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.

The flexible clock: predictive and reactive homeostasis, energy balance and the circadian regulation of sleep–wake timing

“…Conversely, the mean 24 h T burrow temperature in a dark-soiled rocky location, or near geothermally active sites, might exceed the average 24 h air temperature (e.g. see Tomotani et al, 2012). However, a burrowing animal would also spend less energy on thermoregulation under these conditions when diurnal, as long as T burrow and T surface fall below the animals' thermoneutral zone (TNZ; see below).…”

“…First, we would like to stress that, in their natural habitat, many small rodent species typically considered as nocturnal can indeed show extensive periods in which a substantial or even the dominant fraction of their daily activity occurs during the light phase. This includes rats, mice, hamsters and even several subterranean species Gattermann et al, 2008;Harper and Bunbury, 2015;Levy et al, 2007;Tomotani et al, 2012;Urrejola et al, 2005;reviewed in Hut et al, 2012). We propose that the danger of predation might be an important factor in favoring nocturnality in rodents and other herbivorous prey species whenever they can afford it.…”

“…Some rodent species that are diurnal in the field become nocturnal under certain laboratory conditions, including degus (Ebensperger et al, 2004;Kas and Edgar, 1999), hamsters (Gattermann et al, 2008), tuco-tucos (Tomotani et al, 2012), cururos (Urrejola et al, 2005), Nile grass rats (Blanchong et al, 1999) and golden spiny mice (Cohen et al, 2010). These discrepancies in circadian rhythms between the laboratory and field raise the question of whether a 'default' circadian phenotype actually exists.…”

Sincronização fótica e não fótica dos ritmos circadianos em roedores subterrâneos (Ctenomys aff. knighti) e roedores modelo de laboratório (Mus musculus)

“…During my master's dissertation, our group has verified the temporal light exposure pattern of tuco-tucos in the field, by means of visual observations of behavior in seminatural field enclosures. We saw that animals leave their burrows and expose to light during the day, but at varying times day after day (Tomotani et al, 2012). We could not predict whether this irregular light exposure pattern would synchronize the circadian rhythms of tuco-tucos.…”

“…We had previously obtained data for the temporal light exposure pattern of tuco-tucos, by means of visual observations (Tomotani et al, 2012). In those earlier stages, we managed to register episodes of light exposure by observing three animals in one seminatural field enclosure, during the light phase, and recording the times when they presented aboveground activity.…”