Population “cycles” and other fluctuations of arvicoline rodents (Cricetidae) challenge concepts and hypotheses of “population regulation.”  Relevant studies have been undertaken since the emergence of ecology as a discipline, but largely with respect to the “mechanics” of cycles.  Longitudinal study of montane grassland species [24 y, montane voles (Microtus montanus, 18 sites) and longtail voles (M. longicaudus, 3 sites)] and of Laurentian forest species [34 y, 19-22 sites, Boreal redback voles (Myodes gapperi), rock voles (Microtus chrotorrhinus,), southern bog lemmings (Synaptomys cooperi)] suggest that large populations are adaptive, not epiphenomenal.  They allow 1) the annual expansion of populations in spring into contiguous seasonally flooded land, 2) the dispersion of individuals into segments of the metapopulation (sensu stricto), 3) the expansion of the species into appropriate peripheral and isolated habitat patches (e.g., isolated boulder fields), and 4) the expansion of the species into novel habitats (e.g., from bog and fen to the upland ericaceous shrub community).  This paradigm is discussed in the context of reproductive plasticity and should lead to more investigations of reproductive physiology and genetics.