Sex differences in infant rhesus macaque separation-rejection vocalizations and effects of prenatal androgens

Infant and juvenile rhesus macaques exhibit many sexually dimorphic behaviors, including rough and tumble play, mounting, and time spent with nonmother females. This study investigated sex differences in infant rhesus monkey separation-rejection vocalizations (SRVs), and the effects of altering the prenatal hormone environment on these differences. Pregnant females received exogenous androgen (testosterone enanthate), an androgen antagonist (flutamide), or vehicle injections for 30 or 35 days during the second (early) or third (late) trimester of pregnancy. Control females used a greater percentage of coos and arched screams than did control males. In contrast, males used a greater percentage of geckers and noisy screams than did females. Females also had longer SRV bouts, used more calls, and used more types of vocalizations than did males. Mothers were more likely to respond to the SRVs of male infants than to the SRVs of female infants. Prenatal flutamide treatment early in gestation reduced the likelihood that mothers would respond to their male offspring, but prenatal androgen treatment had no effect on response rates of mothers to female offspring. Early, but not late, androgen treatment produced females who vocalized in a male-typical manner. Similarly, early flutamide treatment produced males who displayed more female-typical SRVs. Late flutamide treatments of females produced as much masculinization of SRVs as did early androgen treatment in females. These results demonstrate sex differences in highly emotional vocalizations in infant rhesus macaques and provide evidence that the timing and form of prenatal hormonal exposure influence such vocalizations.     

Sex differences in interest in infants in juvenile rhesus monkeys: relationship to prenatal androgen

Yearling rhesus monkey females interact more with infants than do males. However, the continuity of this sex difference throughout the juvenile period is unknown. Human females display similar sexually differentiated interest in infants, and studies of girls with congenital adrenal hyperplasia suggest that this sex difference may be modulated by prenatal androgen exposure. We investigated infant interest in 1- to 3-year-old juvenile rhesus monkeys. Hormonal influences on this behavior were investigated by treating pregnant females with an androgen-receptor blocker (flutamide), testosterone enanthate, or vehicle, early or late in gestation. Subjects were reared in their well-established natal groups, composed of species-typical matrilineal social structures, including members of all ages. Yearling control females interacted with infants more than did yearling control males. At 2 and 3 years of age, the magnitude of the sex difference in interactions with infants increased markedly, producing effect sizes of more than 2.5 standard deviations. These effects are larger than those reported in humans. Androgen treatment did not affect male or female interactions with infants, but late gestation flutamide masculinized aspects of females' interest in infants. Although early flutamide prevented complete masculinization of male genitalia, this treatment was not accompanied by any alterations in the masculine pattern of infant interest. We found no evidence that the robust juvenile sex difference in frequency of infant interactions results from socialization. However, it was largely unaffected by our hormone manipulations. Whether this reflects characteristics of our specific treatments or is evidence of nonhormonal influences on infant interest remains unanswered.     

Effects of nonmaternal restraint on the vocalizations of infant rhesus monkeys (Macaca mulatta)

This study explored the effects of restraint by females other than the mother on the vocalizations of infant rhesus macaques (Macaca mulatta) in a captive social setting. In this species, females are very attracted to young infants and will frequently approach, groom, and hold the infant. Incompetent handling, abusive behavior, or extended periods away from the mother that prevent the infants from nursing represent potentially significant risks for infants. Vocal responses to such risky conditions appear to be the infants' only means to promote a return to their mothers. We examined the association between the severity of the threat posed to the infant and the nature of its vocal response to restraint, and whether the infants' calls influenced the behavior of their mothers or captors. The results suggest that situations posing greater risks for the infants, i.e., longer periods of restraint, were associated with a greater use of noisy screams. Furthermore, mothers' responding was associated with a greater use of noisy screams as well. The mothers' reactions, however, could be described as cautious, and consisted mostly of closer monitoring; such tempered response might be related to the risk of injury to the infant that could result from a more forceful and direct attempt at retrieval.     

