Human communication is strongly informed by body odour (BO) [1
]. Odours provide information on the emotional state of the sender [3
], mediate attractiveness perception [4
], signal familiarity [5
], and are a source of comfort [6
]. Those features are of particular relevance in intimate relationships, such as between parents and children or between romantic mates. However, the investigation of BO poses methodological challenges. A comprehensive study preparation requires careful consideration of standardisation protocols for the participants, the sampling medium and duration, and how the samples should be presented experimentally. The selection of a suitable body site with regard to sweat physiology and sampling feasibility according to the developmental stages of the participants needs to be considered in particular. Experimental investigations on that regard are however still pending.
In the following, a short overview of previous methodological assessment on body odour sampling is presented (see Table 1
). Standard odour sampling protocols aim at controlling various external and internal sources of odour contamination to keep the odour as clean as possible. First, the influence of perfume and fragranced hygiene products which are widely used in Western 21st century civilisation must be minimized. Odour donors are therefore asked to refrain from using deodorants, antiperspirants, or other perfumed hygiene products and are only allowed to use odourless products, such as specific soap (e.g., [7
]). Second, many studies set a range of eating rules, as some foods change perceived BO quality. For example, Havlicek and Lenochova [10
] found that the consumption of red meat negatively affects BO quality. Conversely, participants of Zuniga and colleagues [11
] report that meat, in addition to tofu, eggs, and fat, would induce a pleasant BO. Although research has been inconsistent on that topic, a number of specific instructions are frequently applied, which include avoidance of garlic, blue cheese, and alcohol [7
]. Third, odour donors are instructed not to smoke during the sampling period. Fourth, odour sampling has to be conducted with garments being pre-washed with odourless detergent, in order to prevent contamination with perfumed detergents or fabric softener. In addition, clothes being worn together with the sampling garment have to be prepared in the same way [8
The next question to address is which medium to use for sampling. Yao and colleagues [15
] investigated the absorbance capability of volatile compounds of BO in different fabrics. They found that polyester absorbed more odour compounds than cotton and wool. Similarly, Callewaert et al. [16
] reported that BOs that had been sampled with polyester smelled more intense and less pleasant, which could be linked with dissimilar bacterial growth when compared to cotton. Nonetheless, according to this study, the microbial profile of polyester clothing rather leads to malodour [16
]. In the previous literature, the most common sampling mediums are thus either cotton pads [13
] or cotton t-shirts [8
]. While t-shirts are simply worn by participants, cotton pads are usually fixed directly to their skin with surgical tape. This impairs feasibility when sampling infants’ BOs, as the tape can lead to itching or removal of the tape, as well as to skin irritation during removal.
Further, the duration of BO sampling should be carefully considered, as it affects perceived BO quality. According to Havlicek et al. [20
], shorter worn cotton pads (12 h) are perceived as more pleasant, more attractive, and less intense than longer worn cotton pads (24 h). The sampling durations to be found in literature vary from 30 min [21
] up to 7 consecutive nights [22
]. An additional influencing factor for assessing BOs in women is the menstrual cycle. The female BO leads to a more pleasant odour imprint in the fertility phase of the cycle, irrespective of whether men [23
] or women [24
] rate the BO. In line with these findings, Lobmaier et al. [19
] have shown that a higher level of oestradiol and a lower level of progesterone relate to the attractiveness ratings of female BOs.
The experimental presentation of the odour samples involves further methodological difficulties. Using freshly sampled BOs (e.g., [25
]) involves organizational and logistical challenges to coordinate sampling and assessments accordingly. Therefore, it is more feasible to store the BO samples until experimental presentation. As Lenochova et al. [7
] investigated, BOs are conservable in a frozen state (at least −32 °C) for up to 6 months without substantial reduction of BO quality. After this time period, perceived intensity of the thawed BO samples decreases, while other odour qualities are not affected.
Finally, the addressed body sites must be chosen.
In view of human sweat physiology, the stage of development of the odour donor and, in particular, secretion of glands must be considered. In adults, the axilla is found as body site of choice when assessing odour-related perception with experimental paradigms [7
Overview of methodological approaches to body odour sampling.
