Dominance hierarchy formation in juvenile crayfish Procambarus clarkii

The formation of social dominance hierarchies was studied in groups of five juvenile crayfish, 1.3-1.8 cm in length, Animals were grouped together in a small, featureless aquarium after having lived in isolation for more than a month. The occurrence of each of four behavior patterns ('attack', 'appr oach', 'retreat' and 'escape') was recorded for each animal, together with the frequency of encounters and the frequency of wins and losses. The frequ encies of wins and losses were used to calculate the relative dominance val ue of each animal in the group. High levels of fighting developed immediate ly upon grouping the animals, and a positive feedback relationship between attacking and winning enabled one animal in each group to emerge quickly as the superdominant. If that animal was the largest, it remained as the supe rdominant; otherwise, it was replaced as superdominant within the first few days by the largest animal. This form of dominance hierarchy, with one sup erdominant and four subordinates, persisted throughout the duration of the grouping. Fighting declined over the first hour and by 24 h had dropped to low levels. After the first day, approaches were used together with attacks , and retreats replaced escapes. Attack and approach were the behavior patt erns displayed most frequently by animals with high dominance values, where as retreat and escape were performed by animals of low dominance. All these trends continued to develop over the next 2 weeks as the number of agonist ic encounters declined to a low level.