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Play fighting during the juvenile period has been shown to be an important experience for the development of sociocognitive skills and the underlying neural mechanisms that support them. Various paradigms have been used to deprive rats of play while still providing social contact. We used the paradigm of rearing a playful rat with a low-playing Fischer 344 (F344) partner to limit the play experienced by Long Evans (LE) rats during the juvenile period. This rearing paradigm has previously been shown to cause sociocognitive impairments in adulthood. In the present paper, we examined the play of same sex LE rats with LE or F344 partners at the peak juvenile period (around 35 days of age). F344 rats launched fewer playful attacks and when attacked, defended atypically compared to how LE do in LE-LE pairs. Playing with an F344 partner afforded LE rats fewer opportunities to engage in prolonged wrestling and fewer opportunities to ward off counterattacks (in which the defending rat becomes the attacker). In addition, there are fewer vocalizations emitted during the encounters in LE-F344 pairs and the types of calls most often emitted differed to those between LE-LE pairs. The altered play and communication experiences were equally present in male and female pairs. These findings are consistent with the hypothesis that, in such rearing paradigms, it is impoverished play experiences in the juvenile period that lead to impaired sociocognitive skills in adulthood.

Several studies on rats and hamsters, across multiple laboratories, have shown that limiting play in the juvenile period leads to adults that have physiological and anatomical changes in the medial prefrontal cortex (mPFC) and reduced socio-cognitive skills. Peers raised with playful peers have better socio-cognitive skills than animals raised with adult partners. Using Long Evans hooded rats - a commonly used domesticated strain - this relationship has been replicated multiple times. However, when the same paradigm was used with laboratory-reared wild rats, no differences were found between rats reared with peers and ones reared with adults. It has been shown that the key play-generated experiences involved are those related to actively wrestling with a partner and turn taking (as measured by role reversals), which give both partners opportunity to gain the advantage during play fighting. In the present study, we tested the hypothesis that wild rat adults provide juveniles more such experiences than do adult Long Evans rats. The asymmetry in the play interactions in adult-juveniles pairs was compared between the two strains. As predicted, wild rat adults initiated more play with the juveniles, wrestled more and provided more opportunities for role reversals. The findings thus support the hypotheses for the observed strain differences in the effects of rearing condition on the mPFC.

Play fighting in many species involves partners competing to bite one another while avoiding being bitten. Species can differ in the body targets that are bitten and the tactics used to attack and defend those targets. However, even closely related species that attack and defend the same body target using the same tactics can differ markedly in how much the competitiveness of such interactions is mitigated by cooperation. A degree of cooperation is necessary to ensure that some turn-taking between the roles of attacker and defender occurs, as this is critical in preventing play fighting from escalating into serious fighting. In the present study, the dyadic play fighting of captive troops of 4 closely related species of Old World monkeys, 2 each from 2 genera of Papio and Mandrillus, was analyzed. All 4 species have a comparable social organization, are large bodied with considerable sexual dimorphism, and are mostly terrestrial. In all species, the target of biting is the same – the area encompassing the upper arm, shoulder, and side of the neck – and they have the same tactics of attack and defense. However, the Papio species exhibit more cooperation in their play than do the Mandrillus species, with the former using tactics that make biting easier to attain and that facilitate close bodily contact. It is possible that species differences in how rigidly dominance relationships are maintained are expressed in the play of juveniles by altering the balance between competition and cooperation.

Juvenile animals of many species engage in social play, but its functional significance is not well understood. This is especially true for a type of social play called fair play (Fp). Social play often involves behavioral patterns similar to adult behaviors (e.g., fighting, mating, and predatory activities), but young animals often engage in Fp behaviors such as role-reversals and self-handicapping, which raises the evolutionary problem of why Fp exists. A long-held working hypothesis, tracing back to the 19th century, is that social play provides contexts in which adult social skills needed for adulthood can be learned or, at least, refined. On this hypothesis, Fp may have evolved for adults to acquire skills for behaving fairly in the sense of equitable distribution of resources or treatment of others. We investigated the evolution of Fp using an evolutionary agent-based model of populations of social agents that learn adult fair behavior (Fb) by engaging in Fp as juveniles. In our model, adults produce offspring by accumulating resources over time through foraging. Adults can either behave selfishly by keeping the resources they forage or they can pool them, subsequently dividing the pooled resources after each round of foraging. We found that fairness as equitability was beneficial especially when resources were large but difficult to obtain and led to the evolution of Fp. We conclude by discussing the implications of this model, for developing more rigorous theory on the evolution of social play, and future directions for theory development by modeling the evolution of play.

