Behavioral development and maternal care in tufted capuchins (Cebus apella) and squirrel monkeys (Saimiri sciureus) from birth through seven months

A longitudinal assessment of behavioral development in wild capuchins: Personality is not established in the first 3 years

Animal personality is defined as consistent individual differences across time and situations, but little is known about how or when those differences are established during development. Likewise, several studies described the personality structure of adult capuchin monkeys, without assessing the ontogeny of these personality traits. We analyzed the behavioral repertoire of 12 wild infants (9 males, 3 females) yellow-breasted capuchin monkeys (Sapajus xanthosternos), in Una Biological Reserve (Bahia, Brazil). Each infant was observed and filmed weekly from birth until 36 months, through daily focal sampling. We analyzed the behavior of each individual in 10 developmental points. By means of component reduction (principal component analysis), we obtained four behavioral traits: Sociability, Anxiety, Openness, and Activity. We investigated whether there were developmental effects on those traits by fitting regression models for the effect of time on behavioral traits, controlling for monkey identity, sex, and cohort. Sociability (decreasing) and Anxiety (increasing) changed significantly along development. By means of repeatability analysis, we did not find intra-individual consistency across time in those traits, so we cannot discriminate stable personality traits in early ontogeny. Our results show that the personality structure of capuchin monkeys is not established during early development, in agreement with the literature on human personality.

Face-to-face interactions between mothers and female infants in wild bearded capuchin monkeys (Sapajus libidinosus)

Once considered uniquely human, mother-infant face-to-face interactions (FF) were observed in a few captive primates. In these studies, FF were correlated to physical contact suggesting a mechanism mediating proximity between mother and infant, as is the case for humans. We investigated this hypothesis in wild capuchin monkeys (Sapajus libidinosus) during the first year of life of eight female infants. Data were weekly focal-day videos of infants from which we recorded FF with mothers. We expected FF would increase with infants' age (as time in contact with mothers decreased) and would more likely occur in the absence of physical contact between the dyad. There was no effect of age in the proportion of interaction time spent in FF, nor in types of FF. A quarter of FF episodes occurred in the absence of physical contact between the dyad, and in most of them physical contact was resumed following the FF. Contrary to predictions, the stability in the first year, mainly when mothers-female infants were in contact, indicates that FF act primarily promoting opportunities for affective communication and intuitive care. However, we found some supportive evidence for the hypothesis that FF regulate proximity between mother and infant, mainly in resume physical contact.

Estilos de cuidado materno em primatas: considerações a partir de uma espécie do Novo Mundo

Resumo Quando buscamos entender o comportamento humano, comparações com primatas não humanos são especialmente relevantes para identificar homoplasias (características semelhantes que evoluem independentemente em diferentes espécies). Neste artigo, apresentamos um estudo longitudinal de dois anos sobre o comportamento materno de macacos-prego (Sapajus spp.) em condições naturalísticas. Nossos resultados permitiram identificar estilos de cuidado distintos dentro de um contínuo de permissividade a proteção. O desenvolvimento observado do vínculo entre mães e filhotes sugere que o período de dependência de filhotes de macaco-prego envolve, além de processos de maturação física, o estabelecimento e desenvolvimento de processos psicológicos associados ao sistema de apego. É possível que a variabilidade de estilos maternos resultante da combinação de características de mães, filhotes e contextos socioecológicos, aliada ao prolongamento do vínculo de apego, pavimente caminhos para diferentes trajetórias de desenvolvimento. Como em humanos, esse pode ser um dos mecanismos pelos quais surgem e se consolidam as diferenças interindividuais nas populações adultas.

Cognitive consequences of cooperative breeding in primates?

