Empathy and pro-social behavior in rats

Abstract

Whereas human pro-social behavior is often driven by empathic concern for another, it is unclear whether nonprimate mammals experience a similar motivational state. To test for empathically motivated pro-social behavior in rodents, we placed a free rat in an arena with a cagemate trapped in a restrainer. After several sessions, the free rat learned to intentionally and quickly open the restrainer and free the cagemate. Rats did not open empty or object-containing restrainers. They freed cagemates even when social contact was prevented. When liberating a cagemate was pitted against chocolate contained within a second restrainer, rats opened both restrainers and typically shared the chocolate. Thus, rats behave pro-socially in response to a conspecific's distress, providing strong evidence for biological roots of empathically motivated helping behavior.

Fig. 1

Rats showed activity focused on the restrainer (red box in A) only if it contained a trapped cagemate. Rats were placed in an arena with a trapped cagemate (n=30, 6 females), an empty restrainer (n = 20, 6 females) a restrainer containing an object (n=8 males) or with an empty restrainer and a free cagemate located across a perforated screen (n=12 males). (A) The locations, plotted at 0.5 Hz, of a representative free rat from each of the 4 groups on the first day of testing are illustrated. (B) The mean (±SEM) amount of time that a rat was >5 cm away from the arena wall is shown for each of the 12 days of testing. Rats in the trapped cagemate condition spent more time in the arena center than did rats in the control conditions (P<0.001, mixed model analysis, post-hoc PLSD). (C) The mean (±SEM) activity across the testing session (averaged across all days) for each group of rats is shown. Rats in the trapped cagemate condition moved significantly faster than rats in all other conditions (P<0.001, mixed model analysis, post-hoc PLSD). (D) The mean ratio (±SEM) of the average activity during the second half of sessions relative to the average activity during the first half is shown for each day of testing. On days 1–6, rats in the trapped cagemate condition stayed significantly more active in the second half relative to the first half of sessions than did rats in the control conditions (P<0.001 mixed model analysis, PLSD).

Fig. 2

Rats placed with a trapped cagemate learned to open the restrainer door in a goal-directed fashion. (A) The proportion of rats in the trapped cagemate condition that opened the door increased across the days of testing. (B) The median time to door-opening is shown across all days of testing. Only rats in the trapped cagemate condition opened the door at decreasing latencies. The dashed line indicates the time point when the door was opened half way by the experimenter. (C) Rats in the trapped cagemate condition showed a sharp increase in activity at the moment that the restrainer door was opened (time 0). Activity from all trials, regardless of whether the door was opened before or after the experimenter opened the door halfway, was averaged. (D) The count of opening type (with head, from side, from top) is shown across days of testing. As free rats learned to open the door, they developed a consistent style, suggesting goal-directed behavior. (E) Initially, rats showed freezing behavior immediately after the door fell over, but as they learned to open the door, they were no longer startled by the door-opening and no longer froze. (F) More alarm calls were recorded in the trapped cagemate condition (n=67 sample files) than in the empty (n=64) and object (n= 67) conditions (p<0.05 ANOVA, PLSD<0.05).

Fig. 3

(A) Females in the trapped cagemate condition (n=6) opened the door at consistently shorter latencies than did males (n=24) on days 7–12 (P<0.01, Mixed model analysis). When median door-opening latencies on days 7–12 were averaged, there was no difference between males (n=14) and females (n=6) in the empty condition. (B) The mean (±SEM) activity was greater for females than males in the trapped cagemate condition (P<0.001, ANOVA) but there was no gender-related difference in the empty condition.

Fig. 4

(A) In the separated experiment, rats (n=9) opened the door for a trapped cagemate even when no contact was possible between the two animals after door-opening. The latencies to door-opening are plotted across all the days of the study. Rats extinguished door-opening when the restrainer was empty, but opened the door when the restrainer contained a cagemate. This pattern was observed regardless of whether rats were tested first with an empty restrainer (n=4; top) or with a trapped cagemate (n=5; bottom; P<0.001, mixed model analysis, PLSD). (B) In the chocolate experiment, rats (n=9) opened the door for a trapped cagemate as well as for a restrainer containing 5 chocolate chips. The median time to door-opening is shown across all days of testing. On days 6–12, rats opened both restrainers at similarly short latencies. (C) Rats in the chocolate empty condition (n=6) opened the empty restrainer at significantly longer latencies than the chocolate restrainer (P<0.01). The dashed lines in B–C indicate the time point when the door was opened half way by the experimenter.