A dear enemy relationship in a territorial cichlid: evidence for the threat-level hypothesis

in Behaviour
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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.

A dear enemy relationship in a territorial cichlid: evidence for the threat-level hypothesis

in Behaviour



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    Experimental tanks and procedures. (a) Experimental tank with the white separator sheet. The plastic shelter tube and aeration stone are shown. (b) Experiment 2: Capital letters indicate the tanks of the focal, neighbour, stranger, and control (empty tank) fish. Arrows show the exchange of the tanks containing the fish. (c) Experiment 3. A stranger fish is placed into exchanged tank E after removal of the neighbour fish.

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    Results of Experiment 1. Average duration (± SE, N=8) of aggressive behaviour by the focal fish against an introduced neighbour and against a stranger (solid bar) during 5 min on days 1 and 5 (white bar). ∗∗∗p<0.001; NS, not significant.

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    Results of Experiment 2 (a) and Experiment 3 (b). Average duration (± SE) of aggressive behaviour by the focal fish against its neighbour (white bar), against the same neighbour shifted to the opposite side (grey bar) (a), and against a stranger on the opposite side (black bar) during 5 min on days 1 and 5. N=8 in (a) and N=9 in (b). ∗∗∗p<0.001; NS, not significant.

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    Change in aggression intensity of the focal fish during 5 min of observation: against a neighbour on day 1 in Experiment 2 (broken line), against a stranger on the opposite side (thin solid line) in Experiment 3, and against the same neighbour shifted to the opposite side (thick solid line) on day 6 in Experiment 2. p<0.05; ∗∗p<0.01; ∗∗∗p<0.001; NS, not significant.


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