Several hypotheses propose that cooperative breeding leads to increased cognitive performance, in both nonhuman and human primates, but systematic evidence for such a relationship is missing. A causal link might exist because motivational and cognitive processes necessary for the execution and coordination of helping behaviors could also favor cognitive performance in contexts not directly related to caregiving. In callitrichids, which among primates rely most strongly on cooperative breeding, these motivational and cognitive processes include attentional biases toward monitoring others, the ability to coordinate actions spatially and temporally, increased social tolerance, increased responsiveness to others' signals, and spontaneous prosociality. These processes are likely to enhance performance particularly in socio-cognitive contexts. Therefore, cooperatively breeding primates are expected to outperform their independently breeding sister taxa in socio-cognitive tasks. We evaluate this prediction by reviewing the literature and comparing cognitive performance in callitrichids with that of their sister taxa, i.e. squirrel monkeys, which are independent breeders, and capuchin monkeys, which show an intermediate breeding system. Consistent with our prediction, this review reveals that callitrichids systematically and significantly outperform their sister taxa in the socio-cognitive, but not in the non-social domain. This comparison is complemented with more qualitative evaluations of prosociality and cognitive performance in non-primate cooperative breeders, which suggest that among mammals, cooperative breeding generally produces conditions conducive to socio-cognitive performance. In the hominid lineage, however, the adoption of extensive allomaternal care presumably resulted in more pervasive cognitive consequences, because the motivational consequences of cooperative breeding was added to an ape-level cognitive system already capable of understanding simple mental states, which enabled the emergence of shared intentionality.

Ontogeny of joint mechanics in squirrel monkeys (Saimiri boliviensis): functional implications for mammalian limb growth and locomotor development

Juvenile animals must often compete against adults for common resources, keep pace during group travel and evade common predators, despite reduced body size and an immature musculoskeletal system. Previous morphometric studies of a diverse array of mammals, including jack rabbits, cats and capuchin monkeys, have identified growth-related changes in anatomy, such as negative allometry of limb muscle mechanical advantage, which should theoretically permit young mammals to overcome such ontogenetic limits on performance. However, it is important to evaluate the potential impact of such ;compensatory' growth trajectories within the context of developmental changes in locomotor behavior. I used standard kinematic and kinetic techniques to investigate the ontogenetic scaling of joint postures, substrate reaction forces, joint load arm lengths and external joint moments in an ontogenetic sample of squirrel monkeys (Saimiri boliviensis). Results indicated that young squirrel monkeys were frequently able to limit forelimb and hind limb joint loading via a combination of changes in limb posture and limb force distribution, potentially compensating for limited muscularity at younger ages. These results complement previous morphometric studies and suggest that immature mammals may utilize a combination of behavioral and anatomical mechanisms to mitigate ontogenetic limits on locomotor performance. However, ontogenetic changes in joint posture, not limb length per se, explained most of the variation in load arm lengths and joint loading in growing squirrel monkeys, indicating the importance of incorporating both anatomical and performance measures when studying the ontogeny of limb joint mechanics.

Sex differences in play behavior in juvenile tufted capuchin monkeys (Cebus apella)

According to the motor training hypothesis, play behavior in juvenile primates improves motor skills that are required in later adult life. Sex differences in juvenile play behavior can therefore be expected when adult animals assume distinct sexually dimorphic roles. Tufted capuchin monkeys show sexually dimorphic levels of physical antagonism in both inter- and intra-group encounters. Accordingly, it can be predicted that juvenile capuchins also show sex differences in social play behavior. To test this hypothesis, the play behavior of nine juvenile and two infant capuchins was examined. As predicted, juvenile males showed significantly higher levels of social play (wrestle, chase) than juvenile females, but no differences were found in nonsocial play (arboreal, object). Levels of infant play behavior were comparable to that of juveniles. These results lend support to the motor training hypothesis and highlight the need for more detailed investigations of individual differences in play behavior.

Facial Displays in Young Tufted Capuchin Monkeys (Cebus apella): Appearance, Meaning, Context and Target

Facial displays are important for communication, and their ontogeny has been studied primarily in chimpanzees and macaques. We investigated the ontogeny, communicative function and target of facial displays in Cebus apella. Our results show that facial displays are absent at birth and develop as infants grow older. Lip-smacking appears first (at about 1 month of age), followed by scalp-lifting, relaxed open-mouth, silent bared-teeth, open-mouth silent bared-teeth displays and finally the open-mouth threat face. Infants perform most facial displays in the same contexts as adults, with the exception of the silent bared-teeth display that young capuchins use primarily, or exclusively, in affiliative contexts. Interestingly, facial displays are exchanged very often with peers, less frequently with adults and almost never with the mother.

