Locomotor play drives motor skill acquisition at the expense of growth: A life history trade-off

Ecological variation in adult social play reveals a hidden cost of motherhood for wild chimpanzees

Though common among humans, social play by adults is an uncommon occurrence in most animals, even between parents and offspring.1,2,3 The most common explanation for why adult play is so rare is that its function and benefits are largely limited to development, so that social play has little value later in life.3,4,5,6 Here, we draw from 10 years of behavioral data collected by the Kibale Chimpanzee Project to consider an alternative hypothesis: that despite its benefits, adult play in non-humans is ecologically constrained by energy shortage or time limitations. We further hypothesized that, since they may be the only available partners for their young offspring, mother chimpanzees pay greater costs of play than other adults. Our analysis of nearly 4,000 adult play bouts revealed that adult chimpanzees played both among themselves and with immature partners. Social play was infrequent when diet quality was low but increased with the proportion of high-quality fruits in the diet. This suggests that adults engage in play facultatively when they have more energy and/or time to do so. However, when diet quality was low and most adult play fell to near zero, play persisted between mothers and offspring. Increased use of play by adult chimpanzees during periods of resource abundance suggests that play retains value as a social currency beyond development but that its costs constrain its use. At the same time, when ecological conditions constrain opportunities for young to play, play by mothers fills a critical role to promote healthy offspring development.

Locomotor play behavior evolves by random genetic drift but not as a correlated response to selective breeding for high voluntary wheel-running behavior

Locomotor play is vigorous and seemingly purposeless behavior, commonly observed in young mammals. It can be costly in terms of energy expenditure, increased injury risk, and predator exposure. The main hypothesized benefit of locomotor play is enhancement of neuromuscular development, with effects persisting into adulthood. We hypothesized that levels of locomotor play would have evolved as a correlated response to artificial selection for increased voluntary exercise behavior. We studied mice from 4 replicate lines bred for voluntary wheel running (High Runner or HR) at 6-8 weeks of age and four non-selected Control (C) lines. Mice were weaned at 21 days of age and play behavior was observed for generations 20 (22-24 days old), 68 (22-23 days old), and 93 (15 days old). We quantified locomotor play as (1) rapid, horizontally directed jerk-run sequences and (2) vertical "bouncing." We used focal sampling to continuously record behavior in cages containing 4-6 individuals during the first 2-3 h of the dark cycle. Observations were significantly repeatable between observers and days. A two-way, mixed-model simultaneously tested effects of linetype (HR vs. C), sex, and their interaction. Contrary to our hypothesis, HR and C lines did not differ in any generation, nor did we find sex differences. However, differences among the replicate HR lines and among the replicate C lines were detected, and may be attributed to the effects of random genetic drift (and possibly founder effects). Thus, play behavior did evolve in this selection experiment, but not as a correlated response to selection for voluntary exercise.

Early life drought predicts components of adult body size in wild female baboons

OBJECTIVES

In many taxa, adverse early-life environments are associated with reduced growth and smaller body size in adulthood. However, in wild primates, we know very little about whether, where, and to what degree trajectories are influenced by early adversity, or which types of early adversity matter most. Here, we use parallel-laser photogrammetry to assess inter-individual predictors of three measures of body size (leg length, forearm length, and shoulder-rump length) in a population of wild female baboons studied since birth.

MATERIALS AND METHODS

Using >2000 photogrammetric measurements of 127 females, we present a cross-sectional growth curve of wild female baboons (Papio cynocephalus) from juvenescence through adulthood. We then test whether females exposed to several important sources of early-life adversity-drought, maternal loss, low maternal rank, or a cumulative measure of adversity-were smaller for their age than females who experienced less adversity. Using the "animal model," we also test whether body size is heritable in this study population.

RESULTS

Prolonged early-life drought predicted shorter limbs but not shorter torsos (i.e., shoulder-rump lengths). Our other measures of early-life adversity did not predict variation in body size. Heritability estimates for body size measures were 36%-67%. Maternal effects accounted for 13%-17% of the variance in leg and forearm length, but no variance in torso length.

DISCUSSION

Our results suggest that baboon limbs, but not torsos, grow plastically in response to maternal effects and energetic early-life stress. Our results also reveal considerable heritability for all three body size measures in this study population.

Effects of reduced kinematic and social play experience on affective appraisal of human-rat play in rats

BACKGROUND

Play is a common and developmentally important behaviour in young mammals. Specifically in Norway rats (Rattus norvegicus), reduced opportunity to engage in rough-and-tumble (RT) play has been associated with impaired development in social competence. However, RT play is a complex behaviour having both a kinematic aspect (i.e., performing complex 3D manoeuvres during play fights) and a social aspect (interacting with a playful partner). There has been little research so far on disentangling the two aspects in RT play, especially on how these two aspects affect the affective appraisal of the intense physical contact during play.

RESULTS

To examine the developmental effects of kinematic and social play reduction on affective appraisal in rats, we subjected male Long-Evans rats from 21 days old to RT play experience that was reduced either kinematically (through playing in a low ceiling environment) or socially (through playing with a less playful Fischer-344 rat). Starting at 35 days, we measured their production of positively (50-kHz) and negatively (22-kHz) valenced ultrasonic vocalisations (USVs) in a 2-min standardised human-rat play procedure that mimicked the playful sequences of nape contact, pinning, and belly stimulation ('tickling') for ten days. We hypothesised that the rats with kinematically or socially reduced play would perceive the 'tickling' less positively and thus emit positive ultrasonic vocalisations at lower rates compared to control rats with non-reduced play experience. Our results confirmed that each of the treatments reduced play differently: while the kinematic reduction abolished playful pinnings entirely, the social reduction decreased the pinnings and made play highly asymmetric. During the tickling procedure, rats mostly produced 50 kHz USV, indicating that they appraised the procedure as positive. There was a wide inter individual variance and high individual consistency in rats' USV responses to 'tickling'. Crucially, neither the kinematically nor the socially reduced play experience affected either type of USV production when rats were 'tickled'.