Overview of methodological approaches to body odour sampling.
|Study||N||Design||Assessed Factor||Medium and Body Site||Notes||Results|
|Fialová et al. 2019 ||n = 12 female odour donors, n = 56 male odour raters||within subject||Caloric intake, BO samples 3x from each donor (during habitual diet, during 48 h of caloric restriction, 72 h after caloric restauration||Cotton pads, axilla||Examination of general health status (blood and urine tests), dietary restrictions||Hedonic ratings: odour samples from restoration phase were significantly more pleasant, attractive, less intense compared with previous phases|
|Lenochova et al. 2009 ||n = 9 male odour donors, n = 14 to 21 female odour raters||within subject||Exp. 1: frozen storage (immediately after sampling, after 2 weeks, 4 weeks, 16 weeks) and repeated thawing (1, 2, and 3 times) of BO samples; Exp. 2: frozen storage for 6 months ||Cotton pads, axilla||Dietary restrictions||Exp. 1: frozen storage: effect on intensity (higher intensity after 4 weeks of storage compared with fresh and 16 weeks of storage); repeated thawing: lower intensity in repeatedly thawed samples; for both storage and thawing no effect on hedonic qualities|
| ||n = 25 male BO donors, n = 27 female odour raters||within subject||Exp. 2: frozen storage of BO samples for 6 months without thawing||Cotton pads, axilla||Identical restrictions as in Exp. 1||Exp. 2: no effect on intensity or hedonic qualities (pleasantness, attractiveness, masculinity)|
|Kohoutová et al. 2012 ||n = 11 male odour donors, n = 30 female odour raters||within subject and control cond.||Shaving of axillary hair (either regularly shaved or unshaved)||Cotton pads, axilla||Dietary restrictions||Effect of shaving: shaved armpits were perceived more pleasant, more attractive, less intense; perceived intensity was increasing with growing back axillary hair|
|Havlíček and Lenochova 2006 ||n = 17 male odour donors, n = 30 female odour raters||within subject||Meat consumption||Cotton pads, axilla|| ||Samples in the non-meat phase were rated more pleasant, more attractive, and less intense|
|Havlíček et al. 2011 ||n = 7 male BO donors, |
n = 25 female BO raters
|within subject||Sampling length: 12 h versus 24 h of sampling||Cotton pads, axilla||Dietary restrictions||Shorter worn cotton pads (12 h) were perceived more pleasant, more attractive, and less masculine and intense|
|Fialová et al. 2016 ||n = 42 male BO donors, n = 82 female BO raters||within subject||Garlic consumption||Cotton pads, axilla|| ||The odours of subjects in the garlic condition were rated as more pleasant, attractive, and less intense|
|Roberts et al. 2013 ||n = 92 male stimulus providers (olfactory, visual), n = 63 female raters||within subject||Repeatability of odour perception||Cotton t-shirts, whole garment||Dietary restrictions||The perceptions of all stimuli were repeatable|
|Callewaert et al. 2014 ||n = 26 odour donors (13 male), n = 7 odour assessors (selected and screened)||between subject||Microbial odour profile after 28 h and qualitative ratings for different fabrics (cotton versus polyester)||Subject of investigation, complete T-shirt ||T-shirts were stored at room temperature, so that bacteria could grow||Cotton t-shirts smelled more pleasant and less intense than the polyester t-shirt; different growth of bacteria in the fabrics: micrococci occurred almost exclusively in polyester, staphylococci occurred in both fabrics, and corynebacterial in neither of the two|
|Zuniga et al. 2017  ||n = 43 male odour donors, n = 9 female raters||between subject||Diet quality (fruit and vegetable intake, and skin yellowness)||Cotton t-shirt||Diet quality was measured (i) directly with self-report questionnaire on dietary habits and (ii) indirectly using skin yellowness||Skin yellowness correlated significantly with positive affective odour ratings, no relation of yellowness and BO intensity.|
Self-reports: consumption of eggs, tofu, oils, and fats was associated with positive ratings; seafood and carbohydrate consumption was associated with more negative and more intense ratings
|Gildersleeve et al. 2012 ||n = 41 female odour donors, n = 112 male odour raters||within subject||Menstrual cycle (high fertility versus low fertility state)||Cotton pads||Fertility state was validated with ovulation tests||Men were able to reliably discriminate between high- and low-fertility samples; high-fertility odours were preferred; discrimination ability was associated with even more pleasant evaluation of the high-fertility samples|
] describes the axilla as a unique source of BO due to the range of different secretions and the highest density of glands. Additionally, the axillary odour seems to be particularly relevant for communication via pheromones and is a prominent odour source in humans [29
]. In addition to the axilla, research hints at another relevant body site in adult women: Varendi and Porter [30
] demonstrated that newborns strongly orient and move towards the maternal breast odour compared with a neutral odour. Similarly, Doucet et al. [31
] found that maternal breast odour links to infant behaviour as infants were more likely to open their eyes and cried less when they could smell the breast odour than when they were exposed to a covered and thus odourless breast. Apart from the infants’ odour perception, this suggests that the maternal breast provides a characteristic body odour, which emerges from the Montgomery glands. Those glands are distributed on the areolae and enlarge during pregnancy and lactation and are believed to play a key-role in initiating the milk transfer processes [32
When it comes to investigation of body odours in children, it is important to consider the different secretions of glands emerging during development. While eccrine and apocrine sweat glands already exist at birth, only eccrine glands are already active at birth. Apocrine and eccrine sweat glands become active during puberty and are distributed over genital, perineal, and axillary areas [33
]. These glands secrete into the canals of hair and thus become active, parallel to emergence of pubic hair during puberty [34
]. Barzantny and colleagues [35
] describe that the development of strong BO is linked with secretion of apocrine glands rather than of eccrine glands. These findings lead to the conclusion that the sampling of BOs from axillary, perineal, and genital areas of the body from (post-) pubertal children who already passed this stage provides olfactory impressions with comparable intensity to adult BO.
