Evolution and Human Behavior 27 (2006) 259 – 269 Male steroid hormones and female preference Markus J. Rantalaa,*,1, C.J. Peter Erikssonb, Anssi Vainikkaa, Raine Korteta,2 aDepartment of Biological and Environmental Science, University of Jyva¨skyla¨, P.O. Box 35, bDepartment of Mental Health and Alcohol Research, National Public Health Institute, Helsinki, Finland Initial receipt 8 September 2005; final revision received 15 November 2005 It has been suggested that human scent works as a signal in mate selection, but the empirical evidence is scarce. Here, we examined whether women’s olfactory preferences for a man’s scent couldbe correlated with his testosterone, estradiol, or cortisol concentrations, and whether these preferenceschange along with the menstrual cycle. In line with previous studies, women in their most fertile periodgave the highest attractiveness ratings to all men. However, the intensity ratings by women at differentmenstrual phases did not significantly differ statistically. Interestingly, we found that cortisolconcentration in saliva correlated positively with the attractiveness but not with the intensity ratings ofmale T-shirt odor by all women’s groups. However, neither testosterone nor estradiol was significantlyassociated with the ratings of attractiveness or intensity. Thus, our study suggests that there could be anovel mechanism for odor-based selection in humans.
D 2006 Elsevier Inc. All rights reserved.
Keywords: Cortisol; Estradiol; Homo sapiens; Mate choice; Menstrual cycle; Olfaction; Phermones, Sexual 4 Corresponding author. Section of Ecology, Department of Biology, University of Turku, FIN-20014 Turku, E-mail address: [email protected] (M.J. Rantala).
1 Current address: Section of Ecology, Department of Biology, University of Turku, FIN-20014 Turku, 2 Current address: BEET (Integrative Ecology Unit), Department of Biological and Environmental Sciences, University of Helsinki, Helsinki, Finland.
1090-5138/06/$ – see front matter D 2006 Elsevier Inc. All rights reserved.
doi:10.1016/j.evolhumbehav.2005.11.002 M.J. Rantala et al. / Evolution and Human Behavior 27 (2006) 259–269 Chemical cues, such as odors and pheromones, have been found to play an important role in sexual selection of many species. Odors and pheromones have been shown to reflect anindividual’s health ) andimmunocompetence in choosy females 2002; ). In humans, odors can reveal the MHCcompatibility with potential mates & Furi, 1997; Wedekind, Seeback, Bettens, & Paepke, 1995), possibly via MHC class Ibinding ligand proteins ). In addition to MHC composition, odorscan convey information about a potential mate’s social or reproductive status. For example, itis suggested that 16-androstenes, 5a-androst-16-en-3h-ol and 5a-androst-16-en-3-one,modulate social and sexual behavior as well as the levels of steroid hormones in circulation(see 1998; Rothardt & Beier, 2001). Thus, circulatory hormones might contribute to theinformation transmitted by odors.
It has been suggested that whereas men rate visual appearance and odor of a mate as somewhat equally important, women may find olfactory cues of a mate very important intheir sexual responsivity and mate choice ).
Furthermore, in experiments where women do not see the men, it has been found thatwomen in the fertile phase of their cycle prefer the body odor of dominant men Roberts, & Flegr, 2005) and men with attractive faces and low fluctuating asymmetry1999). These studies suggest that odors might reflect preferable male qualities to a womanlooking for a mate. There is also evidence that olfactory sensitivity changes across themenstrual cycle and that ovulating women evaluate androsterone (a substance that gives amusky smell to sweat; see review by ) more favorably nearovulation ). Thus, it has beensuggested that symmetric males with attractive faces have higher androstenol levels (e.g.,). Androstenol, a chemicalprecursor of androsterone, is a major contributor to body odor ).
However, so far it is not known whether females judge males differently in relation to highor low testosterone levels by using their scent.
