Sex differences in the play behavior of immature spotted hyenas, Crocuta crocuta

The development of sex differences in play in wild white-faced capuchins (Cebus capucinus)

Many mammalian species display sex differences in the frequency of play behavior, yet the animal literature includes few longitudinal studies of play, which are important for understanding the developmental timing of sex differences and the evolutionary functions of play. We analyzed social play, solitary play, and grooming using an 18-year data set on 38 wild white-faced capuchin monkeys (Cebus capucinus) followed since infancy. Rates of each behavior were measured as the proportion of point samples taken during focal follows in which the individual engaged in each behavior. To determine sex differences in these rates, we ran a series of generalized linear mixed models, considering both linear and quadratic effects of age, and chose the optimal model for each of the three behavioral outcomes based on information criteria. Rates of both social play and solitary play decreased with age, with the exception of social play in males, which increased in the early juvenile period before decreasing. Male and female capuchins had different developmental patterns of social play, with males playing more than females during most of the juvenile period, but they did not display meaningful sex differences in solitary play rates. Additionally, males and females had different patterns of grooming over the lifespan: males participated in grooming at low rates throughout their lives, while adult females participated in grooming at much higher rates, peaking around age 11 years before declining. We suggest that male and female white-faced capuchins may adopt alternative social bonding strategies, including different developmental timing and different behaviors (social play for males vs. grooming for females). Our results were consistent with two functional hypotheses of play, the practice and bonding hypotheses. This study demonstrates that play behavior may be critical for the development of sex-specific social strategies and emphasizes the importance of developmental perspectives on social behaviors.

Juvenile social dynamics reflect adult reproductive strategies in bottlenose dolphins

The juvenile period is a challenging life-history stage, especially in species with a high degree of fission–fusion dynamics, such as bottlenose dolphins, where maternal protection is virtually absent. Here, we examined how juvenile male and female bottlenose dolphins navigate this vulnerable period. Specifically, we examined their grouping patterns, activity budget, network dynamics, and social associations in the absence of adults. We found that juveniles live in highly dynamic groups, with group composition changing every 10 min on average. Groups were generally segregated by sex, and segregation was driven by same-sex preference rather than opposite-sex avoidance. Juveniles formed strong associations with select individuals, especially kin and same-sex partners, and both sexes formed cliques with their preferred partners. Sex-specific strategies in the juvenile period reflected adult reproductive strategies, in which the exploration of potential social partners may be more important for males (which form long-term alliances in adulthood) than females (which preferentially associate with kin in adulthood). Females spent more time alone and were more focused on foraging than males, but still formed close same-sex associations, especially with kin. Males cast a wider social net than females, with strong same-sex associations and many male associates. Males engaged in more affiliative behavior than females. These results are consistent with the social bonds and skills hypothesis and suggest that delayed sexual maturity in species with relational social complexity may allow individuals to assess potential associates and explore a complex social landscape without the risks associated with sexual maturity (e.g., adult reproductive competition; inbreeding).

Accurate Sex Identification of Ancient Elephant and Other Animal Remains Using Low-Coverage DNA Shotgun Sequencing Data

Sex identification of ancient animal biological remains can benefit our understanding of historical population structure, demography and social behavior. Traditional methods for sex identification (e.g., osteological and morphometric comparisons) may be ineffective when animal remains are not well preserved, when sex distinguishing characteristics have not yet developed, or where organisms do not exhibit sex-associated phenotypic dimorphisms. Here we adapt a method developed for human sex determination so that it can be used to identify the sex of ancient and modern animal taxa. The method identifies sex by calculating the ratio of DNA reads aligning to the X chromosome to DNA reads aligning to autosomes (termed the Rx ratio). We tested the accuracy of this method using low coverage genomes from 15 modern elephants (Loxodonta africana) for which sex was known. We then applied this method to ancient elephant ivory samples for which sex was unknown, and describe how this method can be further adapted to the genomes of other taxa. This method may be especially useful when only low-coverage genomic data are obtainable. Furthermore, because this method relies on only the X and not the Y chromosome, it can be used to determine the sex of organisms for which a reference genome was obtained from a female or for which only the X chromosome is reported. Such taxa include the domestic cat, sheep, goat, and horse; and non-domesticated animals such as the Sumatran orangutan, western lowland gorilla and meerkat.

