Pellis, Sergio

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Browsing Pellis, Sergio by Subject "Juvenile rats"

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    The development of juvenile-typical patterns of play fighting in juvenile rats does not depend on peer-peer play experience in the peri-weaning period
    (eScholarship Publishing, University of California, 2015) Himmler, Brett T.; Himmler, Stephanie M.; Stryjek, Rafal; Modlinska, Klaudia; Pisula, Wojciech; Pellis, Sergio M.
    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, samecondition 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.
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    Groups of familiar male rats form unstable partner preferences when play fighting during the juvenile period
    (Cell Press, 2025) Ham, Jackson R.; Pellis, Sergio
    Rats are social animals living in large groups. Within these groups, juveniles engage in vigorous rough-and-tumble play. Despite their natural ecology, most current studies of their play behavior involve pairs not groups. To investigate play under more naturalistic settings, we examined the play of eight groups of juvenile male rats, with each group comprising six, same-aged peers that lived together. Each group was tested on multiple days over the peak play period (30–40 postnatal days). On any given day, rats showed partner preferences for certain individuals in the group, however, preferences varied from day to day. Despite changes in partner preferences, rats chose to play with partners that engaged in more turn taking and with partners with whom they had more symmetrical play relationships. That some individuals within the group were consistently preferred as play partners while others were consistently avoided may have developmental consequences, with those who are favored gaining greater benefits from their juvenile play experiences.
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    Juvenile male rats form preferences based on strain when playing in groups but not in pairs
    (Frontiers Media, 2025) Ham, Jackson R.; Jaiswal, Diya; Waner-Mariquito, Renata; Pellis, Sergio; Achterberg, E. J. Marijke
    Like many young mammals, juvenile rats engage in rough-and-tumble play. Play occurs naturally both in wild and laboratory rats, making it a suitable, ethologically relevant behavior to investigate. In the laboratory, rats are typically housed and tested in dyads, despite living in large colonies in the wild. Consequently, when tested in the lab, rats do not have a choice of partners and are instead paired with whomever the researcher selects. Given that both the amount and style of play rats engage in varies considerably depending on the strain of rat being studied, we tested whether rats select play partners based on strain. To do so, juvenile male Long Evans (LE) focal rats (n = 8) were subjected to three play contexts: (1) group play; (2) dyadic play; and (3) social conditioned place preference. During group play, the LE subject rats were given the choice to play with an LE, a Sprague Dawley (SD), or Fischer 344 rat (F344), simultaneously. During dyadic play, focal rats played one-on-one with an LE, SD, or F344 partner. Finally, the rats were conditioned to a context and a social stimulus, with the context either being paired with an LE (preferred stimulus) or F344 (unpreferred stimulus) partner. We found that, when given a choice in a group setting, LE focal rats prefer to play with same-strain partners over both SD and F344 partners. However, when playing under dyadic conditions (i.e., with an assigned partner), LE rats played with each strain equally. Finally, in the socially conditioned place preference test, we found that the focal rats formed preferences for a particular enclosure, but not for the strain. Together, these results suggest that when given a choice, LE rats prefer to play with their own strain, but when they do not have a choice, any strain will do. Given that the testing paradigm can greatly influence the results obtained and the conclusions drawn, our findings highlight the need to consider the research question(s) being asked when determining the most appropriate paradigms to employ.
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    Specific 50-kHv vocalizations are tightly linked to particular types of behavior in juvenile rats anticipating play
    (Public Library of Science, 2017) Burke, Candace J.; Kisko, Theresa M.; Swiftwolfe, Hilarie; Pellis, Sergio M.; Euston, David R.
    Rat ultrasonic vocalizations have been suggested to be either a byproduct of physical movement or, in the case of 50-kHz calls, a means to communicate positive affect. Yet there are up to 14 distinct types of 50-kHz calls, raising issues for both explanations. To discriminate between these theories and address the purpose for the numerous 50-kHz call types, we studied single juvenile rats that were waiting to play with a partner, a situation associated with a high number of 50-kHz calls. We used a Monte-Carlo shuffling procedure to identify vocalization-behavior correlations that were statistically different from chance. We found that certain call types (“split”, “composite” and “multi-step”) were strongly associated with running and jumping while other call types (those involving “trills”) were more common during slower movements. Further, non-locomotor states such as resting and rearing were strongly predictive of a lack of vocalizations. We also found that the various sub-types of USVs can be clustered into 3–4 categories based on similarities in the way they are used. We did not find a one-to-one relationship between any movements and specific vocalizations, casting doubt on the motion byproduct theory. On the other hand, the use of specific calls during specific behaviors is problematic for the affect communication hypothesis. Based on our results, we suggest that ultrasonic calls may serve to coordinate moment-to-moment social interactions