To date, however, it how this applies to prepubertal odours has never been investigated. Examining the BO of prepubertal children, a number of studies used axillary sampling, as this has been state of the art across BO research and also proven to transport olfactory information [8
]. Moreover, other studies investigating parental perception of children’s odours lack information on sampling methodology and do not specify the exposed body sites at which the odour was sampled [38
Typically, two ways of BO assessment in order to capture human BO perception have been used: rating of BO quality and BO recognition. First, BO quality is measured by pleasantness and intensity perception [38
], which differs according to, e.g., familiarity of an odour [5
]. It yet has been unclear how BO quality differs with respect to different body sites. Second, self-recognition of one’s own BO has been shown in adults [21
] but has not been compared for different body sites. The recognition of one’s own child by chemosignals is considered olfactory kin recognition and has been studied in view of its behavioural relevance. Among other purposes, this serves as insurance of the survival of the offspring because in this way rearing resources can be invested in a targeted manner [36
] and the formation of an affectionate bond is facilitated [42
]. In previous literature, it was shown that olfactory kin recognition is a reliable phenomenon in healthy individuals: Mothers are able to identify their children above chance level [8
]. Therefore, a reliable kin recognition performance may indicate the quality of BO sampling. The authors of Kaitz [25
] found a link between the capability of kin recognition and the length of exposure to their infant’s BO. A particularly exposed part of an infant’s body is the head, as shown in the questionnaire study by Okamoto et al. [44
]: Mothers reported that they actively seek their infants’ BO in everyday rearing, with the head providing the most common source of affective experiences. These results suggest evaluating the perceived BO quality of infant’s heads, its recognizability, but also its methodological feasibility.
The current study therefore investigates whether different BO sites affect perceived BO quality and BO recognition for both maternal and infantile BOs. The axilla, the breast, and the head were compared as assumed areas of relevant odour sources. The participating subjects rated their own and their child’s odour, as well as unfamiliar female and infant odour samples. We hypothesized that the infantile odours are perceived as being equally intense at all body sites and most pleasant on the head. Regarding the mothers, we expected the highest intensity ratings for the axilla and lowest pleasantness for the axilla. Further, we aimed to replicate that the familiar odour (one’s own and one’s own infant’s odour) is perceived as being less intense and more pleasant than unfamiliar odours [5
]. In an exploratory manner, we tested the relationship between pleasantness and intensity ratings for each body site. We assumed that (i) the recognition performance of one’s own infant’s odour is highest for the head odour and that recognition performance of one’s own odour is highest at the axilla. Furthermore, we aimed to replicate that (ii) recognition exceeds chance level at all body sites.
Regarding the infants’ BOs, the head odour was, contrary to our expectations, perceived as least pleasant by trend. In line with our hypothesis, body site did not influence intensity ratings, but familiarity affected the perceived intensity, as one’s own infant smelled less intense than unfamiliar infants. In addition, mothers were able to recognise the BO of their own infants at every site and did so equally well. Body site did not impact pleasantness perception of the maternal Bos, yet the breast was perceived as least intense. As with the infants, we showed that mothers evaluated their own BO as less intense than BOs of other women. According to our expectations, the mothers could recognise their own odour above chance, and performance did not differ between body sites.
This study aimed at exploring BO perception in relation to maternal and infantile odours sampled at different sites of the body. According to a questionnaire study, the head of young infants is a prominent source of pleasant, affective feelings [44
]. In line with these findings, 23 (84.14%) mothers stated in the present study that they liked their infant’s odour the most at the head. However, this statement differed from the experimental results. In view of other experimental studies on children’s BOs, it is not surprising that the infantile axillary odour was perceived as very pleasant [8
]. Hence, the findings presented here might point to differences between questionnaire and experimental studies. While questionnaire studies are, e.g., prone to memory bias [49
], the mothers might underestimate familiarity of the axilla or breast odour of their child, and it has been unknown how those odour sources differ in their perceived quality. While the memory of the child on the arm and therefore the head as a prominent BO source is probably more accessible, the BO of axilla and breast may be similarly familiar through cuddling and changing or smelling the child’s clothes.