Developmental origins of sex differences in the neural circuitry of play

Social play consists of reciprocal physical interactions between conspecifics with many features conserved across species, including the propensity for males to engage in play more frequently and with higher physical intensity. Animal models, such as the laboratory rat, reveal that the underlying neural circuitry of play is subject to sexual differentiation during a critical period early in life. In this review, we discuss the developmental processes that produce distinct neural nodes which modulate both shared and sex-specific aspects of play with a focus on the medial amygdala, lateral septum, and prefrontal cortex. While the cellular mechanisms determining sex differences in play are beginning to be uncovered, the ultimate advantages of play continue to be debated.

Organizational and activational androgens, lemur social play, and the ontogeny of female dominance

The role of androgens in shaping "masculine" traits in males is a core focus in behavioral endocrinology, but relatively little is known about an androgenic role in female aggression and social dominance. In mammalian models of female dominance, including the ring-tailed lemur (Lemur catta), links to androgens in adulthood are variable. We studied the development of ring-tailed lemurs to address the behavioral basis and ontogenetic mechanisms of female dominance. We measured behavior and serum androgen concentrations in 24 lemurs (8 males, 16 females) from infancy to early adulthood, and assessed their 'prenatal' androgen milieu using serum samples obtained from their mothers during gestation. Because logistical constraints limited the frequency of infant blood sampling, we accounted for asynchrony between behavioral and postnatal hormone measurements via imputation procedures. Imputation was unnecessary for prenatal hormone measurements. The typical sex difference in androgen concentrations in young lemurs was consistent with adult conspecifics and most other mammals; however, we found no significant sex differences in rough-and-tumble play. Female (but not male) aggression increased beginning at approximately 15 months, coincident with female puberty. In our analyses relating sexually differentiated behavior to androgens, we found no relationship with activational hormones, but several significant relationships with organizational hormones. Notably, associations of prenatal androstenedione and testosterone with behavior were differentiated, both by offspring sex and by type of behavior within offspring sexes. We discuss the importance of considering (1) missing data in behavioral endocrinology research, and (2) organizational androgens other than testosterone in studies of female dominance.

Beyond aggression: Androgen-receptor blockade modulates social interaction in wild meerkats

In male vertebrates, androgens are inextricably linked to reproduction, social dominance, and aggression, often at the cost of paternal investment or prosociality. Testosterone is invoked to explain rank-related reproductive differences, but its role within a status class, particularly among subordinates, is underappreciated. Recent evidence, especially for monogamous and cooperatively breeding species, suggests broader androgenic mediation of adult social interaction. We explored the actions of androgens in subordinate, male members of a cooperatively breeding species, the meerkat (Suricata suricatta). Although male meerkats show no rank-related testosterone differences, subordinate helpers rarely reproduce. We blocked androgen receptors, in the field, by treating subordinate males with the antiandrogen, flutamide. We monitored androgen concentrations (via baseline serum and time-sequential fecal sampling) and recorded behavior within their groups (via focal observation). Relative to controls, flutamide-treated animals initiated less and received more high-intensity aggression (biting, threatening, feeding competition), engaged in more prosocial behavior (social sniffing, grooming, huddling), and less frequently initiated play or assumed a 'dominant' role during play, revealing significant androgenic effects across a broad range of social behavior. By contrast, guarding or vigilance and measures of olfactory and vocal communication in subordinate males appeared unaffected by flutamide treatment. Thus, androgens in male meerkat helpers are aligned with the traditional trade-off between promoting reproductive and aggressive behavior at a cost to affiliation. Our findings, based on rare endocrine manipulation in wild mammals, show a more pervasive role for androgens in adult social behavior than is often recognized, with possible relevance for understanding tradeoffs in cooperative systems.

