Male Norway rats cooperate according to direct but not generalized reciprocity rules

Evolution of reciprocity with limited payoff memory

Direct reciprocity is a mechanism for the evolution of cooperation in repeated social interactions. According to the literature, individuals naturally learn to adopt conditionally cooperative strategies if they have multiple encounters with their partner. Corresponding models have greatly facilitated our understanding of cooperation, yet they often make strong assumptions on how individuals remember and process payoff information. For example, when strategies are updated through social learning, it is commonly assumed that individuals compare their average payoffs. This would require them to compute (or remember) their payoffs against everyone else in the population. To understand how more realistic constraints influence direct reciprocity, we consider the evolution of conditional behaviours when individuals learn based on more recent experiences. Even in the most extreme case that they only take into account their very last interaction, we find that cooperation can still evolve. However, such individuals adopt less generous strategies, and they cooperate less often than in the classical setup with average payoffs. Interestingly, once individuals remember the payoffs of two or three recent interactions, cooperation rates quickly approach the classical limit. These findings contribute to a literature that explores which kind of cognitive capabilities are required for reciprocal cooperation. While our results suggest that some rudimentary form of payoff memory is necessary, it suffices to remember a few interactions.

Human and nonhuman norms: a dimensional framework

Human communities teem with a variety of social norms. In order to change unjust and harmful social norms, it is crucial to identify the psychological processes that give rise to them. Most researchers take it for granted that social norms are uniquely human. By contrast, we approach this matter from a comparative perspective, leveraging recent research on animal social behaviour. While there is currently only suggestive evidence for norms in nonhuman communities, we argue that human social norms are likely produced by a wide range of mechanisms, many of which we share with nonhuman animals. Approaching this variability from a comparative perspective can help norm researchers expand and reframe the range of hypotheses they test when attempting to understand the causes of socially normative behaviours in humans. First, we diagnose some of the theoretical obstacles to developing a comparative science of social norms, and offer a few basic constructs and distinctions to help norm researchers overcome these obstacles. Then we develop a six-dimensional model of the psychological and social factors that contribute to variability in both human and potential nonhuman norms. This article is part of the theme issue 'Social norm change: drivers and consequences'.

Evidence suggesting that reindeer mothers allonurse according to the direct reciprocity and generalized reciprocity decision rules

Allonursing is the nursing of the offspring of other mothers. Cooperation is an emergent property of evolved decision rules. Cooperation can be explained by at least three evolved decision rules: 1) direct reciprocity, i.e. help someone who previously helped you, 2) kin discrimination, i.e. preferentially direct help to kin than to non-kin, and 3) generalized reciprocity, i.e. help anyone if helped by someone. We assessed if semi-domesticated reindeer, Rangifer tarandus, mothers allonursed according to the decision rules of direct reciprocity, generalized reciprocity and kin discrimination over 2 years. To assess if reindeer mothers allonursed according to the direct reciprocity decision rule, we predicted that mothers should give more help to those who previously helped them more often. To assess if reindeer mothers allonursed according to the kin discrimination decision rule, we predicted that help given should increase as pairwise genetic relatedness increased. To assess if reindeer mothers allonursed according to the generalized reciprocity decision rule, we predicted that the overall number of help given by reindeer mothers should increase as the overall number of help received by reindeer mothers increased. The number of help given i) increased as the number of help received from the same partner increased in the 2012 group but not in both 2013 groups, ii) was not influenced by relatedness, and iii) was not influenced by an interaction between the number of help received from the same partner and relatedness. iv) The overall number of help given increased as the overall number of help received increased. The results did not support the prediction that reindeer mothers allonursed according to the kin discrimination decision rule. The results suggest that reindeer mothers may allonurse according to the direct reciprocity and generalized reciprocity decision rules.

Assessment of help value affects reciprocation in Norway rats

Wild-type Norway rats reciprocate help received in a well-replicated experimental food-giving task, but the criteria to appraise the received help's value are unclear. We tested whether quality or quantity of received help is more important when deciding to return help, and whether partner familiarity and own current need affect this evaluation. We experimentally varied recipients of help's hunger state, and familiar or unfamiliar partners provided either higher caloric food (enhanced quantity; carrots) or food higher in protein and fat (enhanced quality; cheese). Reciprocation of received help was our criterion for the rats' value assessment. Familiarity, food type and hunger state interacted and affected help returned by rats. Rats returned less help to familiar partners than to unfamiliar partners. With unfamiliar partners, rats returned more help to partners that had donated preferred food (cheese) than to partners that had donated less preferred food (carrots), and they returned help earlier if they were satiated and had received cheese. With familiar partners, food-deprived rats that had received cheese returned more help than satiated rats that had received carrots. Our results suggest that Norway rats assess the received help's value based on its quality, their current need and partner familiarity before reciprocating received help.