CONCLUSIONS

This finding indicates that the ability to appraise play-like interactions as positive remains unaffected even when the kinematic or the social aspect of play experience was substantially curtailed.

Meager Milk: Lasting Consequences for Adult Daughters of Primiparous Mothers Among Rhesus Macaques (Macaca mulatta)

Among mammals, primipara who initiate reproduction before full maturity can be constrained in their maternal investment, both due to fewer somatic resources and tradeoffs between their own continued development and reproductive effort. Primipara are particularly limited in their capacity to synthesize milk during lactation, the costliest aspect of reproduction for most mammals, especially primates due to long periods of postnatal development. Due to reduced milk transfer, Firstborns may be at elevated risk for long-term consequences of deficits in early life endowment from their primiparous mothers. Here we investigated mass, growth, stature, and lactation performance among N = 273 adult daughters across N = 335 reproductions, who were their own mother's Firstborn or Laterborn progeny, among rhesus macaques (Macaca mulatta) at the California National Primate Research Center. We further explored mass during infancy of the offspring of Firstborn and Laterborn mothers. Firstborns had accelerated growth during infancy, but had slowed growth during juvenility, compared to Laterborns. Although both Firstborns and Laterborns were the same age at reproductive debut, Firstborns had lower body mass, an effect that persisted throughout the reproductive career. Available milk energy, the product of milk energetic density and milk yield, was on average 16% lower for Firstborns compared to Laterborns, a difference that was only partially mediated by their lower mass. Despite differences in their mothers' energy provision through milk, the mass of infants of Firstborn and Laterborn mothers did not differ at peak lactation, suggesting that infants of Firstborns devote a higher proportion of milk energy to growth than infants of Laterborns. To date few studies have explored how early life conditions shape capacities to synthesize milk and milk composition. Our findings contribute new information among primates on how early life maternal endowments are associated with persistent effects long after the period of maternal dependence well into reproductive maturity.

Evidence for adolescent length growth spurts in bonobos and other primates highlights the importance of scaling laws

Adolescent growth spurts (GSs) in body length seem to be absent in non-human primates and are considered a distinct human trait. However, this distinction between present and absent length-GSs may reflect a mathematical artefact that makes it arbitrary. We first outline how scaling issues and inappropriate comparisons between length (linear) and weight (volume) growth rates result in misleading interpretations like the absence of length-GSs in non-human primates despite pronounced weight-GSs, or temporal delays between length- and weight-GSs. We then apply a scale-corrected approach to a comprehensive dataset on 258 zoo-housed bonobos that includes weight and length growth as well as several physiological markers related to growth and adolescence. We found pronounced GSs in body weight and length in both sexes. Weight and length growth trajectories corresponded with each other and with patterns of testosterone and insulin-like growth factor-binding protein 3 levels, resembling adolescent GSs in humans. We further re-interpreted published data of non-human primates, which showed that aligned GSs in weight and length exist not only in bonobos. Altogether, our results emphasize the importance of considering scaling laws when interpreting growth curves in general, and further show that pronounced, human-like adolescent length-GSs exist in bonobos and probably also many other non-human primates.

Developmental and environmental modulation of fecal thyroid hormone levels in wild Assamese macaques (Macaca assamensis)

Thyroid hormones are key modulators of development, as well as mediators of environmental conditions, by regulating developmental processes and metabolism in primates. Hormone measurement in noninvasively collected samples, that is, feces and urine, is a valuable tool for studying the endocrine function of wildlife, and recent studies have demonstrated the feasibility of measuring thyroid hormones in fecal samples of zoo-housed and wild nonhuman primates. Our study aimed to (i) validate the measurement of immunoreactive fecal total triiodothyronine (IF-T3) in wild Assamese macaques (Macaca assamensis) and (ii) to investigate its developmental changes and its response to environmental changes, including stress responses, in immature individuals. Fecal samples and environmental parameters were collected from individuals of three social groups of wild Assamese macaques living at Phu Khieo Wildlife Sanctuary, Northeastern Thailand. Our study confirmed the methodological feasibility and biological validity of measuring IF-T3 in this population. Specifically, the biological validation demonstrated higher IF-T3 levels in immatures compared to adults, and higher levels in females during late gestation compared to the preconception stage. Our analysis of IF-T3 levels in developing immature macaques revealed a significant increase with age. Furthermore, we found a positive association between IF-T3 and immunoreactive fecal glucocorticoid levels, an indicator of the physiological stress response. Neither minimum temperature nor fruit abundance predicted variation in IF-T3 levels in the immatures. Our findings indicate the possibility for differing effects of climatic factors and food availability on thyroid hormone level changes in immature versus adult animals and in wild compared to experimental conditions. Overall, our study provides the basis for further investigations into the role of thyroid hormones in shaping species-specific traits, growth, and overall primate development.

The practicality of practice: A model of the function of play behaviour

The function of play has been a long-debated topic in animal behaviour. One popular class of accounts is that play offers practice for serious adult behaviour, but little has been done to model the circumstances where this could be true. In this paper, we model an individual who, over the juvenile and subadult ontogenetic periods, has a choice between three behaviours: foraging, playing and rest, where playing improves an individual's ability in some component of a serious adult behaviour. Using stochastic dynamic programming, we show that even when play is more energetically costly and an inferior form of practice than foraging itself, it still may be optimal to play under a variety of circumstances. We offer several instantiations of the play as practice concept to show the possibility of play improving a variety of different adult abilities: antipredatory, foraging and reproductive behaviour. These models show the environmental conditions where play might be expected, as well as the predicted occurrences of play throughout ontogeny. This is a first step in showing the ecological feasibility of the practice hypothesis of play and raises further questions about why playful activity is more beneficial than more deliberate directed practice.

