Patterns of female dominance inPropithecus diadema edwardsi of Ranomafana national park, Madagascar

Evidence for female dominance in the Milne-Edwards' sportive lemur (Lepilemur edwardsi)

Female dominance over males is more frequent in Malagasy lemurs than in other primate clades, but patterns of female-male dominance vary among species, and few data are available for one particularly species-rich genus, Lepilemur. We investigated the types, temporal distribution, and outcome of female-male agonistic conflicts in the Milne-Edwards' sportive lemur (Lepilemur edwardsi). Ten L. edwardsi belonging to five social units were equipped with radio collars and observed during 79 focal follows between May and November 1998. We quantified agonistic conflicts, monthly conflict rates, and documented the winner and context for all conflicts. Female-male agonistic conflicts (N = 162) occurred at a mean rate of 0.21 conflicts/hour within groups. Agonistic conflicts peaked during the birth season and occurred mostly in unknown or infant proximity contexts. Females won 96% of all decided agonistic conflicts (N = 154). Other outcomes occurred only when females were with infants during the birth season. In that context, one female sometimes withdrew from her pair partner, and another female withdrew or fled from an extra-group male whose attacks eventually led to infanticide. Our results suggest female dominance in pair-living L. edwardsi. We hypothesize that elevated female aggression may convey fitness benefits to female Lepilemur because it generally allows females to intervene quickly and efficiently on behalf of their infants against nonfather males who may approach the newborn infants when they are parked in vegetation.

Food availability and male deference in the female-dominant ring-tailed lemur, Lemur catta

Female dominance, a trait common to some Malagasy lemurs, has been viewed as an adaptation that decreases intersexual feeding competition. A hypothesized relationship exists between male “deference” (male submission in the absence of female aggression) and food availability. Sauther (1993) suggested that male ring‐tailed lemurs at the Duke Lemur Center (Pereira et al., 1990) show more deference to females than do males in the wild owing to food abundance in captivity. To reexamine the link between food availability and male deference, we studied agonism and foraging in two nonwild ring‐tailed lemur (Lemur catta) populations: the Los Angeles Zoo and St. Catherines Island (SCI). On SCI, we collected data under two feeding conditions: Low Provisions (low food availability) and High Provisions (high food availability). As expected, male deference measures at our study sites were more similar to measures of deference from other studies of L. catta in captivity than in the wild. Additionally, the change at SCI from low to high food availability was associated with increased male deference to females. Interestingly, male proximity to females during foraging at this location did not notably change between the low to high food availability conditions, suggesting that males were food competitors of females just as often under both feeding conditions. The increase in male deference under conditions of high food availability on SCI was due to males withdrawing more rapidly from female approaches during agonistic interactions. Hence, where food is more abundant, male L. catta are more likely to show submission to females, which appears to be a self‐serving means of avoiding female aggression. Lemur males who are well‐fed appear less apt to risk female aggression to obtain resources than more nutritionally stressed males. Our results support the view of female dominance in lemurs as an adaptive evolutionary response to conditions of resource limitation.

As has been hypothesized for this female‐dominant species, male ring‐tailed lemurs act more submissively toward females under conditions of greater food abundance.

Males more rapidly withdraw from approaches by females under conditions of greater food availability.

Male “deference” to females in this species appears to be self‐serving avoidance of aggression by males, not investment in the female.

Variation in Female Leverage: The Influence of Kinship and Market Effects on the Extent of Female Power Over Males in Verreaux’s Sifaka

Female mammals employ reproductive strategies (e.g., internal gestation) that result in power asymmetries specific to intersexual dyads. Because the number of eggs available for fertilization at any given time for most mammals is quite limited, having a fertilizable egg is potentially an important source of economic power for females. Control over mating opportunities is a source of intersexual leverage for female Verreaux’s sifaka (Propithecus verreauxi). We examined economic factors thought to influence the value of mating opportunities, and, thus, the extent of female leverage: kinship and market effects. Using a longitudinal dataset of agonistic interactions collected during focal animal sampling of all adult individuals in 10 social groups from 2008 to 2019, we tested the effects of relatedness, female parity, reproductive season, and adult sex ratio (population and group) on (1) the direction of submissive signaling and (2) which sex won a contested resource. While 96% of the acts of submission were directed from males toward females, females only won a third of their conflicts with males. Thus, our study has implications for evolutionary explanations of female-biased power. If female power evolved due to their greater need for food and other resources, then intersexual conflicts would be expected to result in males more consistently relinquishing control of resources. As expected, males were more likely to chatter submissively toward successful mothers, during the mating season, and when the sex ratio was male-biased. Although females generally had less power to win a conflict when their fertilizable egg was less valuable (when they were nulliparous or unsuccessful mothers or when interacting with male kin) and with an increasing female-bias in the sex ratio, this ability to win additionally was influenced by which sex initiated the conflict. Our study demonstrates that female leverage can be influenced by the supply and demand for mating opportunities, but evoking submission does not translate into winning a resource. Indeed, intersexual power is dynamic, contextual, and dependent on the individuals in the dyad.