Evaluating social influences on food-processing behavior in white-faced capuchins (Cebus capucinus)

Interpopulation variability in patterns of food processing, similar to what is described as "traditional" or "cultural" variation in chimpanzees (Pan troglodytes) and orangutans (Pongo pygmaeus), was identified in white-faced capuchins (Cebus capucinus). However, recent comparisons of food processing in capuchins were conducted only at the population level, with relatively little attention given to variability among groups, age/sex classes, or individuals. This paper examines variability in the processing of specific food types within the context of various social networks (i.e., patterns of association, rank, and kinship) among free-ranging Cebus capucinus at Santa Rosa National Park in Costa Rica. We collected data on two groups of white-faced capuchins in 2001, identifying rates of "food interest" for each individual, as well as forms of processing for specific food types. Juveniles exhibited the most interest in the food-processing behavior of other group members, and food interest was directed most frequently toward adult females. We identified distinctive processing techniques for several food items (Luehea candida pods, Sloanea terniflora fruits, and caterpillars) that facilitated comparisons among individuals within groups. Food-processing techniques for Sloanea fruit and caterpillars appeared to vary independently of the social networks examined in this study. However, we found evidence that variation in Luehea candida processing is to some degree linked to both patterns of association and social rank. The potential influence of these variables on observed food processing patterns warrants further scrutiny.

Response to novel housing in two groups of captive tufted capuchin monkeys (Cebus apella)

The influence of age, maternal status, and the presence of a group male on use of space was assessed in two groups of captive tufted capuchin monkeys that underwent a move from indoor housing to a larger outdoor facility. Both groups originally contained two adult males, but only one group retained a male after the move. Following the move, mothers spent less time on the ground when carrying their infants than they did when not carrying their infants. In the group with no male (1) individuals decreased time spent on the ground relative to pre-move levels, whereas no such difference was noted in the group with the male; (2) females spent more time carrying their infants than did females in the group with a male. In the group with the adult male, juveniles spent less time on the ground than did non-mother adult females, whereas no difference had existed prior to the move. Grooming rates dropped from pre-move to post-move, but the mean number of partners with which each animal was in contact increased. Measures of social behavior varied across post-move observation periods inversely to time spent on the ground. These results are consistent with the view that an individual's relative vulnerability influences behavioral conservatism in novel environments, and suggests a relatively profound role for males in promoting exploration of new space in this species.

The mother-offspring relationship as a template in social development: reconciliation in captive brown capuchins (Cebus apella)

Mother-offspring (MO) relationship quality was investigated to determine its influence on the development of reconciliation--affiliation between opponents shortly after a fight--because it influenceswhat distressed youngsters learn about calming down. Data were longitudinal and cross-sectional observational samples of 38 MO pairs of monkeys across 24 months. An MO relationship quality index (RQI) classified each pair as secure or insecure. Reconciliation emerged in infancy. Secure youngsters had an appeasing conciliatory style, and insecure youngsters had an agitated conciliatory style. Conclusions are that reconciliation develops from the attachment behavior system and MO RQI is related to the particular conciliatory style youngsters develop by affecting how aroused they are by conflict and the subsequent socializing they seek to calm down.

An index of relationship quality based on attachment theory

Two measures are reported of the nature or quality of a mother-offspring (MO) relationship during development using brown capuchin monkeys (Cebus apella) as models. One is a qualitative classification of MO relationships as secure, resistant, or avoidant attachments. The other is an empirical ratio of relative affiliation to agonism called the MO relationship quality, or MORQ, Index. The two methods tapped similar relationship features so relationships high or low of a median split of MORQ values were heuristically labeled secure (n = 22) or insecure (n = 16), respectively. A comparison revealed extensive behavioral differences between secure and insecure MO relationships and suggested MORQ provided an objective, continuous measure of attachment security.

Prehension in infant capuchins (Cebus apella) from six weeks to twenty-four weeks: video analysis of form and symmetry

We analyzed the spontaneous prehensile activity of two infants living with their mothers in social groups, using videotapes taken once weekly from weeks 5 to 24. Prehensile activities were laterally symmetric. Unimanual activity predominated, although bimanual activity appeared at the same ages as unimanual activity. In most bimanual activity the two hands performed the same action, but complementary actions occurred from the onset of bimanual activity. Extrusion of the tongue towards objects out of reach was observed occasionally, as was precision grasping. Early prehension in capuchins is organized, as in human infants, in a matrix of exploratory activity integrated with vision and oral exploration. Capuchins present a useful model system for the study of manipulative development.