Animal-friendly behavioral testing in field studies: examples from ground squirrels

Field studies of behavior provide insight into the expression of behavior in its natural ecological context and can serve as an important complement to behavioral studies conducted in the lab under controlled conditions. In addition to naturalistic observations, behavioral testing can be an important component of field studies of behavior. This mini review evaluates a sample of behavioral testing methods in field studies to identify ways in which behavioral testing can be animal-friendly and generate ethologically relevant data. Specific examples, primarily from studies of ground squirrels, are presented to illustrate ways in which principles of animal-friendly behavioral testing can be applied to and guide testing methods. Tests conducted with animals in their natural habitat and that elicit naturally occurring behavioral responses can minimize stress and disturbance for animals, as well as disruption of the larger ecosystem, and can have high ethological validity. When animals are trapped or handled as part of a study, behavioral testing can be incorporated into handling procedures to reduce overall disturbance. When behavior is evaluated in a testing arena, the arena can be designed to resemble natural conditions to increase the ethological relevance of the test. Efforts to minimize time spent in testing arenas can also reduce disturbance to animals. Adapting a behavioral test to a species or habitat conditions can facilitate reduced disruption to subjects and increased ethological relevance of the test.

Four errors and a fallacy: pitfalls for the unwary in comparative brain analyses

Comparative analyses are the backbone of evolutionary analysis. However, their record in producing a consensus has not always been good. This is especially true of attempts to understand the factors responsible for the evolution of large brains, which have been embroiled in an increasingly polarised debate over the past three decades. We argue that most of these disputes arise from a number of conceptual errors and associated logical fallacies that are the result of a failure to adopt a biological systems-based approach to hypothesis-testing. We identify four principal classes of error: a failure to heed Tinbergen's Four Questions when testing biological hypotheses, misapplying Dobzhansky's Dictum when testing hypotheses of evolutionary adaptation, poorly chosen behavioural proxies for underlying hypotheses, and the use of inappropriate statistical methods. In the interests of progress, we urge a more careful and considered approach to comparative analyses, and the adoption of a broader, rather than a narrower, taxonomic perspective.

Adult play and the evolution of tolerant and cooperative societies

Play is generally considered an immature affair. However, adult play is present in several mammal species living in complex social systems. Here, I hypothesize that adult social play is favored by natural selection in those species characterized by high level of social tolerance and/or by the need of others' cooperation to reach a goal (i.e., leverage). The integration and comparison of bio-behavioral data on non-human primates and wild social carnivores allows drawing a comprehensive picture on the importance of adult play in facing unpredictable, novel social situations and in overcoming stressful experiences. The ability to cope with potentially competitive interactions through play can favor the emergence of egalitarian societies. A further interesting and beneficial aspect of adult play is its role in synchronizing group activities and favoring collective decision making by renovating the motivation to cooperate in groupmates. As a last step, some considerations about the presence of adult play in the most egalitarian and cooperative human groups (e.g., hunter-gatherer societies) allows discussing the apparent dichotomy between cultural and biological evolution of certain behavioral traits, including social play in adulthood.

Multiple benefits of juvenile play: A ground squirrel's perspective

Adaptive functions of play can vary across species, and also within species, reflecting behavioral ecology and evolutionary history. We evaluated juvenile play in Belding's ground squirrels (Urocitellus beldingi), a species for which field studies have assessed play behavior in the context of the squirrels' ecology and life history. Social play behavior in U. beldingi appears not to have the range of adaptive benefits related to social behavior apparent in species with more complex social organization. Play in juvenile U. beldingi improves general motor skill, which may translate to more proficient performance of behaviors during and beyond the juvenile period. Social play in juvenile squirrels is associated with refinement of temperament and behavior, promoting behavioral shifts toward less docile responses as well as more cautious behavior. Social play also influences behavior of juvenile squirrels in novel situations, fostering greater exploration and adaptability of responses. Important life events in U. beldingi such as the timing of natal dispersal and defense of maternal territories can be influenced by juvenile play.

The energetics of childhood: Current knowledge and insights into human variation, evolution, and health

How organisms capture and ultimately use metabolic energy-a limiting resource of life-has profound implications for understanding evolutionary legacies and current patterns of phenotypic variation, adaptation, and health. Energetics research among humans has a rich history in biological anthropology and beyond. The energetics of childhood, however, remains relatively underexplored. This shortcoming is notable given the accepted importance of childhood in the evolution of the unique human life history pattern as well as the known sensitivity of childhood development to local environments and lived experiences. In this review, I have three objectives: (1) To overview current knowledge regarding how children acquire and use energy, highlighting work among diverse human populations and pointing to recent advances and remaining areas of uncertainty; (2) To discuss key applications of this knowledge for understanding human variation, evolution, and health; (3) To recommend future avenues for research. A growing body of evidence supports a model of trade-offs and constraint in childhood energy expenditure. This model, combined with advancements on topics such as the energetics of immune activity, the brain, and the gut, provides insights into the evolution of extended human subadulthood and the nature of variation in childhood development, lifetime phenotype, and health.

A systematic review of sex differences in rough and tumble play across non-human mammals

Abstract

It is widely believed that juvenile male mammals typically engage in higher rates of rough and tumble play (RTP) than do females, in preparation for adult roles involving intense physical competition between males. The consistency of this sex difference across diverse mammalian species has, however, not yet been systematically investigated, limiting our current understanding of its possible adaptive function. This review uses narrative synthesis to (i) evaluate the ubiquity of male-biased RTP across non-human mammals, (ii) identify patterns of variation within and between taxonomic groups, and (iii) propose possible predictors of variation in these differences, including methodological and socio-ecological factors, for investigation by future studies. We find that most species studied do exhibit higher rates or RTP in males than females, while female-biased RTP is rare. Sex differences are smaller and less consistent than expected, with many studies finding similar rates of RTP in males and females. We identify multiple potential socio-ecological predictors of variation in sex differences in RTP, such as intrasexual competition and dietary niche. However, variation is not strongly phylogenetically patterned, suggesting that methodological and environmental factors, such as sample size and play partner availability, are important to consider in future comparative analyses.

