Some Relations between Territory, Social Hierarchy, and Leadership in the Green Sunfish (Lepomis cyanellus)

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Abstract
Groups of 4 immature sun-fish (L. cyanellus), observed for periods of 19-25 days in small, bare aquaria, maintain hierarchies or territories or both. Hierarchies are fairly stable and become progressively more so with time, [male] [male] usually dominate [female] [female], and the larger individuals tend to dominate smaller ones. Among these fishes, hierarchy represents distinct levels of aggressiveness, whereas territoriality arises from a balance between almost equally aggressive individuals. This balance may be struck at 1st encounter; but, when it occurs later within a hierarchy, territories are first established by the 2 highest-ranking individuals, then by the 3d, and, rarely, by the lowest. 17 of 44 groups kept in bare aquaria developed territoriality. In 12 cases, 3 of the 4 fishes held territories, while the 4th was subordinate to all. In 1 group each member held territory; here few aggressive contacts were observed. Subordinates frequently defended areas against all but the most dominant fish. This "partial" territory seems intermediate between hierarchy and territory. Groups tested in a simple maze-learning situation were superior to isolated controls. Leadership through the maze shifted at almost every trial, but the dominant individual tended also to be the "leader." Aquaria divided into compartments by means of wire-screen partitions with square gates favored formation of territories, in direct proportion to the complexity of the subdivision. Subordinates appear to lessen tension among territory-holding fishes; removal of the omega from aquaria with 3 territories markedly increased aggressive activity among those remaining; the introduction of a new fish led to their attacking it instead of one another.
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