Honest olfactory ornamentation in a female-dominant primate

Sniffing behavior of semi free-ranging Barbary macaques (Macaca sylvanus)

Olfaction is one of the evolutionarily oldest senses and plays a fundamental role in foraging and social interactions across mammals. In primates, the role of olfaction is now well recognized, but better investigated in strepsirrhine and platyrrhine primates than in catarrhines. We observed the sniffing behavior of semi-free ranging Barbary macaques, Macaca sylvanus, at Affenberg Salem, Germany, to assess how frequently macaques sniff and in which contexts, and how sniffing is affected by sex and age. Focal observations of 24 males and 24 females aged 1-25 years showed that Barbary macaques sniffed, on average, 5.24 times per hour, with more than 80% of sniffs directed at food. Irrespective of the context, younger individuals sniffed more often than older ones. Females' sniffs were more often directed at food than male sniffs, while males sniffed more often than females in a social context. Sniffs at conspecifics occurred primarily in a sexual context, with 70% of social sniffs directed at female anogenital swellings performed by males. Of the observed 176 anogenital inspections, 51 involved sniffing of the swelling. Olfactory inspections were followed by copulation significantly less often than merely visual inspections, suggesting that anogenital odors may play a role in male mating decisions, but the role of olfaction in sexual interactions warrants further investigations. In sum, results show that Barbary macaques routinely use olfaction during feeding, but also in a socio-sexual context, corroborating the relevance of the olfactory sense in the lives of catarrhine primates.

Behavioural Correlates of Lemur Scent-Marking in Wild Diademed Sifakas (Propithecus diadema) in the Maromizaha Forest (Madagascar)

Scent-marking through odours from excreta and glandular secretions is widespread in mammals. Among primates, diurnal group-living lemurs show different deployment modalities as part of their strategy to increase signal detection. We studied the diademed sifaka (Propithecus diadema) in the Maromizaha New Protected Area, Eastern Madagascar. We tested whether the scent-marking deposition occurred using a sequential rubbing of different body parts. We also tested if glands (i.e., deposition of glandular secretions) were more frequently rubbed than genital orifices (i.e., deposition of excreta) by comparing different kinds of rubbing behaviour. We then investigated if the depositor's rank and sex affected the sequence of rubbing behaviour, the height at which the scent-marking happened, and the tree part targeted. We found that glandular secretions were often deposited with urine, especially in dominant individuals. The probability of anogenital and chest marking was highest, but chest rubbing most frequently occurred in dominant males. Markings were deposited at similar heights across age and sex, and tree trunks were the most used substrate. Males exhibited long and more complex scent-marking sequences than females. Our results indirectly support the idea that diademed sifakas deploy a sex-dimorphic mixture of glandular secretions and excreta to increase the probability of signal detection by conspecifics.

Chemical cues of identity and reproductive status in Japanese macaques

Olfactory communication plays an important role in the regulation of socio-sexual interactions in mammals. There is growing evidence that both human and nonhuman primates rely on odors to inform their mating decisions. Nevertheless, studies of primate chemical ecology remain scarce due to the difficulty of obtaining and analyzing samples. We analyzed 67 urine samples from five captive female Japanese macaques (Macaca fuscata) and 30 vaginal swabs from three of these females using gas chromatography-mass spectrometry and examined the relationship between odor (compounds identified, richness, intensity, and diversity) and female identity as well as cycle phase. We found a total of 36 urine compounds of which we identified 31, and 68 vaginal compounds of which we identified 37. Our results suggest that urine and vaginal odor varied more between individuals than within cycle phases. However, we found that within a female cycle, urine samples from similar phases may cluster more than samples from different phases. Our results suggest that female odor may encode information about identity (vaginal and urine odor) and reproductive status (urine odor). The question of how conspecifics use female urine and vaginal odor remains open and could be tested using bioassays. Our results and their interpretation are constrained by our limited sample size and our study design. Nonetheless, our study provides insight into the potential signaling role of female odor in sexual communication in Japanese macaques and contributes to our understanding of how odors may influence mating strategies in primates.

Territorial scent-marking effects on vigilance behavior, space use, and stress in female Columbian ground squirrels

Social environments can profoundly affect the behavior and stress physiology of group-living animals. In many territorial species, territory owners advertise territorial boundaries to conspecifics by scent marking. Several studies have investigated the information that scent marks convey about donors' characteristics (e.g., dominance, age, sex, reproductive status), but less is known about whether scents affect the behavior and stress of recipients. We experimentally tested the hypothesis that scent marking may be a potent source of social stress in territorial species. We tested this hypothesis for Columbian ground squirrels (Urocitellus columbianus) during lactation, when territorial females defend individual nest-burrows against conspecifics. We exposed lactating females, on their territory, to the scent of other lactating females. Scents were either from unfamiliar females, kin relatives (a mother, daughter, or sister), or their own scent (control condition). We expected females to react strongly to novel scents from other females on their territory, displaying increased vigilance, and higher cortisol levels, indicative of behavioral and physiological stress. We further expected females to be more sensitive to unfamiliar female scents than to kin scents, given the matrilineal social structure of this species and known fitness benefits of co-breeding in female kin groups. Females were highly sensitive to intruder (both unfamiliar and kin) scents, but not to their own scent. Surprisingly, females reacted more strongly to the scent of close kin than to the scent of unfamiliar females. Vigilance behavior increased sharply in the presence of scents; this increase was more marked for kin than unfamiliar female scents, and was mirrored by a marked 131% increase in free plasma cortisol levels in the presence of kin (but not unfamiliar female) scents. Among kin scents, lactating females were more vigilant to the scent of sisters of equal age, but showed a marked 318% increase in plasma free cortisol levels in response to the scent of older and more dominant mothers. These results suggest that scent marks convey detailed information on the identity of intruders, directly affecting the stress axis of territory holders.