Early maternal recognition of offspring vocalizations in rhesus macaques (Macaca mulatta)

This study investigated the development of maternal recognition of infant calls in rhesus macaques (Macaca mulatta). Fifteen mothers and their offspring, of age ranging from several hours to 28 days, served as subjects of an experiment in which the offspring's distress vocalizations were recorded and then played back to their mothers simultaneously with those of an age-matched control infant. The proportion of time looking at, but not the proportion of orientations to the speaker playing the offspring's vocalizations increased significantly as a function of infant age. Specifically, mothers of infants older than 1 week of age responded longer to the playback of their own infant's calls than did mothers of younger infants. These findings provide the firt evidence that offspring recognition in macaques develops between the first and second week of the infant's life and are consistent with the hypothesis that mothers need to be exposed to their infants' calls in order to learn their acoustic characteristics.     

Sex differences in otoacoustic emissions measured in rhesus monkeys (Macaca mulatta)

Click-evoked otoacoustic emissions (CEOAEs) and distortion-product OAEs (DPOAEs) were measured in about 60 rhesus monkeys. CEOAE strength was substantially greater in females than in males, just as in humans. DPOAE strength was generally slightly stronger in females, just as in humans. In males, CEOAEs were weaker (more masculine) in the fall breeding season and in winter than in the summer. In females, CEOAEs were slightly stronger (more feminine) in the fall, when sex steroids are elevated in females (and males), than in the summer when rhesus monkeys are reproductively quiescent. Thus, the sex differences in CEOAEs were greater in the fall than in the summer. We presume that the seasonal fluctuations in OAEs reflect activational hormonal effects, while the basic sex differences in OAEs likely reflect organizational effects of prenatal androgen exposure. Some monkeys of both sexes had been treated with additional testosterone or the anti-androgen flutamide during prenatal development. In accord with expectations, prenatal androgen treatment weakened CEOAEs in females, and prenatal flutamide treatment strengthened CEOAEs in males. For DPOAEs, the differences between treated and untreated groups were mostly small and often inconsistent. Taken as a whole, the data from both rhesus monkeys and humans suggest that the linear, reflection-based mechanism of OAE production that underlies CEOAEs is more sensitive to prenatal androgenic processes than is the nonlinear distortion mechanism that underlies DPOAEs.     

Cognitive performance in rhesus monkeys varies by sex and prenatal androgen exposure

Men and women differ on performance and strategy on several spatial tasks. Rodents display similar sex differences, and manipulations of early hormone exposure alter the direction of these differences. However, most cognitive testing of nonhuman primates has utilized sample sizes too small to investigate sexually differentiated behaviors. This study presents an investigation of sex differences and the effects of prenatal androgen on spatial memory and strategy use in rhesus monkeys. Monkeys prenatally exposed to vehicle, testosterone, or the androgen receptor blocker flutamide performed a search task in which 5 of 12 goal boxes contained food rewards. Spatial consistency and the presence of local landmarks were varied. Performance when both spatial and marker cues were available did not differ by sex or prenatal treatment. Contrary to predictions, females easily solved the task when local markers were removed, and their performance outscored males. Although eliminating spatial consistency and requiring subjects to use local markers impaired performance by all monkeys, females continued to locate correct goal boxes at higher than chance levels and scored better than males. Blocking prenatal androgen exposure in males improved use of local markers. These findings suggest that the tendency to attend to landmarks and to use them in solving spatial problems is typical of females across many species, including rodents, humans, and rhesus monkeys. In rhesus monkeys and rodents, developmental androgen eliminates this specialization. However, these results are the only known example of better performance of females than males when salient markers are removed.     

The development of behavioural sex differences in infant rhesus macaques (Macaca mulatta)

Studies of infant rhesus macaques have generally reported sex differences in the frequency of expression of some behaviour patterns, such as rough-and-tumble play and socio-sexual mounting. In contrast, sex differences in other behaviour patterns, such as proximity to the mother, have been less consistantly reported. Using data on the behavioural development of infant rhesus macaques living in captive social groups, we have attempted to provide further evidence for, or against, sex differences in behaviour and to investigate the possible influence of maternal rank and parity on sex differences in infant behaviour and mother-infant interactions. The behaviour of 14 male and 20 female infants and their mothers was studied during the first six months of life, including measures of play behaviour socio-sexual mounting, and mother-infant interactions. Our data reveal that, on average, male infants exhibited more rough-and-tumble play and mounting than female infants, and also exhibited stationary play, chasing play, and initiated play more frequently than females. Such sex differences appear to be robust in macaques and have been reported in a variety of housing conditions. male and female infants did not differ in the amount of time spent at particular distances from their mothers, and mothers were not found to behave differently towards sons and daughters, using measures of restraint, rejection, and grooming. These results are in contrast to previous studies on singly-housed mother-infant pairs but similar to those on free-ranging populations. Mothers did behave differently towards their infants depending upon the mother's rank and previous number of offspring. These maternal characteristics may have significant consequences for the behavioural development of both male and female infant primates.     