Significance statement

Rough and tumble play (RTP) is thought to be vital for developing physical skills necessary for aggressive competition in adulthood, explaining an apparently widespread sex difference in RTP in mammals whereby immature males are more likely to engage in this behaviour than females. However, no prior study has systematically investigated the extent to which a male bias in RTP is consistent across diverse mammalian species. We find that although RTP is commonly male biased, findings were highly variable both within- and between-species, and equal participation in RTP by males and females is more common than widely assumed. Our review suggests several potential predictors of variation in sex differences in RTP, particularly levels of intrasexual competition in both males and females. However, our findings also suggest the importance of considering methodological in addition to socio-ecological factors for future research.

Application of a parallel laser apparatus to measure forearms and flanges of wild Bornean orangutans (Pongo pygmaeus wurmbii)

We constructed a parallel laser photogrammetry apparatus constructed from commercially available parts, and measured forearm lengths and flange widths of 16 wild Bornean orangutans. Our objectives were to validate our method and apparatus, discuss issues encountered, and construct preliminary growth curves. For adult males, we also compared flange width to forearm length as a way to investigate the relationship between body size and flange development. We used a camera cage around a DSLR camera, on top of which we attached two parallel green lasers. We estimated error with repeatability, accuracy, and interobserver reliability measures, and measured forearm lengths in three different ways to see which was most consistent. The longest forearm measure was the most repeatable (CV = 1.64%), and was similar to flange repeatability (3.50%). Accuracy measurements of a known object were high (error = 0.25%), and Interobserver discrepancy low (3.74%). Laser spacing increased with distance to the subject, but we corrected for this using calibration photos after each session. We transparently discuss the issues we encountered with the aim that this accessible method can help expand the use of laser photogrammetry. Preliminary measurements show that male flange widths and forearm length do not reliably increase in tandem, and that female growth plateaus at around the age at first birth (15 years old). We conclude with suggested improvements to the apparatus and method to ensure the lasers remain parallel.

Growth trajectories of wild Assamese macaques (Macaca assamensis) determined from parallel laser photogrammetry

Socioecological factors are associated with life-history patterns and growth trajectories among primates. Under certain conditions, selection may favor a temporal decoupling of growth and major life-history events such as sexual maturation or natal dispersal. Yet, empirical tests of these associations in wild populations remain scarce owing to the lack of non-invasive methods to capture growth trajectories. In this study, we first compared two non-invasive methods of digital photogrammetry. Then, we used parallel laser photogrammetry to investigate forearm growth of wild Assamese macaque males and females in their natural habitat at Phu Khieo Wildlife Sanctuary, Thailand to test life-history and socio-ecological hypotheses. Across 48 males and 44 females, we estimated growth trajectories and pseudo-velocity curves by applying quadratic plateau models and non-parametric LOESS regressions. We assessed the development of sexual dimorphism by comparing the sexes at five different ages. Females had completed 96% of their growth at the age at first birth (5.9 years) and ceased growing at 7.1 years of age. Males, in contrast, grew until well after their average age of natal dispersal: they reached 81% of their size at the age of natal dispersal (4.0 years), and ceased growing only at 9.0 years of age, much later than females. Sexual dimorphism in forearm length was driven by an extended growth period in males, which is expected for males dispersing between multimale and multifemale groups and not facing the risk of being ousted by other larger males. Our results contradict the neonatal investment hypothesis that predicts a desynchronization of investment in growth and reproduction only in female baboons, but not other papionins producing cheaper neonates. Furthermore, male Assamese macaques do not delay natal dispersal until they are fully grown, in accordance with predictions of the male-career-framework for species with low to medium level of direct competition.

Pattern of play behavior in infant (age 1 to 12 months) white-headed langurs in limestone forests, southwest China

Play behavior is a significant trait of immature nonhuman primates (hereafter primates) that plays an important role in sensory, locomotor, socio-cognitive, and developmental processes. It has been suggested that the function of play is to practice and improve motor skills related to foraging, avoiding predators, attracting mates, raising offspring, and strengthening the skills needed for the formation and maintenance of social bonds. From September 2009 to August 2010, we investigated the play behavior of 1-12 month-old infant white-headed langurs (Trachypithecus leucocephalus) a Critically Endangered primate species endemic to China. We carried out this study in the Guangxi Chongzuo White-headed Langur National Nature Reserve, and recorded 4421 play bouts and 1302 min of play engaged in by seven infants. We found that infants of different ages exhibited different patterns of play behavior. Specifically, nonsocial play behaviors appeared at one month of age, social play behaviors at two months, and an expanded repertoire of social and non-social play behaviors at three months of age. The frequency and duration of nonsocial play peaked at five months of age and then decreased, while social play gradually increased with age. Nonsocial play did not differ between the sexes, whereas social play showed sex specificity, with a higher frequency and longer duration of social play in male infants compared to female infants. In addition, male and female white-headed langur infants appeared to prefer individuals of same sex as social playmates, but no obvious choice preference for a specific individual. In conclusion, we provide the first report of play behavior in a population of wild Critically Endangered white-headed langurs. We suggest that age- and sex-specific differences in play behavior of infants form the bases for age and sex-based differences in the social interactions of adult langurs.