Integrating the female masculinization and challenge hypotheses: Female dominance, male deference, and seasonal hormone fluctuations in adult blue-eyed black lemurs (Eulemur flavifrons)

In the decades since female social dominance was first described in strepsirrhine primates, researchers have sought to uncover the proximate and ultimate explanations for its development. In the females of various female-dominant species, androgens have been implicated as regulators of behavior and/or predictors of seasonal fluctuations in aggression (the 'Female Masculinization Hypothesis'). Males, more generally, respond to changing social demands via seasonal fluctuations in androgen-mediated behavior (the 'Challenge Hypothesis'), that may also entail changes in activation of the hypothalamic-pituitary-adrenal axis. Here, we explore if androgens, glucocorticoids, and intersexual behavior fluctuate seasonally in the female-dominant, blue-eyed black lemur (Eulemur flavifrons), with potential consequences for understanding female aggression and male deference. Across two studies conducted during the breeding and nonbreeding seasons, we assessed rates of mixed-sex, dyadic social behavior (aggression and affiliation) and concentrations of fecal glucocorticoid metabolites (Study 1) and serum sex hormones (androstenedione, testosterone, and estradiol; Study 2). Our results align with several predictions inspired by the Female Masculinization and Challenge Hypotheses for intersexual relations: During the breeding season, specifically, both aggression and androstenedione peaked in females, while female-initiated affiliation decreased, potentially to facilitate female resource access and reproductive control. By comparison, all target hormones (androgens, estrogen, and glucocorticoids) peaked in males, with glucocorticoid concentrations potentially increasing in response to the surge in female aggression, and unusually high estrogen concentrations year-round potentially facilitating male deference via male-initiated affiliation. These results suggest complex, seasonally and hormonally mediated behavior in Eulemur flavifrons.

Males perceive honest information from female released sex pheromone in a moth

There is accumulating evidence that male insects advertise their quality to conspecific females through pheromones. However, most studies of female released sex pheromone assume information transfer regarding merely the species of the female and her mating status. We show that more information is conveyed through the female sex pheromone, positioning it as an honest sexual trait. We demonstrate that females in bad physical conditions (small, starved, or old) lay significantly fewer eggs than females in good conditions (large, fed, or young). The ratio of components in the sex pheromone blend in gland extracts of the female pink bollworm moths provided honest information on most of the phenotypic conditions tested, whereas the pheromone amount in the glands provided an honest signal of quality for extreme phenotypes only. Moreover, males used the information conveyed by the female pheromone to choose their mates, approaching females that signaled higher reproductive potential. In addition, when simulating the female effect, using the synthetic pheromone blend that represents higher quality females (0.6:0.4 ZZ:ZE), more males were attracted to this blend than to the blend representing the population mean (0.5:0.5 ZZ:ZE). Both, female advertisement for males and the male choosiness, suggest that pheromones have evolved as sexual traits under directional, sexual selection. We suggest that the pheromone blend may serve as a multicomponent signal whereby each component adds information concerning the current condition of the female, and all are necessary to elicit a mate searching response.

Experimental Evaluation of Spontaneous Olfactory Discrimination in Two Nocturnal Primates (Microcebus murinus and M. lehilahytsara)

Solitary species often employ chemocommunication to facilitate mate localization. In the solitarily foraging, nocturnal mouse lemurs (Microcebus spp.), females advertise their short period of estrus acoustically and by increased scent marking, whereas males search widely for receptive females. Both sexes can be trained by operant conditioning to discriminate conspecific from heterospecific urine scent. However, it is not known, if males during and outside the reproductive season show different spontaneous interest in conspecific female urine, and if urine from estrous females elicits a higher investigation response than that from diestrous females. We established a spontaneous discrimination paradigm and quantified olfactory investigation responses of 21 captive male mouse lemurs of M. lehilahytsara and M. murinus when presenting 1 conspecific and 1 heterospecific female urine odor sample simultaneously. Overall, M. murinus investigated stimuli significantly longer than M. lehilahytsara. Moreover, males of M. murinus showed significantly longer olfactory investigation at conspecific urine samples during but not outside the reproductive season. This indicates that female urinary cues are spontaneously discriminated by male M. murinus and that this discrimination is more relevant during the reproductive season. However, males of both species did not show different responses toward urine samples from estrous versus diestrous females. Finally, male age did not correlate with the overall duration of olfactory investigation, and investigation levels were similar when testing with fresh or frozen urine samples. In conclusion, this new spontaneous discrimination paradigm provides a useful additional tool to study olfactory communication of nocturnal primates from the receiver's perspective.

Lack of evidence for pheromones in lemurs

As chemicals that elicit unlearned, functionally specialized, and species-specific responses [1] or 'stereotyped behavior' [2], pheromones differ from mammalian scent signatures that comprise complex, variable mixtures, convey multiple messages via learned chemical combinations, and elicit generalized responses [1]. Studying ring-tailed lemur (Lemur catta) behavior and semiochemistry, a recent study by Shirasu, Ito et al. [2] claimed to have identified "the first sex pheromones in primates." However, reliance on one male in most chemical procedures and on few females in behavioral procedures constrains statistical analyses and challenges the broad applicability of their findings. Also, the non-independent testing of even fewer signaler-recipient dyads downplays the critical role of learning and memory in primate communication [1] - an argument that refuted earlier claims of primate pheromones [3,4]. Here, we challenge each of their four highlighted findings and interpretations.