The immunocompetence handicap hypothesis (ICHH) of sexual selection predicts that the expression of secondary sexual traits should be positively related to the level of testosteroneor other immunosuppressive substance ). It has been found thattestosterone suppresses some parts of the immune system in mammals (reviewed, forexample, ), but a number of studiessuggest that testosterone may actually have a beneficial effect on the immune system(reviewed in ). Thus, it has beensuggested that other potentially immunoregulatory hormones, such as the major stresshormone cortisol, might mask or augment the effects of testosterone Bribiescas, 2005). Likewise, 17h-estradiol has been found to enhance some immune M.J. Rantala et al. / Evolution and Human Behavior 27 (2006) 259–269 functions (e.g., On the other hand, recentstudies suggest that the ICHH mechanism could be mediated by stress hormones such ascorticosterone or cortisol (see et al., 2004), which, in high chronic exposure, are known to have deleterious effects onimmune function (e.g., According tothe ICHH, only genetically immunocompetent (individuals with optimal MHC alleles)individuals can afford to tolerate the costs associated with high concentrations ofimmunosuppressive hormones and thus are able to invest more on sexual advertisementand primary sexual traits (than less immunocompetent individuals(see also Consequently, it has been suggested that male odor associated with circulatory levels ofimmunosuppressive hormones might contain information on the presence of bgood genesQfor females (e.g., Gangestad, 1999).
Thus, in this study, we tested whether attractiveness or intensity of male body odor by female raters is correlated with salivary concentrations of testosterone, estradiol, or cortisol.
Concentrations of testosterone and cortisol in saliva are highly correlated with concentrationsof the respective free hormones in blood (e.g., Hellhammer, 1994; Wang, Plymate, Nieschlag, & Paulsen, 1981). If odors work as honestsexual signals about male quality for choosy women, we expect to find that attractiveness of aman’s odor correlates with that man’s testosterone or cortisol levels in saliva. In addition, wetested whether women’s preference for the characteristics of male scent changes during themenstrual cycle approximated by a day of menstrual cycle-based method or by the use ofcontraceptives. We predicted that females should find male’s scent most attractive at the mostfertile window of their menstrual cycle.
A total of 19 men aged 20 to 35 (mean=27.2F3.33 years) voluntarily participated in the study. Most of the men were students at the University of Jyv7skyl7, Finland. As inprevious studies, body odors were collected by bT-shirt experimentsQ (e.g., Thornhill, 1998; Kuukasja¨rvi et al., 2004; Rikowski & Grammer, 1999; Thornhill &Gangestad, 1999). The white T-shirts were prepared by washing them with nonperfumedsoap powder. Participants had been told to avoid perfumes and deodorants on the day ofthe experiment and to refrain from smoking and eating odorous dishes. Just before theexperiments, all participants received one clean T-shirt and they were required to washtheir body and hair with provided unscented soap and shampoo before wearing the shirt.
These procedures ensured that the personal hygiene of the participants differed minimally.
Each participant wore the T-shirt 5 h while watching movies in an auditorium. This wasconducted during the evening hours to control for within-day variation on hormone levels.
Males were required to wear a light disposable raincoat on top of their T-shirt in orderto prevent acquisition of other distracting scents. We collected three saliva samples from M.J. Rantala et al. / Evolution and Human Behavior 27 (2006) 259–269 each participant: the first one at the beginning of the experiment, the second after 2.5 h,and the third at the end of the odor collection session. After sampling, all saliva sampleswere immediately frozen at À208C until assayed. At the end of this session, eachparticipant folded the T-shirt and placed it in the plastic bag; the T-shirts were then frozenat À208C for later assessments. This procedure was performed in two parts in consecu-tive weeks.
The three saliva samples collected from each male participant (beginning, 2.5 h; end of one time, 5 h) were analysed for testosterone, estradiol, and cortisol concentrations induplicates using the radioimmunoassay technique with commercial kits (DiaSorin, Italy).
The average of all six (3Â2) determinations per hormone was used in later analyses. Intra-assay variation for testosterone was 10.28%; for estradiol, 8.71%; and for cortisol, 6.06%.
