In this study we present evidence that 20-hydroxyecdysone (20E) affects agonistic behavior in male lobsters and that male and female animals differ in their aggressive behaviors. Thirty-minute staged fights were conducted between large males exposed to either artificial seawater (ASW) or 20E and small, anosmic opponents. The nephropores of both combatants were blocked. Fights were videotaped and quantitatively analyzed for aggressive, defensive, avoidance and all behaviors using an ethogram in which behaviors are ranked according to aggressiveness (Mello et al, 1999).

Twenty-hydroxyecdysone-exposed male animals performed more defensive behaviors than ASW-exposed controls, while the small opponents of 20E-exposed animals performed more aggressive behaviors than small control counterparts, in response to changes in their large opponent. Lobsters in fights with 20E-exposed animals performed more behaviors than lobsters in fights with ASW-exposed animals; the differences in frequency of all behaviors between combatants was less in fights with 20E-exposed animals. A post-hoc analysis revealed that large ASW-exposed males performed more defensive behaviors than large ASW-exposed females, and small ASW-exposed males performed more total, aggressive and defensive behaviors than small ASW-exposed females. The total aggressive intensity in fights with female lobsters was higher than in fights with male lobsters. Our results indicate that 20E affects the agonistic behavior of both male and female lobsters. The findings also indicate that the patterns of agonistic behavior are different for males and females, but that it may be incorrect to characterize either sex as more aggressive.