Design, delivery and perception of condition-dependent chemical signals in strepsirrhine primates: implications for human olfactory communication

The study of human chemical communication benefits from comparative perspectives that relate humans, conceptually and empirically, to other primates. All major primate groups rely on intraspecific chemosignals, but strepsirrhines present the greatest diversity and specialization, providing a rich framework for examining design, delivery and perception. Strepsirrhines actively scent mark, possess a functional vomeronasal organ, investigate scents via olfactory and gustatory means, and are exquisitely sensitive to chemically encoded messages. Variation in delivery, scent mixing and multimodality alters signal detection, longevity and intended audience. Based on an integrative, 19-species review, the main scent source used (excretory versus glandular) differentiates nocturnal from diurnal or cathemeral species, reflecting differing socioecological demands and evolutionary trajectories. Condition-dependent signals reflect immutable (species, sex, identity, genetic diversity, immunity and kinship) and transient (health, social status, reproductive state and breeding history) traits, consistent with socio-reproductive functions. Sex reversals in glandular elaboration, marking rates or chemical richness in female-dominant species implicate sexual selection of olfactory ornaments in both sexes. Whereas some compounds may be endogenously produced and modified (e.g. via hormones), microbial analyses of different odorants support the fermentation hypothesis of bacterial contribution. The intimate contexts of information transfer and varied functions provide important parallels applicable to olfactory communication in humans. This article is part of the Theo Murphy meeting issue 'Olfactory communication in humans'.

Organizational and activational androgens, lemur social play, and the ontogeny of female dominance

The role of androgens in shaping "masculine" traits in males is a core focus in behavioral endocrinology, but relatively little is known about an androgenic role in female aggression and social dominance. In mammalian models of female dominance, including the ring-tailed lemur (Lemur catta), links to androgens in adulthood are variable. We studied the development of ring-tailed lemurs to address the behavioral basis and ontogenetic mechanisms of female dominance. We measured behavior and serum androgen concentrations in 24 lemurs (8 males, 16 females) from infancy to early adulthood, and assessed their 'prenatal' androgen milieu using serum samples obtained from their mothers during gestation. Because logistical constraints limited the frequency of infant blood sampling, we accounted for asynchrony between behavioral and postnatal hormone measurements via imputation procedures. Imputation was unnecessary for prenatal hormone measurements. The typical sex difference in androgen concentrations in young lemurs was consistent with adult conspecifics and most other mammals; however, we found no significant sex differences in rough-and-tumble play. Female (but not male) aggression increased beginning at approximately 15 months, coincident with female puberty. In our analyses relating sexually differentiated behavior to androgens, we found no relationship with activational hormones, but several significant relationships with organizational hormones. Notably, associations of prenatal androstenedione and testosterone with behavior were differentiated, both by offspring sex and by type of behavior within offspring sexes. We discuss the importance of considering (1) missing data in behavioral endocrinology research, and (2) organizational androgens other than testosterone in studies of female dominance.

Genetic variation at MHC class II loci influences both olfactory signals and scent discrimination in ring-tailed lemurs

BACKGROUND

Diversity at the Major Histocompatibility Complex (MHC) is critical to health and fitness, such that MHC genotype may predict an individual's quality or compatibility as a competitor, ally, or mate. Moreover, because MHC products can influence the components of bodily secretions, an individual's body odors may signal its MHC composition and influence partner identification or mate choice. Here, we investigated MHC-based signaling and recipient sensitivity by testing for odor-gene covariance and behavioral discrimination of MHC diversity and pairwise dissimilarity in a strepsirrhine primate, the ring-tailed lemur (Lemur catta).

METHODS

First, we coupled genotyping of the MHC class II gene, DRB, with gas chromatography-mass spectrometry of genital gland secretions to investigate if functional genetic diversity is signaled by the chemical diversity of lemur scent secretions. We also assessed if the chemical similarity between individuals correlated with their MHC-DRB similarity. Next, we assessed if lemurs discriminated this chemically encoded, genetic information in opposite-sex conspecifics.

RESULTS

We found that both sexes signaled overall MHC-DRB diversity and pairwise MHC-DRB similarity via genital secretions, but in a sex- and season-dependent manner. Additionally, the sexes discriminated absolute and relative MHC-DRB diversity in the genital odors of opposite-sex conspecifics, suggesting that lemur genital odors function to advertise genetic quality.

CONCLUSIONS

In summary, genital odors of ring-tailed lemurs provide honest information about an individual's absolute and relative MHC quality. Complementing evidence in humans and Old World monkeys, we suggest that reliance on scent signals to communicate MHC quality may be important across the primate lineage.

Chemical composition of glandular secretions from a pair-living monogamous primate: Sex, age, and gland differences in captive and wild owl monkeys (Aotus spp.)

