Mice choose altruistic actions over selfish decisions
To test whether mice are capable of acting for the intentional benefit of conspecifics, we first devised a social decision-making task (SDM) for mice that was equivalent to the human “dictator game”, one of the most prominent game-theoretical paradigms that have been designed to test altruism37. We expanded a standard operant cage with an adjacent compartment, separated by a metal mesh, in which to host a ‘recipient’ that would receive food rewards depending on the choice made by the ‘dictator’ (hereafter referred to as the ‘actor’). The recipient was a passive player with a chance to receive a food reward from a magazine, depending on the actor’s choice. To promote food-seeking behavior, at the start of the test, both the actors and the recipients were maintained at 90% of their free-feeding body weights. The actors were presented with a two-choice decision-making paradigm, in which nose poking resulted in either food rewards for themselves only (selfish choice) or for both themselves and the recipient (altruistic choice; Fig. 1a). We compared this condition against a control group of actor mice without the presence of a recipient. The structure of the task was identical between these two conditions (‘with recipient’ and ‘no recipient’). Thus, any differences in the response could be attributed to the influence of the recipient.
Adult mouse littermates, three to six months-old, both males and females, were housed in same sex-pairs for at least two weeks before the start of testing. Animals were tested for five days, until they reached a stable performance for three consecutive days. At the group level, we found that actor mice with recipients preferred to share food rewards (altruistic choices) more frequently than not (selfish choices), exhibiting a positive decision preference index compared with that of mice in the ‘no recipient ‘condition, which did not display any choice preference (Fig. 1b). The location of the nose poke associated with altruistic or selfish response did not modified the preference for altruistic choices (Fig. 1b). Mice showed an increased number of altruistic over selfish responses when a recipient was present, whereas the mice in the ‘no recipient’ condition mice chose similarly between two nose pokes (Fig. 1c). Following the last session (day 5), we replaced the recipient mice with an inanimate object and tested the actors to determine whether any changes to their preference could be detected in the absence of social motivation. During this condition, the actors decreased their preference (both altruistic and selfish) in the presence of an inanimate object when compared against their behavior in the presence of the recipient (Fig. 1d). These results confirmed that the expression of the preference for altruistic or selfish choice was contingent on the presence of a conspecific.
We observed marked individual differences in the responses of the mice across days. We analyzed the performance of each actor separately and found that eleven of sixteen mice showed a significant increase in altruistic responses, more frequently than could be explained by chance (Fig. 1e, f), whereas the remaining five mice showed a significant decrease in altruistic responses (Fig. 1e, f). Altruistic and non-altruistic mice in the test condition that included a recipient showed significantly different choices starting on the second day of testing (Fig. 1d). The distribution of mouse preferences indicated that greater than 80% of the responses were altruistic for the majority of mice (Fig. 1e).
Next, we asked whether sex influenced altruistic behaviors during the SDM. We analyzed pairs of males and females separately. All actors, both males and females, displayed a significant preference (Supplementary Fig. 1a). At the group level, males displayed a significant preference for altruistic over selfish responses (Supplementary Fig. 1a) and only one male mouse of eight did not prefer to allocate food rewards to his recipient (Supplementary Fig. 1b). In contrast, the females did not show an overall preferential choice (Supplementary Fig. 1a). Among the eight tested pairs, half of the females displayed a preference for altruistic choices, whereas the other half made selfish choices (Supplementary Fig. 1b). Compared with the performance of sex-matched actors that performed the task in the absence of a recipient, only males showed a preference for the altruistic responses (Supplementary Fig. 1c, d).
To determine whether social interactions in the proximity of the divider between the actor and recipient compartments might have influenced the actors’ decisions, we measured the time spent on social exploration in both mice and found that altruistic actor mice spent more time exploring their recipient than selfish actor mice (Fig. 1g). This was evident from the first session of testing, and this pattern was maintained until the last session (Day 5). In contrast, we did not observe any differences in the social exploration by the recipients (Fig. 1h). Importantly, we found that social exploration of the actor mice during the first day of testing was positively correlated with the altruistic responses on the last day of testing, at which point the actors display a consistent behavioral preference (Fig. 1h). To test whether actor-recipient social interactions were necessary for the actors’ social choices, we replaced the metal mesh with an opaque partition dividing the two compartments, which prevented social contact but allowed the passage of auditory and olfactory stimuli. We used a new cohort of mice and tested two conditions, one group of actor mice were tested with the opaque partition and the other group with the mesh. Mice tested in the presence of an opaque partition showed a significant decrease in altruistic responses compared with actor mice who were tested with a metal mesh that allowed social contact (Fig. 1h). These findings suggested that mice use social visual cues or social contacts to establish their decision preferences.
