Field and Laboratory Studies Provide Insights into the Meaning of Day-Time Activity in a Subterranean Rodent (Ctenomys aff. knighti), the Tuco-Tuco

Abstract

South American subterranean rodents (Ctenomys aff. knighti), commonly known as tuco-tucos, display nocturnal, wheelrunningbehavior under light-dark (LD) conditions, and free-running periods .24 h in constant darkness (DD). However,several reports in the field suggested that a substantial amount of activity occurs during daylight hours, leading us toquestion whether circadian entrainment in the laboratory accurately reflects behavior in natural conditions. We comparedcircadian patterns of locomotor activity in DD of animals previously entrained to full laboratory LD cycles (LD12:12) withthose of animals that were trapped directly from the field. In both cases, activity onsets in DD immediately reflected theprevious dark onset or sundown. Furthermore, freerunning periods upon release into DD were close to 24 h indicatingaftereffects of prior entrainment, similarly in both conditions. No difference was detected in the phase of activity measuredwith and without access to a running wheel. However, when individuals were observed continuously during daylight hoursin a semi-natural enclosure, they emerged above-ground on a daily basis. These day-time activities consisted of foragingand burrow maintenance, suggesting that the designation of this species as nocturnal might be inaccurate in the field. Ourstudy of a solitary subterranean species suggests that the circadian clock is entrained similarly under field and laboratoryconditions and that day-time activity expressed only in the field is required for foraging and may not be time-dictated bythe circadian pacemaker.