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, wheel-running behavior 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 to question whether circadian entrainment in the laboratory accurately reflects behavior in natural conditions. We compared circadian patterns of locomotor activity in DD of animals previously entrained to full laboratory LD cycles (LD12:12) with those of animals that were trapped directly from the field. In both cases, activity onsets in DD immediately reflected the previous dark onset or sundown. Furthermore, freerunning periods upon release into DD were close to 24 h indicating aftereffects of prior entrainment, similarly in both conditions. No difference was detected in the phase of activity measured with and without access to a running wheel. However, when individuals were observed continuously during daylight hours in a semi-natural enclosure, they emerged above-ground on a daily basis. These day-time activities consisted of foraging and burrow maintenance, suggesting that the designation of this species as nocturnal might be inaccurate in the field. Our study of a solitary subterranean species suggests that the circadian clock is entrained similarly under field and laboratory conditions and that day-time activity expressed only in the field is required for foraging and may not be time-dictated by the circadian pacemaker.

Figure 1. Comparison of activity phase in constant lab conditions of animals entrained previously to lab and field conditions.

Double-plotted actograms show infrared motion detected general activity of five individuals (left panel), and wheel-running activity of another five individuals (middle panel) that were trapped directly from the field. Laboratory animals (right panel) show wheel-running activity entrained to LD 12∶12 and then released into constant conditions. In all cases, activity was concentrated in the phase corresponding to night in the field or dark phase in the lab (from 19:00 to 07:00). Vertical arrows indicate the time when the animal was released into its lab cage and activity measurement began. The light gray background represents the constant darkness of the lab, while the dark gray and white backgrounds represent the timing of natural light/dark cycles (civil twilights according to the U.S. Naval Oceanography Portal, www.usno.navy.mil).

Figure 2. Phase of activity of the three enclosure entrained animals transferred from the semi-natural enclosure to constant lab conditions.

A) March 2010 animal; B) July 2010 animal; C) March 2011 animal. Each actogram consists of two sections: upper section shows aboveground emergence times (black marks) of one individual animal during enclosure observation; lower section shows subsequent infrared-detected activity under constant laboratory conditions. Vertical arrows indicate the moment when the animal was released into its lab cage and infrared-based motion detection was initiated. In the upper sections of each figure, the dark gray and white backgrounds represent the timing of natural light/dark cycles. In the lower sections, the light gray background represents constant darkness (dim red light). Vertical lines show the astronomical (A) and civil (C) twilights according to the U.S. Naval Oceanography Portal (www.usno.navy.mil). Differences in the interval between the last observed activity in the enclosure and the first detected activity in the laboratory are caused by the differences in the time needed to trap each animal.

Figure 3. Total time spent in each above-ground activity component (soil removal or foraging) during the entire time of observations.