Broadening our knowledge of olfactory communication in strictly monogamous systems can inform our understanding of how chemosignals may facilitate social and reproductive behavior between the sexes. Compared to other social and mating systems, relatively little is known about olfactory communication in strictly monogamous non-human primates. Furthermore, platyrrhines are not well represented in chemical analyses of glandular secretions. We conducted semi-quantitative headspace gas chromatography with mass spectrometry to investigate the chemical components of glandular secretions from the subcaudal and pectoral glands of a strictly pair-living platyrrhine, the owl monkey (Aotus spp.). In this study, the first chemical analysis of a wild platyrrhine population, our goals were to (1) conduct a robust analysis of glandular secretions from both captive and wild owl monkey populations and (2) identify whether biologically relevant traits are present in glandular secretions. We also compared and contrasted the results between two Aotus species in different environmental contexts: wild Aotus azarae (N = 33) and captive A. nancymaae (N = 104). Our findings indicate that secretions from both populations encode sex, gland of origin, and possibly individual identity. These consistent patterns across species and contexts suggest that secretions may function as chemosignals. Our data also show that wild A. azarae individuals are chemically discriminated by age (adult or subadult). Among the captive A. nanycmaae, we found chemical differences associated with location, possibly caused by dietary differences. However, there was no noticeable effect of contraception on the chemical profiles of females, nor evidence that closely related individuals exhibit more similar chemical profiles in A. nancymaae. Overall, our data suggest that glandular secretions of both wild and captive Aotus convey specific information. Future studies should use behavioral bioassays to evaluate the ability of owl monkeys to detect signals, and consider whether odor may ultimately facilitate social and sexual relationships between male and female owl monkeys.

Female Chemical Signalling Underlying Reproduction in Mammals

Chemical communication plays many key roles in mammalian reproduction, although attention has focused particularly on male scent signalling. Here, we review evidence that female chemical signals also play important roles in sexual attraction, in mediating reproductive competition and cooperation between females, and in maternal care, all central to female reproductive success. Female odours function not only to advertise sexual receptivity and location, they can also have important physiological priming effects on male development and sperm production. However, the extent to which female scents are used to assess the quality of females as potential mates has received little attention. Female investment in scent signalling is strongly influenced by the social structure and breeding system of the species. Although investment is typically male-biased, high competition between females can lead to a reversed pattern of female- biased investment. As among males, scent marking and counter-marking are often used to advertise territory defence and high social rank. Female odours have been implicated in the reproductive suppression of young or subordinate females across a range of social systems, with females of lower competitive ability potentially benefiting by delaying reproduction until conditions are more favourable. Further, the ability to recognise individuals, group members and kin through scent underpins group cohesion and cooperation in many social species, as well as playing an important role in mother-offspring recognition. However, despite the diversity of female scent signals, chemical communication in female mammals remains relatively understudied and poorly understood. We highlight several key areas of future research that are worthy of further investigation.

Costs of injury for scent signalling in a strepsirrhine primate

Honesty is crucial in animal communication when signallers are conveying information about their condition. Condition dependence implies a cost to signal production; yet, evidence of such cost is scarce. We examined the effects of naturally occurring injury on the quality and salience of olfactory signals in ring-tailed lemurs (Lemur catta). Over a decade, we collected genital secretions from 23 (13 male, 10 female) adults across 34 unique injuries, owing primarily to intra-group fights. Using gas chromatography-mass spectrometry, we tested for differences in the chemical composition of secretions across pre-injury, injury and recovery, in animals that did and did not receive antibiotics. Lemur genital secretions were significantly dampened and altered during injury, with patterns of change varying by sex, season and antibiotics. Using behavioural bioassays (excluding odorants from antibiotic-treated animals), we showed that male 'recipients' discriminated injury status based on scent alone, directing more competitive counter marking towards odorants from injured vs. uninjured male 'signallers.' That injured animals could not maintain their normal signatures provides rare evidence of the energetic cost to signal production. That conspecifics detected olfactory-encoded 'weakness' suggests added behavioural costs: By influencing the likelihood of intra- or inter-sexual conflict, condition-dependent signals could have important implications for socio-reproductive behaviour.

"Stink flirting" in ring-tailed lemurs (Lemur catta): Male olfactory displays to females as honest, costly signals

Sexual selection for honest behavioral displays of quality has driven the development of remarkably complex courtship behavior in many animal species. Olfactory displays are often overlooked as an area of inquiry compared to auditory and visual displays. Ring-tailed lemur (Lemur catta) scent marking of substrates has been studied extensively, although the male olfactory displays of anointing and wafting tails to females has received relatively little attention. We studied the role of male olfactory displays to females, evaluating whether such signals function as honest, costly signals of male dominance status in two groups of wild L. catta at Berenty Reserve, Madagascar. Our results suggest that male tail anointing and tail wafting displays to pre-estrous and estrous females are correlated with male dominance rank, and moreover appear to operate as costly signals, as such displays increase aggression received from males and females while other types of scent marking do not. Furthermore, females showed greater mating preference (as measured by sexual presents) for resident males who performed the "anoint tail" and "waft tail" displays towards them. When males perform the "anoint tail" and "waft tail" displays to females, they receive higher levels of aggression than if they were to perform other types of scent marking. Interestingly, immigrating (peripheral) males performed the "anoint tail" and "waft tail" displays at higher rates than resident males, which could honestly indicate their quality or may simply be associated with the alternative mating strategy of transferring between groups to gain mating opportunities. Our finding that tail anointing and tail wafting displays function as honest signals of dominance for resident males-and that these costly displays appear to positively affect female mate choice-is the first evidence of this function for this particular olfactory signal in L. catta.

