Colour and contrast of female faces: attraction of attention and its dependence on male hormone status in Macaca fuscata

Assessing color cues of development, breeding status and reproductive condition in captive golden lion tamarins (Leontopithecus rosalia)

Color signals play an important role in intraspecific communication and are well studied in catarrhine primates, which exhibit uniform trichromatic vision that is well suited to detecting such signals. Platyrrhine primates exhibit polymorphic color vision with different individuals possessing different color vision types in most species. Intriguingly, some platyrrhine species exhibit bare faces, which are convergent with those of catarrhines. However, putative functions of bare-faced color signals in platyrrhines remain largely unexplored. We measured facial skin color of five captive golden lion tamarins (Leontopithecus rosalia) using color-calibrated digital photography and modeled these colors to the visual systems of the species. Our results show that facial coloration is different between infant and older adults and varies across reproductive condition, but not between breeding and nonbreeding adults. While preliminary, our study suggests that facial coloration may be involved in sociosexual signaling in golden lion tamarins, and provides intriguing evidence that we hope might stimulate more studies of bare-faced signaling in platyrrhines.

Evaluating genital skin color as a putative sexual signal in wild saddleback (Leontocebus weddelli) and emperor (Saguinus imperator) tamarins

Coevolution between signalers and receivers has played a significant role in the diversity of animal signals and sensory systems. Platyrrhines (monkeys in the Americas) exhibit a remarkable color vision polymorphism that may have been selected by both natural and sexual selection, but sociosexual color signaling among platyrrhines has received almost no attention. Here, we study the color of reproductive skin among different reproductive classes in free-ranging female saddleback (Leontocebus weddelli) and emperor (Saguinus imperator) tamarins, modeling color spaces, and contrasts for the different visual systems. We find that the chromatic saturation and luminance of genital color vary between reproductive classes in saddleback tamarins. Chromatic contrast between the vulva and belly is lower in the parous females (PFs) relative to adult but not currently breeding females, while achromatic contrast is higher in PFs in saddleback tamarins relative to nonparous females. However, in emperor tamarins, genital color (saturation, hue, and luminance) does not vary between reproductive classes. Overall, genital skin color variation is present in tamarins and may play a role in sexual signaling. Nevertheless, the patterns are inconsistent between species, suggesting interspecific variation. Future studies should integrate the perceiver's behavioral responses and the physical and social signaling environments into comprehensive studies of communication as well as consider the role and interaction between multiple sensory modalities.

Twenty-three-year demographic history of the Affenberg Japanese macaques (Macaca fuscata), a translocated semi-free-ranging group in southern Austria

Demographic studies on translocated primate groups provide a unique opportunity to study population dynamics, social strategies, and reproductive parameters of a species adapting to new environments. In 1996, 38 Japanese macaques (Macaca fuscata) of the Minoo-H group (Osaka Prefecture, Japan) were translocated to Affenberg Landskron, a four-hectare naturally forested park in southern Austria. By January 2020, the population had increased to 160 individuals, and a total of 223 births were recorded. Births peaked in late April to late May, and the timing was influenced by neither offspring sex nor parity status of the mother. Infant mortality was low (8.97%), mostly involving primiparous females, and the average interbirth intervals were shorter following the death of an infant (1.10 years) than a surviving infant (1.77 years). Females rarely had offspring with the same males repeatedly, and the reproductive success among males declined with increasing years of presence in the group. The main aspects of reproduction, mortality, and mate choice are consistent with published data on natural and provisioned populations in Japan and those translocated to other countries. The life expectancy for females, however, was relatively high (11.72% chance of reaching the age of 20), whereas birth control prevented them from using their lifetime reproductive potential. By January 2020, the number of old individuals (> 18 years; 17.5%) was close to that of juveniles (< 4 years; 22.5%). The specific group composition, along with the inability of males to emigrate out of their natal group, may affect the social dynamics of the population, which merits further attention in future studies.