Finally, we tested whether sharing food with recipients could motivate a change in decision preference. The actors were first trained to trigger one of the two nose pokes, which both delivered the same food reward. After the mice displayed a stable preference for one nose poke, a recipient mouse was introduced to the adjacent compartment, and nose poking into the non-preferred hole resulted in the delivery of rewards to both mice, whereas nose poking into the preferred hole delivered rewards only to the actor (Supplementary Fig. 2a). The location of the recipient compartment did not biased actor mice preferences as nose poke responses during baseline training in the right and left nose-poke were not different (Supplementary Fig. 2d). At the group level, we found that actor mice displayed a positive change from their baseline preference across days, which suggested that the mice shifted their responses to share food rewards with their recipients (Supplementary Fig. 2b). Although we observed individual differences, the majority of mice displayed a switch from their preference (“altruistic,” 8/13 mice, Supplementary Fig. 2b, c). Under this condition, the day following the last session, we replaced the recipient mouse with an inanimate object and found a decrease in preference compared with the preference expressed when the recipient was present (Supplementary Fig 2f). These results suggest that mice learned or were willing to change their behaviors to share a positive experience, such as a food reward, with their conspecifics.
Actor mice are willing to take altruistic actions even under costly conditions
To challenge the motivation of actor mice to allocate food rewards to their cage mates, we increased the cost of the altruistic decisions by reinforcing the responses at a fixed ratio of 2 (FR2, Fig. 2a). Under this condition, two nose pokes were required to receive food together with the recipient, whereas only one poke was necessary for selfish responses (FR1, Fig. 2a). We tested only those males and females mice that had previously demonstrated a significant preference above chance for altruistic responses after five days in the SDM (Supplementary Fig. 1). We similarly tested mice in the ‘no recipient’ condition, in which their natural preference was set to FR2, whereas the other nose poke option was maintained at FR1 (Fig. 2a).
Both males and females displayed an increased number of altruistic responses over selfish responses, even when additional effort was required (Fig. 2b, c). Moreover, male FR2 responses were higher than those performed by mice tested without a recipient (Fig. 2d). This difference was not confounded by the baseline number of nose poke responses (Fig. 2b, inset). We then further increased the effort necessary to perform an altruistic action by increasing these responses to a FR4 (Fig. 2a). Under this condition, males showed increased altruistic responses compared to both females and to mice without recipients (Fig. 2b, c). Females did not show a preference between the two responses (Fig. 2c), and mice without a recipient switched their preference to nose poke reinforced at FR1 (Fig. 2d). When the altruistic responses were reinforced to FR6, the females switched their preference to the nose poke that delivered food rewards more easily (FR1, Fig. 2d), whereas males continued to prefer altruistic responses (Fig. 2c), making more responses using the FR6 nose poke compared with both female mice and mice without recipients (Fig. 2b, c). Finally, we tested males only in additional sessions to observe when a switch to the non-preferred response would occur. At FR8, male mice switched their preference to selfish responses, although they performed a similar number of nose pokes for both FR1 and FR8 (Fig. 2d). These results suggest that in the presence of a social motivation, male mice preferred to shared food rewards to benefit their cage mates, even under costly conditions.