Seventy-six nonsmoking Caucasian women [age (meanFS.D.)=23.18F3.85 years] partici-pated in the study by sniffing the T-shirts (in addition to these, two women were excludedfrom the study since they reported to have extraordinarily short, 20 days, or long, 50 days,cycle length). The participants were mainly students from the Faculties of Science and Artsof the University of Jyv7skyl7. Before rating, women were given a brief questionnaire tofill out and asked to report their (1) age, (2) whether the woman currently used acontraceptive pill, (3) the first day of the woman’s last menstrual period, and (4) thetypical length (in days) of their menstrual cycle. The raters were instructed to pick one jarat time, open it and smell it, and rate the odor for intensity (1=not intense at all to10=very intense) and sexual attractiveness (À5=very aversive, 0=neutral, to 5=veryattractive). It has been found that female perception of male body odor changes with cyclephase and the use of pills (e.g., ,1999). Therefore, the included female raters having average phase length 28.41F2.91 days(meanFS.D.; min 21, max 38 days) were divided into four groups: noncontraceptive users:in (1) the follicular phase (days from the first day of the cycle to 20th day from the end ofthe cycle, n=9), in (2) the fertile phase (Days 20–14 from the end of the cycle, n=11), in(3) the luteal phase (the last 14 days of the cycle, n=16); and (4) contraceptive users(n=40). This method to assign women to groups is based on the information that the lutealphase lasts usually 14 days, and fertile period does not exceed 6 days Wilcox, & Weinberg, 1999).
Relationships of male hormones with female scores were analysed using Spearman’s rank correlation. Pearson’s product moment correlation was used to study correlationsbetween hormone concentrations. All variables (testosterone after it was transformed using pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi lnðx þ 1Þ) were normally distributed according to Kolmogorov– Smirnov test of normality. However, nonparametric tests were used because the rankingscale between women is rather ordinal than true ratio scale, and nonparametric statisticsshould yield better reliability in such cases. Friedman’s test with Tukey’s honest significantdifference (HSD) tests for several dependent samples was used to analyse differencesbetween women’s groups if Friedman’s nonparametric ANOVA indicated significantdifferences among groups. Wilcoxon’s signed rank sum test was used to compare theattractiveness and intensity ratings by contraceptive users and noncontraceptive users acrossmenstrual phases.
Spearman’s correlations (N=19) of cortisol, estradiol, and testosterone with the ratings of attractiveness and intensity by female raters The number of women in each group is in parentheses.
M.J. Rantala et al. / Evolution and Human Behavior 27 (2006) 259–269 The mean testosterone, estradiol, and cortisol concentrations measured in saliva were 0.33F0.34 ng mlÀ1, 8.36F3.42 pg mlÀ1, and 6.47F2.30 Ag lÀ1 (meanFS.D.), respectively,which represent normal saliva values for adult men (e.g., Penton-Voak and Chen, Schommer, Hellhammer, McEwen, & Kirchbaum, 2001; ).
Testosterone and estradiol were not correlated significantly with either the intensity or the attractiveness of the T-shirts ). Instead, salivary cortisol concentration stronglycorrelated positively with women’s rating of male scent attractiveness in all women’s groups, ), but not with intensity ratings in any women’s groups (). Testosteroneconcentration did not correlate significantly with estradiol (Pearson’s r=À.308, N=19,p=.199), but the correlation with cortisol was nearly significant (Pearson’s r=.439, N=19,p=.060). Estradiol did not correlate with cortisol (Pearson’s r=À.328, N=19, p=.171).
Average male scent attractiveness ratings of women not using contraceptive pills showed significant variation across the menstrual cycle (Friedman’s test, v 2 and women at the most fertile phase of the cycle gave the highest ratings of attractiveness forall males (Tukey’s HSD tests performed post hoc indicated that all groups differedfrom each other at the .05 level (minimum significant difference between rank sums, 14.43;lowest observed difference, 71.00).
Average male scent intensity ratings of women not using contraceptive pills did not change Average attractiveness ratings did not differ significantly between women using contra- ceptive pills and women not using contraceptive pills (Wilcoxon’s test Z=À1.13, p=.260).
Fig. 1. Relationship between male (N=19) saliva cortisol concentration and average attractiveness rating of aman’s T-shirt by female raters at different phases of their menstrual cycle (number of females per group: follicularphase, 9; fertile phase, 11; luteal phase, 16; and contraceptive users, 40).
M.J. Rantala et al. / Evolution and Human Behavior 27 (2006) 259–269 Fig. 2. Average attractiveness ratings of a man’s T-shirt by female raters at different phases of their menstrualcycle. Number of women in each group is given in parentheses.
