Arginine vasotocin reduces levels of cooperative behaviour in a cleaner fish Sónia C. Cardoso a,b , José R. Paitio a , Rui F. Oliveira a,c , Redouan Bshary d , Marta C. Soares b, ⁎ a Unidade de Investigação em Eco-Etologia, ISPA — Instituto Universitário, Lisboa, Portugal b CIBIO, Centro de Investigação em Biodiversidade e Recursos Genéticos, Universidade do Porto, Campus Agrário de Vairão, 4485-661 Vairão, Portugal c Champalimaud Neuroscience Programme, Instituto Gulbenkian de Ciência, Oeiras, Portugal d Université de Neuchâtel, Institut de Biologie, Eco-Ethologie, Rue Emilie-Argand 11, 2000 Neuchâtel, Switzerland HIGHLIGHTS • In nature, cleaner fish need to invest in unrelated partners to yield current and future benefits. • We confirm the importance of the AVT/AVP system as an agent affecting levels of cooperation. • AVT offers a potential mechanistic pathway for the reported flexible service quality. abstract article info Article history: Received 25 July 2014 Received in revised form 13 November 2014 Accepted 14 November 2014 Available online 20 November 2014 Keywords: Cleaner fish Cooperative levels Labroides dimidiatus Neuropeptides Arginine vasotocin Isotocin Decision-making Cooperation between unrelated individuals usually involves investments that often mean a decrease in immediate payoffs, but ensure future benefits. Here we investigated the potential role of the neuropeptides Arginine-vasotocin (AVT) and Isotocin (IT) as proximate agents affecting individuals' cooperative levels in the Indo-pacific bluestreak cleaner wrasse Labroides dimidiatus. Their ‘client’ reef fish partners only benefit from interacting if cleaners eat ecto- parasites and refrain from gleaning preferred client mucus. Thus, cleaners must control their impulse to eat accord- ing to their preference, and eat less preferred items to maintain ongoing interactions and avoid clients' leaving or punishing. We found that solely the experimental transient higher dosage of AVT led to a decrease of cleaners' will- ingness to feed against their preference, while IT and AVT antagonists had no significant effects. The sole effect of AVT on cleaner's performance may imply a link between AVT's influence and a potential activation of a stress response. Our results confirm the importance of the AVT/AVP system as an agent affecting levels of cooperation, offering a potential mechanistic pathway for the reported flexible service quality that cleaners provide their clients. © 2014 Elsevier Inc. All rights reserved. 1. Introduction Cooperation between unrelated individuals often involves invest- ments, which means a decrease in immediate payoffs (for the actor), in order to contribute to the enhancement of benefits in another individual [1]. Evolutionary models usually focus on questions related to potential strategies, which may ensure that investments yield future benefits and hence stabilise cooperation [1–3]. However, current models are ag- nostic about proximal mechanisms that need to be in place to enhance the individuals' ability to decide whether or not to invest. Knowledge on how changes in an individual's physiological/neuro- logical state affect cooperative and social behaviour is needed [4,5], in order to understand variation within and between individuals as well as between species. The neuropeptides arginine vasopressin (AVP) and oxytocin (OT) are well known modulators of a diverse range of ver- tebrate social processes and emotions, including that of humans [6–8]. For example, within humans, experimental setups aiming to increase OT levels have demonstrated that these mediate rises in prosociality, which include trust [9,10] generosity [11,12], empathy [12], and social memory [13], while behavioural manifestations of prosociality have now been linked to individual differences in rs53576 genotype of the OT receptor [14]. Partner support is also a good facilitator of increases in OT plasma levels in both men and women [15]. Regarding AVP, stud- ies have now examined its effects (via intranasal administration) on human facial responses linked to social communication, revealing that AVP influences the response to ambiguous social stimuli [16] and that its effects are sex specific with respect to responses towards same-sex faces, i.e. agonistic in men and affiliative in women [17]. Finally, in a re- cent study, Rilling and colleagues [18], demonstrate that intranasal AVT and IT administration mediate biassed effects in human males and Physiology & Behavior 139 (2015) 314–320 ⁎ Corresponding author. E-mail address: marta.soares@cibio.up.pt (M.C. Soares). http://dx.doi.org/10.1016/j.physbeh.2014.11.052 0031-9384/© 2014 Elsevier Inc. All rights reserved. Contents lists available at ScienceDirect Physiology & Behavior journal homepage: www.elsevier.com/locate/phb