High level of circulating testosterone abolishes decline in scent attractiveness in antigen-treated male mice

Regulation of Sexually Dimorphic Expression of Major Urinary Proteins

Male house mice excrete large amounts of protein in their urinary scent marks, mainly composed of Major Urinary Proteins (MUPs), and these lipocalins function as pheromones and pheromone carriers. Here, we review studies on sexually dimorphic MUP expression in house mice, including the proximate mechanisms controlling MUP gene expression and their adaptive functions. Males excrete 2 to 8 times more urinary protein than females, though there is enormous variation in gene expression across loci in both sexes. MUP expression is dynamically regulated depending upon a variety of factors. Males regulate MUP expression according to social status, whereas females do not, and males regulate expression depending upon health and condition. Male-biased MUP expression is regulated by pituitary secretion of growth hormone (GH), which binds receptors in the liver, activating the JAK2-STAT5 signaling pathway, chromatin accessibility, and MUP gene transcription. Pulsatile male GH secretion is feminized by several factors, including caloric restriction, microbiota depletion, and aging, which helps explain condition-dependent MUP expression. If MUP production has sex-specific fitness optima, then this should generate sexual antagonism over allelic expression (intra-locus sexual conflict) selectively favoring sexually dimorphic expression. MUPs influence the sexual attractiveness of male urinary odor and increased urinary protein excretion is correlated with the reproductive success of males but not females. This finding could explain the selective maintenance of sexually dimorphic MUP expression. Producing MUPs entails energetic costs, but increased excretion may reduce the net energetic costs and predation risks from male scent marking as well as prolong the release of chemical signals. MUPs may also provide physiological benefits, including regulating metabolic rate and toxin removal, which may have sex-specific effects on survival. A phylogenetic analysis on the origins of male-biased MUP gene expression in Mus musculus suggests that this sexual dimorphism evolved by increasing male MUP expression rather than reducing female expression.

Parasite-Mediated Mate Preferences in a Cooperatively Breeding Rodent

Females of many species discriminate among males when choosing a mate and this can bear indirect and direct benefits including the avoidance of parasite transmission from infested males. In rodents, this may be mediated by androgen hormones that affect the expression of urinary odors. Female choosiness may also vary with a female’s infestation status, with infested females being less choosy. In the current study we tested the preference of cooperatively breeding highveld mole-rat (Cryptomys hottentotus hottentotus) females for male urinary odors from healthy males and those naturally infested with a cestode (Mathevotaenia sp.). Thirty females (15 healthy, 15 infested) were allowed to explore a Y-maze with urine samples from healthy and infested males and the frequency of entering choice arms and chambers as well as the duration spend with each odor sample was recorded. Infestation status did neither affect male body mass, urinary testosterone, nor cortisol levels or the body condition of females. Although overall female activity was not affected by infestation status, infested females entered choice arms and chambers significantly less frequently than healthy females. Surprisingly, healthy females preferred odors from infested males while the opposite was true for infested females, independent of male hormone levels. As the study species lives in groups that tend to share the same infestation status, we suggest that highveld mole-rat females may exhibit a preference for unfamiliar odors, possibly as an indicator of genetic diversity, rather than discriminate between infestation status of males. Similar mechanisms may also play a role in other social species.

Regulation of volatile and non-volatile pheromone attractants depends upon male social status

We investigated the regulation of chemical signals of house mice living in seminatural social conditions. We found that male mice more than doubled the excretion of major urinary proteins (MUPs) after they acquired a territory and become socially dominant. MUPs bind and stabilize the release of volatile pheromone ligands, and some MUPs exhibit pheromonal properties themselves. We conducted olfactory assays and found that female mice were more attracted to the scent of dominant than subordinate males when they were in estrus. Yet, when male status was controlled, females were not attracted to urine with high MUP concentration, despite being comparable to levels of dominant males. To determine which compounds influence female attraction, we conducted additional analyses and found that dominant males differentially upregulated the excretion of particular MUPs, including the pheromone MUP20 (darcin), and a volatile pheromone that influences female reproductive physiology and behavior. Our findings show that once male house mice become territorial and socially dominant, they upregulate the amount and types of excreted MUPs, which increases the intensities of volatiles and the attractiveness of their urinary scent to sexually receptive females.

[The role of the vomeronasal system in the formation of the human sexual behaviour]

This review deals with the structure and function of the vomeronasal system and evaluation of its influence on the sexual sphere of humans and animals. Special attention is given to the role of pheromones in the regulation of the sexual behaviour. The data concerning the function of the vomeronasal organ following surgical interventions in the nasal cavity are discussed.

