Aggression can be modulated by both genetic and environmental factors. Here, we analyse how the serotonin transporter (5-HTT) genotype and the environmental situation in which a contest takes place shape the display of offensive aggression. Therefore, male wildtype, heterozygous, and homozygous 5-HTT knockout mice, which are known to differ in inborn levels of anxiety, were confronted three times with a docile opponent in one of three environmental situations: own territory, opponent's territory or neutral area. The main findings were: The frequency of approaching the contestant in order to gather information about him depended significantly on the venue but not on the genotype with lowest frequencies in the opponent's territory. The decision how quickly to attack the opponent was significantly influenced by the 5-HTT genotype but not by the venue: Homozygous 5-HTT knockout mice showed longest latencies. The sum of offensive aggression was significantly influenced by the 5-HTT genotype, the environmental situation, and a genotype by environment interaction. It is likely that, due to their varying genetic predisposition for anxiety, mice of the three genotypes were differentially affected by the aversiveness of the respective venue and the opponent's behaviour, which influenced their decision to display offensive aggression. As a consequence, the amount of aggression shown by homozygous 5-HTT knockout mice was influenced by the venue and the opponent's behaviour, whereas heterozygotes reacted only to the venue. Strikingly, wildtypes behaved always the same way, irrespective of venue and opponent.
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