Stratégies de reproduction des mâles et des femelles chez le macaque rhésus (Macaca mulatta)

Contrairement à d’autres groupes animaux, chez les primates, la hiérarchie de dominance ne détermine pas systématiquement le succès reproductif des mâles. Afin de comprendre pourquoi, j’ai étudié les stratégies de reproduction des mâles et des femelles dans un groupe de macaques rhésus de la population semi-libre de Cayo Santiago (Porto Rico), collectant des données comportementales, hormonales et génétiques pendant deux saisons de reproduction. Les résultats se résument en cinq points. 1. Les nouveaux mâles qui ont immigré dans le groupe d’étude occupaient tous les rangs les plus subordonnés de la hiérarchie de dominance et ont monté en rang suite au départ de mâles plus dominants. Ainsi, l’acquisition d’un rang supérieur s’est faite passivement, en absence de conflits. Par conséquent, les mâles dominants étaient généralement d’âge mature et avaient résidé plus longtemps dans le groupe que les mâles subordonnés. 2. L’accès des mâles aux femelles est en accord avec le « modèle de la priorité d’accès » selon lequel le nombre de femelles simultanément en œstrus détermine le rang de dominance du mâle le plus subordonné qui peut avoir accès à une femelle (p. ex. le mâle de rang 4 s’il y a quatre femelles en œstrus). Bien que les mâles dominants aient eu plus de partenaires et aient monopolisé les femelles de qualité supérieure (dominance, parité, âge) pendant leur période ovulatoire (identifiée grâce au profil hormonal de la progestérone), le rang de dominance n’a pas déterminé le succès reproductif, les mâles intermédiaires ayant engendré significativement plus de rejetons que prédit. Il est possible que ces jeunes adultes aient produit un éjaculat de meilleure qualité que les mâles dominants d’âge mature, leur donnant un avantage au niveau de la compétition spermatique. 3. Les mâles dominants préféraient les femelles dominantes, mais cette préférence n’était pas réciproque, ces femelles coopérant plutôt avec les mâles intermédiaires, plus jeunes et moins familiers (c.-à-d. courte durée de résidence). Au contraire, les femelles subordonnées ont coopéré avec les mâles dominants. La préférence des femelles pour les mâles non familiers pourrait être liée à l’attrait pour un nouveau bagage génétique. 4. L’intensité de la couleur de la peau du visage des femelles pendant le cycle ovarien était corrélée au moment de la phase ovulatoire, une information susceptible d’être utilisée par les mâles pour maximiser leur probabilité de fécondation. 5. Les femelles retiraient des bénéfices directs de leurs liaisons sexuelles. En effet, les femelles en liaison sexuelle bénéficiaient d’un niveau de tolérance plus élevé de la part de leur partenaire mâle lorsqu’elles étaient à proximité d’une source de nourriture défendable, comparativement aux autres femelles. En somme, bien que les mâles dominants aient bénéficié d’une priorité d’accès aux femelles fertiles, cela s’est avéré insuffisant pour leur garantir la fécondation de ces femelles parce que celles-ci avaient plusieurs partenaires sexuels. Il semble que l’âge et la durée de résidence des mâles, corrélats de leur mode d’acquisition du rang, aient confondu l’effet du rang de dominance.In contrast to most animal groups, dominance hierarchy does not systematically determine male reproductive success in primates. In order to investigate why, I studied male and female reproductive strategies in a group of free-ranging rhesus macaques on Cayo Santiago, Puerto Rico. I collected behavioural, genetic, and hormonal data during two consecutive mating seasons. My results are summarized below. 1. All new males who immigrated into the study group occupied the lowest-ranking position in the dominance hierarchy and rose in rank as the higher-ranking males left the group. Achieving a higher dominance rank occurred passively, without physical conflict. Thus, dominant males were mature individuals who resided longest in the group. 2. Male access to oestrus females followed the predictions of the ‘priority of access’ model, in which the number of females in oestrus determines the rank of the lowest-ranking male who can access a female (e.g. the fourth ranking male if four females are in oestrus). Even though dominant males obtained more mating partners and monopolised higher quality females (dominance, parity, age) during the ovulation window (as identified using progesterone profiles), dominance rank did not determine reproductive success, as intermediate-ranking males sired significantly more infants than predicted. It is likely that those young, intermediate-ranking adult males produced high quality ejaculate, giving them an advantage in sperm competition. 3. Dominant males preferred high-ranking females, but this preference was not reciprocal; high-ranking females cooperated with younger and less familiar intermediate-ranking males. Conversely, subordinate females cooperated with dominant males. Female preference for non-familiar males (i.e. short residency in the group) may be explained by an attraction to a novel genetic pool. 4. Female facial color intensity during the ovarian cycle was correlated with the timing of the ovulation window. This information may be used by males in order to maximize their fertilisation probability. 5. Consort females enjoyed a higher level of tolerance from their male partner when they were in proximity to a monopolisable food source, compared to other, non-consort females. This suggests that females obtained direct benefits from their sexual consorts. In conclusion, even though dominant males had priority access to ovulating females in the group, this was insufficient to guarantee fertilisation when females had several sexual partners. It appears that males’ age and length of residency, both correlates of their rank acquisition mode, may have been confounding factors in dominance rank

