Arrested development of secondary sexual adornments in subordinate adult male mandrills (Mandrillus sphinx).

Previous studies of semifree-ranging mandrills identified two morphological and social variants of the adult male, based on behavioral and secondary sexual characteristics. "Fatted" males are social, with highly developed sex skin coloration, large testes, high plasma testosterone levels, and fat rumps; while "nonfatted" males are peripheral or solitary, with paler sex skin, smaller testes, lower plasma testosterone, and slimmer rumps. We present a detailed study of morphology and group association for 10 adult male mandrills, living in two semifree-ranging groups in Gabon, in order to relate differences between males to dominance rank. The results show that rather than existing as two distinct morphotypes, male mandrills represent a continuous spectrum of possibilities between social males with fully developed secondary sexual characteristics, and solitary males with muted secondary sexual characteristics. Alpha males (N = 2) had the highest testosterone levels, the most colorful sex skin, and the most active sternal glands, and were the only males to spend 100% of their time with the social group. Rank relationships between nonalpha males (N = 8) were not always clear, but all subordinate males had lower testosterone levels and less development of the secondary sexual adornments, and were less group-associated than alpha males. These findings suggest that only alpha males have sufficient testosterone to develop full secondary sexual characteristics, and we propose possible socioendocrine mechanisms underlying the suppression of testosterone and secondary sexual development in subordinate adults. We discuss differences in secondary sexual development in relation to reproductive strategies, and discuss the evolution of alternative reproductive morphs in primates.     

Circannual changes in the secondary sexual adornments of semifree-ranging male and female mandrills (Mandrillus sphinx)

Male mandrills (Mandrillus sphinx) have spectacular secondary sexual adornments. These include red and blue sexual skin on the face, rump, and genitalia; a sternal scent-marking gland; and a "fatted" rump. Mandrills are seasonal breeders, and in other seasonally-breeding primate species members of both sexes may show increased expression of secondary sexual characteristics during the mating season. We examined changes in male secondary sexual adornments and testosterone levels, in relation to seasonal changes in the female reproductive cycle and sexual skin morphology, in two semifree-ranging mandrill groups. Females showed circannual changes in sexual skin tumescence, and periods of tumescence peaked from May-July in a long-established group. However, formation of a second, smaller group, two years previous to commencement of the study, disrupted the seasonal pattern of sexual skin tumescence and births. As the groups occupied adjacent enclosures, it appears that social factors, as well as physical environment, affected the seasonal patterning of reproduction in females. Male mandrills, by contrast, did not exhibit marked circannual changes in secondary sexual traits. Although adult male testicular volume and circulating testosterone levels increased significantly during the mating season, sexual skin coloration and rump "fattedness" showed no consistent changes with season. There was some evidence to suggest that maturing males (ages 5-8 yr) showed increased development of red sexual skin during mating periods, but once males had fully developed secondary sexual adornments, they remained stable throughout the year. The possible reasons for this are discussed in relation to intermale competition and social organization in mandrills.     

Reproductive success in the mandrill, Mandrillus sphinx: correlations of male dominance and mating success with paternity, as determined by DNA fingerprinting

Considerable controversy exists concerning possible effects of sexually selected phenotypes via intermale competition on reproductive success. The mandrill ( Mandrillus sphinx ) is an extreme example of evolution by sexual selection, and hence we have studied a semi-free-ranging colony of mandrills in Gabon to gather information on male rank, mating success and paternity, as determined by DNA fingerprinting. Two morphological variants or adult male were identified; 'fatted' males, with maximum secondary sexual coloration, which occupied dominant positions in the social group, and 'non-fatted' males, with muted secondary sexual adornments, smaller testes and lower plasma testosterone levels, which lived as peripheral/solitary individuals. DNA fingerprinting analyses on infants born over five successive years showed that only the two most dominant, fatted males in the group had fathered off spring. Throughout the annual mating season these males attempted to mate-guard and copulate with females during periods of maximal sexual skin tumescence. Male rank and mating success were strongly positively related and the alpha male sired 80–100% of the resulting offspring during three consecutive years. Non-fatted adult males and group associated subadult males engaged in infrequent, opportunistic matings and did not guard females. Loss of alpha status resulted in a fall in reproductive success, but the effect was gradual; the deposed alpha male continued to father 67% and 25% of infants born during the next two years. Thus these results of behavioural and genetic studies on mandrills demonstrate unequivocally that clear-cut relationships exist between male secondary sexual development, social dominance, copulatory behaviour and reproductive success in the social group.     

