Body Size and the Maintenance of Reproductive Isolation in Stickleback, GenusGasterosteus

Abstract

AbstractThe blackspotted sticklebackGasterosteus wheatlandiand the widely studied threespine sticklebackG. aculeatusare sympatric throughout the former’s range and share many aspects of life history and reproductive behaviour. These two species differ significantly in size, withG. wheatlandiof both sexes measured at approximately 60% of the standard length of theirG. aculeatuscounterparts. This study concentrated onG. wheatlandicourtship behaviour and investigated its role in the maintenance of reproductive isolation withG. aculeatus. Specifically, the roles that (1) female body size plays in influencing male courtship preferences and (2) male body size and behaviour play in female courtship preferences were investigated through dummy and live conspecific and heterospecific stimulus presentations. MaleG. wheatlandicourtship preferences are consistent with previously described patterns forG. aculeatus. Males of both species preferentially approach and court the larger of two simultaneously presented live or dummy females. Thus, the smallerG. wheatlandimales are indiscriminate with respect to assortative mate choice; not only preferring to approach and court more fecund conspecific females but, more significantly,G. aculeatus‐sized females. In contrast, females of both species demonstrate strong assortative courtship preferences. When presented with pairs of flask‐enclosed males, females of both species preferentially orient and court the conspecific male over the heterospecific. Similarly, when presented with a conspecific male and a heterospecific male presented singly, females prefer to enter the nest of the conspecific. Systematic analysis of the interactions between these pairs of fish (one male, one female) demonstrates that the breakdown of courtship in heterospecific courtship occurs late in the courtship sequence when the widely differing forms of male leading behaviour results in drastically differing female responses. I suggest that, as previously described inG. aculeatus, the supernormality effect plays a significant role in mediating adaptive mate choice behaviour inG. wheatlandi. However, the added element of a larger sympatric species introduces a possible cost in time and energy devoted to courting heterospecific, and sympatric, females that the largerG. aculeatusdo not likely incur. There is substantial evidence from many sympatricG. aculeatusspecies pairs that there is assortative mate choice based on size and/or courtship behaviour. Courtship trials suggest a more pervasive role for females in assortative mate choice. Whether it is male body sizeper se, or in combination with behaviour, morphology or other cues, is unresolved in the present study.