Timing of prenatal androgen exposure: anatomical and endocrine effects on juvenile male and female rhesus monkeys

Prenatal androgen shapes genital differentiation. In humans, genital anatomy determines sex of rearing and subsequent behavioral development. Rhesus monkey genital anatomy and neuroendocrine function are sexually differentiated, and behavioral development occurs in a complex social environment. We investigated prenatal hormonal influences on sexual differentiation by suppressing or increasing androgens in male and female rhesus monkeys. Pregnant multiparous female rhesus monkeys received 35-40 days of testosterone enanthate (TE) treatment, androgen antagonist (flutamide, FL) treatment, or vehicle starting on gestation day (GD) 35 or 40 (early) or GD 110 or 115 (late). Exogenous androgen increased neonatal LH secretion in females when given early and altered female genital differentiation when administered either early or late. TE treatment, early or late in gestation, had no measurable effects on male genital differentiation or neuroendocrine function. Early FL treatment, however, radically altered male genital differentiation, producing in two cases males with a urethral opening separate from the glans. In females, early FL treatment produced detectable alterations in genitalia consistent with a reduced exposure to prenatal androgen, suggesting that female rhesus monkeys are naturally exposed prenatally to meaningful levels of T. Late FL treatment reduced male penis size and increased neonatal T secretion, but had no effect in females. This is the first study to block endogenous prenatal testosterone in rhesus monkeys, thereby altering sexual differentiation. These findings illustrate the complexity of prenatal influences on anatomical and neuroendocrine development. The relationship between the anatomical changes reported here and sex differences in behavior is currently under investigation.     

Maternal gestational androgens are associated with decreased juvenile play in white-faced marmosets (Callithrix geoffroyi)

Exposure to androgens during prenatal development shapes both physiological and behavioral developmental trajectories. Notably, in rhesus macaques, prenatal androgen exposure has been shown to increase rough-and-tumble play, a prominent behavioral feature in males during the juvenile period in primates. While macaques are an Old World, polygamous species with marked sexually dimorphic behavior, New World callitrichine primates (marmosets and tamarins) live in cooperative breeding groups and are considered to be socially monogamous and exhibit minimal sexual dimorphism in social play, which suggests that androgen may affect this species in different ways compared to macaques. In addition, we previously described considerable variation in maternal androgen production during gestation in marmosets. Here we tested the association between this variation and variation in offspring rough-and-tumble play patterns in both males and females. We measured testosterone and androstenedione levels in urine samples collected from pregnant marmoset mothers and then observed their offspring's play behavior as juveniles (5-10 months of age). In contrast to findings in rhesus macaques, hierarchical regression analyses showed that higher gestational testosterone levels, primarily in the second semester, were associated with decreased rough-and-tumble play in juveniles, and this relationship appears to be driven more so by males than females. We found no reliable associations between gestational androstenedione and juvenile play behavior. Our findings provide evidence to suggest that normative variation in levels of maternal androgen during gestation may influence developmental behavioral trajectories in marmosets in a way that contradicts previous findings in Old World primates.     

Females without infants: mating strategies in two species of captive macaques

Bonnet (Macaca radiata) and rhesus (Macaca mulatta) macaque females were observed during the breeding season to evaluate differences in mating strategies between females without infants and females with dependent offspring. Rhesus nonmothers participated in consortships earlier in the breeding season and initiated and received more approaches with adult males than rhesus mothers did. Bonnet nonmothers entered estrus earlier, began copulating earlier, and approached and presented to adult males more often than bonnet mothers did. Interacting with males frequently and early in in the breeding season may improve nonmothers' chances of conceiving and bringing offspring to term, with little competition from mother peers.     

