Information about an opponent can decrease aggression in male common gobies competing for nest sites

In the common goby, Pomatoschistus microps, males compete aggressively for nest sites when these are in short supply. I investigated whether prior information about the opponent influenced aggression levels during nest competition. Males that, through a one-way mirror, had been observing two other males compete for a nest were placed together with either the winner or the loser. The new male pairs also received nest sites to compete for, and their behaviour was videorecorded. As a control, naïve males, which had not been watching the earlier contest, were also recorded when competing with previous winners or losers. Both bystanders and naïve males acquired the nest more often when competing with a loser than with a winner. The male that got the nest was generally more aggressive than his opponent. However, in trials with bystanders and losers, the male obtaining the nest was less aggressive than in contests between bystanders and winners and in contests between naïve males and losers. Furthermore, when bystanders competed with losers, they were less aggressive than were naïve males competing with losers. There was no difference in aggression between bystanders and naïve males competing with winners. Information about the opponent did not increase the probability of obtaining the nest, but the benefit, when applicable, might instead have consisted of a decrease in aggression, and thus a lowered energetic cost of nest competition.