To Play or Not to Play? Effects of Playmate Familiarity and Social Isolation on Social Play Engagement in Three Laboratory Rat Strains

Abstract

Social play is a motivating and rewarding behavior displayed by juveniles of many mammalian species, including humans and rats. Social play is vital to the development of social skills. Autistic children show less social play engagement which may contribute to their impairments in social skills. There is limited knowledge about what external conditions may positively or negatively influence social play engagement in humans or other animals. Therefore, we determined how two common external conditions, playmate familiarity and social isolation, modulate social play levels and social play defense tactics in juveniles of three common laboratory rat strains: Long-Evans, Sprague-Dawley, and Wistar. Males and females were socially isolated for either 2h or 48h prior to social play testing and were then exposed to either a familiar (cage mate) or novel playmate, creating four testing conditions: 2h-Familiar, 48h-Familiar, 2h-Novel, and 48h-Novel. Both playmate familiarity and social isolation length influenced social play behavior levels and tactics in juvenile rats, but did so differently for each of the three rat strains. Long-Evans played most with a familiar playmate, irrespective of time isolated, Sprague-Dawley played most in the 48h-Familiar condition, and Wistar played the least in the 2h-Familiar condition, but Wistar played more with a novel playmate than Long-Evans and Sprague-Dawley. Analysis of social play tactics by the playmates in response to nape attacks by the experimental rats revealed strain differences with novel playmates. Here, Sprague-Dawley and Wistar defended more nape attacks than Long-Evans. Sprague-Dawley evaded these attacks, thereby shortening body contact. In contrast, Wistar turned to face their playmate attacker and showed more complete rotations, thereby extending body contact and wrestling longer. Role reversals, which increase social play reciprocity and reflect the quality of social play, were higher in Long-Evans and Sprague-Dawley with familiar playmates. Role reversals decreased for Sprague-Dawley but increased for Wistar after 48h isolation. The effects of playmate familiarity or social isolation length on social play levels and tactics were similar across sex for all three strains. In conclusion, we showed that two common external factors (playmate familiarity and social isolation length) that largely vary across social play studies have a major impact on the level and quality of social play in the three rat strains. Strain differences indicate higher level and quality of social play with familiar playmates in Long-Evans, with familiar playmates after short isolation in Sprague-Dawley, and with novel playmates after longer isolation for Wistar. Future research could determine whether strain differences in neuronal mechanisms underlie these condition-induced variations in social play engagement. Our findings are also informative in suggesting that external conditions like playmate familiarity and social isolation length could influence social play levels and social play quality in typical and atypical children.

Figure 1.. Decision tree showing the diverse social play defense tactics of the experimental rat’s playmate in response to a nape attack by the experimental rat.

When attacked, a rat can either defend itself (defense) or ignore the playful attack (no response). If the attacked rat defends itself, it can either show evasion (i.e., run away) or engage in a facing defense. If it turns to face the attacker rat, it either rotates along the vertical body axis or horizontal body axis. Vertical axis rotations end in a mutual upright or standing defense. In contrast, horizontal axis rotations result in either a partial or complete rotation, both of which are commonly referred to as a pin. Only attacks that are defended can result in a role reversal. For an in-depth read of the microstructure of rat play, see Pellis et al. (2022). Adapted from Stark (2021).

Figure 2.. Experimental design to determine the effects of playmate familiarity (familiar or novel) and social isolation (2h or 48h) on social play engagement in juvenile male and female Long-Evans, Sprague-Dawley and Wistar rats.