The Island of Female Power? Intersexual Dominance Relationships in the Lemurs of Madagascar

The extant primates of Madagascar (Lemuriformes) represent the endpoints of an adaptive radiation following a single colonization event more than 50 million years ago. They have since evolved a diversity of life history traits, ecological adaptations and social systems that rivals that of all other living primates combined. Their social systems are characterized by a unique combination of traits, including the ability of adult females to dominate adult males. In fact, there is no other group of mammals in which female dominance is so widespread. Yet, recent research has indicated that there is more interspecific variation in lemur intersexual relationships than previously acknowledged. Here, we therefore review and summarize the relevant literature, quantifying the extent of sex-bias in intersexual dominance relations documented in observational and experimental studies in captivity and the wild. Female dominance is often, but not always, implemented by spontaneous male submission in the absence of female aggression and linked to female sexual maturation. We connect the available evidence to the hypotheses that have been proposed to explain the evolution of female dominance among lemurs. The occurrence of female dominance in all lemur families and the interspecific variation in its extent indicate that it has evolved soon after lemurs colonized Madagascar – presumably in response to particular ecological challenges – and that it has since been reduced in magnitude independently in some taxa. Our study contributes important comparative information on sex roles from an independent primate radiation and provides general insights into the conditions, opportunities and obstacles in the evolution of female-biased power.

The ontogeny of sexual dimorphism in free-ranging rhesus macaques

Objective

Reconstructing the social lives of extinct primates is possible only through an understanding of the interplay between morphology, sexual selection pressures, and social behavior in extant species. Somatic sexual dimorphism is an important variable in primate evolution, in part because of the clear relationship between the strength and mechanisms of sexual selection and the degree of dimorphism. Here, we examine body size dimorphism across ontogeny in male and female rhesus macaques to assess whether it is primarily achieved via bimaturism as predicted by a polygynandrous mating system, faster male growth indicating polygyny, or both.

Methods

We measured body mass in a cross-sectional sample of 364 free-ranging rhesus macaques from Cayo Santiago, Puerto Rico to investigate size dimorphism: 1) across the lifespan; and 2) as an outcome of sex-specific growth strategies, including: a) age of maturation; b) growth rate; and c) total growth duration, using regression models fit to sex-specific developmental curves.

Results

Significant body size dimorphism was observed by prime reproductive age with males 1.51 times the size of females. Larger male size resulted from a later age of maturation (males: 6.8-7.8 years versus females: 5.5-6.5 years; logistic model) and elevated growth velocity through the pre-prime period (LOESS model). Though males grew to larger sizes overall, females maintained adult size for longer before senescence (quadratic model).

Discussion

The ontogeny of size dimorphism in rhesus macaques is achieved by bimaturism and a faster male growth rate. Our results provide new data for understanding the development and complexities of primate dimorphism.

Female rule in lemurs is ancestral and hormonally mediated

Female social dominance (FSD) over males is unusual in mammals, yet characterizes most Malagasy lemurs, which represent almost 30% of all primates. Despite its prevalence in this suborder, both the evolutionary trajectory and proximate mechanism of FSD remain unclear. Potentially associated with FSD is a suite of behavioural, physiological and morphological traits in females that implicates (as a putative mechanism) 'masculinization' via androgen exposure; however, relative to conspecific males, female lemurs curiously show little evidence of raised androgen concentrations. By observing mixed-sex pairs of related Eulemur species, we identified two key study groups--one comprised of species expressing FSD and increased female scent marking, the other comprised of species (from a recently evolved clade) showing equal status between the sexes and the more traditional pattern of sexually dimorphic behaviour. Comparing females from these two groups, we show that FSD is associated with more masculine androgen profiles. Based on the widespread prevalence of male-like features in female lemurs and a current phylogeny, we suggest that relaxation of hormonally mediated FSD emerged only recently and that female masculinization may be the ancestral lemur condition, an idea that could revolutionize our understanding of the ancient socioecology and evolution of primate social systems.

