Smell of Success and Failure: the Role of Intrinsic and Extrinsic Chemical Signals on the Social Behavior of Crayfish1, The

Integrative and Comparative Biology, Sep 2005 by Moore, Paul A, Bergman, Daniel A

SYNOPSIS.

Animals commonly modify their behavior in the presence of a conspecific or in response to signals. This is particularly true in the context of aggressive exchanges, which animals use to form networks of social relationships and to communicate social status associated with those relationships. Although hierarchical structures are a widespread phenomenon that has been studied extensively, the dynamic communication processes, specifically chemical communication in this review, is relatively overlooked. In particular, it is the exchange of information during agonistic interactions that mediates hierarchies and/or alters the outcomes of agonistic interactions. Given the theoretical appeal of these interactions, and the evolutionary importance and taxonomic diversity associated with social hierarchies, it is not surprising that the sensory mechanisms involved in the formation and maintenance of hierarchical structures have received recent attention. In crayfish, dominance is thought to be largely determined by physical superiority, where encounters are largely dyadic and fighting behavior is highly stereotyped. However, recent evidence has shown that the outcome of dyadic encounters are dependent upon a number of factors other than physical size, that include the exchange of chemical information during encounters, previous social history, and the intrinsic neurochemical state of opponents. We have attempted to provide a comprehensive analysis of the extrinsic chemical processes (previous history, sensory communication, etc.) and intrinsic chemical processes (neurochemical state) that produce and maintain dominance relations and social hierarchies in crayfish. We hope that this review will bring together a global picture of the processes that determine a crayfish's social standing and how intrinsic and extrinsic chemicals have substantial effects on aggressive states and agonistic bouts.

INTRODUCTION

Social behavior

Aggressive acts between conspecifics are termed agonistic interactions. It is through these types of interactions that dominance hierarchies are established (Bell and Gorton, 1978; Copp, 1986; Mesterton-Gibbons and Dugatkin, 1995). A higher social status within a social hierarchy can allow for increased access to food, mates, and shelters (Maynard Smith and Price, 1973). The use of a dominant status to achieve a higher fitness is found in many invertebrates and vertebrates (Tilson and Hamilton, 1984; Beacham and Newman, 1987; Moore et al., 1997; Schuett, 1997; Lopez and Martin, 2002; Correa et al., 2003). More specifically, decapods can acquire shelters by defeating residents in agonistic bouts and thereby increase access to available food and mates, and to reduce predator pressure (Capelli and Hamilton, 1984; Karnofsky et al., 1989; Cromarty et al., 1999; Vorburger and Ribi, 1999; Usio et al., 2001). Consequently, dominant individuals experience increased fitness over subordinate individuals due to the utilization of these benefits (Wilson, 1975).

When using agonistic interactions to establish and reinforce social hierarchies, the energetic cost can be great and possibly even result in injury. However this energetic cost in acquiring resources can be offset by the benefits afforded by these resources that can lead to increased growth and mating opportunities, and decreased predation risk involved (Wilson, 1975). By detecting or communicating with conspecifics within a social hierarchy, dominant or subordinate animals can avoid injury by reducing the number of competitive interactions or intensity of conflicts over resources. Communication of social position when resources are in conflict involves the exchange of information between a sender and a receiver, and then the subsequent use of this information by the receiver to make decisions on the appropriate response (Dawkins and Krebs, 1978; Bradbury and Vehrencamp, 1998). Consequently, the signaling of status level can be important because it functions in reducing fight intensity and duration, thus resulting in a reduced risk of injury (Enquist, 1985; Beecher, 1989; Adamo and Hanlon, 1996). Within this framework, agonistic behavior may be regarded as a behavioral tool, i.e., a means of interacting with other conspecifics, to enhance the survival and/or reproductive effort of the individuals involved in the interaction (Wilson, 1975). Communication of one's status may be used to influence the outcome of an encounter with or without direct physical interaction (Wilson, 1975). During agonistic encounters, a contestant may signal whether the encounter should escalate or decrease its fighting intensity.

Social behavior from largely asocial animals

Fighting behavior of clawed decapod crustaceans has attracted considerable interest in the broad behavioral literature on aggression due to the presence of conspicuous visual displays and potentially lethal weaponry (Thorp and Ammerman, 1978; Smith and Dunham, 1990; Caldwell, 1992; Rutherford et al., 1995). The ultimate goal of any aggressive encounter is to obtain an elevated social status that confers an advantage to an individual in obtaining a resource (Wilson, 1975). Conversely, a subordinate individual may lose access to resources through unsuccessful bouts, but may obtain a net benefit by avoiding such costs as increased energy expenditure, injury from a conspecific or increased predation risk (Wilson, 1975). If a subordinate does not gain a benefit, then th e lower status will have a negative effect on fitness by reducing the number of resources available.