Treatment with the glucocorticoid antagonist RU486 reduces cooperative cleaning visits of a common reef fish, the lined bristletooth Albert F.H. Ros ⁎, Philippe Vullioud, Redouan Bshary Department of Biology, University of Neuchâtel, Emile-Argand, 11, 2000 Neuchâtel, Switzerland abstract article info Article history: Received 5 May 2011 Revised 21 September 2011 Accepted 23 September 2011 Available online xxxx Keywords: Cortisol Teleost Cleaning mutualism RU486 Mifepristone Stress Cooperation often involves a conflict of interest. This is particularly true in situations where one individual seeks out a service but cannot properly control the quality of the service given by the partner who would gain from defecting. An example is cleaning mutualism involving the bluestreak cleaner wrasse (Labroides dimidiatus) and its reef-fish ‘clients’. These cleaners may reduce the stress experienced by their clients by re- moving parasites; however they occasionally cheat clients (i.e. defect) by eating mucus and other living tis- sues. Here we present experimental support for the hypothesis that stress responses increase the motivation for clients to seek out such risky asymmetric interactions. We manipulated the stress response by blocking glucocorticoid receptors with the antagonist RU486 in a species that is a regular visitor of cleaner fish, the lined bristletooth (Ctenochaetus striatus). Field observations 1 week after treatment with RU486 showed that antagonist treatment led to a reduction in cleaning duration compared to control treatment. This was not explained by a general effect on client behavior as intraspecific social behavior appeared unaffected. We propose that antagonist treatment reduced stress responses to the presence of ectoparasites, which in turn reduced the client's perception of benefits from seeking out cleaning interactions. The results demon- strate a hitherto overlooked variable role of stress and stress responses on cooperative behavior. © 2011 Elsevier Inc. All rights reserved. Introduction Cooperation generally involves an exchange of goods and services (Noë, 2001). Cooperative acts are often an investment: they reduce cur- rent payoffs of the actor and increase the current payoff of the recipient, as exemplified in a prisoner's dilemma payoff matrix (Axelrod and Hamilton, 1981). Conflicts of interest arise in that individuals would benefit from reducing their investment as long as the partner does not alter its cooperative behavior in response. In the iterated prisoner's di- lemma game a defecting individual risks that the partner will defect in return in the next round. However when the partner lacks this option to reciprocate, the temptation to defect should be particularly strong. Such asymmetric games are commonly found in nature (Bshary and Bronstein, 2004; Bshary and Bronstein, 2011). In these cases, individ- uals with fewer options to control their partner (including via punish- ment and partner choice) should seek out interactions with this partner only if the costs of defection are relatively low in comparison to the benefits of cooperation (Johnstone and Bshary, 2002). A related question is how individuals make appropriate decisions about when to interact with a partner. This question can be addressed by investigating the role of cognition in decision making, for example by studying how prior experience affects partner choice. Alternatively, it can be addressed by studying how physiological processes are affect- ed by partners and whether these processes influence subsequent deci- sions to interact with those partners (Brosnan and Bshary, 2010). Recently the latter approach has been used to study whether hormones play a role in cooperative games in humans. For example trust, which af- fects the likelihood of individuals to invest in potentially risky coopera- tive interactions, has been shown to be promoted by oxytocin (Kosfeld et al., 2005; Baumgartner et al., 2008). One mechanism by which oxyto- cin might exert this effect on trust is by suppressing the standard physi- ological response to a risky situation, namely the glucocorticoid stress response (Ditzen et al., 2009). Glucocorticoids are released by the adre- nals and target a wide range of peripheral and neural tissues to optimize behavioral stress responses when needed (Selye, 1936; Korte et al., 2005; Oliveira and Galhardo, 2009). Consequently, in humans the release of glucocorticoids (cortisol) in response to stress is associated with lowered levels of interpersonal trust (Takahashi et al., 2005). Therefore, stress may reduce an individual's trust and thereby the motivation to invest in cooperative interactions that bear the risk of exploitation by an unco- operative partner. While human studies have typically investigated dyadic interac- tions, more complicated interactions exist in multi-player interactions. This is the case in cleaning mutualism involving the bluestreak cleaner wrasse and its reef-fish clients which can be thought of as triadic since it involves the cleaner fish, the client and the client's ectoparasites (Eibl-Eibesfeldt, 1955; Limbaugh, 1961; Bshary and Côté, 2008). The health of clients (player 1) is constantly challenged by ectoparasites Hormones and Behavior xxx (2011) xxx–xxx ⁎ Corresponding author at: University of Neuchâtel, Department of Behavioral Ecology, Rue Émile-Argand 11, 2000 Neuchâtel, Switzerland. Fax: +41 327183001. E-mail address: albert.ros@unine.ch (A.F.H. Ros). YHBEH-03270; No. of pages: 7; 4C: 0018-506X/$ – see front matter © 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.yhbeh.2011.09.013 Contents lists available at SciVerse ScienceDirect Hormones and Behavior journal homepage: www.elsevier.com/locate/yhbeh Please cite this article as: Ros, A.F.H., et al., Treatment with the glucocorticoid antagonist RU486 reduces cooperative cleaning visits of a com- mon reef fish, the lined bristletoo..., Horm. Behav. (2011), doi:10.1016/j.yhbeh.2011.09.013