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Freshwater threespine stickleback fall along a benthic (bottom feeding)–limnetic (plankton feeding) continuum of ecotypes. Although trophic and courtship behaviour are well-described, the full reproductive cycle has not been characterized in the ecotypes. We provide detailed in situ descriptions of the reproductive cycle of males in one population of each ecotype, and a less detailed description in one oceanic population likely to represent the ancestral form that gave rise to the post-glacial freshwater radiation in southern British Columbia. All populations exhibited typical patterns including: pre-spawning nest building, fanning (which peaked in intensity just before hatching of embryos), and gluing (which was most common from nest building through the early parental phase). These elements of behaviour may be fixed by the physiological requirements of the fry. Males in the benthic and oceanic populations interacted frequently with groups and solitary non-neighbours, reflecting the abundance of bottom-feeding adults. Interactions with groups did not occur in limnetic populations and those with solitary non-neighbours were rare. Interactions were common with neighbours in all populations, but in the limnetic population they were conspicuous and ritualized, rather than the simple chases observed in the other populations. Males in the limnetic population also attended to the nest more, in some cases producing a 3-cm mound of sediment following the completion of spawning and often placing rushes around the pit-nest. The general pattern was one in which males in the cannibalistic benthic and oceanic populations appeared to be constrained in their interactions with neighbours and their attention to the nest by the need for vigilance and nest defense, while limnetic males attended more to nests and neighbours, possibly exhibiting the optimal pattern of nest-directed activities for the fry. [ABSTRACT FROM AUTHOR]

At the onset of adaptive differentiation caused by colonization of new environments, patterns of ancestral plasticity can influence trait expression, and thus determine which phenotypes are exposed to selection. This hypothesis can be evaluated in the postglacial adaptive radiation of threespine stickleback, Gasterosteus aculeatus, in northwestern North America. The very large oceanic populations that gave rise to the freshwater radiation are unlikely to have changed significantly since colonization was initiated, so the pattern of plasticity in oceanic fish can be used to infer the ancestral pattern. We demonstrate that courtship behaviour in oceanic sticklebacks is plastic and suggest that its expression is mediated by large cannibalistic foraging groups that attack and consume young in nests defended by males. We then demonstrate that, under laboratory conditions, males from two benthic (bottom-feeding) populations, where group cannibalism is common, zigzag less than males from a limnetic (plankton-feeding) population in which such groups do not form. The difference in courtship behaviour of these ecotypes parallels that seen in the field and the pattern of plasticity observed in oceanic fish. Ecotypic differences were also retained in laboratory-reared fish from one limnetic and one benthic population. Biting was a more common component of courtship in the laboratory than in the field, and males from freshwater populations bit more than those from the oceanic population, a difference that disappeared in laboratory-reared benthic and limnetic males. Our results suggest that ancestral plasticity may have influenced the subsequent evolution of some elements of courtship behaviour in the stickleback radiation. [Copyright &y& Elsevier]