Phylogenetic analysis of the evolution of a signal of aggressive intent in northern swordtail fishes.

The initiation of a coevolutionary relationship between signal and response can be explained by either the receiver taking advantage of information inadvertently provided by the sender or the sender taking advantage of a perceptual bias in the receiver. In addition, once both signal and response are present, the exchange of information may or may not be cooperative. We examined the evolution of a signal of aggressive intent (expression of vertical bars) across all the northern swordtail fishes (Xiphophorus) in a phylogenetic context. We found that the signal was present before responses evolved, which suggests that this coevolutionary relationship was initiated by the receiver taking advantage of information inadvertently provided by the signaler. In addition, we introduce a novel method for examining the cooperative nature of signaling systems and provide some evidence to suggest that in this signaling system, receivers may be exploiting an honest signal in some species.

Evolutionarily labile responses to a signal of aggressive intent.

Males of many swordtail species possess vertical bar pigment patterns that are used both in courtship and agonistic interactions. Expression of the bars may function as a conventional threat signal during conflicts with rival males; bars intensify at the onset of aggression and fade in the subordinate male at contest's end. We used mirror image stimulation and bar manipulations to compare the aggressive responses of the males of four swordtail species to their barred and barless images. We found that having a response to the bars is tightly linked to having genes for bars, while the nature of the response the bars evoked varied across species. Specifically, we report the first known instance where closely related species exhibited differing and contradictory responses to a signal of aggressive motivation. Demonstrating that a signal conveys the same information across species (aggressive intent) while the response to that information has changed among species suggests that the nature of the responses are more evolutionarily labile than the signal.