Another explanation might relate to prior findings, which indicate that the exposure of an odour affects its perception. As Kaitz and colleagues [25
] showed, the duration of a mother’s exposure to her newborn’s BO influences how well she can recognise her new-born through olfactory stimuli. In this study, we did not directly test exposure length, as the infants were already several months old, and hence, the participating mothers were already exposed to their infant’s odour for this time period. However, as mentioned above, the head is assumed to be the most exposed body site of an infant to its mother. Therefore, it is possible that the head is most often mentioned as the site of an infant’s body that smells best because it is the most exposed and not because it actually smells better than some other sites of the body (e.g., axilla and breast).
For the hedonic perception of maternal BO, there was no preference for a specific body site. Nevertheless, our exploratory analyses delivered a trend for the axilla of unfamiliar women: the more intense the axillary odour, the less pleasant it was perceived. This finding is consistent with previous literature on the relation of malodour and axillary odour [35
In addition, the different secretion in the different sites of the body is relevant—especially in adults, as many secretion processes that lead to the development of more severe body odours only come into effect during puberty [33
]. In adults, the axilla is of great importance when it comes to implicit communication, e.g., via androstedadienone, a prominent substance found in the male axilla skin surface that increases a positive emotional state in women [50
]. Additionally, the odour of the maternal breast contributes to implicit communication, e.g., between mother and child. Hence, the breast odour is assumed to be characteristic enough to be recognised, which could be caused, like at the axilla, by the presence of apocrine sweat glands [51
] and Montgomery glands [32
]. Regarding the head BO secretion, there is a particularly high density of sebaceous glands there [52
]. According to Rajan and colleagues [53
], the sebum secreted at the head contributes considerably to the individual’s BO, as it is produced rapidly so that BO is quickly regenerated—for example, after taking a bath. In order to find out how different degrees of secretion by sebaceous and other sweat glands are reflected exactly in different body sites, future studies require more precise examination. In particular, BOs should be evaluated according to more differentiated hedonic criteria, e.g., in a chemical expert panel or supplemented with a chemical extraction procedure in order to quantify objective odour compounds. This has been done for infant’s heads [54
] but is still pending for other BO sources.
During testing, some mothers reported that their infants’ beanies fell off at night, which led to an inconsistent wearing duration of the beanies, so that no statement about the general sampling quality can be made. Those reports emerged during the ongoing data collection, so that we were not able to systematically track the sampling quality. Moreover, it is very difficult for mothers to precisely state the sampling quality, as the sampling took place during nighttime, and thus the infants were not constantly watched. To overcome such obstacles, sampling during daytime might be a solution. When a mother is awake, she can put the beanie back on her infant’s head more easily. Another option would be to sample the odour with a bandage wrapped around the head, which could stick a little better. In this way, the problem of the different head circumferences of the infants, which may have led to the beanies falling off, could also be solved. The above explained methodological issues may be the reason why there was no clear evidence in the Bayesian model for our hypothesis on the hedonic perception of infantile odours at different body sites. To test whether a greater rating uncertainty might have been due to lower intensity of BO samples, we compared variances of low- and high-intensity groups. This did not affect perception of infantile odours but did affect maternal odours, as pleasantness ratings showed a greater variance for intensely perceived BOs. This might have been due to more polarizing hedonic ratings in strong Bos, whether positive or negative, since odour perception shows huge interindividual difference, e.g., relating to genotypic variation [55
]. The study presented here replicated previous findings on human BO perception. First, we found that mothers perceive infantile odours as generally pleasant [8
] and that the odour of one’s own infant was rated as being less intense than unfamiliar odours [5
]. Second, the same pattern was observed for the perception of familiar versus unfamiliar adult BOs. Third, according to prior studies, mothers were well able to identify their own infant and themselves by olfactory cues. Furthermore, we expanded previous findings on the olfactory-recognition abilities of one’s own kin [56
] and of oneself [21
] (by showing that this effect can be applied to different parts of the body.
We are aware of the limitations of the data presented here based on the small sample size. The resulting insufficient sensitivity of the sample could, e.g., have contributed to the lack of findings regarding a maternal preference for the BO of the own infant: While we investigated 28 mothers and observed only a tendency towards higher pleasantness ratings for one’s own child’s BO compared to unfamiliar BOs, prior studies reporting a strong preference effect included N = 50 mothers [43
] and N = 167 mothers [8