Effects of prenatal treatment with antiandrogens on luteinizing hormone secretion and sex steroid concentrations in adult spotted hyenas,Crocuta crocuta

Prenatal androgen treatment can alter LH secretion in female offspring, often with adverse effects on ovulatory function. However, female spotted hyenas (Crocuta crocuta), renowned for their highly masculinized genitalia, are naturally exposed to high androgen levels in utero. To determine whether LH secretion in spotted hyenas is affected by prenatal androgens, we treated pregnant hyenas with antiandrogens (flutamide and finasteride). Later, adult offspring of the antiandrogen-treated (AA) mothers underwent a GnRH challenge to identify sex differences in the LH response and to assess the effects of prenatal antiandrogen treatment. We further considered the effects of blocking prenatal androgens on plasma sex steroid concentrations. To account for potential differences in the reproductive state of females, we suppressed endogenous hormone levels with a long-acting GnRH agonist (GnRHa) and then measured plasma androgens after an hCG challenge. Plasma concentrations of LH were sexually dimorphic in spotted hyenas, with females displaying higher levels than males. Prenatal antiandrogen treatment also significantly altered the LH response to GnRH. Plasma estradiol concentration was higher in AA-females, whereas testosterone and androstenedione levels tended to be lower. This trend toward lower androgen levels disappeared after GnRHa suppression and hCG challenge. In males, prenatal antiandrogen treatment had long-lasting effects on circulating androgens: AA-males had lower T levels than control males. The sex differences and effects of prenatal antiandrogens on LH secretion suggest that the anterior pituitary gland of the female spotted hyena is partially masculinized by the high androgen levels that normally occur during development, without adverse effects on ovulatory function.     

Agonistic screams differ among four species of macaques: the significance of motivation-structural rules

We compared screams of four species of macaques (rhesus monkey, Macaca mulatta; pigtailed monkey, M. nemestrina; Sulawesi crested black macaque, M. nigra; stumptailed macaque, M. arctoides) with respect to predictions of Morton's motivation-structural rules (Morton 1977, American Naturalist, 111, 855-869). We examined screams produced by victims of attack that involved contact aggression (pulling, pushing, slapping, grappling and biting) from a higher-ranking opponent. For each macaque species, we digitized 100 screams from females 3 years of age or older and measured acoustic features of each call. We used discriminant function analysis to determine whether the 400 vocalizations could be assigned to the correct caller species on the basis of their acoustic structure. Calls were assigned to the correct species at a significantly higher rate (93.5%) than expected by chance (25%). Each of the four macaque species used acoustically distinct screams in a shared context. While the differences in the macaque species' vocalizations suggest no simple correlation between immediate context and the acoustic forms of screams, there was general correspondence between the acoustic structure predicted by motivation-structural rules and inferences about the internal state of the vocalizer derived from the typical intensity of aggressive patterns that characterize each of the four species. Copyright 2000 The Association for the Study of Animal Behaviour.     

Nature Needs Nurture: The Interaction of Hormonal and Social Influences on the Development of Behavioral Sex Differences in Rhesus Monkeys

Thirty years of research on early social and hormonal environments and their relationship to the expression of behavioral sex differences in rhesus monkeys are reviewed. These studies demonstrate that whether aggressive and submissive behaviors are sexually dimorphic depends primarily on the social and not the hormonal environment. Early rearing environments without mothers or allowing brief periods of peer interaction produced higher levels of male aggression and female submission. Presenting behavior was expressed more by females than males in environments with high male aggressivity and female submissiveness. No sex differences in presenting occurred in low aggressivity environments, unless monkeys were reared isosexually, when males presented more than females. Rough and tumble play and foot-clasp mounting were consistently exhibited more by males than females across all rearing environments studied, but rearing environment affected the degree of the sex difference. When reared isosexually males displayed less, and females more, foot-clasp mounting than when heterosexually reared. No social environment increased the low frequency of female rough and tumble play. Suppressing neonatal androgen in males did not effect any sexually dimorphic behavior. Prenatal androgen administration to genetic females masculinized many aspects of their juvenile behavior, consistently increasing rough and tumble play and foot-clasp mounting across different social environments. Thus the sexually dimorphic behaviors which showed the smallest variability across social contexts were the most profoundly affected by the prenatal hormonal environment. These studies demonstrate that the expression of consistent juvenile behavioral sex differences results from hormonally induced predispositions to engage in specific patterns of juvenile behavior whose expression is shaped by the specific social environment experienced by the developing monkey.     

