2412 Introduction Agonistic behaviors that regulate conflict situations between conspecifics have shaped sociality across evolution (Lorenz, 1963; King, 1973). Conflicts arise because animals compete for different resources (territory, food, mates, breeding sites, etc.) and they are solved when one individual keeps the resource in struggle (dominant) and the other loses it (subordinate). Though the resources individuals compete for may be different and the behavioral patterns involved may be extremely diverse, agonistic encounters always follow the same phases (evaluation, contest and post-conflict resolution). Conflicts may be solved without escalated contests, but animals usually have to fight to elucidate which will become the winner or the loser. Therefore, aggression, defined as an overt behavior that leads to displacing, dominating or harming another individual (Nelson, 2006), is part of the contest phase of agonistic encounters. Given the diversity of aggressive behaviors and the variety of contexts in which aggression is expressed, it has not been possible to reach a general consensus in the definition of aggression and its classification, though several authors have provided useful theoretical frameworks in this concern (Moyer, 1968; Brain, 1979; Wingfield et al., 2006). Within the agonistic context, individuals can aggressively compete either for space or for a mate, or aggressively establish hierarchies in a flock. Thus, depending on the resources individuals compete for and the motivation towards combat, at least three distinctive types of aggression can be observed during agonistic encounters: territorial aggression, reproduction-related aggression and rank-related aggression (Wingfield et al., 2006). The type or types of aggression displayed by one species are correlated with its social structure. For example, it is not likely for gregarious species to display exclusive territorial aggression, nor for solitary species to display rank-related aggression. However, it is not always easy to discern between types of aggression, and to obtain model systems to test clear-cut examples of a distinctive type of aggression. Besides, both in natural performing animals and in laboratory settings, different types of aggression often intermingle, resulting, for example, in territorial aggression only observed between males during breeding while they are competing for mates, or rank-related aggression established by territory defense (Brown, 1964). Several brain areas are known to control aggression, though it has not been possible to dissect a fixed neural circuit responsible for aggressive behavior. Rather, following Newman’s hypothesis (Newman, 1999), we interpret aggressive behaviors as emergent properties of a complex social behavior network (SBN) that includes the medial preoptic area, lateral septum, anterior hypothalamus, ventromedial hypothalamus, periaqueductal gray, Summary Agonistic behavior has shaped sociality across evolution. Though extremely diverse in types of displays and timing, agonistic encounters always follow the same conserved phases (evaluation, contest and post-resolution) and depend on homologous neural circuits modulated by the same neuroendocrine mediators across vertebrates. Among neuromodulators, serotonin (5-HT) is the main inhibitor of aggression, and arginine vasotocin (AVT) underlies sexual, individual and social context differences in behavior across vertebrate taxa. We aim to demonstrate that a distinct spatio-temporal pattern of activation of the social behavior network characterizes each type of aggression by exploring its modulation by both the 5-HT and AVT systems. We analyze the neuromodulation of aggression between the intermale reproduction-related aggression displayed by the gregarious Brachyhypopomus gauderio and the non-breeding intrasexual and intersexual territorial aggression displayed by the solitary Gymnotus omarorum. Differences in the telencephalic activity of 5-HT between species were paralleled by a differential serotonergic modulation through 1A receptors that inhibited aggression in the territorial aggression of G. omarorum but not in the reproduction-related aggression of B. gauderio. AVT injection increased the motivation towards aggression in the territorial aggression of G. omarorum but not in the reproduction-related aggression of B. gauderio, whereas the electric submission and dominance observed in G. omarorum and B. gauderio, respectively, were both AVT-dependent in a distinctive way. The advantages of our model species allowed us to identify precise target areas and mechanisms of the neuromodulation of two types of aggression that may represent more general and conserved strategies of the control of social behavior among vertebrates. Key words: territorial aggression, reproduction-related aggression, serotonin, AVT, social behavior network, Gymnotus omarorum, Brachyhypopomus gauderio. Received 25 October 2012; Accepted 31 January 2013 The Journal of Experimental Biology 216, 2412-2420 © 2013. Published by The Company of Biologists Ltd doi:10.1242/jeb.082180 REVIEW Neuromodulation of the agonistic behavior in two species of weakly electric fish that display different types of aggression Ana C. Silva 1,2, *, Rossana Perrone 1 , Lucía Zubizarreta 1 , Gervasio Batista 1 and Philip K. Stoddard 3 1 Unidad Bases Neurales de la Conducta, Instituto de Investigaciones Biológicas Clemente Estable, Montevideo 11600, Uruguay, 2 Laboratorio de Neurociencias, Facultad de Ciencias, Universidad de la Republica, Montevideo 11400, Uruguay and 3 Department of Biological Sciences, Florida International University, Miami, FL 33199, USA *Author for correspondence (asilva@iibce.edu.uy) THE JOURNAL OF EXPERIMENTAL BIOLOGY