Contournement de l'ordre de dominance en réponse à la compétition alimentaire chez le macaque crabier (Macaca fascicularis)

Mémoire numérisé par la Direction des bibliothèques de l'Université de Montréal

Male immigration triggers increased growth in subordinate female meerkats.

There is increasing evidence that some vertebrates can adjust their growth rate in relation to changes in the social context that affect their probability of breeding. Here, we show that, in meerkats (Suricata suricatta), which are singular cooperative breeders, subordinate females increase in body mass after their father is replaced as the dominant male in their natal group by an immigrant male, giving them regular access to an unfamiliar and unrelated mating partner, while their brothers showed no similar increase nor did subordinate females living in other stable groups (where male immigration did not occur did) in this time period. Moreover, subordinate females showed a greater increase in growth rate when their father was succeeded by an unfamiliar immigrant male than when he was replaced by a familiar male who was already resident. These results suggest that female meerkats can adjust their rate of growth to changes in the kinship composition of their groups that provide them with increased access to unrelated breeding partners, which may occur in other mammals as well when breeding opportunities change.Natural Environment Research Council European Research Counci

Experimental evidence that female rhesus macaques (Macaca mulatta) perceive variation in male facial masculinity

Among many primate species, face shape is sexually dimorphic, and male facial masculinity has been proposed to influence female mate choice and male-male competition. However, whether conspecifics pay attention to facial masculinity has only been assessed in humans. Here, working with free-ranging rhesus macaques, Macaca mulatta, we used a two-alternative look-time experiment to test whether females perceive male facial masculinity. We presented 107 females with pairs of images of male faces – one more masculine and one more feminine – and recorded their looking behaviour. Females looked toward the masculine face longer than the feminine face in more trials than predicted by chance. Although there was no overall difference in average look-time between masculine and feminine faces across all trials, females looked significantly longer at masculine faces in a subset of trials for which the within-pair difference in masculinity was most pronounced. Additionally, the proportion of time subjects looked toward the masculine face increased as the within-pair difference in masculinity increased. This study provides evidence that female macaques perceive variation in male facial shape, a necessary condition for intersexual selection to operate on such a trait. It also highlights the potential impact of perceptual thresholds on look-time experiments

Effect of Mating Activity and Dominance Rank on Male Masturbation Among Free-Ranging Male Rhesus Macaques

Abstract The adaptive function of male masturbation is still poorly understood, despite its high prevalence in humans and other animals. In non-human primates, male masturbation is most frequent among anthropoid monkeys and apes living in multimale-multifemale groups with a promiscuous mating system. In these species, male masturbation may be a non-functional by-product of high sexual arousal or be adaptive by providing advantages in terms of sperm competition or by decreasing the risk of sexually transmitted infections. We investigated the possible functional significance of male masturbation using behavioral data collected on 21 free-ranging male rhesus macaques (Macaca mulatta) at the peak of the mating season. We found some evidence that masturbation is linked to low mating opportunities: regardless of rank, males were most likely to be observed masturbating on days in which they were not observed mating, and lower-ranking males mated less and tended to masturbate more frequently than higher-ranking males. These results echo the findings obtained for two other species of macaques, but contrast those obtained in red colobus monkeys (Procolobus badius) and Cape ground squirrels (Xerus inauris). Interestingly, however, male masturbation events ended with ejaculation in only 15% of the observed masturbation time, suggesting that new hypotheses are needed to explain masturbation in this species. More studies are needed to establish whether male masturbation is adaptive and whether it serves similar or different functions in different sexually promiscuous species

Female ornaments: is red skin color attractive to males and related to condition in rhesus macaques?