Do Female Mandrills Prefer Brightly Colored Males?

Although secondary sexual adornments are widespread in male primates, few studies have examined female choice for these characters. Mandrills (Mandrillus sphinx) present an extreme example of sexual dimorphism, with males exhibiting an array of striking adornments. The most dominant adult male in a group exhibits the brightest and most extensive red coloration, while the other males are less brightly colored. I examined whether female mandrills prefer brightly colored males using data on periovulatory sexual behavior during the 1996 mating season for all males 8 years old (n = 5) and all parous females (n = 9) in a semifree-ranging colony at CIRMF, Gabon. Brightness of male coloration is significantly positively correlated with time spent within 2 m of females, female responsibility for proximity, number of sexual presentations received, % approaches accepted by females, and % inspections with which females cooperated. Females also groomed only the brightest male. Behaviors indicating female preference are not correlated significantly with male dominance rank, and partial correlations confirm that the influence of male color on female behavior is stronger than that of male rank. With the influence of male dominance rank controlled, correlation coefficients between female behaviors and male mating success are high and positive. In further support of the hypothesis that females show mate choice for brightly colored males, independent of dominance rank, I report an unusual case wherein the alpha male fell in rank without loss of coloration. He experienced no significant change in female responsibility for proximity, sexual presentations received, or female reaction to approaches or inspections, though he was no longer observed to mate. Accordingly, female mandrills attend to differences in male secondary sexual characters and favor brightly colored males. As brightly colored males are also dominant this reinforces the influence of male-male competition on male reproductive success and may explain the very high reproductive skew in mandrill males and their extraordinary appearance.     

Male dominance and genetically determined reproductive success in the mandrill (Mandrillus sphinx)

Darwin referred to the adult male mandrill (Mandrillus sphinx) as the most brightly coloured of all mammals, citing the brilliant red and blue pigmentation of the face, rump, and genitalia as extreme examples of evolution by sexual selection. Considerable controversy exists concerning possible effects of sexually selected phenotypes via intermale competition on reproductive success. Behavioural and genetic studies of a large, semi-free ranging mandrill colony in Gabon have now demonstrated that clear-cut relationships exist between male secondary sexual development, social dominance, copulatory behaviour, and reproductive success in this primate species. Two morphological variants of adult male were identified; “fatted” males, with maximum secondary sexual coloration, which occupied dominant positions in the social group, and “non-fatted” males, with muted secondary sexual adornments, smaller testes and lower plasma testosterone levels, which lived as peripheral/solitary individuals. DNA fingerprinting analyses on infants born over five successive years showed that only the two most dominant, fatted males in the group had fathered offspring. Throughout the annual mating season these males attempted to mate-guard and copulate with females during periods of maximal sexual skin tumescence. Male rank and mating success were strongly positively related and the alpha male sired 80 – 100% of the resulting offspring during three consecutive years. Non-fatted adult males and group associated subadult males engaged in infrequent, opportunistic matings and did not guard females. Loss of alpha status resulted in a fall in reproductive success, but the effect was gradual; the deposed alpha male continued to father 67% and 25% of infants born during the next two years. Thus, whilst claims that male dominance determines mating success and paternity in primates have caused considerable debate, these results on mandrills provide unequivocal evidence for the existence of such effects.     