Neurophysiological mechanisms underlying affiliative social behavior: insights from comparative research

Humans are intensely social animals, and healthy social relationships are vital for proper mental health (see Lim and Young, 2006). By using animal models, the behavior, mental, and physiological processes of humans can be understood at a level that cannot be attained by studying human behavior and the human brain alone. The goals of this review are threefold. First, we define affiliative social behavior and describe the primary relationship types in which affiliative relationships are most readily observed--the mother-infant bond and pair-bonding. Second, we summarize neurophysiological studies that have investigated the role of neurohypophyseal nanopeptides (oxytocin and vasopressin) and the catecholamine dopamine in regulating affiliative social behavior and the implications of said research for our understanding of human social behavior. Finally, we discuss the merits and limitations of the using a comparative approach to enhance our understanding of the mechanisms underlying human affiliative social behavior.

Dominance as adaptive stressing and ranking of males, serving to allocate reproduction by differential self-suppressed fertility: towards a fully biological understanding of social systems

Dominance is a biological concept of an asymmetric 'power' relationship between (any pair of) individuals, as a result of previous encounters with others biasing likelihood of contesting. That this requires dedicated neural structure shows that dominance is adaptive; and it is usually thought that fitness is increased through dominance (hierarchy) minimising mutually unproductive contest over resources, and/or determining access to or control over resources. But highly inconsistent data indicates that this operational definition is too wide, and given clear evidence that dominance is invariably same-sex, it would seem instead to function primarily to allocate reproduction. Dominance contest exposes individual differences in metabolic vigor especially, but also in various other, including sophisticated attributes; and by a self-organising process there is ranking of same-sex individuals in a hierarchy. But this achieves nothing in itself without an integral mechanism of corresponding individual variable self-suppression of the physiology re reproduction--and mate choice with rank as the criteria. Reproductive suppression would appear to vary along a continuum, from in some species (most 'cooperative breeders') a 100% reproductive skew with total suppression of all individuals bar the sole breeder to, in most others, a gradient down the length of the dominance hierarchy. The mechanism in most species is directly either hormonal or pheromonal, on top of an indirect consequence of the stress caused by relatively low rank. Dominance would seem to have evolved as a major instrument of the proposed 'genetic filter' function of the male, whereby in effect accumulated deleterious genetic material is 'quarantined' in the male half of the lineage from where it is purged, so as to keep this source of reproductive logjam away from females, thereby to avoid amplifying the problem of the female being necessarily the limiting factor in reproduction. The theory makes predictions mutually exclusive of the consensus model, that dominance/DH is: same-sex only; present whenever, within one or both sexes, there is potential conflict over reproduction, and there is no mechanism to preclude this, but otherwise is absent; always associated with some degree of differential physiological reproductive suppression. This new conceptualization of dominance has major implications for the social as well as biological sciences, in that resource-competition models of the basis of sociality will have to give way to a thoroughgoing biological understanding that places centre-stage not resources but reproduction; with consequent radical revision of notions of 'power'.

Chest staining variation as a signal of testosterone levels in male Verreaux's Sifaka

Male Verreaux's sifaka (Propithecus verreauxi) exhibit variation in the staining of chest hair in association with the activity of the sternal gland. Scent-marking behavior and social relationships have been shown to vary with the state of chest staining. Research on other mammals suggests that sternal gland activity is modulated by testosterone. The goal of this study was to examine the relationship among fecal testosterone, testes mass, and chest staining in sifaka. I predicted that for sifaka (1) testosterone and testes mass are positively correlated, (2) stained males have higher testosterone and larger testes than clean-chested males, (3) testes mass is greater during the mating season than the birth season, and (4) testes mass is comparable to species with multimale mating systems. Eighty fecal samples were collected from eight adult males in five social groups in the Kirindy Mitea National Park of Madagascar during the mating season and analyzed using enzyme immunoassays. Males were captured during the mating season and the subsequent birth season to measure body mass and testes size and to document chest staining. Stained males had significantly higher fT and larger testes mass than clean-chested males. Testes mass was significantly greater during the mating season than the birth season for all males. However, the stained males exhibited less testes mass reduction during birth season than clean-chested males. These results suggest that the activity of the sternal gland is regulated by testosterone and that sexual selection has shaped male morphological variation in Verreaux's sifaka.