Social interactions are often analyzed by scoring segments of predefined behavior and then statistically assessing numerical and sequential patterns to identify the structure of the encounters. However, this approach can miss the dynamics of the animals' relationship over the course of the encounter, one that often involves invariant bonds, say a nose-to-nose orientation, with many different movements performed by both partners acting to counteract each other's attempts to break or maintain the relationship. Moreover, these invariant bonds can switch from one configuration to another during an interaction, leading from one stable configuration to another. It is this stepwise sequence of configurational stabilities that lead to functional outcomes, such as mating, aggression, or predation. By focusing on the sequence of invariant relational configurations, the deep structure of interactions can be discerned. This deep structure can then be used to differentiate between compensatory movements, no matter how seemingly stereotyped they may appear, from movement patterns which are restricted to a particular form when more than one option is available. A dynamic perspective requires suitable tools for analysis, and such tools are highlighted as needed in describing particular interactions. [ABSTRACT FROM AUTHOR]

Studies of rats and some primates show that rough-and-tumble play among juveniles improves social competence, cognition, and emotional regulation later in life. Most critically, such play makes animals better able to respond to unexpected situations. But not all animals engage in play, and not all animals that play appear to gain these benefits. Using a model developed by Burghardt (2005), the authors argue that there are enabling conditions--such as how behavior systems develop and the presence of surplus resources--that make play-like behavior possible. Once such behavior emerges, other enabling conditions help transform it into more exaggerated patterns of play that can be co-opted for various functions. For species living in complex social systems with an extended juvenility, play has become a tool to refine the control that the prefrontal cortex has over other neural circuits. Such control permits these animals to have more nuanced responses to a variety of situations. In short, the juvenile experience of play refines the brain to be more adaptable later in life.

During play fighting, rats attack and defend the nape and during the course of this competitive interaction, they may adopt a configuration in which one animal stands over its supine partner (i.e., pin). Because the pin configuration is typically frequent and relatively easy to identify, it has been widely used as a marker to detect the effects of experimental treatments. In the present study, the frequency of pinning during standardized, 10min trials in three strains of rats, Long Evans hooded (LE), Sprague-Dawley (SD) and wild (WWCPS), was compared. LE and SD had higher rates than WWCPS rats (#/min: 6.5, 5.5, 1.5, respectively). When adjusted for strain differences in the frequency of attacks, SD as well as WWCPS rats had lower rates of pinning compared to LE rats. Both SD and WWCPS rats were less likely to use tactics of defense that promote pinning. Moreover, while the majority of the pins achieved in LE rats arose from the defender actively rolling over onto its back, the majority of pins in WWCPS rats arose because one partner pushed the other onto its back. SD rats were intermediate in this regard. Finally, once they do adopt the pin configuration, SD rats are less likely to remain supine than LE and WWCPS rats. That is, both SD and WWCPS rats have significantly fewer pins than LE rats, but a different combination of factors account for this. These data highlight the need to use a battery of measures for ascertaining the effects of experimental manipulations on play. Some suggested guidelines are provided.

Rough-and-tumble play, or play fighting, is common in the young of many mammals. Research on play fighting among rats shows that there are many levels of neural control over this behavior: subcortical mechanisms mediate the motivation and behavior of such play, and the cortex provides mechanisms by which the play changes with age and context. The cortical mechanisms help to explain the advantages playing offers the brain. The cortically induced modulations of the content of play with age ensure that exposure to particular kinds of experiences are enhanced during the critical juvenile period. These experiences, in turn, modify the development of other areas of the cortex. Such cortical changes appear to mediate the effects of play on the refinement of social skills. As a result, rats that play as juveniles are more socially competent as adults. This work was supported by the Natural Sciences and Engineering Research Council of Canada and the Alberta Heritage Foundation for Medical Research.