Sexual Selection and the differences between the sexes in Mandrills (Mandrillus sphinx)

Sexual selection has become a major focus in evolutionary and behavioral ecology. It is also a popular research topic in primatology. I use studies of mandrills (Mandrillus sphinx), a classic example of extravagant armaments and ornaments in animals, to exemplify how a long-term, multidisciplinary approach that integrates field observations with laboratory methods can contribute to on-going theoretical debates in the field of sexual selection. I begin with a brief summary of the main concepts of sexual selection theory and the differences between the sexes. I then introduce mandrills and the study population and review mandrill life history, the ontogeny of sex differences, and maternal effects. Next, I focus on male-male competition and female choice, followed by the less well-studied questions of female-female competition and male choice. This review shows how different reproductive priorities lead to very different life histories and divergent adaptations in males and females. It demonstrates how broadening traditional perspectives on sexual selection beyond the ostentatious results of intense sexual selection on males leads to an understanding of more subtle and cryptic forms of competition and choice in both sexes and opens many productive avenues in the study of primate reproductive strategies. These include the potential for studies of postcopulatory selection, female intrasexual competition, and male choice. These studies of mandrills provide comparison and, I hope, inspiration for studies of both other polygynandrous species and species with mating systems less traditionally associated with sexual selection.

Mix it and fix it: functions of composite olfactory signals in ring-tailed lemurs

Animals communicating via scent often deposit composite signals that incorporate odorants from multiple sources; however, the function of mixing chemical signals remains understudied. We tested both a 'multiple-messages' and a 'fixative' hypothesis of composite olfactory signalling, which, respectively, posit that mixing scents functions to increase information content or prolong signal longevity. Our subjects-adult, male ring-tailed lemurs (Lemur catta)-have a complex scent-marking repertoire, involving volatile antebrachial (A) secretions, deposited pure or after being mixed with a squalene-rich paste exuded from brachial (B) glands. Using behavioural bioassays, we examined recipient responses to odorants collected from conspecific strangers. We concurrently presented pure A, pure B and mixed A + B secretions, in fresh or decayed conditions. Lemurs preferentially responded to mixed over pure secretions, their interest increasing and shifting over time, from sniffing and countermarking fresh mixtures, to licking and countermarking decayed mixtures. Substituting synthetic squalene (S)-a well-known fixative-for B secretions did not replicate prior results: B secretions, which contain additional chemicals that probably encode salient information, were preferred over pure S. Whereas support for the 'multiple-messages' hypothesis underscores the unique contribution from each of an animal's various secretions, support for the 'fixative' hypothesis highlights the synergistic benefits of composite signals.

Baby on board: olfactory cues indicate pregnancy and fetal sex in a non-human primate

Olfactory cues play an integral, albeit underappreciated, role in mediating vertebrate social and reproductive behaviour. These cues fluctuate with the signaller's hormonal condition, coincident with and informative about relevant aspects of its reproductive state, such as pubertal onset, change in season and, in females, timing of ovulation. Although pregnancy dramatically alters a female's endocrine profiles, which can be further influenced by fetal sex, the relationship between gestation and olfactory cues is poorly understood. We therefore examined the effects of pregnancy and fetal sex on volatile genital secretions in the ring-tailed lemur (Lemur catta), a strepsirrhine primate possessing complex olfactory mechanisms of reproductive signalling. While pregnant, dams altered and dampened their expression of volatile chemicals, with compound richness being particularly reduced in dams bearing sons. These changes were comparable in magnitude with other, published chemical differences among lemurs that are salient to conspecifics. Such olfactory 'signatures' of pregnancy may help guide social interactions, potentially promoting mother-infant recognition, reducing intragroup conflict or counteracting behavioural mechanisms of paternity confusion; cues that also advertise fetal sex may additionally facilitate differential sex allocation.

D'scent of man: a comparative survey of primate chemosignaling in relation to sex

This article is part of a Special Issue (Chemosignals and Reproduction). As highly visual animals, primates, in general, and Old World species (including humans), in particular, are not immediately recognized for reliance in their daily interactions on olfactory communication. Nevertheless, views on primate olfactory acuity and the pervasiveness of their scent signaling are changing, with increased appreciation for the important role of body odors in primate social and sexual behavior. All major taxonomic groups, from lemurs to humans, are endowed with scent-producing organs, and either deposit or exude a wealth of volatile compounds, many of which are known semiochemicals. This review takes a comparative perspective to illustrate the reproductive context of primate signaling, the relevant information content of their signals, the sexually differentiated investigative responses generated, and the behavioral or physiological consequences of message transmission to both signaler and receiver. Throughout, humans are placed alongside their relatives to illustrate the evolutionary continuum in the sexual selection of primate chemosignals. This ever-growing body of evidence points to a critical role of scent in guiding the social behavior and reproductive function throughout the primate order.