Life history in male mandrills (Mandrillus sphinx): physical development, dominance rank, and group association

We assess life history from birth to death in male mandrills (Mandrillus sphinx) living in a semifree-ranging colony in Gabon, using data collected for 82 males that attained at least the age of puberty, including 33 that reached adulthood and 25 that died, yielding data for their entire lifespan. We describe patterns of mortality and injuries, dominance rank, group association, growth and stature, and secondary sexual character expression across the male lifespan. We examine relationships among these variables and investigate potential influences on male life history, including differences in the social environment (maternal rank and group demography) and early development, with the aim of identifying characteristics of successful males. Sons of higher-ranking females were more likely to survive to adulthood than sons of low-ranking females. Adolescent males varied consistently in the rate at which they developed, and this variation was related to a male's own dominance rank. Males with fewer peers and sons of higher-ranking and heavier mothers also matured faster. However, maternal variables were not significantly related to dominance rank during adolescence, the age at which males attained adult dominance rank, or whether a male became alpha male. Among adult males, behavior and morphological development were related to a male's own dominance rank, and sons of high-ranking females were larger than sons of low-ranking females. Alpha males were always the most social, and the most brightly colored males, but were not necessarily the largest males present. Finally, alpha male tenure was related to group demography, with larger numbers of rival adult males and maturing adolescent males reducing the time a male spent as alpha male. Tenure did not appear to be related to characteristics of the alpha male himself.     

Accessory gland size influences time to sexual maturity and mating frequency in the stalk-eyed fly, Cyrtodiopsis dalmanni

Age at first reproduction is an extremely important life-historytrait. Several factors such as nutritional state and age-specificfecundity have been shown to influence time to sexual maturity;however, little work has been done in insects. We addressedthis in a stalk-eyed fly (Cyrtodiopsis dalmanni), by testingthe hypothesis that time to sexual maturity is associated withthe development of male internal reproductive structures. Wefound that sexual maturity was attained after an increased rateof growth in the accessory glands, several days after maturesperm bundles, and motile sperm were observed in the testes.Although testis development is essential, the results suggestthat accessory gland growth is more closely associated withthe time taken to reach sexual maturity than is testis growth.When we manipulated the growth of testes and accessory glandsvia a dietary manipulation, we found that delayed growth ratesincreased the time taken to reach sexual maturity. Among thedelayed individuals, sexually mature males had larger accessoryglands, but not testes, than did immature males. In adult males,mating frequency was significantly positively correlated withaccessory gland size, but not with testis length or body size.We conclude that accessory gland size is a critical determinantof sexual maturity and male mating frequency in this species.     

Development from birth to sexual maturity in a semi-free-ranging colony of mandrills (Mandrillus sphinx) in Gabon.

This report presents information collected over 7 years (1983-1990) in Gabon, on a breeding group of 14, increasing to 45, mandrills maintained in a rainforest enclosure. Under these conditions, a seasonal cycle of mating (June-October) and birth (January-May) occurred. Females began to exhibit sexual skin swellings at age 2.75-4.5 years (3.6 +/- 0.6 years, mean +/- SD; n = 10) and first delivered offspring when 3.25-5.5 years old (4.4 +/- 0.8 years; n = 9). Gestation periods ranged from 152 to 176 days (167 +/- 9 days; n = 6 accurately dated pregnancies) and interbirth intervals from 11 to 15 months (12.4 +/- 1.3 months; n = 15). Females could reproduce 2 years before attaining adult body weight (10-15 kg) and complete dental eruption by 5.0-5.5 years. Males, by contrast, developed more slowly, reaching adult body weight (30-35 kg) and testicular volume (volume of left testis: 25-30 ml) at 8 years. Consistently high circulating testosterone concentrations (8.17 +/- 2.0 ng ml-1) could be measured by 9 years of age. Fully developed males exhibited fatting of the rump and flanks, as well as striking sexual skin coloration and an active sternal cutaneous gland; little expression of these features was evident during pubertal development. Marked individual age differences occurred with regard to the onset and complete development of these features, suggesting possible interactions between social environment and physical maturation.     