Aggression and hierarchical steepness inhibit social play in adult wolves

In canids, play dynamics seem to be more affected by dominance hierarchy rather than cooperative social bonds. To test this hypothesis we studied a colony of grey wolves (Canis lupus lupus). We quantified the dynamics of aggression and hierarchical changes in two periods (Sample 1 and Sample 2). Sample 2 was characterized by higher level of aggressiveness and by a more strict and steep linear hierarchy. The negative correlation between rank distance and play frequency characterizing both periods and the higher play asymmetry in Sample 2 suggest that rank rules dictate play rules thus highlighting the competitive side of wolf play behaviour. The overall affiliation rates showed no variation between the two samples. Yet, play performance was modified. In Sample 2, wolves reduced playful activity, limited the number of players per session and avoided playing during high competition contexts. Our findings support the hypothesis that wolf play is modulated by dominance relationships more than by cooperative social bonds.

Juvenile social play and yearling behavior and reproductive success in female Belding’s ground squirrels

Benefits of play behavior have been described for individuals during the juvenile period; however, it is less clear whether benefits of juvenile play commonly extend beyond the period of juvenile development. I evaluated possible associations between juvenile social play and yearling maternal territorial behavior and reproductive success in female Belding’s ground squirrels (Urocitellus beldingi). Higher rates of juvenile social play were associated with greater intensity of territorial aggressive and vigilant behavior during gestation but not later phases of the yearling reproductive cycle, suggesting that both juvenile play and later experience may influence maternal territorial behavior in yearling females. Juvenile social play was also a reliable predictor of whether a yearling female successfully weaned a litter, raising the possibility that juvenile play behavior influences yearling reproductive success via effects on maternal territorial behavior in female U. beldingi. Other factors were also related to reproductive success in yearling females. Body mass at the beginning of the yearling reproductive period was not correlated with juvenile social play but was associated with the likelihood of successfully weaning a litter. Yearling females whose mother was still alive were more likely to wean a litter than those whose mother was not alive. Mothers and yearling daughters tended to have maternal territories near each other, and mothers with a surviving yearling daughter expressed aggressive behavior at elevated rates and directed aggression toward intruders on the daughter’s territory. Overall, results of the study suggest that juvenile social play is among the variables that influence maternal territorial behavior and reproductive success in yearling female U. beldingi, and are consistent with the idea that juvenile play has benefits beyond the juvenile period for female U. beldingi.

Double helix: reciprocity between juvenile play and brain development

This review summarizes what is presently known about the function, sexual differentiation, and neural circuitry of juvenile rough-and-tumble play. Juvenile rough-and-tumble play is a unique motivated behavior that is widespread throughout the mammalian order and usually occurs more often in males. Immediate early gene studies indicate that cortical and subcortical circuits, many of which are sensitive to sex steroid hormones, mediate juvenile play. Sex differences in rough-and-tumble play are controlled in part by neonatal exposure to androgens or their estrogenic metabolites. Studies indicate that testicular androgens during play are also necessary to stimulate male-like levels of play initiation. The resemblance of rough-and-tumble play to aggression and sexual behavior has led some to question whether male-typical adult behavior is contingent upon the experience of play. Attempts to control the amount of play through social isolation show that social experience during adolescence is critical for male-typical adult behaviors to be expressed. This well-established finding, together with evidence that play induces neural plasticity, supports the hypothesis that juvenile play contributes to male-typical brain development that ultimately enables the expression of adult social and reproductive behavior.

Age-related differences in the performance, diffusion, and maintenance of stone handling, a behavioral tradition in Japanese macaques