The molecular evolutionary dynamics of the vomeronasal receptor (class 1) genes in primates: a gene family on the verge of a functional breakdown

Olfaction plays a critical role in both survival of the individual and in the propagation of species. Studies from across the mammalian clade have found a remarkable correlation between organismal lifestyle and molecular evolutionary properties of receptor genes in both the main olfactory system (MOS) and the vomeronasal system (VNS). When a large proportion of intact (and putatively functional) copies is observed, the inference is made that a particular mode of chemoreception is critical for an organism’s fit to its environment and is thus under strong positive selection. Conversely, when the receptors in question show a disproportionately large number of pseudogene copies, this contraction is interpreted as evidence of relaxed selection potentially leading to gene family extinction. Notably, it appears that a risk factor for gene family extinction is a high rate of nonsynonymous substitution. A survey of intact vs. pseudogene copies among primate vomeronasal receptor Class one genes (V1Rs) appears to substantiate this hypothesis. Molecular evolutionary complexities in the V1R gene family combine rapid rates of gene duplication, gene conversion, lineage-specific expansions, deletions, and/or pseudogenization. An intricate mix of phylogenetic footprints and current adaptive landscapes have left their mark on primate V1Rs suggesting that the primate clade offers an ideal model system for exploring the molecular evolutionary and functional properties of the VNS of mammals. Primate V1Rs tell a story of ancestral function and divergent selection as species have moved into ever diversifying adaptive regimes. The sensitivity to functional collapse in these genes, consequent to their precariously high rates of nonsynonymous substitution, confer a remarkable capacity to reveal the lifestyles of the genomes that they presently occupy as well as those of their ancestors.

Maintenance of familiarity and social bonding via communal latrine use in a solitary primate (Lepilemur leucopus)

Latrine use (i.e., the repeated use of specific defecation/urination sites) has been described for several mammals, including carnivores, ungulates, and primates. However, the functional significance of latrine use in primates has not been studied systematically yet. We, therefore, followed 14 radio-collared individuals of the pair-living white-footed sportive lemur (Lepilemur leucopus) for 1097 hours of continuous focal observations to investigate latrine distribution, seasonality of latrine use, as well as age and sex of users to test various hypotheses related to possible functions of latrine use, including territory demarcation, resource defense, signaling of reproductive state, social bonding, and mate defense. All individuals of a social unit exhibited communal use of latrines located in the core area of their territory, supporting the social boding hypothesis. Latrine use seems to facilitate familiarity and social bonding within social units via olfactory communication in this primate that lives in family units but exhibits low levels of spatial cohesion and direct social interactions. In addition, frequency of latrine visitation was higher during nights of perceived intruder pressure, supporting the mate defense hypothesis. However, animals did not react to experimentally introduced feces from neighboring or strange social units, indicating that urine may be the more important component of latrines than feces in this arboreal species. Based on a survey of latrine use and function in other mammals, we conclude that latrines facilitate communication particularly in nocturnal species with limited habitat visibility and in species where individuals are not permanently cohesive because they constitute predictable areas for information exchange.

Molecular evolutionary characterization of a V1R subfamily unique to strepsirrhine primates

Vomeronasal receptor genes have frequently been invoked as integral to the establishment and maintenance of species boundaries among mammals due to the elaborate one-to-one correspondence between semiochemical signals and neuronal sensory inputs. Here, we report the most extensive sample of vomeronasal receptor class 1 (V1R) sequences ever generated for a diverse yet phylogenetically coherent group of mammals, the tooth-combed primates (suborder Strepsirrhini). Phylogenetic analysis confirms our intensive sampling from a single V1R subfamily, apparently unique to the strepsirrhine primates. We designate this subfamily as V1Rstrep. The subfamily retains extensive repertoires of gene copies that descend from an ancestral gene duplication that appears to have occurred prior to the diversification of all lemuriform primates excluding the basal genus Daubentonia (the aye-aye). We refer to the descendent clades as V1Rstrep-α and V1Rstrep-β. Comparison of the two clades reveals different amino acid compositions corresponding to the predicted ligand-binding site and thus potentially to altered functional profiles between the two. In agreement with previous studies of the mouse lemur (genus, Microcebus), the majority of V1Rstrep gene copies appear to be intact and under strong positive selection, particularly within transmembrane regions. Finally, despite the surprisingly high number of gene copies identified in this study, it is nonetheless probable that V1R diversity remains underestimated in these nonmodel primates and that complete characterization will be limited until high-coverage assembled genomes are available.

Individual recognition through olfactory-auditory matching in lemurs

Individual recognition can be facilitated by creating representations of familiar individuals, whereby information from signals in multiple sensory modalities become linked. Many vertebrate species use auditory-visual matching to recognize familiar conspecifics and heterospecifics, but we currently do not know whether representations of familiar individuals incorporate information from other modalities. Ring-tailed lemurs (Lemur catta) are highly visual, but also communicate via scents and vocalizations. To investigate the role of olfactory signals in multisensory recognition, we tested whether lemurs can recognize familiar individuals through matching scents and vocalizations. We presented lemurs with female scents that were paired with the contact call either of the female whose scent was presented or of another familiar female from the same social group. When the scent and the vocalization came from the same individual versus from different individuals, females showed greater interest in the scents, and males showed greater interest in both the scents and the vocalizations, suggesting that lemurs can recognize familiar females via olfactory-auditory matching. Because identity signals in lemur scents and vocalizations are produced by different effectors and often encountered at different times (uncoupled in space and time), this matching suggests lemurs form multisensory representations through a newly recognized sensory integration underlying individual recognition.

Wake up and smell the conflict: odour signals in female competition

Odour signals used in competitive and aggressive interactions between males are well studied in the context of sexual selection. By contrast, relatively little is known about comparable signals used by females, despite current interest in the evolution of female ornaments and weaponry. Available evidence suggests that odour signals are important in competitive interactions between female mammals, with reductions or reversals of male-biased sexual dimorphism in signalling where female competition is intense. Scent marking is often associated with conflict between females over access to resources or reproductive opportunities. Female scent marks may therefore provide reliable signals of competitive ability that could be used both by competitors and potential mates. Consistent with this hypothesis, we report that aggressive behaviour of female house mice is correlated with the amount of major urinary protein (MUP) excreted in their urine, a polymorphic set of proteins that are used in scent mark signalling. Under semi-natural conditions, females with high MUP output are more likely to produce offspring sired by males that have high reproductive success, and less likely to produce offspring by multiple different sires, suggesting that females with strong MUP signals are monopolized by males of particularly high quality. We conclude that odour signals are worthy of more detailed investigation as mediators of female competition.