Maternal stress effects on infant development in wild Verreaux's sifaka (Propithecus verreauxi)

Abstract

Maternal effects mediated by nutrients or specific endocrine states of the mother can affect infant development. Specifically, pre- and postnatal maternal stress associated with elevated glucocorticoid (GC) output is known to influence the phenotype of the offspring, including their physical and behavioral development. These developmental processes, however, remain relatively poorly studied in wild vertebrates, including primates with their relatively slow life histories. Here, we investigated the effects of maternal stress, assessed by fecal glucocorticoid output, on infant development in wild Verreaux’s sifakas (Propithecus verreauxi), a group-living Malagasy primate. In a first step, we investigated factors predicting maternal fecal glucocorticoid metabolite (fGCM) concentrations, how they impact infants’ physical and behavioral development during the first 6 months of postnatal life as well as early survival during the first 1.5 years of postnatal life. We collected fecal samples of mothers for hormone assays and behavioral data of 12 infants from two birth cohorts, for which we also assessed growth rates. Maternal fGCM concentrations were higher during the late prenatal but lower during the postnatal period compared to the early/mid prenatal period and were higher during periods of low rainfall. Infants of mothers with higher prenatal fGCM concentrations exhibited faster growth rates and were more explorative in terms of independent foraging and play. Infants of mothers with high pre- and postnatal fGCM concentrations were carried less and spent more time in nipple contact. Time mothers spent carrying infants predicted infant survival: infants that were more carried had lower survival, suggesting that they were likely in poorer condition and had to be cared for longer. Thus, the physical and behavioral development of these young primates were impacted by variation in maternal fGCM concentrations during the first 6 months of their lives, presumably as an adaptive response to living in a highly seasonal, but unpredictable environment.

Significance statement

The early development of infants can be impacted by variation in maternal condition. These maternal effects can be mediated by maternal stress (glucocorticoid hormones) and are known to have downstream consequences for behavior, physiology, survival, and reproductive success well into adulthood. However, the direction of the effects of maternal physiological GC output on offspring development is highly variable, even within the same species. We contribute comparative data on maternal stress effects on infant development in a Critically Endangered primate from Madagascar. We describe variation in maternal glucocorticoid output as a function of ecological and reproductive factors and show that patterns of infant growth, behavioral development, and early survival are predicted by maternal glucocorticoids. Our study demonstrates how mothers can influence offspring fitness in response to challenging environmental conditions.

Short-Term Effects of Video-Games on Cognitive Enhancement: the Role of Positive Emotions

According to established background knowledge, playing is essential in human development and a power remediation tool in clinical populations. In clinical interventions, the beneficial roles of playing have often been sought and investigated in the specific features of the game, rather than in the positive emotions generated by playing. However, regardless of game specifications, cognitive enhancement could be driven by the emotions linked to play. Establishing the causal connections between play and cognitive enhancement should allow us to determine how to involve play in therapy, prevention and educational programmes. Today, video-gaming is one of the most diffused forms of play. In the first crossover randomized controlled trial, we compared the short-term effects induced by shooting and puzzle video-games in visual perception, sensorimotor and reading skills in children with developmental coordination disorder and dyslexia. The funnier and more activating game enhanced breadth of visual perception and reduced sensorimotor and reading disorders. Visual perception, sensorimotor and reading improvements correlated with fun. In the second crossover randomized controlled trial, comparing the effects of the same shooting with a fighting video-game in healthy young adults, we show that regardless of game characteristics, changes in positive emotions correlated with contextual reading enhancement, while play-driven biochemical activation boosted single word and pseudoword reading. The short-term effects induced by play could be a useful clinical tool for the prevention and treatment of multiple cognitive disorders.

Sex-specific differences in somatic investment and strategies of physical activity among Portuguese schoolchildren

OBJECTIVES

Physical activity (PA) is required for healthy growth, development, and maturation and plays an important role in the prevention of overweight and obesity in childhood and adolescence. Sex-differences in PA levels are well documented, with boys spending more time in PA, especially in moderate-to-vigorous activities. Following the Life History Theory, our aim is to study if PA affects the fat tissues increases during childhood and juvenile phases in both sexes.

METHODS

Time spent in sedentary, light, and moderate-to-vigorous PA levels were measured in a sample of 415 Portuguese children and juveniles (207 females/208 males; aged 6-11 years), using an accelerometer for 7 days. Skinfolds related with body fat were objectively collected and socioeconomic status factors were reported using a parental questionnaire.

RESULTS

The outcomes show that girls' and boys' fat variables increased during the end of the childhood and the juvenile phase. However, these variables were differently affected by PA. Girls increased fat variables with the sedentary activity while boys decreased fat variables with moderate-to-vigorous PA. Alike, active boys but not girls reduced the fat increase tendency with age.

CONCLUSIONS

Although both sexes displayed a general fat increment with age, moderate-to-vigorous PA dampens the increase only in boys. In fact, active girls increased body fat in the same manner as non-active girls. From an evolutionary perspective, it could explain sex-specific somatic strategies related to future reproduction or, with future mating and intrasexual competition.

A cross-species neural integration of gravity for motor optimization

Recent kinematic results, combined with model simulations, have provided support for the hypothesis that the human brain shapes motor patterns that use gravity effects to minimize muscle effort. Because many different muscular activation patterns can give rise to the same trajectory, here, we specifically investigate gravity-related movement properties by analyzing muscular activation patterns during single-degree-of-freedom arm movements in various directions. Using a well-known decomposition method of tonic and phasic electromyographic activities, we demonstrate that phasic electromyograms (EMGs) present systematic negative phases. This negativity reveals the optimal motor plan's neural signature, where the motor system harvests the mechanical effects of gravity to accelerate downward and decelerate upward movements, thereby saving muscle effort. We compare experimental findings in humans to monkeys, generalizing the Effort-optimization strategy across species.

Sex-Specific Variation of Social Play in Wild Immature Tibetan Macaques, Macaca thibetana

Simple Summary

Social play among immature individuals has been well-documented across a wide range of mammalian species. It represents a substantial part of the daily behavioral repertoire during immature periods, and it is essential for acquiring an appropriate set of motor, cognitive, and social skills. In this study, we found that infant Tibetan macaques (Macaca thibetana) exhibited similar patterns of social play between males and females, juvenile males engaged more aggressive play than juvenile females, and juvenile females engaged more affiliative play than juvenile males. Our results provided more evidence to understand the functional differences of social play in immature nonhuman primates.

