Increased aggressiveness in gravid American lobsters,Homarus americanus

Abstract

Reports in the literature suggest that gravid female lobsters (Homarus americanus) are more aggressive than non-gravid female lobsters. In an earlier study [Cromarty SI et al. 1998. Biol Bull 194:63–71], we showed that, with one exception, gravid females did not respond to an unfamiliar stimulus with escape swimming. Here we present quantitative evidence showing that gravid female lobsters are more aggressive than non-gravid females. Gravid and non-gravid females were allowed to fight against larger, non-gravid female opponents in an enclosed arena. The videotaped behaviors were ranked in a behavioral hierarchy (Rank of Aggression) according to their degree of aggressiveness and intensity. The behaviors were classified and ranked as follows: (1) aggression toward opponent, (2) redirected aggressive behavior (toward arena wall), (3) redirected defensive behavior, (4) defensive behavior against opponent, and (5) avoidance behavior. Gravid lobsters performed more behaviors and more aggressive behaviors than did non-gravid controls. The behaviors of gravid females ranked higher on the Rank of Aggression scale than did non-gravid females. The opponents of gravid females performed more aggressive and more intensely aggressive acts than the opponents of non-gravid females; they also performed more defensive acts (i.e., those used to ward off an opponent's attack) than did their non-gravid opponents, although there were no differences in intensity of defensive behaviors between gravid and non-gravid animals. There were no significant differences in the number of avoidance behaviors among the four types of combatants (gravid, non-gravid, gravid opponent, and non-gravid opponent), and none in avoidance tailflipping. However, gravid females were more likely to perform aggressive tailflips and performed significantly more aggressive tailflips than non-gravid animals. Together, this suggests that any physiological inhibition of escape swimming may have been overcome by the heightened intensity of gravid fights and the increased aggressiveness of gravid animals. Aggr. Behav. 25:451–472, 1999. © 1999 Wiley-Liss, Inc.