Relatedness communicated in lemur scent

Lemurs are the most olfactory-oriented of primates, yet there is still only a basic level of understanding of what their scent marks communicate. We analyzed scent secretions from Milne-Edwards' sifakas (Propithecus edwardsi) collected in their natural habitat of Ranomafana National Park, Madagascar. We sought to test whether the scent mark could signal genetic relatedness in addition to species, sex, season, and individuality. We not only found correlations (r (2) = 0.38, P = 0.017) between the total olfactory fingerprint and genetic relatedness but also between relatedness and specific components of the odor, despite the complex environmental signals from differences in diet and behavior in a natural setting. To the best of our knowledge, this is the first demonstration of an association between genetic relatedness and chemical communication in a wild primate population. Furthermore, we found a variety of compounds that were specific to each sex and each sampling period. This research shows that scent marks could act as a remote signal to avoid inbreeding, optimize mating opportunities, and potentially aid kin selection.

The "secret" in secretions: methodological considerations in deciphering primate olfactory communication

Olfactory communication in primates is gaining recognition; however, studies on the production and perception of primate scent signals are still scant. In general, there are five tasks to be accomplished when deciphering the chemical signals contained in excretions and secretions: (1) obtaining the appropriate samples; (2) extracting the target organic compounds from the biological matrix; (3) separating the extracted compounds from one another (by gas chromatography, GC or liquid chromatography, LC); (4) identifying the compounds (by mass spectrometry, MS and associated procedures); and (5) revealing biologically meaningful patterns in the data. Ultimately, because some of the compounds identified in odorants may not be relevant, associated steps in understanding signal function involve verifying the perception or biological activity of putative semiochemicals via (6) behavioral bioassays or (7) receptor response studies. This review will focus on the chemical analyses and behavioral bioassays of volatile, primate scent signals. Throughout, we highlight the potential pitfalls of working with highly complex, chemical matrices and suggest ways for minimizing problems. A recurring theme in this review is that multiple approaches and instrumentation are required to characterize the full range of information contained in the complex mixtures that typify primate or, indeed, many vertebrate olfactory cues. Only by integrating studies of signal production with those verifying signal perception will we better understand the function of olfactory communication.

Female blue tits with brighter yellow chests transfer more carotenoids to their eggs after an immune challenge

Female ornaments are present in many species, and it is more and more accepted that sexual or social selection may lead to their evolution. By contrast, the information conveyed by female ornaments is less well understood. Here, we investigated the links between female ornaments and maternal effects. In birds, an important maternal effect is the transmission of resources, such as carotenoids, into egg yolk. Carotenoids are pigments with antioxidant and immunomodulatory properties that are crucial for females and developing offspring. In blue tits, we evaluated whether ultraviolet (UV)/blue and yellow feather colouration signals a female's capacity to allocate carotenoids to egg yolk. Because mounting an immune response is costly and trade-offs are more detectable under harsh conditions, we challenged the immune system of females before laying and examined the carotenoid level of their eggs afterward. A positive association between feather carotenoid chroma and egg carotenoid level would be expected if yellow colouration signals basal immunity. Alternatively, if female colouration more generally reflects maternal capacity to invest in reproduction under challenging conditions, then other components of colouration (i.e. yellow brightness and UV/blue colouration) could be linked to maternal capacity to invest in eggs. No association between egg carotenoid levels and UV/blue crown colouration or female yellow chest chroma was found; the latter result suggests that yellow colouration does not signal immune capacity at laying in this species. By contrast, we found that, among females that mounted a detectable response to the vaccine, those with brighter yellow chests transmitted more carotenoids into their eggs. This result suggests yellow brightness signals maternal capacity to invest in reproduction under challenging conditions, and that male blue tits may benefit directly from choosing brighter yellow females.

Eulemur, me lemur: the evolution of scent-signal complexity in a primate clade

Signal complexity has been linked to social complexity in vocal, but not chemical, communication. To address this gap, we examined the chemical complexity of male and female glandular secretions in eight species of Eulemur. In this diverse clade of macrosmatic primates, species differ by social or mating system and dominance structure. We applied principal component and linear discriminate analyses to data obtained by gas chromatography/mass spectrometry. Beyond the significant effects on chemical signals of gland type, sex, season and species, we found effects of social variables and phylogeny. Notably, female odours were more chemically complex in multimale-multifemale species than pair-bonded species, whereas male odours were more chemically complex in codominant species than female-dominant species. Also, the traditional sexual dimorphism, whereby male signal complexity exceeds that of females, was present in codominant species, but reversed in female-dominant species. Lastly, a positive relationship between the species' pairwise chemical distances and their pairwise phylogenetic distances supported a gradual, but relatively fast mode of signal evolution. We suggest that the comparative method can be a powerful tool in olfactory research, revealing species differences relevant to the understanding of current signal utility and evolutionary processes. In particular, social complexity in lemurs may have selected for olfactory complexity.