Social separation in infant Cebus apella: patterns of behavioral and cortisol response

34 infant tufted capuchins (Cebus apella) were separated from their social groups for a 2-hour period and videotaped in isolation at the ages of 6 months and 1 year. Baseline and 2-hour blood samples were measured for levels of serum cortisol. Compared to homecage baseline levels, passivity, locomotion and vocalizations increased during separation, while self-directed behavior and environmental exploration decreased. Both behavioral and cortisol responses to separation showed individual stability over the 6 month period, although both responses were somewhat attenuated at the later age. There was little correlation between cortisol and behavior during separations. Females vocalized more than did males during separations and showed greater cortisol increases at 6 months of age. The pattern of behavioral response seen in the 2 hours following separation appeared to be more passive than the typical 'protest' response described in many nonhuman primates, and may reflect either the physical circumstances of the separation or a characteristic of species with relaxed social bonds and considerable allomothering available to infants.

Application of positron emission tomography to determine cerebral glucose utilization in conscious infant monkeys

Cerebral glucose metabolism has been used as a marker of cerebral maturation and neuroplasticity. In studies addressing these issues in young non-human primates, investigators have used positron emission tomography (PET) and [18F]2-fluoro-2-deoxy-D-glucose (FDG) to calculate local cerebral metabolic rates of glucose (1CMRG1c). Unfortunately, these values were influenced by anesthesia. In order to avoid this confounding factor, we have established a method that permits reliable measurements in young conscious vervet monkeys using FDG-PET. Immature animals remained in a conscious, resting state during the initial 42 min of FDG uptake as they were allowed to cling to their anesthetized mothers. After FDG uptake, animals were anesthetized and placed in the PET scanner with data acquisition beginning at 60 min post-FDG injection. FDG image sets consisted of 30 planes separated by 1.69 mm, parameters sufficient to image the entire monkey brain. Our method of region-of-interest (ROI) analysis was assessed within and between raters and demonstrated high reliability (P < 0.001). To illustrate that our method was sensitive to developmental changes in cerebral glucose metabolism, quantitative studies of young conscious monkeys revealed that infant monkeys 6-8 months of age exhibited significantly higher 1CMRG1c values (P < 0.05) in all regions examined, except sensorimotor cortex and thalamus, compared to monkeys younger than 4 months of age. This method provided high resolution images and 1CMRG1c values that were reliable within age group. These results support the application of FDG-PET to investigate questions related to cerebral glucose metabolism in young conscious non-human primates.

Effect of infants on adult social relations in the squirrel monkey (Saimiri Sciureus)

7 groups of squirrel monkeys were observed to assess the effect of infants on social interactions and interanimal distances among adult members of their natal groups. Each group contained one or two infants, 5 to 7 months of age at the start of the study. Infants initiated affiliative and playful interactions with all adults, whereas adults directed few mostly antagonistic, interactions toward infants. Following the removal of infants from groups, distances between adults decreased and adult affiliative interactions increased more than 100%. The results indicate that infants within the age range examined can have a pervasive, primarily inhibitory, influence on adult social relationships.

Successful Mother- and Group-Rearing of a Newborn Capuchin Monkey (Cebus Capucinus) Following Emergency Major Surgery

During capture, in order to separate him from a possessive adult female and return him to his mother, a newborn male in a laboratory group of Cebus capucinus monkeys was found to have a seriously infected compound fracture of the humerus associated with a deep and extensive slash wound. Amputation of the affected limb was deemed necessary. Shortly after surgery the newborn was returned to his mother, in isolation from the group, with periodic removal for post-surgical care. Three weeks later the mother-newborn pair was returned to the social group and no further intervention occurred. Regular observations revealed mutual behavioural adjustments to the handicap by the mother and newborn. Compared to a normal age-mate, the amputee received more positive social attention from the mother and other group-members. Despite his showing delays in locomotor and manipulatory activities, the handicapped infant showed good behavioural progress. Early resocialization thus appears feasible following emergency surgery in newborn primates.

Development of manipulation in capuchin monkeys during the first 6 months

This study describes the orderly changes in manipulation over the first 6 months in capuchin monkeys (Cebus apella). By 6 months of age, all the basic forms of manipulation seen in adults have appeared. Actions that occur frequently in the first 8 weeks are gentle and involve sustained visual orientation and aimed reaching. Later actions are more vigorous, and involve grasping. Large increases in the rate of activity are evident over the period of development studied. The increase from the first 8 weeks to the second may be due to (a) an increase in the amount of time spent alert and active, (b) a decrease in the amount of time spent in a ventral position, (c) improvements in postural control and stamina and (d) the onset of independent locomotion. Changes in form can be attributed primarily to postural factors and to neuromuscular development (precisely aimed and controlled movements appearing in the 5th and 6th months).