Front Cover

Reciprocal exchanges can be found across the tree of life and can involve different commodities and services. Artwork by Monika Jasnauskaite.

The prisoner's dilemma.

Schematic representation of a typical coordinated predator inspection event. The numbers show order (left-to-right and top to bottom). The larger fish in the top left corner of each panel represents a potential predator, the smaller fish represent prey fish. Fish silhouettes designs from PhyloPic.

Whether individuals reciprocate appears to be influenced by the experimental context, whether there is a reason to reciprocate, the equity of the interaction, and the quality of the relationship, with individuals in close relationships more likely to reciprocate and less likely to base their decision on their partner's recent behavior. (Photograph used with permission from Kelly Leverett)

What explains the long-term cooperative relationships that underlie social grooming in primates and food sharing in vampire bats? Reciprocity and pseudo-reciprocity are often considered alternative explanations, but they are not mutually exclusive. Helping a partner can simultaneously increase their willingness to reciprocate, enhance their ability to reciprocate, and yield additional byproduct benefits from their survival or reproduction. Rather than conceptualizing reciprocity versus pseudo-reciprocity into discrete categories, it is more insightful to conceptualize responsiveness and interdependence as co-existing and interacting effects. Drawings by Javier Lazaro.

Reciprocity is often believed to be rare because of its cognitive requirements, which are thought to exceed the capabilities of most animals. This so-called cognitive constraint hypothesis predicts that, if reciprocation ever occurs among animals, it must necessarily be immediate. In this study, we show that Himalayan tahr, a cognitively unsophisticated ungulate, are able to reciprocate grooming over long time frames, thus falsifying the cognitive constraint hypothesis.

In larger groups, males can support each other in coalitions against male group mates. Here we show with data on Barbary macaques, that males cooperate more with partners that are more loyal to them and that loyalty is reciprocated. We argue that this partner choice mechanism may often affect an individual's coalition network betweenness - depicted here as the size of the node.

Paired worms trade eggs for sperm following reciprocity rules.

Meta-analyses of reciprocity experiments with Norway rats demonstrate, across a wide range of studies, that females help partners according to the direct and generalized reciprocity decision rules, whereas males help by direct, but not generalized, reciprocity. There was no apparent difference in the help given by females between the direct and generalized reciprocity decision rules, suggesting that females may frequently apply the cognitively less demanding decision rule of generalized reciprocity, unless they need to distinguish between individuals differing in helpfulness.

Do kinship, reciprocity and dominance influence prosocial food provisioning in the group service paradigm in azure-winged magpies, carrion crows and common ravens? We found no evidence for an effect of kinship or reciprocity on food provisioning. Contrary to our predictions, the subjects' dominance was not positively correlated with their rate of providing food in any species. Among ravens, dominance was instead positively correlated with receiving food from others.

We present the first direct comparative study on short-term reciprocity in chimpanzees (a) and human children (b). In a novel version of an instrumental helping task, subjects could press a button to help a partner access a tool. Chimpanzees and children helped. However, the extent of short-term reciprocity differed. After receiving help, children always helped in return but helped substantially less when interacting with an unhelpful partner. Chimpanzees showed similar short-term reciprocity only in the first half of the experiment.

Findings of reciprocity in domestic dogs have been inconsistent. We investigated whether pet dogs would provide food to a helpful conspecific who had pressed a button to donate food to them and an unhelpful partner who had not provided food. We found that dogs did not reciprocate the receipt of food and also that the familiarity of the partner had no influence on reciprocity. Moreover, salivary oxytocin concentration was not influenced by receiving help or not.

Individuals sometimes direct help preferentially towards those who have helped others. This is typically explained in terms of indirect reciprocity where individuals who give to others increase their own chance of receiving help from third parties. Here, I consider whether a strategy of helping those who help others can spread simply because the strategy benefits other discriminating helpers.