Laboratory and field data relevant to the survival value of the suprachiasmatic nuclei (SCN) pacemaker are presented for 4 related North American squirrel species that evolved in habitats with markedly different environmental conditions. Laboratory studies used free-running activity rhythms under constant conditions as the signature of an endogenous pacemaker. Field studies documented the circadian ecology of the 4 species; survival of intact controls was compared, when possible, with an SCN-lesioned group of free-living animals or with wild-caught animals in a laboratory facility that simulated natural conditions. All 4 species were rhythmic in the laboratory as indicated by precise free-running wheel-turning rhythms under constant conditions, as well as by photo-entrainment of free-runs to a 24-hr light–dark schedule. Southern flying squirrels (Glaucomys volans) presumably minimized predation by sleeping in sheltering tree dens during daylight, with awakening by their SCN timer for nocturnal gliding. Eastern chipmunks (Tamias striatus) were strictly diurnal but avoided predation in elaborate dens as well as by facultative hibernation and winter food caches. Antelope squirrels (Ammospermophilus leucurus) were also diurnal with dens for minimizing predation at night. Golden-mantled ground squirrels (Spermophilus lateralis) were diurnal, fossorial species with protective dens; during the extended hibernation in winter, the SCN played a role in limiting the time spent in arousal episodes. Thus, in the evolution of the physiology and behavior of these species, the SCN has become involved in diverse functions ranging from regulation of activity to reduction of energy expenditure during arousal episodes of hibernating ground squirrels.