Testing the Dear Enemy Hypothesis in a group-living cichlid fish : Dear Enemy relationships in Neolamprologus multifasciatus in Lake Tanganyika

Sammanfattning: The dear enemy effect is a widespread behavioral phenomenon that promotes selective pacifism in order to minimize costs of territorial defense. The dear enemy hypothesis predicts heightened aggression towards unfamiliar conspecifics compared to familiar neighbors where territorial boundaries are established. The purpose of this study was to test the dear enemy hypothesis in Neolamprologus multifasciatus, a social cichlid fish endemic to the shell beds of Lake Tanganyika. They establish breeding groups that control distinct territories, each governed by a dominant male. Territories can be very densely spaced, and conspecific border intrusions generally trigger aggressive responses from the focal group. The dominant males are responsible for the majority of territory defense. In this field experiment, 27 breeding groups were each given two treatments, each consisting of artificial introduction of an out-of-group dominant male. One male originated from an immediately neighboring group, the other from a distant group.  A downward-facing camera recorded group members’ aggressivity exhibited towards the experimentally presented male. The behavior was manually scored according to a behavior ethogram and analyzed with either linear mixed effects models or generalized linear mixed effects models. No clear dear enemy effect was detected within Neolamprologus multifasciatus. Several reasons are proposed for this result. Firstly, a contingent “ceiling effect” in terms of aggressive behavior imaginably deterred efforts of dear enemy detection. Conversely, perhaps negligible necessity of stranger-neighbor recognition within Neolamprologus multifasciatus has resulted in discriminatory behavior being absent. In conclusion, no dear enemy effect was detected and future research will address possible alternative explanations.