Circadian rhythms of locomotor activity in captive Emin’s mole-rats,Heliophobius emini(Rodentia: Bathyergidae)

Abstract

Many African mole-rats (Bathyergidae) exhibit distinct circadian rhythms, despite the fact that they are strictly subterranean and are very infrequently exposed to light. We investigated the circadian system of Emin’s mole-rats ( Heliophobius emini ) by exposing 13 individuals to different lighting regimes. The mole-rats were subjected to a standard 12-h light:12-h dark light cycle, followed by a constant darkness (DD) cycle to evaluate entrainment and endogenous rhythms of locomotor activity. After allowing the mole-rats to re-entrain to a 12L:12D light cycle, the light cycle was inverted to examine the duration of re-entrainment following a drastic change in the light cycle. Furthermore, we investigated the effect of different ambient temperatures on the locomotor activity of the mole-rats, and they were thus subjected to a 12D:12L light cycle at 20°C, 25°C, and 30°C. Lastly, mole-rats were presented with a long-day (16L:8D) and a short-day (8L: 16D) light cycle to look at the effect on preferred active time. The animals showed a preference for diurnal activity with 9 mole-rats entraining to the light cycle, and maintaining clear endogenous rhythms throughout DD. Subjects were excessively slow to re-entrain their activity to the inverted light cycle. When subjected to different ambient temperatures, individuals showed marked increases in average activity at 20°C, with the lowest average activity occurring at 30°C. The mole-rats maintained their diurnal preference regardless of day length, condensing their activity during the short-day light cycle, while extending their active period during the long-day cycle. These results suggest that Emin’s mole-rats are able to perceive light and entrain their locomotor activity to light cycles, albeit weakly. In addition, ambient temperature influenced the level of overall activity, indicating that these animals may use behavioral thermoregulation to maintain a constant body temperature.