Despite competing for resources such as space, food and mates, many territorial animals are less aggressive towards neighbours who rarely go beyond their territorial boundaries. This so-called dear enemy phenomenon is advantageous in territorial defence, but it has not been well studied in fish. In this work, we tested the ‘correct–incorrect boundary paradigm’ of the dear enemy phenomenon using the territorial cichlid fish Neolamprologus pulcher, which exhibits dear enemy relationships. When the fish was placed in a small experimental tank, in which fish established its territory, it was initially very aggressive against a neighbouring fish in an adjacent tank, but the aggression level decreased rapidly (within 4 days). When the tank containing the neighbour was shifted to the opposite side, the focal fish was more aggressive than the day before, but it exhibited less aggression than it did against a stranger placed on the shifted side. This lower level of aggression suggested that the focal fish did not regard the shifted neighbour as a stranger. Our observations provide support for the threat-level hypothesis, according to which territory owners will modulate aggression intensity based on the threat level.
When are neighbours ‘dear enemies’ and when are they not? The responses of territorial male variegated pupfish, Cyprinodon variegatus, to neighbours, strangers and heterospecifics. —