due to
paternal uncertainty and the limited temporal reproductive
interaction. This development would implicate the male fear
of cuckoldry as an evolutionary pressure [50]. The outcome
would be that the female’s ability to secure paternal care is
affected by mechanisms that increase temporal aspects of
the pair bond and enhance male confidence of paternity.
In contrast with this line of argument, Benshoff and
Thornhill [54] and Symons [53] have proposed a second
evolutionary scenario in which hidden oestrous evolved to
increase the chances of successful cuckoldry by females so
they ‘‘can escape the negative consequences of being pawns
in marriage games’’ ([55] p. 350). Once monogamy is
established, a female’s best strategy would be to copulate
outside the pair bond because she can then obtain superior
genes with a certain expectation of paternal investment. In
this case the outcome is genetically superior offspring.
These two hypotheses imply different impacts of
heritable traits. If those genes which induce paternal care
were relevant for offspring success, a male paternitysecuring
function for lost oestrous would be possible. If
there are other relevant traits not related to paternal care but
relevant to offspring survival, then hidden oestrous could
allow females to exploit occasional opportunities to mate
outside the pair bond [56]. In both cases, male knowledge of
ovulation may be selected against because it would hinder
the female’s mating strategies [52,57].
Recently, the second hypothesis has received considerable
support. Bellis and Baker [58] conducted a study of
2708 females and found those 13.8% of 145 ‘unprotected’
extra-pair copulations (EPC) occurred during the fertile
period and were preceded in most cases by intra-pair
copulations (IPC). EPCs were rarely followed by IPCs.
According to his study EPC and thus female infidelity peaks
at ovulation. The authors conclude that these results hint at
female-induced sperm competition, which would be
expected by the second hypothesis of the evolutionary
function of concealed ovulation discussed above. Still it is
unclear what proximate mechanism or mechanisms cue
female EPC at ovulation. The assumption that concealed
ovulation serves to deceive males is common to all these
theories. Supposedly, females deceive males about the fertile
phase of the menstrual cycle to help ensure male parental
investment, which yields an optimal number of offspring.
Additionally, concealed ovulation helps females to monopolize
reproduction and, as a consequence, forces males to
develop reproductive strategies for gaining access to
ovulating females. It is reasonable to expect male counter
strategies would develop against the deceptive attempt by
females to conceal ovulation. Grammer [21] described a
possible male counter strategy: the evolution of the
androstenone–androstenol signalling system. In his study,
290 female subjects rated the odour of androstenone. A
change in assessment throughout the menstrual cycle was
found: at the time of ovulation the women found the scent of
androstenone, the most dominant odour of the male armpit,
to be more pleasant than on the other days of the menstrual
cycle. These results suggest that there is a change in the
emotional evaluation of males triggered by the reaction to
androstenone. The findings support previous results by
Maiworm [59], which were of borderline significance. Male
body odour is usually perceived as unattractive and
unpleasant by females but this evaluation changes at the
point in the menstrual cycle when conception is most likely.
This finding is underlined by the fact that anosmia to
androstenone also varies with cycle. At the conceptual
optimum we find fewer anosmic females. It could be
suggested that changes in anosmia during the cycle could
also be a female strategy, although more data need to be
gathered to prove this hypothesis. Thus the change in female
attitudes towards male body odour could have a strong
impact on mate selection and perhaps self-initiated
copulations by females. If we regard the androstenol–
androstenone-signaling system, the situation for androstenol
seems clear, it makes males more attractive for females.
Female advantage in this case is nil, unless fitter males
produce more androstenol. The situation is more complicated
because producing androstenol inevitably produces
androstenone. The androstenone production has a disadvantage
in its unpleasantness. Hence attractiveness-enhancing
androstenol immediately oxidizes to androstenone,
which repels females. A non-producing male could do quite
well in a population of producers, because females would
not be repelled by his body odour. So the attractivenessenhancing
component of the smell does not seem to be the
main, or at least only, function of the signalling system.
Regarding androstenone, the fact that females assessed its
odour as more pleasant at the time of ovulation could be of
advantage for males, as odorous males will be more
successful when approaching ovulating females, rather than
non-ovulating females. This suggests that males use a kind
of passive ‘ovulation-radar’ for the detection of the actually
hidden ovulation.
Females faced with an evolved male strategy to detect
hidden ovulation would be likely to develop a counter
strategy. One possible strategy could be to manipulate male
cognition and thus adaptive male information processing in
mate selection. Research on many species of non-human
primates (especially on rhesus monkeys) has shown the
ability to perceive ovulation by smell. Although normally
motivated to copulate, when sexually inexperienced rhesus
males were made anosmic they showed no further sexual
motivation despite a powerful visual cue: the female’s
swelling [60]. Furthermore, rhesus males show no interest in
ovariectomized rhesus females, presumably because ovariectomized
rhesus females lose the odour characteristic of
ovulation. Rhesus males regain interest in copulation when
the vaginal secretions from non-ovariectomized females are
applied to ovariectomized females. Studies on menstrual
cycle fluctuations in the fatty-acid composition of women’s
vaginal fluids indicated that a similar type of signalling
system might also exist in humans [16, 61–63]. For example,
human vaginal secretions have a composition that is similar
to the vaginal secretions of female rhesus monkeys. The
application to ovariectomized female rhesus monkeys, either
of human, or rhesus vaginal secretions, induced similar
activation of rhesus male sexual interest [64].
The behaviourally active fraction of the rhesus vaginal
secretions—referred to as ‘Copulins’—consists of volatile,
short-chained fatty acids [65]. These same substances (i.e.,
the short-chained fatty acids: acetic, propanoic, butanoic,
methylpropanoic, methylbutanoic, methylpentanoic acid)
occur in human vaginal secretions, albeit in slightly different
amounts [16]. In addition, the composition of these copulins
varies during the menstrual cycle [62].
Cowley et al. [66] found that rhesus vaginal secretions
change peoples’ assessment of other people, and that the
application of copulins tends to yield a more positive
impression of females. Doty et al. [67] used a questionnaire
to evaluate the intensity and pleasantness of different vaginal
fluids from a complete menstrual cycle. They found that
odour at ovulation was both the most intense odour and the
least unpleasant.
In a study by Juette (unpublished data) synthesized
female vaginal secretions (‘Copulins’) were tested for their
ability to act as signals for males. Menstrual, ovulatory and
pre-menstrual fatty acid compositions of Copulins and an
odourless water control were presented to 60 non-smoking
male subjects for 25 min in a double-blind experiment. To
control for changes in sex hormones that were induced by
copulins, saliva-samples were taken before and after presentation.
While inhaling either a composition of copulins or a
control, males rated pictures of females for attractiveness. It
was shown that ovulatory fatty acid compositions stimulated
male androgen secretion and changed the discriminatory
cognitive capacities of males with regard to female attractiveness
in that males became less discriminating. As we can
learn from the above examples, human pheromones seem to
work as beautifully balanced ‘strategic weapons’ in the
‘battle of the sexes’ and the ‘war of signals’ resulting from
asymmetric investment theory.
2. Conclusion
As we can learn from the reviewed studies on
pheromones, the model of humans being only optical
animals has to be revised. Human sociosexual interactions
are influenced by pheromones, even if they cannot be
detected consciously. Pheromones have the potential to
influence human behaviour and physiology and so there has
to be asked the question, in which way the modern striving
for cleanliness and odourlessness affects our everyday social
lives and human reproductive success in the future. What we
know at the moment, as many studies in the last few years
have pointed out, is that the human sense of smell has by far
been underestimated in the past and that humans, like other
animals, use olfactory signals for the transmission of
biologically relevant information.
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