Vivien C. Pellis

The impairments in reaching and the movements of compensation in rats with motor cortex lesions: an endpoint, videorecording, and movement notation analysis

Reaching for food by rats, with the limb contralateral to limb area motor cortex damage, was analyzed using end-point scores, videoanalysis, and Eshkol-Wachmann Movement Notation (EWMN). End point results from groups of rats with small, medium, and large lesions showed reaching success and amount of food grasped per reach decreased with increases in lesion size. Videoanalysis and EWMN showed that the impairments were attributable to: (1) an inability to pronate the paw over the food by abduction of the upper arm, and (2) an inability to supinate the paw at the wrist to orient the food to the mouth. There were no obvious impairments in locating food using olfaction, in positioning the body in order to initiate a reach, or in clasping the digits to grasp food. There were only mild impairments in lifting, aiming, and advancing the limb. In rats with medium and large lesions, loss of pronation and supination were compensated for by a variety of whole body movements. These findings are discussed in reference to neural and behavioral mechanisms underlying recovery of function and the contribution of the motor cortex to skilled movements in the rat and other species.

Play-fighting differs from serious fighting in both target of attack and tactics of fighting in the laboratory rat Rattus norvegicus

Play-fighting is often difficult to differentiate from inhibited or immature serious fighting because both may utilize many of the same behavior patterns. In the rat the two behaviors involve different targets of attack. During play-fighting, snout or oral contact is directed at the opponents nape of the neck, whereas during serious fighting, male residents mostly direct their bites at the intruders rump. Although similar to those used in serious fighting, the behavior patterns used during play-fighting are modified to achieve the different targets of attack. Even though the tactics of attack and defense appear more adult-like with increasing age, the playful targets persist well into adulthood.

Multiple Differences in the Play Fighting of Male and Female Rats. Implications for the Causes and Functions of Play

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

Rough-and-Tumble Play and the Development of the Social Brain

Social play—that is, play directed toward others—is a readily recognizable feature of childhood. In nonhuman animals, social play, especially seemingly competitive rough-and-tumble play or play fighting, has been the most studied of all forms of play. After several decades of study, researchers of play fighting in laboratory rats have pieced together the rudiments of the neural mechanisms that regulate the expression of this behavior in the mammalian brain. Furthermore, the understanding of the organization, development, and neural control of play in rats has provided a model with which to examine how the experiences accrued during play fighting can lead to organizational changes in the brain, especially those areas involved in social behavior.

The Role of the Cortex in Play Fighting by Rats: Developmental and Evolutionary Implications

Play is a distinctive behavior of young mammals, especially mammals with a well-developed forebrain. For this reason it is thought that there may be a relation between forebrain evolution and highly elaborated play behavior. This study investigated the contribution of the cortex to play behavior by comparing play in control and neonatally decorticated rats (Rattus norvegicus). Play fighting in rats involves the combination of attack by one rat and defense by the recipient, with pinning arising when specific patterns of defense are used. Whether paired with another decorticate or with an intact pairmate, decorticates attacked pairmates as frequently as did intacts, and they were just as likely to defend against playful attacks as were intacts. Where decorticates differed from intacts was on a measure of pinning, in which one rat stands over a supine partner, decorticate rats displayed a reduction of 50% relative to control rats during the juvenile stage in which play is most pronounced (days 25 to 40). Juvenile decorticate rats adopted types of defensive responses which were less likely to result in the pinning configuration. Thus, a reduced pinning frequency reflects an altered pattern of defense, not a reduced level of play fighting. Rather, the decorticate patterns of defense were typical of those defensive responses displayed by adult rats. That is, decorticate juveniles exhibit a precociously mature pattern of playful defense. As intact controls mature, they come to resemble the decorticates in their defensive responses, and hence the difference in pinning frequency between decorticate and intact pairs diminishes. This suggests that the cortex may inhibit the escalation of defense in juveniles and thus promote prolonged ventral-ventral contact during play fighting. The results further suggest that the cortex is involved in the development of adult behavioral skills by facilitating juvenile play.

The prejuvenile onset of play fighting in laboratory rats (Rattus norvegicus).