Abstract

Theories proposed to explain social play have centered on its function in establishing social relationships critical for adulthood, its function in developing motor skills needed to survive, and promoting cognitive development and social learning. In this study, we compared variations in social play among infant and juvenile male and female Macaca thibetana. Given that this species is characterized by female philopatry and male dispersal, we hypothesized that immature females use social play as a mechanism to develop bonds that persist through adulthood whereas immature males use play to develop social skills needed to successfully enter new groups. The results indicated that social play steadily increased during the infant period and peaked at approximately 12 months of age. There were no significant differences in the frequency or types of social play exhibited between infant males and infant females. During the juvenile period, however, social play was found to decrease with age, with males engaging in social play more frequently than juvenile females. Moreover, whereas juvenile males engaged in more aggressive forms of play, juvenile females engaged in more affiliative forms of play. In addition, juvenile females engaged in higher rates of grooming than juvenile males. These results provide evidence of sex-specific differences and imply the functional variation of social play in Tibetan macaques, with immature males using social play to develop skills needed to enter and enhanced their dominance rank in a new social group and immature females using social play to develop long-term same-sex social bonds in their natal group.

Let's unite in play! Play modality and group membership in wild geladas

Two of the main hypotheses put forth to explain the function of immature social play are the Social Skill Hypothesis and the Motor Training Hypothesis focussing on whether play can improve social competence to develop cooperative social networks or physical abilities to outcompete others, respectively. Here, we tested these hypotheses on a monkey species, the wild gelada (Theropithecus gelada) from the Kundi plateau, Ethiopia. This species is organized in bands divided in One-Male Units (OMUs), united only via social play. Immatures form 'play units' in which individuals from the same and different OMUs interact. We analysed the potential differences between inter- and intra-OMU play to verify which of the two hypotheses (Social Skill or Motor Training Hypothesis) best explains the function of play in geladas. We analysed 527 video-recorded social play sessions and found mixed support for both hypotheses. In agreement with the Social Skill Hypothesis, we found that play in geladas shows scarce social canalization being similarly distributed across age, sex and group membership. In line with the Motor Training Hypothesis, we detected higher levels of competition (shorter and more unbalanced sessions) in inter-OMU compared to intra-OMU play. Hence, in geladas play can be a tool for both the development of social relationships and the improvement of the physical skills necessary to cope with either future mates or competitors. In conclusion, neither hypothesis can be discarded and both hypotheses concur in explaining why immature geladas peculiarly form 'play units' embracing both ingroup and outgroup members.

Early-life behaviour predicts first-year survival in a long-distance avian migrant

Early-life conditions have critical, long-lasting effects on the fate of individuals, yet early-life activity has rarely been linked to subsequent survival of animals in the wild. Using high-resolution GPS and body-acceleration data of 93 juvenile white storks (Ciconia ciconia), we examined the links between behaviour during both pre-fledging and post-fledging (fledging-to-migration) periods and subsequent first-year survival. Juvenile daily activity (based on overall dynamic body acceleration) showed repeatable between-individual variation, the juveniles' pre- and post-fledging activity levels were correlated and both were positively associated with subsequent survival. Daily activity increased gradually throughout the post-fledging period, and the relationship between post-fledging activity and survival was stronger in individuals who increased their daily activity level faster (an interaction effect). We suggest that high activity profiles signified individuals with increased pre-migratory experience, higher individual quality and perhaps more proactive personality, which could underlie their superior survival rates. The duration of individuals' fledging-to-migration periods had a hump-shaped relationship with survival: higher survival was associated with intermediate rather than short or long durations. Short durations reflect lower pre-migratory experience, whereas very long ones were associated with slower increases in daily activity level which possibly reflects slow behavioural development. In accordance with previous studies, heavier nestlings and those that hatched and migrated earlier had increased survival. Using extensive tracking data, our study exposed new links between early-life attributes and survival, suggesting that early activity profiles in migrating birds can explain variation in first-year survival.

Play behavior in immature moor macaques (Macaca maura) and Japanese macaques (Macaca fuscata)

Play is widespread across mammalian taxa, but species strongly vary in the ways they play. In less despotic primate species (i.e., with less steep dominance hierarchies, less severe conflicts, and more reconciliation), play has been described as being more frequent, cooperative, and freely expressed. To study the link between social play and dominance style, we compared play behavior in free-ranging infants, juveniles and subadults of more despotic Japanese macaques (Macaca fuscata, N = 24) and less despotic moor macaques (Macaca maura, N = 17). We found interspecific differences in play behavior that corresponded with the contrasting dominance styles of the study species, largely confirming our predictions. In particular, moor macaques spent a larger proportion of time in solitary and social play than Japanese macaques, while Japanese macaques spent a larger proportion of time in grooming interactions. In moor macaques, play sessions included more players, a larger variety of play behaviors, greater play face rates, a greater proportion of time in contact play, and a higher rate of reciprocal play-biting than in Japanese macaques. Aggressive escalations were not common, but more frequent in Japanese macaques. Finally, a higher frequency of play faces during play sessions predicted the occurrence of more reciprocal play-bites, but not the proportion of time spent in contact play behaviors. Additional studies on other groups and species will allow a better understanding of the link between dominance style and social play.

Bringing up baby: Maternal responsiveness, secondary attachments, and the development of infant social competence in wild olive baboons (Papio anubis)

Caregiver responsiveness and presence of secondary attachments play a crucial role in children's socio-cognitive and emotional development, but little is known of their effect on the development of non-human primates. Here we present the results of a 16-month behavioral study conducted on 22 wild infant olive baboons (Papio anubis) at the Uaso Ngiro Baboon Project, Kenya. This is the first study to examine the effects of maternal responsiveness and secondary attachments on the development of infant social behavior in a wild primate species that does not breed cooperatively. The data track maternal responsiveness and the rates of two behavioral indicators of infant social competence-orienting toward interactions and social play-over the course of the first year of life. Maternal responsiveness decreased as infants grew older, while infant orientation toward interactions and play behavior increased. Infants with poorly responsive mothers were more likely to have secondary attachments, and infants with secondary attachments to siblings oriented more frequently to social interactions than those with secondary attachments to adult/subadult males or with no secondary attachments. These findings indicate that variation in maternal responsiveness and presence of secondary attachments can influence the development of social competence in olive baboon infants.