Sexual selection produces extravagant male traits, such as colorful ornaments, via female mate choice. More rarely, in mating systems in which males allocate mating effort between multiple females, female ornaments may evolve via male mate choice. Females of many anthropoid primates exhibit ornaments that indicate intraindividual cyclical fertility, but which have also been proposed to function as interindividual quality signals. Rhesus macaque females are one such species, exhibiting cyclical facial color variation that indicates ovulatory status, but in which the function of interindividual variation is unknown. We collected digital images of the faces of 32 rhesus macaque adult females. We assessed mating rates, and consortship by males, according to female face coloration. We also assessed whether female coloration was linked to physical (skinfold fat, body mass index) or physiological (fecal glucocorticoid metabolite [fGCM], urinary C-peptide concentrations) condition. We found that redder-faced females were mated more frequently, and consorted for longer periods by top-ranked males. Redder females had higher fGCM concentrations, perhaps related to their increased mating activity and consequent energy mobilization, and blood flow. Prior analyses have shown that female facial redness is a heritable trait, and that redderfaced females have higher annual fecundity, while other evidence suggests that color expression is likely to be a signal rather than a cue. Collectively, the available evidence suggests that female coloration has evolved at least in part via male mate choice. Its evolution as a sexually selected ornament attractive to males is probably attributable to the high female reproductive synchrony found in this species

The influence of stress hormones and aggression on cooperative behaviour in subordinate meerkats

In cooperative breeders, aggression from dominant breeders directed at subordinates may raise subordinate stress hormone (glucocorticoid) concentrations. This may benefit dominants by suppressing subordinate reproduction but it is uncertain whether aggression from dominants can elevate subordinate cooperative behaviour, or how resulting changes in subordinate glucocorticoid concentrations affect their cooperative behaviour. We show here that the effects of manipulating glucocorticoid concentrations in wild meerkats (Suricata suricatta) on cooperative behaviour varied between cooperative activities as well as between the sexes. Subordinates of both sexes treated with a glucocorticoid receptor antagonist (mifepristone) exhibited significantly more pup protection behaviour (babysitting) compared to those treated with glucocorticoids (cortisol) or controls. Females treated with mifepristone had a higher probability of exhibiting pup food provisioning (pup-feeding) compared to those treated with cortisol. In males, there were no treatment effects on the probability of pup-feeding, but those treated with cortisol gave a higher proportion of the food they found to pups than those treated with mifepristone. Using 19 years of behavioural data, we also show that dominant females did not increase the frequency with which they directed aggression at subordinates at times when the need for assistance was highest. Our results suggest that it is unlikely that dominant females manipulate the cooperative behaviour of subordinates through the effects of aggression on their glucocorticoid levels and that the function of aggression directed at subordinates is probably to reduce the probability they will breed

Assessing dominance hierarchies: validation and advantages of progressive evaluation with Elo-rating

Highlights ► Elo-rating generates reliable dominance hierarchies and circumvents drawbacks of established ranking methods. ► It allows visualizing dominance relationships and the detection of rank dynamics. ► An index to objectively assess the stability of a dominance hierarchy is proposed

Testing the priority-of-access model in a seasonally breeding primate species

In mammals, when females are clumped in space, male access to receptive females is usually determined by a dominance hierarchy based on fighting ability. In polygynandrous primates, as opposed to most mammalian species, the strength of the relationship between male social status and reproductive success varies greatly. It has been proposed that the degree to which paternity is determined by male rank decreases with increasing female reproductive synchrony. The priority-of-access model (PoA) predicts male reproductive success based on female synchrony and male dominance rank. To date, most tests of the PoA using paternity data involved nonseasonally breeding species. Here, we examine whether the PoA explains the relatively low reproductive skew in relation to dominance rank reported in the rhesus macaque, a strictly seasonal species. We collected behavioral, genetic, and hormonal data on one group of the free-ranging population on Cayo Santiago (Puerto Rico) for 2 years. The PoA correctly predicted the steepness of male reproductive skew, but not its relationship to male dominance: the most successful sire, fathering one third of the infants, was high but not top ranking. In contrast, mating success was not significantly skewed, suggesting that other mechanisms than social status contributed to male reproductive success. Dominance may be less important for paternity in rhesus macaques than in other primate species because it is reached through queuing rather than contest, leading to alpha males not necessarily being the strongest or most attractive male. More work is needed to fully elucidate the mechanisms determining paternity in rhesus macaques