Play fighting in rats is most frequent in the juvenile phase (30-40 days) and then wanes following puberty. During the juvenile phase, the most commonly used defensive tactic to block access to the nape (i.e., the play target) is to rotate around the longitudinal axis to supine. From this position of lying on its back, the defender uses its limbs to hold off the attacking pup. With the onset of puberty males, but not females, switch to more adultlike patterns of defense. The adultlike pattern involves partially rotating around the longitudinal axis of the body, but retaining ground contact with the hindpaws. In this position, the defender is able to rear to a defensive upright posture, or can slam into the partner with its hip. In this study, the manner of onset of play fighting and its components was investigated by daily videotaped observations of six litters of Long Evans hooded rats, starting at 15 or 16 days and ending at 30 days postnatally. The predominant form of social interaction in the midteens was allogrooming, but by 20 days, playful attacks to the nape were the most common form of contact. Allogrooming was most often directed to the head, not the nape. With respect to playful defense, the more adultlike tactics matured first, with all tactics present in the repertoire by 20 days. The fully juvenile pattern of defense did not mature until 25-30 days with both males and females exhibiting the same developmental pattern. These data lead to several conclusions. First, play fighting is a separate category of behavior from the outset, and does not emerge from allogrooming. Second, the adultlike defense patterns do not emerge gradually from the juvenile ones at puberty, but rather, all are present in the repertoire from infancy; and third, both males and females have the same pattern of onset of play fighting. These conclusions are discussed with respect to the possible functions of juvenile play fighting.

On knowing it's only play: The role of play signals in play fighting

Abstract Play fighting in many species contains behavioral elements of direct aggression; that is, those behavior patterns that are used to threaten and contact opponents. Although many investigators have alleged that there are play signals that can be used to unambiguously distinguish playful from nonplayful aggression, the existence of such signals is mainly anecdotal and correlational. For most species, such signals are either not present, or are not present with sufficient regularity to function as unambiguous markers of play. In most playful encounters, participants seem able to use contextual cues to assertain whether the actions of the partner are affiliative or not. Where play promoting signals do appear to exist, they are often general purpose affiliative signals rather than ones specific to play. Such signals are most often associated with situations where the playful intentions of the performer may be ambiguous to the recipient. In these situations, such signals appear to be crucial in avoiding escalation to serious aggression. The danger of escalating from play to aggression is most often reported as animals become sexually mature and stronger. Also, it is in early adulthood that members of a social group compete for dominance. In these situations, a signal “this is play” may not be sufficient. Instead, signals that can retroactively appease a play partner with “I was only kidding, ” may be very important so as to manipulate, and thus obfuscate, the true intent of the action. When present, the signals occurring in play are highly variable in form and context, which is what would be expected for such subtle social manipulation. We suggest that species with more complex social relationships, where individuals are likely to probe and test their relationships with play fighting, are most likely to be the ones that make the most sophisticated use of such signals.

Role reversal changes during the ontogeny of play fighting in male rats : attack vs. defense

From weaning until sexual maturity, the rates at which young male rats hold each other supine during play fighting appear to become progressively asymmetrical. These changes have been previously thought to reflect an initial lack of dominance and a later development of dominance-subordinance relationships. In this paper it is shown that pairs of male rats exhibit asymmetries in playful attack and playful defense throughout development. The changes, resulting in greater asymmetry of pinning rates, are shown to result from age-dependent changes in defensive tactics; the relationship, therefore, remains constant while the form of the behavior changes. Furthermore, it is not the animals showing the highest rates of playful attack who become dominant in older ages.

Attack and Defense During Play Fighting Appear to be Motivationally Independent Behaviors in Muroid Rodents

Play fighting is composed of two subcomponent behaviors-attack and defense. If these two components are controlled by the same motivational system, it is expected that they will co-vary. Therefore, it is expected that (1) when species are compared, those that exhibit high levels of playful attack should also exhibit high levels of playful defense; (2) when individuals are compared within a species, those that exhibit high levels of playful attack should also exhibit high levels of playful defense; and (3) developmental, as play fighting wanes, both playful attack and playful defense should wane simultaneously. A comparison of playful attack and playful defense among six species of muroid rodents reveals that they are not correlated; high rates of defense can occur with low rates of attack, and vice versa. A comparison of individuals from one species (i.e., rats) reveals that those exhibiting the highest levels of attack are not necessarily those exhibiting the highest levels of defense. Finally, a developmental analysis of rats and other species of muroid rodents shows that attack wanes with age whereas defense does not. These data indicate that playful attack and playful defense are motivationally distinct behaviors in these species.

Identification of the possible origin of the body target that differentiates play fighting from serious fighting in syrian golden hamsters (Mesocricetus auratus)

Play fighting in the Syrian Golden hamster Mesocricetus auratus can be distinguished from serious fighting by the targets attacked in each case. In play fighting, the animals attack and defend the cheeks and cheek pouches, whereas in serious fighting they attack and defend the rump and lower flanks. Since play typically involves the use of behaviors borrowed from other functional contexts, this paper investigates the origin of the cheek target during play fighting. Comparison of resident-intruder serious fighting with awake and anesthetized intruders does not reveal the cheek to be an inhibited target for serious attack. Similarly, analysis of social investigation and allog-rooming, while revealing the ears to be important targets, do not show the cheeks to be targets in these behaviors. Sniffing, licking, and nibbling of the cheek area appear to occur mainly during sexual encounters by males. This area, seemingly a sexual target, may be the one utilized during play fighting.