Identifying the sources of behavioral diversity in non-human primates is vital to understanding the evolution of human behavior. Stone handling (SH, hereafter) is a form of object play consisting of the manipulation of stones by performing various behavioral patterns. This behavior is socially transmitted from generation to generation in Japanese macaques (Macaca fuscata), as a behavioral tradition. SH behavior in particular may reflect on the origin and evolution of stone-tool material culture. The objective of this study was to assess how group size, age structure, and age- and sex-related differences may account for the substantial intra- and inter-troop variations in SH reported in ten troops of Japanese macaques. Our results supported the hypothesis that patterns of variation in SH across troops reflected variability in group size and composition in age classes. We found that troop size was correlated with the proportion of troop members exhibiting SH simultaneously. The effect of troop size on the synchronized performance of SH may reveal the contagious nature of play. Our results suggest that the age structure of the group may affect the diffusion of SH. As predicted by the surplus energy hypothesis, a major functional hypothesis about play, intra-group variation in SH reflected more age- than sex-related differences. SH mainly occurred and was more frequent in younger than in older individuals, whereas no significant sex differences were found. SH episodes were shorter, more vigorous, and SH patterns were more diverse and less complex in immature than in mature individuals. The present findings reveal that age-related factors and group size may constrain the performance, diffusion, and maintenance of SH within a troop. Contrary to most other socially transmitted stone-tool using behaviors in non-human primates and early hominids, there is no optimal SH pattern. Provided some form of social learning, the non-adaptive nature of SH may allow particular SH pattern preferences to emerge at the group level. Our findings urge the use of an inter-populational comparative approach based on multivariate analyses when addressing the question of the evolution of behavioral traditions in primate and human populations.

Endocrine and energetic mediation of play behavior in free-living Belding's ground squirrels

Many juvenile mammals play, and rates and patterns of play behavior often differ between young males and females. The sexual dimorphisms typical of mammalian play suggest that it might be influenced by gonadal hormones. Moreover, because play competes with growth, physical development, and acquisition of fat reserves for available energy, play behavior should theoretically be influenced by energetic variables. We examined patterns of social play behavior and endocrine and energetic mediation of social play in free-living juvenile Belding's ground squirrels (Spermophilus beldingi). Bouts of social play in young S. beldingi resembled adult copulation and fighting, and young males initiated sexual play but not play fighting at much higher rates than did young females. To elucidate the proximal causes of play, we altered early androgen exposure by treating females with testosterone (T) at birth and used females treated with oil vehicle as controls. We concurrently manipulated energy availability by provisioning with extra food and used unprovisioned squirrels as controls. Hourly rates of play behavior were highest near the time of weaning and declined thereafter among both experimental and control groups of juveniles. Thus, we observed no influence of either T treatment or food provisioning on the temporal patterning of play behavior. Perinatal T treatment had no effect on play fighting, but caused rates of sexual play behavior initiated by young females to increase to near those observed for young males, suggesting that T organizes a masculine tendency to initiate sexual play behavior but not play fighting. Food provisioning increased rates of play among males and females from both T-treated and control litters, suggesting that energy availability limits play behavior.

How mothers influence the development of litter-mate preferences in Belding's ground squirrels

We performed three experiments to examine the role of mothers in the development of litter-mate preferences in captive juvenile Belding's ground squirrels, Spermophilus beldingi. First, when observed in the absence of mothers, juveniles did not play preferentially with litter-mates, which suggests that mothers somehow influence the ontogeny of litter-mate preferences. Second, when mothers were present but unable to intervene in social interactions, juveniles displayed litter-mate preferences, which suggests that mothers do not influence their offsprings' social development by directly intervening in social interactions. In another group, mothers were removed daily, a few hours before nocturnal immergence, and returned the following morning. Juveniles in this group did not display litter-mate preferences and at night they occupied burrows with many more non-litter-mates than litter-mates. These results suggest that associating with non-litter-mates can compromise the development of litter-mate preferences, and implies that mothers indirectly influence social development by affecting the identities of sleeping partners. Third, newly emergent juveniles that interacted only with litter-mates for 3 days in the absence of mothers subsequently preferred litter-mates over non-litter-mates as play partners. This result demonstrates that once litter-mate preferences are instilled, due in part to social experiences during juveniles' initial days above-ground, the preferences are expressed even in the absence of mothers. Collectively, the results demonstrate that the presence of S. beldingi mothers is important to juvenile social development, but that mothers do not actively direct the ontogeny of their offsprings' social relationships. Copyright 1998 The Association for the Study of Animal Behaviour. Copyright 1998 The Association for the Study of Animal Behaviour.