Play in Elephants: Wellbeing, Welfare or Distraction?

Simple Summary

Animal play is a subject of great interest and some enduring controversy. Why do animals play, when do they play and if they do not play much, does this indicate that they may be physically or emotionally stressed? We explore these questions for elephant calves ranging in age from birth to five years old, and we compare play in captivity with that observed in the wild for two species. Against our general expectation that calves might play less play in captivity, we found that wild elephants spent the least time in play, probably because wild calves have to solve other social problems and be on the move constantly in order to find enough food, escape from predators, and keep up with their mothers and other relatives. Play is a diverse and subtle potential indicator of wellbeing for young animals, and we suggest that its presence needs to be interpreted with caution as it could represent either a distraction from a constant or unchallenging environment or provide arousal. Play appears to act as a behavioural mechanism for creating physical and social challenges for elephants of all ages, irrespective of their environment.

Abstract

We explore elephant play behaviour since (a) play has been proposed to represent a potential welfare indicator; and (b) play has been associated with long-term survival in the wild. We categorised play into four types, and investigate both social (gentle, escalated-contact) and non-social (lone-locomotor, exploratory-object) play from observations made on wild (Asian N = 101; African N = 130) and captive (Asian N = 8; African N = 7) elephant calves ranging in age from birth to five years. Social play was the most frequent type of play among immature elephants, accounting for an average of 3%–9% of active time. Non-social play accounted for an additional 1%–11% of time. The most time spent in play was seen in captive Asian calves, particularly at the ages of 1–6 months, while wild African calves spent the least time in play overall, even though they had the greatest number and most diverse range of play partners available. We assessed calf energetics using time spent suckling, resting, moving and independent feeding. Time spent playing was unrelated to time spent suckling but negatively associated with time spent independently feeding. There were no associations with time spent moving or resting. Maternal energy via lactation was unrelated to play early in life, but energy acquired independently may constrain or enable play. Play, while a potential indicator of compromised welfare for many species when absent, can act as a highly stimulating activity for captive elephants in the absence of other forms of arousal.

Developmental responses to early-life adversity: Evolutionary and mechanistic perspectives

Adverse ecological and social conditions during early life are known to influence development, with rippling effects that may explain variation in adult health and fitness. The adaptive function of such developmental plasticity, however, remains relatively untested in long-lived animals, resulting in much debate over which evolutionary models are most applicable. Furthermore, despite the promise of clinical interventions that might alleviate the health consequences of early-life adversity, research on the proximate mechanisms governing phenotypic responses to adversity have been largely limited to studies on glucocorticoids. Here, we synthesize the current state of research on developmental plasticity, discussing both ultimate and proximate mechanisms. First, we evaluate the utility of adaptive models proposed to explain developmental responses to early-life adversity, particularly for long-lived mammals such as humans. In doing so, we highlight how parent-offspring conflict complicates our understanding of whether mothers or offspring benefit from these responses. Second, we discuss the role of glucocorticoids and a second physiological system-the gut microbiome-that has emerged as an additional, clinically relevant mechanism by which early-life adversity can influence development. Finally, we suggest ways in which nonhuman primates can serve as models to study the effects of early-life adversity, both from evolutionary and clinical perspectives.

Does Juvenile Play Programme the Equine Musculoskeletal System?

Simple Summary

Locomotor play is a common behaviour expressed across a diverse range of species. As a cursorial animal, the horse is capable of locomotor activity within a relatively short time after birth. In the foal, spontaneous locomotor play occurs early in life and has an obvious role in the development of locomotor skills. The intensity and vigour of locomotor play increases with age and this, in turn, provides cumulative increases in the loads the musculoskeletal system experiences. These progressive cumulative loading cycles (bouts of locomotor play), in both the timing and magnitude, reflect the microstrain required to stimulate bone development based on the mechanostat theorem. Data from the published literature were presented to provide empirical support for this hypothesis. Thus, spontaneous locomotor play may be vital to ensure optimal bone development in the horse. Modern production systems need to provide appropriate opportunities for foals to perform spontaneous locomotor play to optimise bone development and reduce the risk of future musculoskeletal injury later in life.

Abstract

In mammals, play behaviour appears innate and, because of this, may provide insight into the frequency and intensity of load that is required to stimulate positive musculoskeletal development. The objective of this review was to explore the interaction between play and tissue (bone) development at a molecular through to whole-animal level, with specific focus on the horse as a model. The basis of our understanding of the response of bone to loading is the mechanostat theorem. This assumes that at a tissue level, bone attempts to keep localised strain within the physiological range of 1500–2500 microstrain. Loads above this range result in a modelling response to reduce strain, and strain below this threshold results in remodelling to maintain the localised physiological range. In foals, locomotor play is dramatic and vigorous, with cumulative increases in both intensity and complexity. Based on published literature describing locomotor play in foals and the microstrain at different gaits in the horse, it was proposed that locomotor play in foal aligns with the mechanostat theorem in both the magnitude and frequency of load cycles applied. The cumulative increases in the complexity and intensity of locomotor play as the foal develops, in turn, ensure the strain rates associated with play remain above the local physiological range and promote material and architectural changes in the distal limb bones. Thus, spontaneous locomotor play may be vital to ensure optimal bone development in the horse. Modern management systems need to provide appropriate opportunities for foals to perform spontaneous locomotor play to optimise bone development and reduce the risk of future musculoskeletal injury later in life.