Changes in the secondary sexual adornments of male mandrills (Mandrillus sphinx) are associated with gain and loss of alpha status

Two semifree-ranging mandrill groups, inhabiting large, naturally rainforested enclosures in Gabon, were studied to measure morphological, endocrine, and behavioral changes that occurred when adult males rose, or fell, in dominance rank. Gaining alpha rank (N = 4 males) resulted in increased testicular size and circulating testosterone, reddening of the sexual skin on the face and genitalia, and heightened secretion from the sternal cutaneous gland. Blue sexual skin coloration was unaffected. New alpha males increased in rump "fattedness," but not in body mass, and spent more time associated with other group members, rather than ranging alone. Loss of alpha position (N = 4 males) resulted in less pronounced effects than those that occurred after males had risen to alpha positions. Deposed alpha males showed decreased testicular volume, decreased body mass, a reduction in the extent of red (but not blue) sexual skin coloration, and decreased sternal gland activity. Deposed males did not decrease in the brightness of sex skin coloration. These results demonstrate that male-male competition and rank reversals have remarkable effects upon testicular function, secondary sexual traits, and behavior in the adult male mandrill. Secondary sexual traits respond to changes in male social status and therefore may be important as intrasexual signals of dominance rank.     

A preliminary investigation of urinary testosterone and cortisol levels in wild male mountain gorillas

Urinary steroid hormone levels were measured in wild male mountain gorillas (Gorilla gorilla beringei) to determine how levels of testosterone and cortisol corresponded with age and social rank. Urine samples were collected noninvasively from 18 males, ranging in age from 3–26 years, in three groups of wild mountain gorillas at the Karisoke Research Center, Rwanda, Africa, and samples were analyzed using radioimmunoassay procedures. Males were classified as being immature (<7 years), maturing (10–13 years), or adult (+13 years of age). Immature males had significantly lower levels of testosterone and higher levels of cortisol than both maturing and adult males. No differences in testosterone or cortisol levels were found between maturing and adult males. Dominant males exhibited a trend toward significantly higher levels of testosterone than subordinate males, but no difference was found between cortisol levels of dominant and subordinate males. These results suggest that the increase in testosterone associated with puberty occurs prior to any outward sign of development of secondary sexual characteristics. Within-group male–male competition may affect testosterone levels, but the lack of difference in cortisol levels between dominant and subordinate males suggests that subordinate males are not socially stressed, at least as measured by cortisol. Am. J. Primatol. 43:51–64, 1997. © 1997 Wiley-Liss, Inc.     

Maturation of social reward in adult male Syrian hamsters does not depend on organizational effects of pubertal testosterone

The rewarding value of female sexual stimuli develops across puberty, as sexually-naïve adult, but not prepubertal, male hamsters show a conditioned place preference (CPP) for both vaginal secretions and a receptive female. Similarly, only adults show an endogenous testosterone surge when they encounter vaginal secretions. Testosterone by itself can condition a place preference in male rodents. Therefore, Experiment 1 assessed whether the endogenous testosterone surge elicited by vaginal secretions is necessary to show a CPP. Both gonad-intact and gonadectomized, testosterone-treated adult males showed a CPP for vaginal secretions, indicating that the rewarding value of this social cue is independent of an endogenous testosterone surge. However, organizational effects of pubertal testosterone could be necessary for adolescent development of social reward, as pubertal testosterone organizes adult-typical expression of sexual behavior. To investigate this possibility, in Experiment 2, sexually-naïve prepubertal and adult male hamsters were gonadectomized and received testosterone-filled capsules four weeks later. Testing began after two weeks of testosterone replacement. Adult males showed a CPP for both vaginal secretions and a receptive female, whether or not they experienced pubertal testosterone. Thus, the acquisition of positive valence of sexual stimuli is not organized by pubertal testosterone. Taken together, the ability of female sexual stimuli to serve as an unconditioned reward to adult male hamsters is independent of the chemosensory-induced endogenous testosterone surge and also organizational effects of pubertal testosterone. Instead, sexual reward may be dependent either on activational effects of testosterone or gonadal hormone-independent mechanisms.     

