Influences of Prior Agonistic Experiences On Aggression Measures in the Male Swordtail (Xiphophorus Helleri)

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Abstract
The influences of prior agonistic experience on the aggressive behaviour of swordtail males were studied in standardized aggression tests. In the "standard-opponent" test the test subject fights against a much smaller opponent behind a transparent partition, whereas in the mirror test it attacks its own mirror image. Both tests are appropriate to measure mean biting rates in an adequate sample of individuals. If reproducible estimates of the aggressiveness of single individuals are needed, the mirror test is the most appropriate method, for it gives correlation coefficients up to 0.96 between a first and a second measurement. When fish had been involved in a series of escalating activities without final decision of the encounters, in subsequent mirror tests their biting frequencies were elevated for at least 24 h after the last fight. Winning a rank order fight led to an increase and losing to a decrease in biting frequency 24 h later. By contrast, after social settlement in a long-term stabilized social hierarchy the relation is totally reversed: Dominant males showed much lower biting frequencies than subordinate males in the mirror test. The hypothesis is proposed that even under natural conditions high ranking males tend to use their energy potential in a more economic way than low ranking males. The existence of rank-dependent fighting strategies could be due to differences in risk: The fighting activities of dominant males are extremely risky because they can lose their high social status, which inevitably would lead to a lowered genetic fitness, whereas the aggressive actions of subordinate males are not very risky because their social status is low anyway. The influences of prior agonistic experience on the aggressive behaviour of swordtail males were studied in standardized aggression tests. In the "standard-opponent" test the test subject fights against a much smaller opponent behind a transparent partition, whereas in the mirror test it attacks its own mirror image. Both tests are appropriate to measure mean biting rates in an adequate sample of individuals. If reproducible estimates of the aggressiveness of single individuals are needed, the mirror test is the most appropriate method, for it gives correlation coefficients up to 0.96 between a first and a second measurement. When fish had been involved in a series of escalating activities without final decision of the encounters, in subsequent mirror tests their biting frequencies were elevated for at least 24 h after the last fight. Winning a rank order fight led to an increase and losing to a decrease in biting frequency 24 h later. By contrast, after social settlement in a long-term stabilized social hierarchy the relation is totally reversed: Dominant males showed much lower biting frequencies than subordinate males in the mirror test. The hypothesis is proposed that even under natural conditions high ranking males tend to use their energy potential in a more economic way than low ranking males. The existence of rank-dependent fighting strategies could be due to differences in risk: The fighting activities of dominant males are extremely risky because they can lose their high social status, which inevitably would lead to a lowered genetic fitness, whereas the aggressive actions of subordinate males are not very risky because their social status is low anyway.