Water games by mountain gorillas: implications for behavioral development and flexibility-a case report

Functions of play, which may be performed solo or in a social context, include motor training and behavioral flexibility. Play is often more common in infancy and the juvenile period, although it also occurs in adults of many species. In contrast to social play, few studies have investigated solitary play. Here, we present new empirical data on solitary water play in a subadult and two adult mountain gorillas in Bwindi Impenetrable National Park, Uganda, observed on three different days between January and February 2018. Focal sampling was used to record the behavior of the individuals interacting with water. Movements included vigorous rotation of the arms, splashing the water, tilting the head, making a play face, and sweeping with the hands to create waves on the water surface. One of the episodes represents the first vigorous display of splashing water ever reported for Bwindi gorillas. Our observations highlight three significant components of mountain gorilla development and behavior: play, behavioral flexibility, and exploration.

Steroid hormone concentrations in milk predict sex-specific offspring growth in a nonhuman primate

OBJECTIVES

In humans and other mammals, maternal hormones are transferred to offspring during lactation via milk and may regulate postnatal development, including the pace of early growth. Here, we used a nonhuman primate model to test the hypotheses that milk cortisol and dehydroepiandrosterone-sulfate (DHEAS) concentrations reflect maternal characteristics, and that changes in these hormones across lactation are associated with early postnatal growth rates.

METHODS

Demographic information, morphometrics, and milk samples were collected from rhesus macaque mothers and their infants at the California National Primate Research Center in Davis, California. Using linear models, we examined the relationship between maternal traits and milk hormone concentrations (N = 104 females) and explored the effect of milk hormones on the rate of offspring growth (N = 72 mother-infant dyads), controlling for available milk energy.

RESULTS

Contrary to previous studies, we found that milk cortisol concentrations were categorically higher in multiparous females than in primiparous females. However, milk DHEAS concentrations decreased with maternal parity. Neither milk cortisol nor DHEAS were related to maternal rank. Finally, changes in milk hormones predicted offspring growth in a sex-specific and temporal manner: increases in cortisol from peak to late lactation predicted faster female growth, and increases in DHEAS concentrations from early to peak and peak to late lactation predicted faster male growth.

CONCLUSIONS

Our findings shed light on how hormonal components of milk have sex-specific effects on offspring growth during early postnatal life with varying temporal windows of sensitivity.

Adaptive developmental plasticity in rhesus macaques: the serotonin transporter gene interacts with maternal care to affect juvenile social behaviour

Research has increasingly highlighted the role that developmental plasticity-the ability of a particular genotype to produce variable phenotypes in response to different early environments-plays as an adaptive mechanism. One of the most widely studied genetic contributors to developmental plasticity in humans and rhesus macaques is a serotonin transporter gene-linked polymorphic region (5-HTTLPR), which determines transcriptional efficiency of the serotonin transporter gene in vitro and modifies the availability of synaptic serotonin in these species. A majority of studies to date have shown that carriers of a loss-of-function variant of the 5-HTTLPR, the short (s) allele, develop a stress-reactive phenotype in response to adverse early environments compared with long (l) allele homozygotes, leading to the prevalent conceptualization of the s-allele as a vulnerability allele. However, this framework fails to address the independent evolution of these loss-of-function mutations in both humans and macaques as well as the high population prevalence of s-alleles in both species. Here we show in free-ranging rhesus macaques that s-allele carriers benefit more from supportive early social environments than l-allele homozygotes, such that s-allele carriers which receive higher levels of maternal protection during infancy demonstrate greater social competence later in life. These findings provide, to our knowledge, the first empirical support for the assertion that the s-allele grants high undirected biological sensitivity to context in primates and suggest a mechanism through which the 5-HTTLPR s-allele is maintained in primate populations.

Differences in play can illuminate differences in affiliation: A comparative study on chimpanzees and gorillas

Play behaviour reinforces social affiliation in several primate species, including humans. Via a comparative approach, we tested the hypothesis that play dynamics in a group of lowland gorillas (Gorilla gorilla gorilla) are different from those in a group of chimpanzees (Pan troglodytes) as a reflection of their difference in social affiliation and agonistic support. We selected one group of lowland gorillas and one of chimpanzees, hosted at the ZooParc de Beauval (France), managed in a similar way and living in similar enclosures. The same observers video-collected and analysed data on play behaviour in both groups, by applying identical methodological procedures. Data showed that adult play was less frequent in the group of gorillas compare to chimpanzees. Polyadic play, which involves more than two players and is characterised by the most uncertain outcome, was also less frequent in gorillas than chimpanzees. Play sessions were more unbalanced (more unidirectional patterns by one of the player towards the other) in chimpanzees than in gorillas but in the latter play escalated more frequently into serious aggression. Play asymmetry in the gorilla group increased as the number of players increased, which explains why gorillas limited their polyadic playful interactions. In conclusion, our findings on the study groups of apes can be a valuable starting point to expand the study of social play in the great apes to evaluate if inter-individual affiliative relationships really account for the differences in play distribution and dynamics.

Evidence for energetic tradeoffs between physical activity and childhood growth across the nutritional transition

Despite broad implications for understanding human life history, energetics, and health, the impact of physical activity on childhood growth remains unclear. Particularly understudied is the effect of secular changes in physical activity on child development. We address these shortcomings using data spanning the transition from traditional to semi-developed economy among Yucatec Maya agriculturalists. Anthropometric and behavioral observation data were collected from children living in a subsistence-based rural community in 1992 and again in 2012 following the introduction of a school and mechanized technologies but minimal overt dietary change. Multiple regression analyses demonstrate dramatic twenty-year transformations in how children spent their time. This behavioral change was associated with large declines in estimated physical activity level (PAL), associated activity energy expenditure savings of several hundred kilocalories/day, and sizable increases in mean height, weight, and triceps skinfold thickness. Controlling for observed frequency of market food consumption, PAL was inversely related to child body size and subcutaneous fat stores and significantly mediated the effects of data collection year on anthropometric indices. These findings indicate that physical activity can considerably influence childhood growth, highlighting the role of energy allocation tradeoffs between physical activity and competing life tasks in shaping patterns of human ontogeny and health.