Color Can Shorten Breakthrough Times in Continuous Flash Suppression through Increased Salience and Task Relevance

Color can enhance the perception of relevant stimuli by increasing their salience and guiding visual search towards stimuli that match a task-relevant color. Using Continuous Flash Suppression (CFS), the current study investigated whether color facilitates the discrimination of targets that are difficult to perceive due to interocular suppression. Gabor patterns of two or four cycles per degree (cpd) were shown as targets to the non-dominant eye of human participants. CFS masks were presented at a rate of 10 Hz to the dominant eye, and participants had the task to report the target’s orientation as soon as they could discriminate it. The 2-cpd targets were robustly suppressed and resulted in much longer response times compared to 4-cpd targets. Moreover, only for 2-cpd targets, two color-related effects were evident. First, in trials where targets and CFS masks had different colors, targets were reported faster than in trials where targets and CFS masks had the same color. Second, targets with a known color, either cyan or yellow, were reported earlier than targets whose color was randomly cyan or yellow. The results suggest that the targets’ entry to consciousness may have been speeded by color-mediated effects relating to increased (bottom-up) salience and (top-down) task relevance.

Platyrrhine color signals: New horizons to pursue

Like catarrhines, some platyrrhines show exposed and reddish skin, raising the possibility that reddish signals have evolved convergently. This variation in skin exposure and color combined with sex-linked polymorphic color vision in platyrrhines presents a unique, and yet underexplored, opportunity to investigate the relative importance of chromatic versus achromatic signals, the influence of color perception on signal evolution, and to understand primate communication broadly. By coding the facial skin exposure and color of 96 platyrrhines, 28 catarrhines, 7 strepsirrhines, 1 tarsiiform, and 13 nonprimates, and by simulating the ancestral character states for these traits, we provide the first analysis of the distribution and evolution of facial skin exposure and color in platyrrhini. We highlight ways in which studying the presence and use of color signals by platyrrhines and other primates will enhance our understanding of the evolution of color signals, and the forces shaping color vision.

A parameterized digital 3D model of the Rhesus macaque face for investigating the visual processing of social cues

BACKGROUND

Rhesus macaques are the most popular model species for studying the neural basis of visual face processing and social interaction using intracranial methods. However, the challenge of creating realistic, dynamic, and parametric macaque face stimuli has limited the experimental control and ethological validity of existing approaches.

NEW METHOD

We performed statistical analyses of in vivo computed tomography data to generate an anatomically accurate, three-dimensional representation of Rhesus macaque cranio-facial morphology. The surface structures were further edited, rigged and textured by a professional digital artist with careful reference to photographs of macaque facial expression, colouration and pelage.

RESULTS

The model offers precise, continuous, parametric control of craniofacial shape, emotional expression, head orientation, eye gaze direction, and many other parameters that can be adjusted to render either static or dynamic high-resolution faces. Example single-unit responses to such stimuli in macaque inferotemporal cortex demonstrate the value of parametric control over facial appearance and behaviours.

COMPARISON WITH EXISTING METHOD(S)

The generation of such a high-dimensionality and systematically controlled stimulus set of conspecific faces, with accurate craniofacial modelling and professional finalization of facial details, is currently not achievable using existing methods.

CONCLUSIONS

The results herald a new set of possibilities in adaptive sampling of a high-dimensional and socially meaningful feature space, thus opening the door to systematic testing of hypotheses about the abundant neural specialization for faces found in the primate.

The neuroendocrinology of sexual attraction

Sexual attraction has two components: Emission of sexually attractive stimuli and responsiveness to these stimuli. In rodents, olfactory stimuli are necessary but not sufficient for attraction. We argue that body odors are far superior to odors from excreta (urine, feces) as sexual attractants. Body odors are produced by sebaceous glands all over the body surface and in specialized glands. In primates, visual stimuli, for example the sexual skin, are more important than olfactory. The role of gonadal hormones for the production of and responsiveness to odorants is well established. Both the androgen and the estrogen receptor α are important in male as well as in female rodents. Also in primates, gonadal hormones are necessary for the responsiveness to sexual attractants. In males, the androgen receptor is sufficient for sustaining responsiveness. In female non-human primates, estrogens are needed, whereas androgens seem to contribute to responsiveness in women.

