Long-term consequences of agonistic interactions between socially naive juvenile American lobsters (Homarus americanus).

Agonistic behavior of decapod crustaceans occurs in the wild, is highly stereotypical, and is readily quantifiable. Here we explore the persistence of dominance relationships in socially naı̈ve juvenile American lobsters (Homarus americanus) by examining the time frame over which behavior changes as a result of previous fight experience. We paired opponents for three fights: initial; 1 hour later; and after 1, 4, or 7 days of separation. As reported in previous studies, the mean duration of encounters decreased when a dominance relationship had been established. After 1 day of separation, both duration and intensity of encounters remained significantly lower compared to the initial fight. At 4 days of separation, the duration of encounters was still significantly reduced, indicating that the behavior of socially naı̈ve juvenile lobsters is influenced by fight experience for at least 4 days. Though aggression has been studied extensively, little is known about its underlying neural mechanisms. The purpose of the present study was to examine the persistence of behavioral effects of fighting experience in socially naı̈ve juvenile lobsters (Homarus americanus Milne-Edwards, 1837), as a step toward investigating both the neural substrate and ethological relevance of such effects. In socially naı̈ve juvenile lobsters, fighting behavior is highly stereotypical, easily evoked, and readily quantifiable (1). In the wild, fights are often short, as size asymmetries and previous experience play an important role in early decisionmaking (2, 3). In the laboratory, stable dominance relationships form readily between pairs of lobsters (1, 2, 4), and hierarchies form within groups (5, 6, 7, 8). Previous studies have shown that subordinates tend to avoid engaging dominants (1, 3, 5, 6, 9), and if interactions do take place, their duration and intensity are reduced. Thus, with the formation of dominance relationships, fighting tapers off, and the duration and intensity of encounters can be used as measures of “willingness” to fight (1). Studies of adult, socially experienced lobsters indicate that recognition of a dominance relationship is retained for at least 1 day of separation after an initial fight (4). Pairs separated for 7 days showed variable results, and all animals seemed to have forgotten their initial dominance relationships after 14 days. Although the patterns of fighting behavior between pairs of experienced and pairs of socially naı̈ve animals are similar, differences are apparent, for example, in the time taken to establish dominance and in the intensity of fighting. Karavanich and Atema (4) postulate that these differences may be due to shaping of the adult behavior by experience. Naı̈ve lobsters exhibit more ritualized fighting, making it possible to quantify the behavior and to define the consequences of experience on the subsequent patterns of behavior (1). We have characterized the conduct of pairs of socially naı̈ve, juvenile animals over three 30-min fighting periods. The first fight served to establish the dominance relationship, the second (1 h later) was used to confirm that relationship, and the third (either 1, 4, or 7 days later) was used to determine how long the effects of the initial two fighting periods continued to influence subsequent fighting behavior. A preliminary report of these data has been published (10). Juvenile lobsters, that had been isolated visually and physically, but not chemically, from the fourth post-larval Received 9 July 2004; accepted 25 August 2004. * To whom correspondence should be addressed: E-mail: edward_ [email protected] Reference: Biol. Bull. 207: 183–187. (December 2004) © 2004 Marine Biological Laboratory