Aggressive habituation and rim circling in the social organization of bluegill sunfish (Lepomis macrochirus)

Synopsis

The rim circling (RC), and aggressive habituation of nesting bluegills were studied in natural colonies during the six days of the reproductive period following spawning. To examine the stimulus specificity of aggressive habituation, selected test individuals and four neighbors were observed on four occasions during one day. After the first observation period established baseline data on aggressive interactions, the ‘appearances’ of three of the neighbors were manipulated and further observations were made. Two more observation periods followed, separated by a sham manipulation to allow for possible disturbance effects. Fish lengths, inter-nest distances, and positions in the colony were also recorded.

An analysis of the data demonstrated that aggressive interaction increased for the three manipulated neighbors. There were no significant differences among the data for the fourth (control) neighbors. The enhanced aggressive interactions between the test and manipulated males were initiated by the males at least partially in response to the changed appearance of the neighbor. Further, the level of aggressive interaction returned to normal after about 2 hours, indicating that the test male had habituated to the new appearances of his neighbors. Thus visual appearance plays an important role in the habituation of aggression between neighboring bluegills. It was not possible to determine if the behavior of the neighbor was also an important cue used in aggressive habituation, but this seems likely. Advantages of aggressive habituation are discussed. It is noteworthy that aggression decreased as internest distance increased.

Levels of RC varied significantly over the six days, in a time course dissimilar from aggression. The RC rate is dependent on size of male, nest locality, and circling activity of neighbors. Observations on the synchrony of rim circling indicated social facilitation. It is argued that RC probably fulfills an advertisement function important in defending the brood. The motivation for RC is likely complex, and its performance reflects a mixed aggressive and reproductive strategy in the nesting population. Aggressive habituation functions in the social organization of bluegills to facilitate the occurrence of group displays such as RC.