In our setting, the altruistic responses were the results of conditioned learning, supported by a positive outcome for the actor. Thus, to dissect the social motivation to make an altruistic decision from the motivation to collect a food reward, we tested a satiety-induced reward devaluation. Mice were tested for five days in the SDM, and, following the last session, we devalued the reward outcome by pre-feeding the mice to satiety using the same reward pellets prior to test actor mice in a session that did not provide food rewards. We tested one condition in which neither the actor nor the recipient received rewards (‘no reward’), whereas another group of actor mice was tested under conditions in which the actor mice did not receive any reinforcements but were still able to allocate food rewards to the recipient (‘reward to recipient only’). Both groups of mice displayed reward devaluation, as indicated by a decrease in the nose poke response (Fig 2e), compared with previous session without pre-feeding (valued). However, we found that the mice increased their preference for altruistic responses when allocation to a recipient was possible, whereas mice that did not receive rewards and could not allocate rewards to the recipient did not modify their preference (Fig 2e). These results suggested that mice were willing to help their conspecifics, even in the absence of apparent self-interests associated with a positive reward outcome.
Mice display more selfish choices with unfamiliar conspecifics
Familiarity between individuals is known to amplify prosocial behaviors11,19,38.To test whether social closeness affects the willingness to allocate food to others, in a new cohort of mice we tested the actions of actors in response to unfamiliar recipients that were housed in different cages. We found that actors tested in the presence of unfamiliar recipients showed opposite choices compared with actor mice tested in the presence of familiar recipients (Fig. 3a). In particular, actors (both males and females) made fewer altruistic responses in the presence of an unfamiliar compared to mice tested in the presence of cage mates (Fig. 3b). To determine individual differences in the responses across animals, we analyzed the performance of each actor. Under the condition featuring an unfamiliar recipient, we found that nine mice of fifteen showed a significant increase in the number of selfish responses (Fig. 3c, d), whereas only three mice acted altruistically. Three mice did not show any preference (Fig. 3c, d). The distribution of mouse preferences showed that fewer than 20% of responses were altruistic for the majority of mice (Fig. 3d). Thus, when the actor mice were presented with non-cage mates, they acted more selfishly than actors paired with cage mates. These data indicated that familiarity facilitates altruistic choices in mice.
Social hierarchy differentiates preference for altruistic choices
Social animals self-organize into hierarchies, where group members vary in their level of dominance, affecting social relationship24. To determine the impact of the hierarchical relation between animals within the same cage on the preference for altruistic choices we used the tube test, a robust assay in which one mouse forces its opponent out of a narrow tube and classified dominant39. Mice were tested pairwise using a round robin design, on daily sessions after the SDM task, and the social rank of each mouse was calculated on the basis of winning against the other cage mates (Fig. 4a).
We analysed the relation of mice from 9 cages. In all cages the relation between mice was transitive and linear (a is more dominant over b, b more dominant g, g more dominant over d and then a should be dominant over all the others; Fig. 4b). Among the mice that performed the SDM task as actor and displayed preference for selfish choices, only 2 were a dominants, the rest were subordinate (Fig. 4c). Whereas, in the group of actor mice that displayed preference for altruistic choices, 6 were subordinate and 8 were dominant (Fig. 4c). We quantified this difference calculating a David’s score (DS) for each mouse, a measure of dominance in the social hierarchy. Actor mice with preference for selfish choices displayed lower scores, and thus lower social rank, compared to their recipient conspecifics (Fig. 4d). Conversely, we did not detect any differences in DS between actor mice displaying preference for altruistic choices and their recipients (Fig. 4e). Finally, we grouped the actor mice that performed on the SDM task based on their social rank and found that mice in the a rank displayed higher altruistic preference compared to mice in the beta and gamma ranks (Fig. 4f). These results indicates that dominant individuals are more willing to share food rewards with their in-group members.
Emotional state matching motivates altruistic choices in dominant mice
Empathy refers to behavioral reactions to others emotional states, including the motivation to help and the affective tendency to experience the emotions of others40. Thus, we tested the hypothesis that the increased altruistic choices in familiar dominant mice could relate to an increased affective state matching between individuals. To do this we used an observational fear conditioning paradigm (Fig. 4g), where mice can vicariously match the emotional state of their companion41. Following the SDM task, actor mice and their recipients were placed in the two compartments of a double-chambered standard fear-conditioning apparatus, separated by a transparent partition. The actor mouse (observer) was allowed to observe the recipient (demonstrator) receiving repetitive foot shocks (Fig. 4g). We found that freezing behavior, which reflected the observational fear induced by social transmission, was higher in altruistic versus selfish mice (Fig. 4g). Both groups of mice spent similar amount of time in exploration of their conspecific demonstrator (Fig. 4g). Furthermore, the scores obtained in the observational fear learning positively correlated with social dominance (Fig. 4h). Altogether these results indicate that dominant mice showed more empathic-like behaviors, including higher altruism and emotional contagion.
