Inference of evolutionary pattern: diversionary displays of three-spined sticklebacks

In three lakes and one marine lagoon in British Columbia, Canada, groups of three-spined sticklebacks (Gasterosteus aculeatus) cannibalize embryos and fry guarded by conspecific parental males. At the approach of these groups, males perform displays that divert the groups, often causing them to bypass the male's territory. Similar large groups failed to elicit such displays from males in three noncannibalistic populations. The diversionary displays in two lakes are similar, apparently attracting conspecifics through foraging deception, but sticklebacks in a third lake and in a marine lagoon perform a more complex array of display types, some of which appear to incorporate ritualized elements of other behavioral repertoires. Because the freshwater populations are derived from marine populations of Gasterosteus, the complex display repertoire is apparently ancestral relative to the freshwater radiation. This inference is strengthened by comparison with the diversionary display repertoire in an Atlantic marine population (primitive outgroup). Thus, the absence of the display in some populations and the relatively limited display forms in others appear to represent a loss of all or part of the ancestral display. Loss of the display in noncannibalistic populations may be adaptive because it enhances mating success. The significance of partial loss in some cannibalistic populations is less clear. Interpretations of these patterns demonstrate the value of accounting for past historical relationships among populations when inferring historical patterns of behavior evolution.