Play fighting in rats involves attack and defense of the nape. To protect the nape, rats use a variety of defensive tactics, with different strains having specific preferences. Targeting of the nape is established before weaning and defense matures over the course of the week preceding and the week proceeding weaning. Thus, it is possible that experience from engaging in immature forms of play is needed to consolidate the nape as the playful target and for the development of the juvenile-typical pattern of defense. Two experiments were conducted to evaluate this possibility. For the first experiment, male rats were reared over the week post-weaning in either pairs or alone, and their play tested with unfamiliar partners when juveniles (31-34 days). For the second experiment, during the week preceding weaning, male and female rats were placed into one of three conditions: (1) with the mother and no peers, (2) with same-sex siblings but no mother or (3) with both the mother and same-sex siblings. The subjects were tested in same-sex, same-condition pairs when juveniles (31-34 days). Rats from all conditions, in both experiments, attacked the nape during play fighting and developed the same juvenile-typical patterns of playful defense. This suggests that the experience of peer-peer play in the peri-weaning period is not necessary for the development of the attack and defense components of juvenile-typical play.

During play fighting, rats attack and defend the nape, which if contacted is nuzzled with the snout. While all strains of rats can use all defensive tactics to protect the nape, there are strain-typical preferences for using particular tactics This study tests two hypotheses for this strain difference: (1) that each strain has strain-specific thresholds for each tactic, or (2) that each strain attacks differently which leads to strain differences in which defense tactics are used. Juvenile Long-Evans and Sprague-Dawley males were tested with both unfamiliar (experiment 1) and familiar (experiment 2) same-strain and different-strain partners. Experiment two was conducted to determine if familiarity with a different strain might allow rats to modify their strain-typical pattern of play. If hypothesis (1) were true, they would maintain strain-typical defense patterns irrespective of partner strain, whereas for (2) it would vary with partner strain. Hypothesis (1) was supported in the first experiment; all the rats maintained their strain-typical patterns regardless of the partner’s strain. However, the second experiment supported neither hypothesis, as each animal displayed strain-divergent behavior when playing with partners of a different strain as well as with partners of the same strain. Given that in the second experiment subjects were reared in mixed-strain groups, it is possible that, during the early juvenile period, animals are susceptible to discordant social experiences

Complex movements, such as placing food into the mouth, involve coordinating multiple limb segments. Given the degrees of freedom for one limb segment, the computational costs of such complex movements can be high. One way to reduce such costs is to limit the adjusting movements needed to achieve coordination of distal body parts to only one part of the body. For example, for scratching the head, the hand or foot needs to make contact with the head and this involves movements of the head, neck and torso, as well as those of the foot and leg, or hand and arm. In this situation, the foot or hand is raised to a specific location in space and then makes oscillatory movements, but it is movements by the head and neck that ensure appropriate contact is made with the head (Pellis, 2010). In this paper, whether such cost-saving rules apply across functional contexts is tested in the purple swamp hen by comparing head and foot coordination during head scratching and during food reaching and handling. This species uses its foot to grasp and hold a wide range of food items that are picked up in its bill. Comparison of hundreds of videotaped sequences revealed that, in both cases, the bird uses the same rule: that of making the accommodating movements with only one of those body parts, even when coordination requires movements of disparate parts of the body. These data show that there are likely common computational cost-saving rules that widely apply to movements occurring in many different functional contexts.