Game theory in biology: 50 years and onwards

Game theory in biology gained prominence 50 years ago, when Maynard Smith & Price formulated the concept of an evolutionarily stable strategy (ESS). Their aim was to explain why conflicts between animals of the same species usually are of a 'limited war' type, not causing serious injury. They emphasized that game theory is an alternative to previous ideas about group selection, which were used by ethologists to explain limited aggression. Subsequently, the ESS concept was applied to many phenomena with frequency dependence in the evolutionary success of strategies, including sex allocation, alternative mating types, contest behaviour and signalling, cooperation, and parental care. Both the analyses of signalling and cooperation were inspired by similar problems in economics and attracted much attention in biology. Here we give a perspective on which of the ambitions in the field have been achieved, with a focus on contest behaviour and cooperation. We evaluate whether the game-theoretical study of the evolution of cooperation has measured up to expectations in explaining the behaviour of non-human animals. We also point to potentially fruitful directions for the field, and emphasize the importance of incorporating realistic behavioural mechanisms into models. This article is part of the theme issue 'Half a century of evolutionary games: a synthesis of theory, application and future directions'.

When deciding to cooperate by direct reciprocity, Norway rats sometimes benefit from olfactory competence and seem not impaired by insufficient cognitive abilities

Direct reciprocity requires the ability to recognize and memorize social partners, and to remember their previous actions. 'Insufficient cognitive abilities' have been assumed to potentially impair the ability to cooperate by direct reciprocity. Here we compare the propensity of rats to use direct reciprocity with their ability to memorize and recognize sensory cues in a non-social task. Female rats enriched in one of three sensory modalities (visual, olfactory or auditory) performed better in a learning task when they were tested with the specific sensory modality in which they have been enriched. For the cooperation test, during three subsequent reciprocity experiments the rats could provide two partners differing in their previous helpfulness with food. Individuals performing better in the non-social learning task that involved olfactory cues applied direct reciprocity more successfully in one experiment. However, in the experiment preventing visual cues and physical contact, rats applied direct reciprocity rules irrespective of their performance in the learning task with olfactory cues. This indicates that an enhanced olfactory recognition ability, despite being beneficial, is not a prerequisite for the rats' ability to cooperate by direct reciprocity. This might suggest that when rats have all types of information about their social partner, individuals may apply other criteria than the reciprocity decision rule when determining how much help to provide, as for instance coercion. Interestingly, when all individuals are constrained to mostly rely on olfactory memory, individuals apply direct reciprocity independently of their ability to memorize olfactory cues in a non-social context. 'Insufficient cognitive abilities' may thus not be the true reason when direct reciprocity is not observed.

Helping behavior in prairie voles: A model of empathy and the importance of oxytocin

Several studies suggest that rodents show empathic responses and helping behavior toward others. We examined whether prairie voles would help conspecifics who were soaked in water by opening a door to a safe area. Door-opening latency decreased as task sessions progressed. Female and male voles stayed close to the soaked voles' side at equal rates and opened the door with similar latencies. When the conspecific was not soaked in water, the door-opening latency did not decrease. This suggests that the distress of the conspecific is necessary for learning to open the door and that the door-opening performed by prairie voles corresponds to helping behavior. Additionally, we examined the helping behavior in prairie voles in which oxytocin receptors were genetically knocked out. Oxytocin receptor knockout voles demonstrated less learning of the door-opening behavior and less interest in soaked conspecifics. This suggests that oxytocin is important for the emergence of helping behavior.

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Prairie voles demonstrated helping behavior toward a cagemate in distress

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There was no difference in helping behavior depending on the helper’s sex

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Learning of the helping behavior was prevented when cagemates were not in distress

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Oxytocin receptor knockout prairie voles demonstrated less helping behavior

Rats show direct reciprocity when interacting with multiple partners

Direct reciprocity, where individuals apply the decision rule 'help someone who has helped you', is believed to be rare in non-human animals due to its high cognitive demands. Especially if previous encounters with several partners need to be correctly remembered, animals might either stop reciprocating favours previously received from an individual, or switch to the simpler generalized reciprocity mechanism. Here we tested the decision rules Norway rats apply when interacting with multiple partners before being able to return received help. In a sequential prisoner's dilemma situation, focal subjects encountered four different partners that were either helpful or not, on four consecutive days. On the fifth day, the focal subject was paired with one of the previous four partners and given the opportunity to provide it with food. The focal rats returned received help by closely matching the quantity of help their partner had previously provided, independently of the time delay between received and given help, and independently of the ultimate interaction preceding the test. This shows that direct reciprocity is not limited to dyadic situations in Norway rats, suggesting that cognitive demands involved in applying the required decision rules can be met by non-human animals even when they interact with multiple partners differing in helping propensity.

