In crustaceans, as in most animal species, the amine serotonin has been suggested to serve important roles in aggression. Here we show that injection of serotonin into the hemolymph of subordinate, freely moving animals results in a renewed willingness of these animals to engage the dominants in further agonistic encounters. By multivariate statistical analysis, we demonstrate that this reversal results principally from a reduction in the likelihood of retreat and an increase in the duration of fighting. Serotonin infusion does not alter other aspects of fighting behavior, including which animal initiates an encounter, how quickly fighting escalates, or which animal eventually retreats. Preliminary studies suggest that serotonin uptake plays an important role in this behavioral reversal.Intraspecific encounters among clawed decapod crustaceans are characterized by a distinct shortage of diplomatic skills. With the exception of mating behavior, most interactions are agonistic in nature, escalating until one of the combatants withdraws. Success is based largely on physical superiority (1-3). Thus, resident populations are bound by a system of dominant/subordinate relationships based on initial agonistic encounters (4, 5). Fights escalate according to rules closely matching predictions of game theory (i.e., sequential assessment strategies), in which animals acquire information about an opponent's strength and fighting abilities in a stepwise manner (6-10). In this context, the timing of the decision to withdraw by either animal becomes the key element in determining the duration and progress of a fight (6,8,9). Decisions may be made after only a brief encounter (seen particularly in the wild) or after prolonged periods of fighting when the physical asymmetries between animals are small. The presence of a highly structured, quantifiable behavioral system in these animals, combined with the potential to bring the analysis to the level of individual neurons (11-16), offers unique vistas in crustaceans for a search for the proximate roots of aggression.The amine serotonin [5-hydroxytryptamine creatinine sulfate complex (5HT)] has been linked to aggression in a wide and diverse range of species, including humans (17-20). The nature of the linkage, however, is not simple, and it has proven difficult to unravel the role of the amine in the behavior. In vertebrates, lowered levels of 5HT (endogenous or experimentally induced) or changes in amine neuron function that lower the effectiveness of serotonergic neurons generally correlate with increased levels of aggression (19,20) whereas in invertebrates, the converse is believed to be true (11-13). Genetic alterations of amine neuron function also can change aggressive behavior in animals (21-24) and in people (25-27) although, again, in most cases, it is not clear how the genetic change is linked to the behavior. For example, in humans, a mutation leading to inactivation of one form of the enzyme monoamine oxidase leads to a particular form of explosive violent beha...