Regulation of highly homologous major urinary proteins in house mice quantified with label-free proteomic methods

Major urinary proteins (MUPs) are highly homologous proteoforms that function in binding, transporting and releasing pheromones in house mice. The main analytical challenge for studying variation in MUPs, even for state-of-the-art proteomics techniques, is their high degree of amino acid sequence homology. In this study we used unique peptides for proteoform-specific identification. We applied different search engines (ProteinPilot™vs. PEAKS®) and protein databases (MUP database vs. SwissProt + unreviewed MUPs), and found that proteoform identification is influenced by addressing background proteins (unregulated urinary proteins, non-MUPs) during the database search. High resolution Q-TOF mass spectrometry was used to identify and precisely quantify the regulation of MUP proteoforms in male mice that were reared in standard housing and then transferred to semi-natural enclosures (within-subject design). By using a designated MUP database we were able to distinguish 19 MUP proteoforms, with A2CEK6 (a Mup11 gene product) being the most abundant based on spectral intensities. We compared three different quantification strategies based on MS1- (from IDA and SWATH™ spectra) and MS2 (SWATH™) data, and the results of these methods were correlated. Furthermore, three data normalization methods were compared and we found that increased statistical significance of fold-changes can be achieved by normalization based on urinary protein concentrations. We show that male mice living in semi-natural enclosures significantly up-regulated some but not all MUPs (differential regulation), e.g., A2ANT6, a Mup6 gene product, was upregulated between 9-fold (MS1) and 13-fold (MS2) using the designated MUP database. Finally, we show that 85 ± 7% of total MS intensity can be attributed to MUP-derived peptides, which supports the assumption that MUPs are the primary proteins in mouse urine. Our results provide new tools for assessing qualitative and quantitative variation of MUPs and suggest that male mice regulate the expression of specific MUP proteoforms, depending upon social conditions.

The vomeronasal system mediates sick conspecific avoidance

Although sociability offers many advantages, a major drawback is the increased risk of exposure to contagious pathogens, like parasites, viruses, or bacteria. Social species have evolved various behavioral strategies reducing the probability of pathogen exposure. In rodents, sick conspecific avoidance can be induced by olfactory cues emitted by parasitized or infected conspecifics. The neural circuits involved in this behavior remain largely unknown. We observed that olfactory cues present in bodily products of mice in an acute inflammatory state or infected with a viral pathogen are aversive to conspecifics. We found that these chemical signals trigger neural activity in the vomeronasal system, an olfactory subsystem controlling various innate behaviors. Supporting the functional relevance of these observations, we show that preference toward healthy individuals is abolished in mice with impaired vomeronasal function. These findings reveal a novel function played by the vomeronasal system.

Test of immunocompetence handicap hypothesis in mice (Mus musculus) infected with Trichinella spiralis

The immunocompetence handicap hypothesis (ICHH) proposes that testosterone enhances the expression of sexual traits but suppresses immune function. However, studies to test the hypothesis have shown mixed results. Alternatively, sexual traits, immune function, and parasite susceptibility may be mediated by the stress hormone corticosterone. Here, we report an experimental test of the ICHH that included the manipulation of both testosterone and parasites in male laboratory mice (Mus musculus L., 1758). We conducted a factorial experiment, injecting each individual mouse with testosterone or not and infecting them with the nematode parasite Trichinella spiralis (Owen, 1835) or not. As predicted, testosterone enhanced the scent attractiveness of male mice, whereas parasite infection reduced it, but only in male mice not injected with testosterone. However, we found no evidence that corticosterone is involved in mediating the effects of testosterone. These results confirm that maintaining high testosterone levels entails the cost of increased parasite abundance. This study provides direct evidence supporting the ICHH.

Reintroducing domesticated wild mice to sociality induces adaptive transgenerational effects on MUP expression

When brought into captivity, wild animals can adapt to domestication within 10 generations. Such adaptations may decrease fitness in natural conditions. Many selective pressures are disrupted in captivity, including social behavioral networks. Although lack of sociality in captivity appears to mediate domestication, the underlying mechanisms are not well understood. Additionally, determining the contribution of genetic inheritance vs. transgenerational effects during relaxed selection may provide insight into the flexibility of adaptation. When wild-derived mice kept under laboratory conditions for eight generations were reintroduced to sociality and promiscuity (free mate choice), they adapted within two generations. Fitness assessments between this promiscuous lineage and a monogamous laboratory lineage revealed male-specific effects. Promiscuous-line males had deficits in viability, but a striking advantage in attracting mates, and their scent marks were also more attractive to females. Here, we investigate mechanistic details underlying this olfactory signal and identify a role of major urinary protein (MUP) pheromones. Promiscuous-line males inherit higher MUP expression than monogamous-line males through transgenerational inheritance. Sociality-driven maternal and paternal effects reveal intriguing conflicts among parents and offspring over pheromone expression. MUP up-regulation is not driven by hormone-driven transduction pathways, but rather is associated with reduction in DNA methylation of a CpG dinucleotide in the promoter. This reduction in methylation could enhance transcription by promoting the binding of transcription factor USF1 (upstream stimulatory factor 1). Finally, we experimentally demonstrate that increased MUP expression is a female attractant. These results identify molecular mechanisms guiding domestication and adaptive responses to fluctuating sociality.