Age-dependent changes in sperm production, semen quality, and testicular volume in the black-footed ferret (Mustela nigripes)

The black-footed ferret (Mustela nigripes), which was extirpated from its native North American prairie habitat during the 1980s, is being reintroduced to the wild because of a successful captive-breeding program. To enhance propagation, the reproductive biology of this endangered species is being studied intensively. The typical life span of the black-footed ferret is approximately 7 yr. Female fecundity declines after 3 yr of age, but the influence of age on male reproduction is unknown. In this study, testis volume, seminal traits, sperm morphology, and serum testosterone were compared in 116 males from 1 to 7 yr of age living in captivity. Results demonstrated that testes volume during the peak breeding season was similar (P > 0.05) among males 1 to 5 yr of age, reduced (P < 0.05) among males 6 yr of age, and further reduced (P < 0.05) among males 7 yr of age. Motile sperm/ejaculate was similar in males 1 to 6 yr of age but diminished (P < 0.05) in those 7 yr of age. Males at 6 and 7 yr of age produced fewer (P < 0.05) structurally normal sperm than younger counterparts; however, serum testosterone concentrations were not reduced (P > 0.05) in older males. Histological comparison of testicular/epididymal tissue from 5- and 7-yr-old black-footed ferrets confirmed that the interval between these two ages may represent a transitional period to reproductive senescence. In summary, functional reproductive capacity of male black-footed ferrets exceeds that of females by at least 2 yr. Testes and seminal quality are indistinguishable among males 1 to 5 yr of age, with progressive reproductive aging occurring thereafter.     

Stress, social behaviour, and secondary sexual traits in a male primate

We examined variation in glucocorticoid levels in the mandrill, a brightly coloured primate species, to identify major social influences on stress hormones, and investigate relationships among glucocorticoid levels, testosterone and secondary sexual ornamentation. We collected a total of 317 fecal samples for 16 adult male mandrills over 13 months, including mating and non-mating periods and periods of both dominance rank stability and instability, and compared fecal glucocorticoid levels with dominance rank, rank stability, presence of receptive females, gastro-intestinal parasite infection, fecal testosterone and facial red coloration. Glucocorticoid levels did not vary systematically with dominance rank, but increased when the dominance hierarchy was unstable, and increased in the presence of receptive females. The relationship between dominance rank and glucocorticoid levels changed direction according to the stability of the dominance hierarchy: glucocorticoid levels were higher in subordinate males under stable conditions, but under conditions of instability higher ranking males had higher glucocorticoid levels. The influence of dominance rank also interacted with the presence of receptive females: glucocorticoids were higher in dominant males than in subordinates, but only during mating periods, suggesting that dominant males are more stressed than subordinates during such periods. These findings support previous studies showing that the relationship between glucocorticoids and dominance rank in male baboons is dependent on the social environment. We also found that males with higher glucocorticoids suffered a higher diversity of gastrointestinal parasite infection, in line with evidence that glucocorticoids suppress the immune system in other species. However, we found no support for the stress-mediated immunocompetence handicap hypothesis for the evolution of condition-dependent ornaments: glucocorticoid and testosterone levels were positively related, rather than the negative relationship predicted by the hypothesis, and we found no relationship between red colour and glucocorticoid levels, suggesting that glucocorticoids do not play a role in translating social conditions or physical health into ornament expression in this species.     

Growth and ontogeny of sexual size dimorphism in the mandrill (Mandrillus sphinx)

We present body mass (N = 419) and crown-rump length (CRL, N = 210) measurements from 38 male and 49 female mandrills born into a semifree-ranging colony in order to describe growth from birth to adulthood, and to investigate maternal influences upon growth. Adult male mandrills are 3.4 times the body mass, and 1.3 times the CRL, of adult females. Body mass dimorphism arises from a combination of sex differences in length of the growth period (females attain adult body mass at 7 years, males at 10 years) and growth rate. Both sexes undergo a subadult growth spurt in body mass, and this is much more dramatic in males (peak velocity 551 g/months +/- 89 SEM at 84-96 months). CRL dimorphism arises from bimaturism (females attain adult CRL at 6 years, males after 10 years), and neither sex shows a particular subadult growth spurt in CRL. Sexual size dimorphism thus represents important time and metabolic costs to males, who mature physically approximately 3-4 years after females. Considerable interindividual variation occurs in the size-for-age of both sexes, which is related to maternal variables. Older mothers have heavier offspring than do younger mothers, and higher-ranking mothers have heavier offspring than do lower ranking mothers. Mass advantages conferred upon offspring during lactation by older and higher-ranking mothers tend to persist postweaning in both sexes. Thus maternal factors affect reproductive success in both sexes, influencing the age at which offspring mature and begin their reproductive career.     

