Parasites often alter the behavior of their hosts in ways that are ultimately beneficial to the parasite or its offspring. Although the alteration of host behavior by parasites is a widespread phenomenon, the underlying neuronal mechanisms are only beginning to be understood. Here, we focus on recent advances in the study of behavioral manipulation via modulation of the host central nervous system. We elaborate on a few case studies, in which recently published data provide explanations for the neuronal basis of parasite-induced alteration of host behavior. Among these, we describe how a worm may influence the nervous system of its cricket host and manipulate the cricket into committing suicide by jumping into water. We then focus on Ampulex compressa, which uses an Alien-like strategy for the sake of its offspring. Unlike most venomous hunters, this wasp injects venom directly into specific cerebral regions of its cockroach prey. As a result of the sting, the cockroach remains alive but immobile, but not paralyzed, and serves to nourish the developing wasp larva.

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...

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This review summarizes a set of experimental approaches with which we explore fighting behavior in crayfish and the importance of aminergic systems in its control. Our results illustrate that agonistic behavior in crustaceans can be characterized within a quantitative framework, that different types of behavioral plasticity in aggressive behavior are in need of physiological explanation, and that pharmacological intervention involving serotonergic systems produces characteristic changes in fighting. Moreover, we attempt to identify changes in neurochemistry during the acquisition of social status. Many of the studies presented here summarize ongoing work. Nonetheless, results to date complement and extend previous detailed physiological, morphological and biochemical studies exploring the roles of amines in aggression.