On Day 1, rats were randomly assigned as experimental rat (E), familiar playmate (F1, F2), or novel playmate (N1–4). During social isolation, the experimental rat remained in its home cage (indicated with a blue lid) while the two cagemates (F1 and F2) were placed in a new cage (‘familiar playmate cage’, indicated with a red lid). Novel playmates were housed in pairs (‘novel playmate cages’, indicated with a green lid). Social play testing occurred in the home cage of the experimental rat at the start of the dark phase (14:00 h). After a 2h or 48h isolation period, experimental rats were exposed to a 10-min Habituation Social Play Test on Day 7 with either a familiar (F1) or novel (N1 or N2) playmate. After this habituation test, the familiar playmates were rehoused with the experimental rats in the 2h Isolation group while the experimental rats in the 48h Isolation group remained singly housed. On Day 8, experimental rats underwent the 10-min Social Play Test by exposing them to the other familiar playmate (F2) or to a novel playmate from the other novel playmate cage (N3 or N4). All familiar and novel playmates remained socially housed during the experiment to prevent any isolation effects on the stimulus rats. Image created with BioRender.com

Fig. 3 |. Experimental condition comparisons for social play behaviors per rat strain – Long-Evans showed most social play with familiar playmates, Sprague-Dawley show most social play with familiar playmates after 48h isolation, and Wistar showed the least social play with familiar playmates after 2h isolation.

A-D: Long-Evans played more with a familiar than a novel playmate, as shown by longer duration of social play under both 2h and 48h isolation (A), more nape attacks under 48-h isolation (B), and more pins under both 2h and 48h isolation (C) while no effects were seen for number of supines (D). E-G: Sprague-Dawley played more with a familiar playmate after 48h isolation than with a familiar playmate after 2h isolation and a novel playmate after 48h isolation, as shown by longer duration of social play (E), more nape attacks (F), and more pins (G). Sprague-Dawley had more supines with a familiar playmate after 2h versus 48h isolation (H). I-J: Wistar played more with a familiar playmate after 48h versus 2h isolation, as shown by longer duration of social play (I) and more nape attacks (J). Wistar played more with a novel than a familiar playmate after 2h isolation (I). K-L: Wistar had more pins (K) and supines (L) after 48h versus 2h isolation. Males are represented by blue squares, females are represented by red circles; Three-way ANOVA analyses per strain to assess effects of Playmate, Isolation, and Sex, followed by Bonferroni post hoc tests to assess interaction effects; * p < 0.05, ** p ≤ 0.01, *** p ≤ 0.001.

Fig. 4 |. Experimental condition comparisons per rat strain for social play defense tactics of the playmates in response to nape attacks by the experimental rat – Most notably are more role reversals by familiar playmates in Long-Evans and Sprague-Dawley and fewer role reversals by Sprague-Dawley but more role reversals by Wistar after 48h isolation.

A-G: Long-Evans familiar playmates showed higher proportions of attacks defended (A), evasions (C), and role reversals (G) than novel playmates irrespective of time isolated by the experimental Long-Evans rats. Long-Evans novel playmates showed a higher proportion of mutual uprights (F) when experimental rats were isolated for 48h versus 2h. Long-Evans familiar and novel playmates showed a higher proportion of role reversals (G) when experimental rats were isolated for 2h versus 48h. There were no experimental condition differences on facing defenses (B), partial rotations (D) or complete rotations (E). H-N: There were no experimental condition differences for Sprague-Dawley playmates on attacks defended (H), facing defenses (I), evasions (J), complete rotations (L), or mutual uprights (M). However, Sprague-Dawley familiar playmates exposed to an experimental Sprague-Dawley rat isolated for 2h showed a lower proportion of partial rotations (K) compared to novel playmates and compared to familiar playmates exposed to an experimental Sprague-Dawley rat isolated for 48h. Sprague-Dawley familiar playmates showed a higher proportion of role reversals (N) than novel playmates irrespective of time isolated by the experimental rats. Moreover, Sprague-Dawley playmates exposed to an experimental rat isolated for 2h showed a higher proportion of role reversals (N) than those exposed to an experimental rat isolated for 48h. O-U: There were no experimental condition differences for Wistar playmates on attacks defended (O), facing defenses (P), evasions (Q), partial rotations (R), or mutual uprights (T). However, Wistar novel playmates showed a higher proportion of complete rotations (S) than familiar playmates, irrespective of isolation time. Wistar playmates exposed to an experimental Wistar rat isolated for 48h showed a higher proportion of role reversals (U) than playmates exposed to an experimental Wistar rat isolated for 2h. Behaviors are expressed as proportions. Males are represented by blue squares, females are represented by red circles; Three-way ANOVA analysis per strain to assess effects of Stimulus, Isolation, and Sex, followed by Bonferroni post hoc tests to assess interaction effects; * p < 0.05, ** p ≤ 0.01, *** p ≤ 0.001.