Androgen-induced yawning in rhesus monkey females is reversed with a nonsteroidal anti-androgen

In the adult rhesus monkey, yawning is an androgen-dependent sexually dimorphic behavior with males yawning more frequently than do females reflecting sex differences in circulating androgens. Studies in a variety of species indicate that yawning is mediated by various neurochemicals including dopamine, serotonin, and oxytocin. In rhesus monkeys, exogenous androgen reliably induces yawning in females to male-like levels. This study investigated whether flutamide, a nonsteroidal anti-androgen, reverses yawning induced by exogenous androgen administration in adult female rhesus monkeys. Six adult female rhesus monkeys were given chronic DHT alone and in combination with daily injections of flutamide and observed for yawning behavior. Treatment with DHT alone significantly increased yawning from 0.3 yawns per 30 min at the pretreatment baseline to 4.7 yawns per 30 min. Concurrent administration of flutamide significantly reduced the rate of yawning to 1.9 yawns per 30 min. These data indicate that flutamide is an effective tool for blocking the central effects of androgens in rhesus monkey females and that androgens regulate yawning similarly in both males and females.     

Intended Receivers and Functional Significance of Grunt and Girney Vocalizations in Free-Ranging Female Rhesus Macaques

We investigated the intended receivers and contexts of occurrence of grunt and girney vocalizations in rhesus macaques (Macaca mulatta) to assess whether these calls are best interpreted as signals of benign intent or as calls that may function to attract the attention of other individuals or induce arousal. We focally observed 19 free‐ranging adult female rhesus macaques. Female calls increased dramatically after infants were born, and most were directed toward mother–infant dyads. When infants were physically separated from their mothers, callers visually oriented toward infants in over 90% of the cases, suggesting that infants were the intended receivers of grunts and girneys. Approaches followed by vocalizations were more likely to lead to the caller grooming the mother, less likely to elicit a submissive response, and more likely to result in infant handling than approaches without calls. Infant handling, however, was not necessarily benign. Vocalizations were often emitted from a distance >1 m and were rarely followed by approaches or social interactions. Our results suggest that grunts and girneys are unlikely to have evolved as signals that encode information about the caller's intention or subsequent behavior. Whereas girneys may be acoustically designed to attract infants’ attention and elicit arousal, grunts may have no adaptive communicative function. Mothers, however, may have learned that other females’ grunts and girneys are unlikely to be associated with significant risk and, therefore, are generally tolerant of the caller's proximity and behavior.     

Expression of adult female patterns of sexual behavior by male, female, and pseudohermaphroditic female rhesus monkeys

Gonadally intact pseudohermaphroditic female and normal female and neonatally castrated male rhesus monkeys were given estrogen treatment as adults and evaluated for attractivity, proceptivity, and receptivity during tests with a tethered stud male. Pseudohermaphrodites were produced by injecting their mothers during pregnancy with either testosterone propionate (TP) or dihydrotestosterone propionate (DHTP). Castrated males had reliably lower attractivity than normal females on all indicator responses shown by the tethered males. Additionally, castrated males showed reliably fewer proceptive responses on 4 of 5 measures than normal females. Receptivity could not be assessed in this situation for castrated males, because tethered males never contacted them unless the castrated males were displaying presentation. No reliable differences were observed between pseudohermaphrodites produced by prenatal treatments with TP or DHTP. Pseudohermaphrodites generally showed reliably less attractivity and proceptivity than normal females and reliably more of these traits than castrated males. Attractivity scores for pseudohermaphrodites were not different from those for normal females until proximity to the tethered male was established. Receptivity was not different in pseudohermaphrodites compared with normal females. Results indicate prenatal androgenization and its developmental sequelae lead to a defeminization in adulthood which, in this testing situation, was principally manifested by a deficiency in the performance of proceptive behaviors. Additionally, defeminization in rhesus monkeys, unlike that demonstrated in rodents, does not depend upon actions of an aromatizable androgen.     