Dispersal Status Influences Hormones and Behavior in the Male Spotted Hyena

Male spotted hyenas (Crocuta crocuta) reach puberty at 24 months of age and then invariably emigrate from their natal clans 1 to 38 months later. Thus there are two classes of reproductively mature males in everyCrocutaclan: adult natal males born in the clan and adult immigrant males born elsewhere. In one free-living hyena population in Kenya, these two groups of males were compared with respect to measures of aggression, social dominance, sexual behavior, and circulating hormone levels. Adult natal males engaged in higher hourly rates of aggression than did immigrants, won all fights with immigrants, and were socially dominant to immigrants. In addition, adult natal males engaged in far lower hourly rates of sexual behavior with resident females than did immigrants, and natal males were never observed to copulate with natal females. Mean basal plasma cortisol values did not differ between the two groups of adult males, but cortisol concentrations in immigrants were positively correlated with tenure in the clan and with immigrant male social rank. Adult natal males had plasma testosterone levels significantly lower than those of immigrants. Social rank and plasma testosterone values were positively correlated among immigrant males. Thus two different relationships appear to exist between circulating testosterone and social rank in maleCrocuta:one apparent in immigrants and the other in natal adult males. Our results suggest that dispersal might disinhibit testosterone secretion in postpubertal male hyenas.     

Monthly patterns of testosterone and behavior in prospective fathers

The individual time patterns of salivary testosterone of adult healthy men, self-reported sexual behavior and their co-occurrence with regular weekly or monthly intervals were studied. Twenty-seven volunteer males (mean age 33 +/- 1 years) collected daily morning saliva over a period of 90 days. Evening questionnaires provided daily information on sexual activity. From the saliva, testosterone immunoreactive substances were determined using enzyme immunoassay. To detect events in which increases of testosterone were associated with sexual activity and at the same time controlling for regular internal patterns in men, data were analyzed using Theme software. First results indicated a varying number of complex nonrandom interaction patterns of testosterone with sexual activity, but also with weekly (i.e., Saturdays) and monthly intervals (i.e., 28-day full-moon intervals). The social context of the occurrence of specific pattern combinations was elaborated using parameters from the men's self-reported general life history profiles. Peak hormone levels occurred around weekends in the majority of the males. The 28-day monthly interval coincided with testosterone peaks only in those of the paired men who reported a current wish for children ("prospective fathers"), but not in unpaired men or in those who did not wish to have children with their current partner. Rather than representing a direct regular pattern of the male testosterone per se, the observed patterns suggest that men have the facultative potential to adjust their testosterone responses to their female partner's cycle. In line with the interactions between behavior and androgens observed in vertebrates in general, this study adds an example of the mutual character of hormone-behavior interactions and, thus, for the social context of testosterone patterns in human males.     

Changes in social interactions during adolescence in male mandrills (Mandrillus sphinx)

The adolescent period in male mandrills extends from the time the testicles descend (at the age of 3-4 years) to the time adult characteristics develop (at the age of 10 years), and is thus one of the longest adolescent periods in cercopithecines. In this short cross-sectional study of 11 male mandrills (4.7-9.1 years old), we analyzed social behavioral patterns to investigate partner preference and changes in types of interactions throughout puberty. The mandrills were separated into two clusters on the basis of all of the social interactions scored, as well as the age and sex of the protagonist. During early to mid puberty, immature individuals interacted preferentially with other male adolescents and juveniles. Playing was an important activity, and some affiliative interactions with juveniles were observed. Older adolescents interacted more with adult males and females. These older adolescents were predominantly aggressive toward other adolescents, females, and juveniles, and showed little or no affiliative or playful behaviors. These results indicate a shift in social partners: older males interacted more frequently with adult males, and their behavior involved more aggression and less submission compared to younger adolescents, which showed predominantly submissive and playful behavior. These changes are consistent with preparation for dispersal and future intermale competition.     

