Abstract
Using pairings of male crayfish Procambarus clarkii with a 3–7% difference in size, we confirmed that physically larger crayfish were more likely to win encounters (winning probability of over 80%). Despite a physical disadvantage, small winners of the first pairings were more likely to win their subsequent conflicts with larger naive animals (winning probability was about 70%). By contrast, the losers of the first pairings rarely won their subsequent conflicts with smaller naive animals (winning probability of 6%). These winner and loser effects were mimicked by injection of serotonin and octopamine. Serotonin-injected naive small crayfish were more likely to win in pairings with untreated larger naive crayfish (winning probability of over 60%), while octopamine-injected naive large animals were beaten by untreated smaller naive animals (winning probability of 20%). Furthermore, the winner effects of dominant crayfish were cancelled by the injection of mianserin, an antagonist of serotonin receptors and were reinforced by the injection of fluoxetin, serotonin reuptake inhibitor, just after the establishment of social order of the first pairings. Injection of octopamine channel blockers, phentolamine and epinastine, by contrast, cancelled the loser effects. These results strongly suggested that serotonin and octopamine were responsible for winner and loser effects, respectively.
Highlights
The establishment of status-dependent dominance hierarchy through conflict between conspecifics is essential for territorial animals to maintain their social stability
The smaller crayfish won in three pairings, while in the remaining pairing no winner was found within a 45 min period (Fig. 1A left)
As the results of our previous study showed, more than 80% of winning crayfish had longer bodies and/or chelae length, and winners were usually heavier in mass, even if their differences were less than 5% of the losing opponents [50]
Summary
The establishment of status-dependent dominance hierarchy through conflict between conspecifics is essential for territorial animals to maintain their social stability. A previously winning experience increases the winning probability of the agonistic encounter, whereas a previous losing experience has the opposite effect. These winner and loser effects have been widely described in both vertebrates [1], [2], [3], [4], [5], [6] and arthropods [7], [8]. The injection of serotonin or octopamine into dominant and subordinate animals modifies aggressiveness during the following encounters but fails to reverse previous hierarchical rank in [25], [26], [27], [28]
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