BLA neuronal silencing abolish emotional contagion
Encoding of information needed for social transfer have been reported to depend on neuronal projections from the anterior cingulate cortex to the BLA34. Thus, we first tested whether BLA downregulation could change the capacity to be affected by others affective state, using the observational fear conditioning paradigm (Fig. 5a, b). We used a chemogenetic approach to target the glutamatergic neurons in the mouse BLA with a virus carrying the inhibitory designer receptors exclusively activated by designer drugs (DREADD) receptor hM4Di (AAV-CaMKIIa-hM4Di-mCherry) an engineered inhibitory G-protein-coupled receptor that can decrease neuronal activity, or a control virus (AAV-CaMKIIa-mCherry, Fig. 5a and Supplementary Fig. 3a, b). To achieve neuronal downregulation in the BLA, hM4Di mice received clozapine-N-oxide (hM4Di CNO), 30 minutes before testing. As control, also the mice injected with AAV-CaMKIIa-mCherry received CNO (control CNO, Fig. 5b). We found a significant reduction of freezing behavior during the conditioning phase in animals treated with CNO for BLA downregulation, compared to control mice (Fig. 5b), suggesting reduced social transmission of emotions. Thus, consistent with a previous study34, we show that the BLA is critically implicated in the emotional state matching.
BLA neuronal silencing revert the preference for altruistic choices
We revealed a correlation between emotional contagion and willingness to make altruistic choices. We then assessed whether BLA functioning can also mediate the latter process. Following the same approach used for the observational fear conditioning paradigm, we tested the effects of BLA neural silencing during the SDM task (Fig. 5c). We analysed the decision preference score of the actor mice of both control CNO and hM4Di CNO groups tested with familiar recipient mice. We found that whereas control mice displayed a positive score, suggesting preference for altruistic choices, BLA-silenced mice failed to show such preference (Fig. 5d). Indeed, control mice showed an increased number of altruistic over selfish responses, whereas mice with BLA-silencing did not show any difference between the two choices (Fig. 5e). Analysis of individual performance showed that six of the eight mice in which BLA was silenced displayed a significant preference for selfish choices (Fig. 5f). On the contrary, in control mice the majority (6/8) showed preference for altruistic choices (Fig. 5f), similarly to what we have shown in naïve animals tested with familiar recipients (Fig. 1b). The reduced preference for altruistic choices in BLA-downregulated mice was not dependent on reduced interest for social exploration (Supplementary Fig. 3d, e). Moreover, BLA silencing did not affect the number of responses and latency to make a choice (Supplementary Fig. 3f, g). Thus, these results indicate that the BLA is required to make decisions that benefit others. Altogether, these results indicate that BLA is crucially implicated in empathic-like behaviors, including increased altruism and emotional contagion.
BLA neuronal silencing reduces dominance
Social hierarchy influenced the preference for altruistic or selfish choices (Fig. 4). However, the implication of the BLA in the expression of social hierarchy is still unexplored. Thus, we tested whether the BLA could be linked also to the representation of social ranks. To test this, mice with CNO- induced BLA-silencing and control CNO mice were tested after the SDM in daily sessions in the tube test for the assessment of hierarchical relations (Supplementary Fig. 4a, b). All the recipient mice received as control AAV-CamKIIa-mCherry virus and were injected with CNO (Supplementary Fig. 4a, b). We found that silencing of the BLA in hM4Di CNO mice significantly decreased the dominance compared to control CNO mice (Fig. 5g). Indeed, a higher number of hM4Di CNO actor mice was subordinate to their recipient conspecific (Fig. 5h and Supplementary Fig. 4c, d). Further, whereas control mice were distributed across all the four ranks (a, b, g, d), mice with downregulated BLA ranked only b and g (Supplementary Fig. 4e-g). Altogether, consistent with our findings linking altruistic decision preference with hierarchy status, these experiments provide initial evidence of BLA as a common hub in the determination of social dominance, emotional contagion and social decision making.