Craniodental features in male Mandrillus may signal size and fitness: an allometric approach

According to a hypothesis in the broader mammalian literature, secondary sexual characteristics that have evolved to signal fitness and size to other conspecifics should exhibit positive allometry across adult males within a species. Here this hypothesis is tested in the genus Mandrillus. The overbuilding of bony features in larger individuals necessitates a functional explanation as bone is metabolically expensive to produce and maintain. Canine size and size of the maxillary ridge are scaled against a body size surrogate in intraspecific samples of male Mandrillus sphinx (mandrills) and Mandrillus leucophaeus (drills). Areal dimensions are weighted more heavily as they represent the size of a feature as it is viewed by individuals. Measures of the maxillary ridge and canine tooth are significantly correlated with the size surrogate and scale with positive allometry in both samples supporting the hypothesis that these features function to advertise a male's body size and fitness to other males competing for mates and potential discerning females. This is the first study in primates to test for intraspecific positive allometric scaling of bony facial features in adult males based on a theory of fitness signaling and sexual selection.     

Laryngeal muscle and motor neuron plasticity in Xenopus laevis: Testicular masculinization of a developing neuromuscular system

In Xenopus laevis, the sexual differentiation of the neuromuscular system responsible for courtship song is controlled by testicular androgen secretion. To explore the sensitivity of this system to androgenic stimulation, male and female frogs were gonadectectomized and given testis transplants at seven different developmental stages between the end of metamorphosis and adulthood, grown to sexual maturity, and the laryngeal muscle fibers and motor axons were counted. Muscle fiber and axon numbers in males were not affected by the testicular transplant at any stage. In females, testicular transplants at all developmental stages increased muscle fiber numbers in adulthood. Values attained were, however, significantly less than those of adult intact or testis-transplanted males. Testis transplantation increased laryngeal axon numbers in females to levels equivalent to those of intact males; this effect was obtained at every stage of postmetamorphic development including adulthood. To further explore androgen regulation in adults, males and females were gonadectomized and implanted with silicone tubes containing testosterone propionate for 1.5–3 years and laryngeal muscle fibers and axon numbers compared to those of gonadectomized or sham-operated adult controls. Neither treatment with exogenous androgen nor gonadectomy had any effect on laryngeal muscle fiber or axon number in either males or females; values did not differ from those of sham-operated controls. We conclude that testicular secretions can induce laryngeal muscle fiber and axon addition in females throughout postmetamorphic life. This degree of plasticity, exhibited after the period when adult values are normally attained, stands in contrast to the effects of administration of synthetic androgen and suggests that the degree of plasticity in adult females may be underestimated if exogenous hormones rather than testicular transplants are provided. © 1993 John Wiley & Sons, Inc.     

Violent coalitionary attack by female mandrills against an injured alpha male

Female contact aggression against males is relatively rare in species in which the adult males are larger than the females, but it has the potential to influence group structure, male group membership, tenure, and dominance rank. We report an incident in which female mandrills living in a semi-free-ranging group in Franceville, Gabon, attacked a male that was apparently incapacitated after a fight with another male and was unable to escape. The attack involved the alpha male and did not occur in a sexual or infanticidal context. Other adult and adolescent males observed the attack, but when one adult male attempted to participate he was chased away by the females. This observation adds to reports of female coalitions excluding unwanted males from primate groups, or even killing them. The fact that this can also occur in mandrills suggests that females have a degree of control over male group membership, despite the large degree of sexual dimorphism in this species, and highlights the importance of coalitions in primate social organization.