COURTSHIP BEHAVIOUR OF MALE THREESPINE STICKLEBACKS (GASTEROSTEUS ACULEATUS) FROM OLD AND NEW HYBRID ZONES

Abstract We compared courtship behaviour of male threespine sticklebacks (Gasterosteus aculeatus) from an 'old' hybrid zone (OldHZ) and 'new' hybrid zones (NewHZ) in southwest British Columbia. High frequencies of phenotypically-intermediate forms occurred in each HZ, between the low-plated freshwater (FW) and high-plated marine (MAR) forms. The OldHZ was formed early in the present post-glacial period and probably has existed for thousands of years. The 'new' HZ (NewHZ) is a system of drainage ditches built in the late nineteenth century. In the laboratory, we quantified and compared courtship behaviour (zigzags, bites, creeping through, fanning) of males from each HZ. We compared these results with those from a previous study that quantified courtship of FW and MAR males. In general, courtship of male from the NewHZ was intermediate between the FW and MAR forms, but zigzag courtship of males from the OldHZ was the significantly less vigorous. In general, other courtship behaviour (biting, fanning, gluing, crawling through and the first response) of hybrid zones males was intermediate between FW and MAR males. Within each HZ, courtship differences were not related to phenotype (lateral plates) or size of males or females. The reduced zigzag courtship of OldHZ males is consistent with the hypothesis that change in courtship behaviour of hybrid phenotypes is evidence of the development of premating isolating mechanisms between the FW and MAR forms. There is no evidence, however, of any form of hybrid inviability although we did suspect that we had less success getting OldHZ males to build nests in our laboratory tanks. Without firm evidence of some form of hybrid inviability, the conclusion that male courtship could serve as an isolating mechanism remains speculative.