Are Men's Perceptions of Sexually Dimorphic Vocal Characteristics Related to Their Testosterone Levels?

Feminine physical characteristics in women are positively correlated with markers of their mate quality. Previous research on men’s judgments of women’s facial attractiveness suggests that men show stronger preferences for feminine characteristics in women’s faces when their own testosterone levels are relatively high. Such results could reflect stronger preferences for high quality mates when mating motivation is strong and/or following success in male-male competition. Given these findings, the current study investigated whether a similar effect of testosterone occurs for men’s preferences for feminine characteristics in women’s voices. Men’s preferences for feminized versus masculinized versions of women’s and men’s voices were assessed in five weekly test sessions and saliva samples were collected in each test session. Analyses showed no relationship between men’s voice preferences and their testosterone levels. Men’s tendency to perceive masculinized men’s and women’s voices as more dominant was also unrelated to their testosterone levels. Together, the results of the current study suggest that testosterone-linked changes in responses to sexually dimorphic characteristics previously reported for men's perceptions of faces do not occur for men's perceptions of voices.

Allelic variation of the COMT gene in a despotic primate society: A haplotype is related to cortisol excretion in Macaca fuscata

Sequence variations in genes of the monoamine neurotransmitter system and their common function in human and non-human primate species are an ongoing issue of investigation. However, the COMT gene, coding for the catechol-O-methyltransferase, has not yet attracted much scientific attention regarding its functional role in non-human primates. Considering that a polymorphism of the human COMT gene affects the enzyme activity and cortisol level in response to a social stressor, this study investigated the impact of COMT on endocrine stress and behavioural parameters in Japanese macaques (Macaca fuscata). The species exemplifies a despotic hierarchy in which males' social rank positions require an adaptation of behaviour strategies. During the mating period steroid secretion and the frequency of aggressive encounters between males increase. We addressed i) whether this species exhibits potential functional COMT variants, ii) whether these variants are associated with faecal cortisol excretion of males, iii) how they are distributed among different social rank positions and iv) whether they are associated with behavioural strategies during times of mate competition. By genotyping 26 males we identified three COMT haplotypes (HT), including a putative splice mutant (HT3). This variant was associated with increased cortisol excretion. Given the observed inverse correlation between cortisol and physical aggression, we assume that different COMT haplotypes may predispose individuals to pursue more or less aggressive strategies. How these gene-stress effects might favour a specific social role is discussed. Our study of non-invasive genotyping in combination with behavioural and endocrine parameters represents an important step towards the understanding of gene-stress effects in a hierarchically organised primate society.

Multimodal Advertisement of Pregnancy in Free-Ranging Female Japanese Macaques (Macaca fuscata)

The role of multiple sexual signals in indicating the timing of female ovulation, and discrimination of this timing by males, has been particularly well studied among primates. However the exhibition of pregnancy signals, and how such signals might modulate male post-conception mating decisions, is still poorly understood. Here we aimed to determine if Japanese macaque males use changes in female sexual signals (behavioral, visual and auditory) to discriminate pregnancy and adjust their socio-sexual behaviors. We combined behavioral observations, digital photography and endocrinological (progestogen and estrogen) data, collected systematically during three one-month periods: the pre-conceptive period, the 1st month of pregnancy and the 2nd month of pregnancy. We analyzed variation in the probability of detecting male and female socio-sexual behaviors and estrus calls, as well as changes in female face color parameters, in relation to female reproductive state. Based on our focal observations, we found that males did not copulate during the pregnancy period, and that female socio-sexual behaviors generally decreased from the pre-conceptive to post-conceptive periods. Female face luminance decreased from the pre-conceptive month to the pregnancy period whereas face color only varied between the 1st and 2nd month of gestation. Our results suggest that Japanese macaque females display sexual cues of pregnancy that males might use to reduce energy wasted on non-reproductive copulations with pregnant females. We hypothesize that females advertize their pregnancy through changes in behavioral, visual and potential auditory signals that males can use to adjust their mating behaviors. We finish by discussing implications for male and female post-conception strategies.