Multiple differences in the play fighting of male and female rats. Implications for the causes and functions of play

Play fighting is the most commonly occurring form of social play in juvenile mammals. Typically, males engage in more play fighting than females, and this difference has been shown to depend on the action of androgens perinatally. It is generally believed that the differences in play fighting between the sexes are quantitative and do not involve qualitative differences in the behavior performed. We show that this is an incorrect characterization of sex difference in play fighting. For example, in laboratory rats, there are at least five different mechanisms that contribute to the observed sex differences in play fighting. These mechanisms involve (I) the motivation to initiate play, (II) the sensory capacity to detect and respond to a play partner, (III) the organization of the motor patterns used to interact with a partner, (IV) age-related changes at puberty in initiating play and in responding to playful contact, and (V) dominance-related changes in adulthood in the pattern of playful interaction. Sex differences in the play fighting of rats are due to an interaction of all of these mechanisms, some of which are sex-typical not play-typical, and involve both quantitative and qualitative differences. This is clearly different from the prevailing view that play fighting is a unitary behavior which is masculinized perinatally. Indeed, even though all five mechanisms are androgenized perinatally, the sensorimotor differences also involve defeminization (i.e. reduction of female-typical qualities). This expanded view of the mechanisms contributing to the sex differences in play fighting has implications for both the analysis of the neural systems involved, and for the functional significance of this activity in childhood and adulthood.

Sexual dimorphism of perineal muscles and motoneurons in spotted hyenas

Female spotted hyenas are known for their male-like genitalia, high levels of aggression, and dominance over males, characteristics which are attributed to exposure to elevated levels of testosterone in utero. Although the nervous system of spotted hyenas has not previously been examined, one might predict that neural systems which are sexually dimorphic in other mammals would be monomorphic in this species. Spinal motoneurons which innervate muscles associated with the phallus are located in Onuf's nucleus and are more numerous in males than in females in a wide array of mammals. Onuf's nucleus was examined in adult and neonatal spotted hyenas and, contrary to expectation, was found to be sexually dimorphic in the typical mammalian pattern: Males have significantly more motoneurons in Onuf's nucleus than do females. This dimorphism was correlated with a previously undescribed dimorphism in the relevant target musculature. Specifically, the morphology of the bulbocavernosus muscle is distinctly different in male and female spotted hyenas. Pregnant hyenas were treated with anti-androgen in an attempt to interfere with the actions of androgen during fetal development. Motoneuron number in Onuf's nucleus and the morphology of the bulbocavernosus muscle were feminized in males exposed to anti-androgen in utero.

A mechanism for virilization of female spotted hyenas in utero

Female spotted hyenas exhibit male-like genitalia and dominance over males. Hyena ovarian tissues incubated in vitro produced large quantities of the steroid hormone precursor androstenedione. The activity of aromatase, which converts androstenedione to estrogen, was one-twentieth as great in hyena versus human placental homogenates. In comparison, the activity of 17 beta-hydroxysteroid dehydrogenase, which converts androstenedione to testosterone, was equal in the two homogenates. The limited aromatase activity may allow the hyena placenta to convert high circulating concentrations of androstenedione to testosterone, which results in virilization of the fetal external genitalia and possibly destruction of fetal ovarian follicles. Androstenedione production by residual ovarian stromal cells during reproductive life accounts for the epigenetic transmission of virilization in female spotted hyenas.

Sex differences in the play fighting activity of golden hamster infants

Play fighting of homosexual and heterosexual dyads in golden hamsters was studied. The behaviors of male-male (MM), female-female (FF), and female-male (FM) dyads were recorded from the 25th to the 31st day of age. Play fighting activity of heterosexual dyads was depressed since MF and FM dyads exhibited a significantly high physical contact time and engaged in less play fighting, with a significantly lower time spent on such activity. Alternation ratio in the pinning posture was very high and subjects did not differ in the mean number of active (top) and passive pinning (under) in each dyad, showing an inexistence of sex dominance. In most cases, differences in the performances of MM and FF dyads were not significant. We have concluded that play fighting in golden hamsters may be predominantly modulated by environmental stimuli (sex of the partner) rather than by endogenous stimuli (androgen level).