Antigen-induced changes in odor attractiveness and reproductive output in male mice

Modulation of social signals by antigen-induced immunoenhancement is a significant component of behavioral and reproductive adaptations of a host population to parasitic pressure. To investigate this concept, we studied odor attractiveness and reproductive output of ICR male mice treated with keyhole limpet hemocyanin (KLH) as an antigenic stimulus. We collected urine samples for olfactory preference tests (control vs. KLH administration) on different days following treatment. We found that the differences in odor attractiveness between control and immunized males, which were observed on the 3rd day, disappeared soon afterwards. Odor attractiveness of male mice positively correlated with their immunoresponsiveness, which was assessed by the sum of anti-KLH IgG1 and anti-KLH IgG2a titers. According to the hypothesis of terminal investment, antigen-treated males had higher reproductive output in comparison with control males and produced more progeny as a result.

Oestradiol level and opportunistic mating in women

The ovarian steroid hormone oestradiol plays a crucial role in female fertility, sexual motivation and behaviour. We investigated the relationship between oestradiol and the likelihood that women would engage in opportunistic mating. Two salivary samples were taken from normally cycling women within the peri-ovulatory and luteal phase of the menstrual cycle. At both testing sessions, participants also completed self-perceived desirability scales and provided subjective reports of sexual and social motivations, and satisfaction with their primary relationship partner. Oestradiol level was positively associated with a woman's self- and other-perceived physical attractiveness and with inclinations to mate outside her current relationship. Oestradiol was marginally negatively associated with a woman's satisfaction with her primary partner and relationship commitment. Results provide support for the relationship between physical beauty and fertility and suggest that physiological mechanisms play a major role in guiding a woman's mating strategies.

Genetic resistance to infection influences a male's sexual attractiveness and modulation of testosterone

Females may be attracted to males genetically resistant to infectious diseases, and one potential mechanism for this mating bias is that such males may be better able to maintain high testosterone. To test these two hypotheses, we collected scent-marks from male house mice (Mus domesticus) genetically resistant and susceptible to Salmonella due to a single locus (Nramp 1, also known as Slc11a1). We tested whether females are more attracted to the scent-marks of resistant males, and whether such males are better able to maintain testosterone concentrations during an experimental Salmonella infection. We found that females preferred the scent-marks of genetically resistant males compared to susceptible ones; but they showed no preferences 5d after males were infected. As predicted, genetically resistant males maintained their testosterone concentrations during the experimental infection, whereas susceptible males showed a significant decline 14 d after inoculation. These differences in the males' ability to modulate testosterone, however, do not explain females' attraction to resistant males. Thus, our results indicate that females sometimes prefer males genetically resistant to infection, and they provide the first evidence that males modulate their testosterone depending upon their genetic resistance to infection; however, we found no evidence to link these two findings.

Women's estradiol predicts preference for facial cues of men's testosterone

A growing body of research has shown that women express stronger attraction to more masculine traits when they are tested near ovulation than when tested during other times in the menstrual cycle. Although these effects have been interpreted as increased preferences for markers of elevated testosterone during times in the cycle when conception is most likely, no previous studies have directly demonstrated that women express stronger attraction to higher testosterone men at different times in the cycle. In addition, little research has addressed which hormonal or other physiological mechanisms may regulate temporal shifts in women's attractiveness judgments. In this research, we demonstrate that women with higher estradiol concentrations exhibit stronger preferences for the faces of men with higher testosterone concentrations, and that women's testosterone preference and estradiol curves track one another across days of the cycle. The findings are the first direct demonstration in humans that hormone concentrations in one sex are associated with attraction to cues of hormonal status in the opposite sex. The results support a functional role for estradiol in calibrating women's mating psychology to indices of their current fertility, analogous to similar processes that have been documented in nonhuman species. A strong correlation between estradiol and testosterone preference specifically during the luteal phase further suggests that women's mate preferences may track their fertility between different cycles in addition to being calibrated to the timing of ovulation within individual cycles.

Recognition and avoidance of the odors of parasitized conspecifics and predators: differential genomic correlates

In many species of animals chemical stimuli are an important source of information about the threats and dangers present in the social and non-social world. Olfactory cues play a fundamental role in modulating social recognition and interactions in a wide variety of mammals. Rodents, in particular, utilize chemical signals, to recognize and avoid conspecifics infected with parasites and other pathogens. Animals also respond to, and utilize, predator odor related information to assess and minimize their risk of predation. In this review, we briefly focus on the relations between odors, parasite recognition and avoidance and consider some of the associated hormonal, neural and genomic mechanisms. We describe how both male and female rodents distinguish between infected and uninfected males on the basis of odors, displaying aversive response to, and avoidance of, the urine odors of infected individuals. We further describe how the recognition and avoidance of the odors of infected individuals involves genes for the neuropeptide, oxytocin, (OT), and estrogenic mechanisms. We show that mice with deletions of the oxytocin gene (OT knockout mice (OTKO)) and mice whose genes for estrogen receptor (ER)-alpha or ER-beta [ER knockout mice (ERKO), alphaERKO and betaERKO] have been disrupted are specifically impaired in their recognition, avoidance, and memory of the odors of infected individuals. We contrast this with the recognition and display of aversive responses to predator (cat) odor that are insensitive to these genetic manipulations. These findings reveal some of the mechanisms associated with the olfactory mediated recognition of parasitized individuals and predators.