However, women using contraceptive pills gave, on average, higher intensity ratings thannormally ovulating women (Wilcoxon’s test Z=À1.97; p=.049; meanFS.D., 3.18F0.98 vs.
2.97F1.18, respectively).
Interestingly, we found a positive correlation between salivary cortisol level and attractiveness ratings of men’s T-shirts by all women’s groups, but no significant evidencethat the primary androgen, testosterone, could be associated with the odor of a man. Thus,in the light of our study it seems that either cortisol or some other hormone connected tothe cortisol metabolism may shape the attractiveness of body odors in humans. Forexample, several androgens and cortisol are metabolised from the same precursor,pregnenolone (e.g., Further studies are needed to confirmour result.
Recently, it has been found that several women’s preferences—males with a masculine face (Penton-Voak et al., 1999), body odor of symmetric men (bodyodor of men with immunocompetent genotypes (and masculinebehavioral displays (change across the menstrual cycle. However, we did not find evidence that the preference forhigh saliva cortisol levels changed significantly during the cycle, but all groups of womenseemed to prefer males with high cortisol levels.
M.J. Rantala et al. / Evolution and Human Behavior 27 (2006) 259–269 Why did women prefer the odors of men who had high cortisol level? Physiologically, it is possible that cortisol has either a direct or indirect link via adrenalin to the activity of theapocrine sweat glands which might have effects on the attractiveness of body odor Hasegawa, 1971; Rothardt & Beier, 2001). Further, metabolism of sweat by commensalmicrobes might make the individual’s odor more pleasant depending on sweating rate (see,for example, ). Evolutionarily, immunosuppressivehormones are said to be essential for the production of good-quality sperm since haploidsperm might be susceptible to autoimmune attacks ). Thus, bypreferring the scent of cortisol, women might be able to gain both good sperm and goodimmune genes for their offspring since only genetically immunocompetent males would beable to maintain high cortisol levels ). Unfortunately, we were notable to measure the amount of perspiration in this study. However, it is important to note thatcortisol concentration did not correlate with the intensity ratings of T-shirts, suggesting thatcortisol affects the scent qualitatively, not quantitatively. Also, it is possible that some of thestudied men might have experienced stress, which could affect their cortisol values. Althoughthe experimental conditions were the same for all participants and were planned to be aspleasant as possible.
Women’s self-reported sexual desire has been reported to peak at the fertile phase of the menstrual cycle (see, for example, when women also exhibit an increasedolfactory sensitivity ). Thus, as indicated in our study, itis likely that the average attractiveness rating of male scent by women not using contraceptivepills changes across the cycle, showing the highest attractiveness at the fertile phase of thecycle. However, the average intensity ratings of women did not change during the cycleamong women not using contraceptive pills. The use of contraceptive pills affected theaverage intensity ratings when analysed as overall difference between contraceptive users andnonusers, suggesting that pills might, in some circumstances, increase the olfactorysensitivity. In this study, we did not collect menstrual cycle diaries or measure salivaryestradiol and progesterone. Thus, these results may suffer from some uncontrolled variation inthe exact timing of ovulation and fertility groups. To minimise these problems, we excludedall women who had a cycle length either shorter than 21 days or longer than 37 days.
Our study suggests that women’s olfactory sensitivity to a man’s scent changes during the menstrual cycle, being highest before and during the fertile window of the cycle. Moreover,our results propose that cortisol or its interplay with other causal factors affecting male scentmay shape the olfactory cues of mate selection in humans. Further studies are needed toexplore the generality of this result in other taxa and the evolutionary significance of thispreference to women’s fitness.
We wish to express our gratitude to the men and women who volunteered for the study. We thank Seppo Kuukasj7rvi, Andrey Stoehr, Annie Leonard, Christina Lynch, Ann Hedrick, andanonymous referees for helpful comments on the draft of this paper. This study was funded by M.J. Rantala et al. / Evolution and Human Behavior 27 (2006) 259–269 the Academy of Finland to MJR (projects: 202624 and 207294) and RK (project: 204837), andby the Biological Interactions Graduate School of the Finnish Ministry of Education to AV.
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