Macaques are the most geographically widespread and behaviorally diverse primate genus, and although macaque species share the same, basic social structure, they display broad inter specificvariation in patterns of adult social behavior. Based on these patterns, macaque species have been arranged along a 4-grade scale for social style. At one end of the scale, there are grade 1 species (e.g., Japanese macaques) that have highly hierarchical and despotic social systems, and at the other end, grade 4 species (e.g., Tonkean macaques) that have more relaxed and egalitarian social systems. We predicted that a species from the more despotic end of the spectrum should have more competitive play fights and that a species from the egalitarian end, more cooperative ones. A detailed analysis of videotaped sequences of juvenile play fighting in Tonkean and Japanese macaques was used to characterize the targets and tactics of attack and defense. Even though the two species have a similar behavioral repertoire, there are significant differences in how that repertoire is used and these differences are consistent with one species having more competitive interactions than the other. Contrasting multi-animal play fights versus pairs showed that the more cooperative style of the Tonkean macaques is further exaggerated. The results suggest that differences in styles of attack anddefense in play fighting may be influenced by differences in the species’ social systems.

While the behavior of many animals can be identified as involving discrete and stereotyped actions, there is a persistent tension between emphasizing the fixedness of the actions (“Fixed Action Patterns”) and emphasizing the variation in the components comprising those actions (“Modal ActionPatterns”). One such action, the back and forward judder of crickets often exhibited in agonistic interactions, was analyzed. Judders occurring on a horizontal surface by Gryllus bimaculatus were compared to those occurring on an inclined platform. Although the body movements involved were variable, that variability occurred in the context of maintaining some features of judder invariant. For example, the crickets maintained their bodies so that they were horizontal relative to the substrate, not to gravity, and most features of the back and forward movement (e.g., distance moved, velocity) were maintained as fixed despite differences in posture and movement. At a theoretical level, what these findings suggest is that behavior patterns involve a combination of fixedness and variation in the service of that fixedness. It becomes an empirical issue to discern these complementary components.

Play is rare in the Animal Kingdom, but relatively common in the larger brained vertebrate taxa. Comparisons at the level of classes, orders, and, in some cases, families, suggest that larger brained taxa are more likely to contain playful species. However, at the species level, such relationships generally disappear. In some well documented mammalian taxa, such as Rodentia, it is clear that there are species which do not play at all, some where the play is quite complex and some showing all grades in between. Comparative methods are used here to supplement proximal analyses of the content of one particular form of play, play fighting, so as to identify the neurobehavioral mechanisms that are needed in rodents to evolve complex play from simpler antecedents. At least five independent neural mechanisms are shown to be necessary to produce the most complex example of play fighting in rodents. The identification of such levels of control provides a new method for systematizing the diversity of play present in mammals. Furthermore, this approach sets the stage for re-evaluating the relationship between brain size and play. That is, the issue can be reconceptualized in terms of whether species with larger brains are more likely to have a greater number of control mechanisms. It is not that larger brained species are more likely to play, but rather, that when they do play, the content of their play is more flexible. Suitable comparative data sets are needed to test these possibilities.

During play fighting, rats often assume a pinning configuration, where one animal stands over its supine partner. The on-top partner can stand on the ground or on its supine partner with its hind paws. When standing on the ground, the rat is more stable and is better able to respond to its partner’s actions. The frequency of this more stable pattern of standing during pinning (called anchoring) is higher following puberty than during the juvenile phase. Two hypotheses explaining this developmental change in anchoring were tested. The first hypothesis maintains that the lower level of anchoring in juvenile rats reflects an immature sensorimotor capability. The second hypothesis suggests that, since rats are more playful as juveniles, such heightened levels of play may interfere with movements otherwise used to maintain the stable anchored position at this age. Neither hypothesis was supported: infants have similar anchoring levels to postpubertal rats, and juveniles have a relatively low level of anchoring irrespective of how frequently they play fight. Therefore, the lower level of anchoring in the juvenile phase appears to be a developmental peculiarity of this age. These findings support the view that play fighting in the juvenile phase may be organized in a manner to increase the occurrence of the experiences that are developmentally beneficial in this activity.

Play behavior is typically most frequent in immature animals and then declines by adulthood. New research reveals a brain mechanism that may underlie this age-related decline in play. [ABSTRACT FROM AUTHOR]