Cooperative responses in rats playing a 2 × 2 game: Effects of opponent strategy, payoff, and oxytocin

The present study tested cooperation in rats playing a 2 × 2 game (2 players, 2 responses) in an operant chamber, where players choose to cooperate or defect without knowledge of their partner's choice. We evaluated cooperative responses in rats (Subjects) playing different games [iterated Prisoner's Dilemma (IPD), Stag Hunt] with a Stooge partner utilizing different response strategies [Tit-for-tat (TFT), Win-stay, Lose-shift (WSLS), Random], and we determined the effects of oxytocin (OT). IPD trial outcomes and payoffs included mutual cooperation (reward, R, 3 sugar pellets each), mutual defection (punishment, P, 1 pellet each), or unilateral defection (temptation, T, 5 pellets) and cooperation (sucker, S, 0 pellets). Stag Hunt was similar, except that T = 2 pellets. We hypothesized that Subjects would make more cooperative responses when playing Stag Hunt vs IPD, when playing IPD with a Stooge using TFT vs WSLS or Random, and when treated with OT. At baseline, Subjects' overall likelihood of cooperation was unaffected by the game (IPD vs SH) or by the Stooges' response strategy (TFT, WSLS, Random). Cooperative responses earned Subjects more pellets, except when playing with a Stooge using a random strategy. Trial outcomes (R, T, S or P) also varied by game and strategy, although the mutual defection (P) was the most common. Systemic pretreatment with OT increased Subjects' cooperative responses, resulting in fewer P and more R outcomes. In particular, IPD-Random Subjects were more cooperative, even at the expense of earning fewer pellets. These results demonstrate that OT increases cooperative behavior in rats playing 2 × 2 games.

The social life of Norway rats (Rattus norvegicus)

The Norway rat has important impacts on our life. They are amongst the most used research subjects, resulting in ground-breaking advances. At the same time, wild rats live in close association with us, leading to various adverse interactions. In face of this relevance, it is surprising how little is known about their natural behaviour. While recent laboratory studies revealed their complex social skills, little is known about their social behaviour in the wild. An integration of these different scientific approaches is crucial to understand their social life, which will enable us to design more valid research paradigms, develop more effective management strategies, and to provide better welfare standards. Hence, I first summarise the literature on their natural social behaviour. Second, I provide an overview of recent developments concerning their social cognition. Third, I illustrate why an integration of these areas would be beneficial to optimise our interactions with them.

Cooperation: How Vampire Bats Build Reciprocal Relationships

In theory, reciprocal relationships should develop gradually, to reduce the risk of helpers being exploited. In a classic case of reciprocity, vampire bats share blood with starving roost-mates. Now it transpires they share food only after first having established grooming relationships.

Rats play tit-for-tat instead of integrating social experience over multiple interactions

Theoretical models of cooperation typically assume that agents use simple rules based on last encounters, such as 'tit-for-tat', to reciprocate help. By contrast, empiricists generally suppose that animals integrate multiple experiences over longer timespans. Here, we compared these two alternative hypotheses by exposing Norway rats to partners that cooperated on three consecutive days but failed to cooperate on the fourth day, and to partners that did the exact opposite. In additional controls, focal rats experienced cooperating and defecting partners only once. In a bar-pulling setup, focal rats based their decision to provide partners with food on last encounters instead of overall cooperation levels. To check whether this might be owing to a lack of memory capacity, we tested whether rats remember the outcome of encounters that had happened three days before. Cooperation was not diminished by the intermediate time interval. We conclude that rats reciprocate help mainly based on most recent encounters instead of integrating social experience over longer timespans.

Reciprocity: Different behavioural strategies, cognitive mechanisms and psychological processes

Reciprocity is probably one of the most debated theories in evolutionary research. After more than 40 years of research, some scientists conclude that reciprocity is an almost uniquely human trait mainly because it is cognitively demanding. Others, however, conclude that reciprocity is widespread and of great importance to many species. Yet, it is unclear how these species reciprocate, given its apparent cognitive complexity. Therefore, our aim was to unravel the psychological processes underlying reciprocity. By bringing together findings from studies investigating different aspects of reciprocity, we show that reciprocity is a rich concept with different behavioural strategies and cognitive mechanisms that require very different psychological processes. We reviewed evidence from three textbook examples, i.e. the Norway rat, common vampire bat and brown capuchin monkey, and show that the species use different strategies and mechanisms to reciprocate. We continue by examining the psychological processes of reciprocity. We show that the cognitive load varies between different forms of reciprocity. Several factors can lower the memory demands of reciprocity such as distinctiveness of encounters, memory of details and network size. Furthermore, there are different information operation systems in place, which also vary in their cognitive load due to assessing the number of encounters and the quality and quantity of help. We conclude that many species possess the psychological processes to show some form of reciprocity. Hence, reciprocity might be a widespread phenomenon that varies in terms of strategies and mechanisms.