Sexual Orientation Affects Neural Responses to Subtle Social Aggression Signals

The current series of studies are the first to examine brain responses to social aggression signals as a function of male and female sexual orientation. For the first set of studies (1a, 1b), axillary sweat had been collected from 17 heterosexual men and 17 heterosexual women aggressively responding to frustrating opponents (aggression condition) and while playing a construction game (control condition). Sweat samples were pooled according to sex and condition, and presented via a constant flow olfactometer to 17 gay and 23 heterosexual men (Study 1a), and 19 lesbian and 25 heterosexual women (Study 1b). Ongoing EEG was recorded from 61 scalp locations, chemosensory event-related potentials (CSERPs; P2, P3-1, P3-2) were analyzed, and neuronal sources calculated (low resolution electromagnetic tomography). Within the second set of studies (2a, 2b), pictures of males' and females' weak angry and neutral facial expressions were presented to 21 gay and 23 heterosexual men (Study 2a), and 19 lesbian and 26 heterosexual women (Study 2b), and ERPs (N170, P3) were analyzed. Gay men showed larger P3-1 amplitudes than heterosexual men upon presentation of male aggression sweat, accompanied by activation of the right inferior frontal gyrus (IFG, BA 10). Gay men also displayed longer N170 latencies in response to men's compared to women's angry facial expressions, while heterosexual men did not. In women, sexual orientation did not affect the processing of aggression sweat or anger expressions. Gay men showed preferential processing of chemosensory aggression signals (P3-1 amplitudes), indicating fine-tuned socioemotional sensitivity, related to activation of brain areas involved in emotion regulation (IFG). They further process the relative relevance of visual aggression signals (N170 latency). These results were in line with theories proposing a common evolutionary pathway for same-sex attraction and traits easing social integration.

Male mice and cows perceive human emotional chemosignals: a preliminary study

Olfactory cues of individuals of the same species or from different species may induce changes in behaviors and physiological reactions in mammals. However, there are few studies on the influence of human odor on animal behavior and welfare, especially those of rodents and farm animals. The present study aimed to investigate whether the odor of a stressed human (in sweat) would modify the behavior of mice and cows. We hypothesized that laboratory and farm animals can perceive human emotions though olfactory cues and that human emotional chemosignals can modify their behavioral reactions and welfare. Two odors of human axillary sweat were collected from engineering students (n = 25, 14 females and 11 males; 21.1 ± 0.7 years old, range: 19-23 years old): a "stress" odor collected after an exam and a "non-stress" odor collected after a standard class. Two experiments were then conducted to test the discrimination of these two odors by male mice (n = 20) under standard conditions and by cows (n = 10) under farm conditions. During the experiments, the behavioral responses of the animals to both odors (through a dispenser for the mice and a bucket for the cows) were observed. The mice produced significantly (p = 0.004) more fecal pellets with the stress odor dispenser than with the non-stress-odor dispenser. The cows spent significantly (p = 0.04) more time smelling the non-stress-odor bucket than control. For both species, the other behaviors observed did not differ significantly between the odors. Mice and cows seemed to perceive and react to stressful human chemosignals. Mice showed physiological reactions that indicated stress in response to the stress odor of humans, while cows showed preference reactions in response to the non-stress odor of humans. This preliminary study showed that laboratory and farm animals, such as male mice and cows, seemed to discriminate certain odors emitted by humans that were likely related to different emotions. Animals may recognize stressful human chemosignals, associate these signals with negative husbandry practices or human-animal relationships, and consequently modify their behavior.

The lasting smell of emotions: The effects of reutilizing fear sweat samples

A growing body of research has shown that human apocrine sweat carries information about the emotional state of its donor. Exposure to sweat produced in a fear-inducing context triggers in its receivers a simulacrum of this emotional state, as evidenced by increased medial frontalis and corrugator supercilii (facial electromyography; fEMG) activity – two facial muscles involved in the display of fear facial expressions. However, despite the increased interest in the effects of emotional sweat, little is known about the properties of these chemical sweat samples. The goal of this study was to examine whether a second application of the same sweat sample would yield reliable results. Specifically, we assessed whether sweat samples collected from Portuguese males (N = 8) in fear (vs. neutral)-inducing contexts would produce similar fEMG activations (i.e., in the medial frontalis and corrugator supercilii) in female receivers (N = 60) across two independent applications (the first with Dutch and the second with Portuguese receivers). Our findings showed that exposure to fear (vs. neutral) sweat resulted in higher activation of both muscles compared with neutral odors, revealing a similar data pattern across the two applications and underlining the feasibility of reusing emotional sweat samples. The implications of these findings for properties of these sweat volatiles are discussed.

Ethnic influences on the perceptual properties of human chemosignals

Individuals of African and Caucasian descent show different chemical signatures in their body odors (BO). Does such biological difference have a perceptual correlate? We tested BO donors and raters of Afro-Portuguese (AP) and Caucasian (C) descent to investigate whether olfactory ratings reveal an ethnic bias and whether olfactory ethnic discrimination is possible. C (vs. AP) women rated the C BO as more pleasant, even when controlling for intensity. The C BO labelled as AP was rated as more intense by C raters. Although discriminability of ethnicity and sex is at chance, a nominal advantage for AP vs. C BO emerges.