Sight or scent: lemur sensory reliance in detecting food quality varies with feeding ecology

Visual and olfactory cues provide important information to foragers, yet we know little about species differences in sensory reliance during food selection. In a series of experimental foraging studies, we examined the relative reliance on vision versus olfaction in three diurnal, primate species with diverse feeding ecologies, including folivorous Coquerel's sifakas (Propithecus coquereli), frugivorous ruffed lemurs (Varecia variegata spp), and generalist ring-tailed lemurs (Lemur catta). We used animals with known color-vision status and foods for which different maturation stages (and hence quality) produce distinct visual and olfactory cues (the latter determined chemically). We first showed that lemurs preferentially selected high-quality foods over low-quality foods when visual and olfactory cues were simultaneously available for both food types. Next, using a novel apparatus in a series of discrimination trials, we either manipulated food quality (while holding sensory cues constant) or manipulated sensory cues (while holding food quality constant). Among our study subjects that showed relatively strong preferences for high-quality foods, folivores required both sensory cues combined to reliably identify their preferred foods, whereas generalists could identify their preferred foods using either cue alone, and frugivores could identify their preferred foods using olfactory, but not visual, cues alone. Moreover, when only high-quality foods were available, folivores and generalists used visual rather than olfactory cues to select food, whereas frugivores used both cue types equally. Lastly, individuals in all three of the study species predominantly relied on sight when choosing between low-quality foods, but species differed in the strength of their sensory biases. Our results generally emphasize visual over olfactory reliance in foraging lemurs, but we suggest that the relative sensory reliance of animals may vary with their feeding ecology.

Female plumage coloration is sensitive to the cost of reproduction. An experiment in blue tits

1. A growing number of studies suggest that female ornaments are linked to maternal quality and influence male mate choice. These findings challenge the traditional male-biased view of sexual selection and the hypothesis that female ornaments are the outcome of a genetic correlation with male ornaments. To further test the hypothesis that female traits have a function, it is now essential to investigate their honesty and to determine how signalling and reproduction interact in females. If female traits are honest indicators of quality, then they are likely to have a specific signalling function. 2. We investigated whether carry-over effects of reproduction might ensure the honesty of plumage colour signalling of a bird species with conspicuous UV-blue and yellow coloration, the blue tit Cyanistes caeruleus. Reproductive effort was manipulated by removing clutches, thereby forcing both sexes to reproduce twice and to raise chicks later in the breeding season when food is less abundant. In the year following this manipulation, we investigated the change in plumage in experimental and control males and females. The change was measured in the two putative feather ornaments, the UV-blue cap and the yellow breast, and another feather trait probably less likely to be sexually selected: the wing length. We also tested whether higher-quality females had their coloration less affected by the experiment. 3. We found that control but not manipulated males and females increased their signal towards UV. In addition, in the manipulated group, females that were able to lay more eggs had their UV-blue coloration less affected by the treatment. For yellow coloration, we found that manipulated yearlings but not manipulated adults decreased their yellow chroma in comparison with control. Lastly, our results show that the condition of the manipulated females tended to be positively correlated with yellow chroma. 4. These results show that the trade-offs between reproduction and signalling can ensure the honesty of conspicuous plumage traits in female and male blue tits. In addition, they suggest that female traits have the potential to evolve under sexual selection in this and other bird species.

Night and day: the comparative study of strepsirrhine primates reveals socioecological and phylogenetic patterns in olfactory signals

Studies of chemical signals in vertebrates typically target single species; however, a broader understanding of olfactory communication may derive from comparative studies. We collected urine from 12 species representing most families of strepsirrhine primates--an excellent model clade because of variation in scent marking and socioecology. Using SPDE/GC-MS, we identified the volatile chemical composition of male and female urine from six 'urine marking' species and six glandular or 'non-urine marking' species. We found no sex differences, but as predicted, urine markers expressed the most chemically complex and distinctive urine. More distantly related species had more dissimilar urinary profiles, suggesting gradual signal evolution. Reconstructing ancestral chemical profiles revealed different evolutionary trajectories for urine and non-urine markers. We suggest that urine marking is an ancestral behaviour related to solitary, nocturnal living and that parallel evolutionary shifts towards greater reliance on derived glandular marking occurred in a family (Lemuridae) characterized by diurnality and sociality.

Smelling wrong: hormonal contraception in lemurs alters critical female odour cues

Animals, including humans, use olfaction to assess potential social and sexual partners. Although hormones modulate olfactory cues, we know little about whether contraception affects semiochemical signals and, ultimately, mate choice. We examined the effects of a common contraceptive, medroxyprogesterone acetate (MPA), on the olfactory cues of female ring-tailed lemurs (Lemur catta), and the behavioural response these cues generated in male conspecifics. The genital odorants of contracepted females were dramatically altered, falling well outside the range of normal female variation: MPA decreased the richness and modified the relative abundances of volatile chemicals expressed in labial secretions. Comparisons between treatment groups revealed several indicator compounds that could reliably signal female reproductive status to conspecifics. MPA also changed a female's individual chemical 'signature', while minimizing her chemical distinctiveness relative to other contracepted females. Most remarkably, MPA degraded the chemical patterns that encode honest information about genetic constitution, including individual diversity (heterozygosity) and pairwise relatedness to conspecifics. Lastly, males preferentially investigated the odorants of intact over contracepted females, clearly distinguishing those with immediate reproductive potential. By altering the olfactory cues that signal fertility, individuality, genetic quality and relatedness, contraceptives may disrupt intraspecific interactions in primates, including those relevant to kin recognition and mate choice.