Fig. 4 |. Experimental condition comparisons per rat strain for social play defense tactics of the playmates in response to nape attacks by the experimental rat – Most notably are more role reversals by familiar playmates in Long-Evans and Sprague-Dawley and fewer role reversals by Sprague-Dawley but more role reversals by Wistar after 48h isolation.

A-G: Long-Evans familiar playmates showed higher proportions of attacks defended (A), evasions (C), and role reversals (G) than novel playmates irrespective of time isolated by the experimental Long-Evans rats. Long-Evans novel playmates showed a higher proportion of mutual uprights (F) when experimental rats were isolated for 48h versus 2h. Long-Evans familiar and novel playmates showed a higher proportion of role reversals (G) when experimental rats were isolated for 2h versus 48h. There were no experimental condition differences on facing defenses (B), partial rotations (D) or complete rotations (E). H-N: There were no experimental condition differences for Sprague-Dawley playmates on attacks defended (H), facing defenses (I), evasions (J), complete rotations (L), or mutual uprights (M). However, Sprague-Dawley familiar playmates exposed to an experimental Sprague-Dawley rat isolated for 2h showed a lower proportion of partial rotations (K) compared to novel playmates and compared to familiar playmates exposed to an experimental Sprague-Dawley rat isolated for 48h. Sprague-Dawley familiar playmates showed a higher proportion of role reversals (N) than novel playmates irrespective of time isolated by the experimental rats. Moreover, Sprague-Dawley playmates exposed to an experimental rat isolated for 2h showed a higher proportion of role reversals (N) than those exposed to an experimental rat isolated for 48h. O-U: There were no experimental condition differences for Wistar playmates on attacks defended (O), facing defenses (P), evasions (Q), partial rotations (R), or mutual uprights (T). However, Wistar novel playmates showed a higher proportion of complete rotations (S) than familiar playmates, irrespective of isolation time. Wistar playmates exposed to an experimental Wistar rat isolated for 48h showed a higher proportion of role reversals (U) than playmates exposed to an experimental Wistar rat isolated for 2h. Behaviors are expressed as proportions. Males are represented by blue squares, females are represented by red circles; Three-way ANOVA analysis per strain to assess effects of Stimulus, Isolation, and Sex, followed by Bonferroni post hoc tests to assess interaction effects; * p < 0.05, ** p ≤ 0.01, *** p ≤ 0.001.

Fig. 5 |. Rat strain comparisons for social play behaviors per experimental condition - Strain differences are most prevalent when rats were exposed to a novel playmate.

A-D: There were no strain differences on social play behaviors when exposed to a familiar playmate after 2h isolation, except for fewer supines in Wistar compared to Long Evans (D). E-H: There were no strain differences on social play behaviors when exposed to a familiar playmate after 48h isolation, except for fewer supines in Sprague-Dawley compared to Long Evans (H). I-K: Long Evans played less than Wistar when exposed to a novel playmate after 2h isolation, as shown by shorter duration of social play (I), fewer nape attacks (J), and fewer pins (K). I: Sprague Dawley played less than Wistar when exposed to a novel playmate after 2h isolation. M-O: Long Evans and Sprague-Dawley played less than Wistar when exposed to a novel playmate after 48h isolation, as shown by shorter duration of social play (M), fewer nape attacks (N), and fewer pins (O). N: Long-Evans had fewer nape attacks than Sprague Dawley when exposed to a novel playmate after 48h isolation. L, P: There were no strain differences on supines when exposed to a novel playmate after 2h or 48h isolation. One-way ANOVA analysis per condition followed by Bonferroni post hoc tests to assess strain differences; * p < 0.05, ** p ≤ 0.01, *** p ≤ 0.001.