Female dominance over males in primates: self-organisation and sexual dimorphism

The processes that underlie the formation of the dominance hierarchy in a group are since long under debate. Models of self-organisation suggest that dominance hierarchies develop by the self-reinforcing effects of winning and losing fights (the so-called winner-loser effect), but according to 'the prior attribute hypothesis', dominance hierarchies develop from pre-existing individual differences, such as in body mass. In the present paper, we investigate the relevance of each of these two theories for the degree of female dominance over males. We investigate this in a correlative study in which we compare female dominance between groups of 22 species throughout the primate order. In our study female dominance may range from 0 (no female dominance) to 1 (complete female dominance). As regards 'the prior attribute hypothesis', we expected a negative correlation between female dominance over males and species-specific sexual dimorphism in body mass. However, to our surprise we found none (we use the method of independent contrasts). Instead, we confirm the self-organisation hypothesis: our model based on the winner-loser effect predicts that female dominance over males increases with the percentage of males in the group. We confirm this pattern at several levels in empirical data (among groups of a single species and between species of the same genus and of different ones). Since the winner-loser effect has been shown to work in many taxa including humans, these results may have broad implications.

Female feeding priority in bonobos,Pan paniscus, and the question of female dominance

The question of whether bonobos show feeding priority and female dominance has been proposed and examined, both in the wild and in captive studies, with differing results. The relationship between female dominance and female feeding priority has been best studied in prosimian primates. These studies use established criteria of females consistently evoking submissive behavior from males in dyadic encounters for determining female dominance. Although the relationship is complex, female dominance in prosimians is associated with preferential access to food. Data from studies of wild habituated bonobos in the Lomako Forest, Democratic Republic of the Congo, are examined for evidence of both female feeding priority and female social dominance using similar criteria as used for prosimians. Bonobos showed evidence of female feeding priority in small, but not in large, food patches. Male-male competition for mating opportunities at the start of the food bout was related to some, but not all, differences in time spent feeding between the sexes. Female dominance similar to that seen in prosimians was not observed in these bonobos. Males were consistently dominant in dyadic interactions. Female feeding priority with male dyadic social dominance implies that male deference during feeding cannot be excluded as one explanation of interpretations of female dominance in bonobos. Additionally, dominance of male bonobos by females appears to require the presence of female coalition partners. As in other primates with female feeding priority, bonobo females express this trait where food is economically defendable. Unlike prosimians, however, bonobo female feeding priority may result from male deference and the importance of female coalitions in nondyadic interactions.

Elevated testosterone levels and social ranks in female rock hyrax

In mammals, males maintain significantly higher testosterone (T, 'the male hormone') levels than females throughout the year and are typically dominant over females. Our study of the cooperatively breeding rock hyrax (Procavia capensis) demonstrated a distinct exception. In most hyrax social groups that we studied, adult females were at the highest social rank within the group and showed higher than or equal T levels to males. To our knowledge, this is the first reported instance of adult female mammals demonstrating higher T levels than adult males. However, although T levels significantly correlated with rank in males, in females such correlations were not detected, suggesting a more complex interplay between behavior and endocrine factors in this species.

Sex differences in scent marking in Propithecus edwardsi of Ranomafana National Park, Madagascar

In this study we compared the scent-marking rates of females with those of males. Specifically, we examined the ability of season, dominance status, and natal status to explain the frequency of scent marking in female sifakas living wild in Ranomafana National Park, Madagascar, and compared the results with those published for males [Pochron et al., American Journal of Primatology, in press]. We also sought to determine whether vulnerability to infanticide affects marking frequency in adults of either sex, and whether female reproductive status affects female marking behavior. We found that males marked at twice the rate of females, and like males, females in single-female groups marked at the highest rates. Dominant females and non-natal females marked at higher rates than did subordinate females and natal females, a pattern also seen in males. This suggests that scent marks may convey important information about status. Neither females nor males varied their marking frequency with the presence of vulnerable infants. Females did not alter marking frequency with reproductive state, and like males, they marked at higher rates in the period prior to the mating season than they did in the mating season itself. This implies that females may use scent marks more for intrasexual aggression or territoriality than for mate attraction.

Patterns of male scent-marking in Propithecus edwardsi of Ranomafana National Park, Madagascar

Scent-marking behavior has been well documented in many primate species. Three common functions attributed to scent-marking in males of multi-male/multi-female lemur species include: 1) advertisement of individual identity, 2) territorial defense, and 3) reproductive suppression. We examined the average number of scent-marks per hour exhibited daily by adult male sifakas (Propithecus edwardsi) and found that patterns of scent-marking changed with season, natal status, and dominance status. Males in single-male groups scent-marked at the highest rate, followed by dominant males, males of equal status, and subordinate males. Non-natal males generally scent-marked at higher rates than natal males, and adult males living in a natal group without a parent marked at higher rates than males living with a parent. All males scent-marked at higher rates in the migration season compared to the other seasons. These patterns were consistent with territorial defense and advertisement to females, and the suggestion that these chemical signals impart information concerning status. Since scent-marking behavior tracked seasons and varied with both dominance and natal status, it may serve multiple functions in males.