Women smelling men's masked body odors show enhanced harm aversion in moral dilemmas

Among the most unnoticeable stimuli providing social information, body odors are powerful social tools that can modulate behavioral and neural processing. It has recently been shown that body odors can affect moral decision-making, by increasing the activations in neural areas processing social and emotional information during the decision process. The aim of the present study was twofold: 1) to test whether body odors selectively affect decisions to real dilemmatic moral scenario (incongruent) vs. fake (congruent) dilemmas, and 2) to characterize whether the impact of masked body odors is modulated by four conceptual factors: personal force, intentionality, benefit recipient and evitability. Women chose between utilitarian (sacrificing a person's life in order to save other lives) or deontological actions (deciding against the harmful action) in 64 moral dilemmas under the exposure of a neutral fragrance (masker) or a masked male body odor. Our results showed that the masked male body odor did not specifically affect the answers to real and fake dilemmas but instead, its effect is modulating whether the agent harms the victim in a direct or indirect manner (personal force) to save herself or only other people (benefit recipient). In particular, when exposed to the masked body odor participants gave more deontological answers when the harm was indirect and only other people were saved. These data support the hypothesis that body odors induce participants to perceive the individuals described in moral dilemmas as more real, triggering harm avoidance.

The scent of security: Odor of romantic partner alters subjective discomfort and autonomic stress responses in an adult attachment-dependent manner

When in a stressful situation, access to adult attachment figures (e.g., romantic partners) is an important means by which adults regulate stress responses. The practice of smelling a partner's worn garment is reported as a self-treatment against stress. Here, we experimentally determined whether exposure to a partner's body odor attenuates adults' subjective discomfort and psychophysiological responses, and whether such effects are qualified by adult attachment security. In a blocked design, participants (N = 34) were presented with their partner's body odor, their own body odor, the odor of a clean t-shirt and rose odor, while exposed to weak electric shocks to induce discomfort and stress responses. Results showed that partner body odor reduces subjective discomfort during a stressful event, as compared with the odor of oneself. Also, highly secure participants had attenuated skin conductance when exposed to partner odor. We conclude that partner odor is a scent of security, especially for attachment-secure adults.

Chemical composition of axillary odorants reflects social and individual attributes in rhesus macaques

Abstract

Scents play an important role in the life of most terrestrial mammals and may transmit valuable information about conspecifics. Olfaction was long considered of low importance in Old World monkeys due to their relative reduction of olfactory structures and low incidence of scent-marking behavior but has been increasingly recognized for mediating social relationships in recent years. Yet, studies investigating the composition of their chemical cues remain scarce. In the present study, we analyzed the potential information content of chemicals present on the skin of rhesus macaques (Macaca mulatta). We collected axillary secretions from 60 animals of the semifree-ranging population on Cayo Santiago (Puerto Rico, USA) with precleaned cotton swabs from which the secretions were subsequently extracted and analyzed by gas chromatography-mass spectrometry. Rhesus macaque axillary odorants varied in their overall similarity and composition. This variation was attributable to differences in sex, group membership, and kinship and further appeared to reflect age and rank in parts of our sample. The compounds most strongly associated with this variation primarily comprised larger molecular weight aldehydes and steroids. Such compounds are considered to be perceivable by the primate olfactory system through close-range interactions or through breakdown into smaller molecules by bacterial fermentation. Overall, our results provide additional evidence that odors of Old World monkeys reflect a wealth of potential information about their carrier, which provides the basis for chemical communication via body odors; however, its use by conspecifics needs to be confirmed in bioassays.

Significance statement

One prerequisite for olfactory communication is the presence of systematic variation in animal odors that is related to attributes such as age, sex, or kinship. The composition of odors has been examined in numerous mammals but, with the exception of humans, remains poorly understood in Old World monkeys and apes, taxonomic groups in which most species do not show scent-marking behavior. In the present study, we show that the composition of axillary secretions of an Old World monkey, the rhesus macaque, reflects sex, group membership, relatedness, and possibly also age and rank. This variation thus provides a basis for olfactory communication in Old World monkeys.

Always follow your nose: the functional significance of social chemosignals in human reproduction and survival

This article is part of a Special Issue "Chemosignals and Reproduction" Across phyla, chemosensory communication is crucial for mediating a variety of social behaviors, which form the basis for ontogenetic and phylogenetic survival. In the present paper, evidence on chemosensory communication in humans, with special reference to reproduction and survival, will be presented. First, the impact of chemosignals on human reproduction will be reviewed. Work will be presented, showing how chemosensory signals are involved in mate choice and partnership formation by communicating attractiveness and facilitating a partner selection, which is of evolutionary advantage, and furthermore providing information about the level of sexual hormones. In addition to direct effects on phylogenetic survival, chemosignals indirectly aid reproductive success by fostering harm protection. Results will be presented, showing that chemosensory communication aids the emotional bond between mother and child, which in turn motivates parental caretaking and protection, leading to infant survival. Moreover, the likelihood of group survival can be increased through the use of stress-related chemosignals. Stress-related chemosignals induce a stress-related physiology in the perceiver, thereby priming a fight-flight-response, which is necessary for an optimum adaption to environmental harm. Finally, effects of sexual orientation on chemosensory communication will be discussed in terms of their putative role in stabilizing social groups, which might indirectly provide harm protection and foster survival. An integrative model of the presented data will be introduced. In conclusion, an outlook, focusing on the involvement of chemosensory communication in human social behavior and illustrating a novel approach to the significance of chemosensory signals in human survival, will be given.