Fig. 6 |. Rat strain comparisons per experimental condition for social play defense tactics of the playmates in response to nape attacks by the experimental rat – Most notable are strain differences with novel playmates with more attacks defended by Sprague-Dawley and Wistar, more evasions by Sprague-Dawley and more facing defenses and complete rotations by Wistar.

A-G: When experimental rats were exposed to a familiar playmate after 2h isolation, there were no strain differences on the proportions of attacks defended (A), facing defenses (B), evasions (C), partial rotations (D), complete rotations (E), or mutual uprights (F) by the playmates. However, Wistar playmates showed fewer role reversals (G) than Long-Evans and Sprague-Dawley playmates. H-N: When experimental rats were exposed to a familiar playmate after 48h isolation, Long Evans playmates showed fewer defended attacks (H) than Sprague-Dawley and Wistar playmates and fewer evasions (J) than Sprague-Dawley playmates. No strain differences were found for the other defense tactics. O-U: When experimental rats were exposed to a novel playmate after 2h isolation, Long Evans playmates showed fewer defended attacks (O) than Sprague-Dawley and Wistar playmates and fewer facing defenses (P) and complete rotations (S) than Wistar playmates. Sprague-Dawley playmates showed more evasions (Q) than Long-Evans and Wistar playmates. No strain differences were found for the other defense tactics. V-Z”: When experimental rats were exposed to a novel playmate after 48h isolation, Long Evans playmates showed fewer defended attacks (V) but more mutual uprights (Z’) than Sprague-Dawley and Wistar playmates. Wistar playmates showed more complete rotations (Z) than Long-Evans and Sprague-Dawley playmates. No strain differences were found for the other defense tactics. Behaviors are expressed as proportions. One-way ANOVA analyses per experimental condition followed by Bonferroni post hoc tests to assess strain differences; * p < 0.05, ** p ≤ 0.01, *** p ≤ 0.001.

Fig. 6 |. Rat strain comparisons per experimental condition for social play defense tactics of the playmates in response to nape attacks by the experimental rat – Most notable are strain differences with novel playmates with more attacks defended by Sprague-Dawley and Wistar, more evasions by Sprague-Dawley and more facing defenses and complete rotations by Wistar.

A-G: When experimental rats were exposed to a familiar playmate after 2h isolation, there were no strain differences on the proportions of attacks defended (A), facing defenses (B), evasions (C), partial rotations (D), complete rotations (E), or mutual uprights (F) by the playmates. However, Wistar playmates showed fewer role reversals (G) than Long-Evans and Sprague-Dawley playmates. H-N: When experimental rats were exposed to a familiar playmate after 48h isolation, Long Evans playmates showed fewer defended attacks (H) than Sprague-Dawley and Wistar playmates and fewer evasions (J) than Sprague-Dawley playmates. No strain differences were found for the other defense tactics. O-U: When experimental rats were exposed to a novel playmate after 2h isolation, Long Evans playmates showed fewer defended attacks (O) than Sprague-Dawley and Wistar playmates and fewer facing defenses (P) and complete rotations (S) than Wistar playmates. Sprague-Dawley playmates showed more evasions (Q) than Long-Evans and Wistar playmates. No strain differences were found for the other defense tactics. V-Z”: When experimental rats were exposed to a novel playmate after 48h isolation, Long Evans playmates showed fewer defended attacks (V) but more mutual uprights (Z’) than Sprague-Dawley and Wistar playmates. Wistar playmates showed more complete rotations (Z) than Long-Evans and Sprague-Dawley playmates. No strain differences were found for the other defense tactics. Behaviors are expressed as proportions. One-way ANOVA analyses per experimental condition followed by Bonferroni post hoc tests to assess strain differences; * p < 0.05, ** p ≤ 0.01, *** p ≤ 0.001.