Development in Japanese macaques (Macaca fuscata): Sexually dimorphic behavior during the first year of life

We have documented several sexually dimorphic patterns of behavior that develop during the first year of life in infant Japanese macaques and their mothers. Mothers treated their infants differently by sex—mothers of males broke contact with them and retrieved them more frequently than did mothers of females. And mothers of male infants moved more frequently than did mothers of female infants. Male infants played more, played in larger groups, and mounted more frequently; female infants groomed and spent more time close to other monkeys in larger social groups than did males. Female infants were also punished by other group members more frequently than were male infants. We conclude that male and female Japanese macaque infants receive differential treatment early in life by both their own mothers and other animals, and males and females in turn treat their mothers and other animals differently. There appears to be a reciprocal relationship between the behavior of infants, mothers and other social partners that contributes to the development of sexually dimorphic patterns of behavior.     

Japanese macaques (Macaca fuscata) social development: Sex differences in Juvenile behavior

We have quantitatively documented the development of sex differences in the behavior of juvenile Japanese macaques (1 to 2 years of age). Mothers treated their offspring differently by sex, i.e., mothers of males broke contact with them more frequently than did mothers of females. Juvenile males played more, and mounted other macaques more frequently; juvenile females groomed their mothers more and were also punished by other group members more frequently than were males. Males showed a pattern of decreasing interactions with their mothers, but females increased the frequency of their maternal interactions. These patterns appear to presage the life histories of the sexes. However, comparisons with other species of nonhuman primates indicate that although sex differences in behavior are common, the variability among species severely limits cross-specific generalizations.     

Factors regulating the timing of puberty onset in female rhesus monkeys (Macaca mulatta): role of prenatal androgens, social rank, and adolescent body weight

This study investigated whether prenatal androgen exposure, social rank, and body weight are factors regulating pubertal development in outdoor-housed female rhesus monkeys. Subjects' mothers received injections of testosterone enanthate (20 mg/ wk), flutamide (an androgen receptor blocker, 30 mg/kg twice daily), or vehicle during Gestational Days 35/40-70 (early) or Days 105/110-140 (late). Monitoring of pubertal development began around 28 mo of age during the fall breeding season, with frequent assessment of menstruation, circulating steroids, and weight. Menarche occurred 1.5 mo later in females treated late in gestation than in females treated early in gestation. This short menarche delay occurred in females treated with androgen, flutamide, or vehicle. No effect of prenatal manipulations on first ovulation were found. Social rank was related to first ovulation but not menarche, with low-ranked females less likely than high- or middle-ranked females to ovulate at 2.5 yr than at 3.5 yr of age. Females ovulating early, around 2.5 yr, had higher pubertal body weights and body mass indexes (BMI) than did females ovulating later, suggesting that better nutritional reserves or positive energy balance affect pubertal development. Thus, social rank and likely nutritional status influenced pubertal development in this study. Hormonal manipulations had no detectable effect; instead, handling late in gestation, which may have increased maternal adrenal activity, delayed menarche. This finding contrasts with earlier studies that showed that prenatal androgens delay menarche by 4-6 mo on average. This study supports late gestation as a period of increased sensitivity to environmental insult and demonstrates that multiple factors, including prenatal programming, modulate the specific timing of pubertal events.     

Primigravidity,body weight,and costs of rearing first offspring in rhesus macaques

Body size is associated with menarche and ovarian function, but the relationship to first conception is rarely examined. We conducted a longitudinal investigation of rhesus macaques, Macaca mulatta, to determine the effect of differences in body weight on both the age at first conception and survivorship of first progeny. Young females who became pregnant weighed significantly more than peers who remained barren, with weight changes for females who successfully raised offspring no different than those for females who did not produce offspring. Infant mortality among primiparae was not significantly greater than that among multiparae, although nearly twice as high. First-born males tended to have lower survivorship than first-born female offspring. We suggest that reproductive costs encountered by primiparous females are more likely to be modulated by immature neuroendocrine function than by inexperience, small body size, or infant suckling patterns. We conclude that body size influences probability of first conception, socioendocrine factors mediate the likelihood of infant survival, and primiparous production of male progeny seems to exert a greater reproductive cost than does production of female progeny. Am. J. Primatol. 46:135–144, 1998. © 1998 Wiley-Liss, Inc.