The shadow of the fut

Humans show great flexibility in adjusting their levels of cooperation to account for current and future circumstances. For example, levels of cooperation are higher if there is more competition at the level of the whole population than with interacting partners [1] and when individuals are likely to gain social prestige [2]. Humans also show the capacity to increase current levels of cooperation to account for future payoffs if it is likely that repeated interactions will occur with the same partner (known as ‘the Shadow of the Future’) [3]. Here, we provide the first evidence for this capacity in a non-human animal, the cleaner fish Labroides bicolor. L. bicolor individuals show uneven frequency of use of different areas within a large home range, which should in turn affect the delay between repeated interactions with individual reef fish ‘clients’. In areas where the frequency of clients encountering cleaners is higher, cleaners are more likely to experience future costs of cheating, so future payoffs are of more concern for current decisions. In line with this, we found a negative correlation between cheating and the frequency of clients encountering cleaners in L. bicolor home ranges. In contrast to the well-studied cleaner fish Labroides dimidiatus, which has small cleaning stations, the closely related L. bicolor roves over much larger areas [4]. Roving is predicted to destabilize cooperative behaviour [5] because it would reduce the frequency of repeated interactions between cleaners and clients. This would undermine the effectiveness of punishment and partner switching [6], which are used by clients of L. dimidiatus to ensure that cleaners do not cheat by feeding on their preferred mucus, but instead cooperate by feeding on client ectoparasites [7]. Accordingly, it has 1Evolutionary Ecology Group, Department of Zo 2Institut de Zoologie, Eco-Ethologie, Université E-mail: [email protected] 1