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 12h light:12h dark light cycle, followed by 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 20oC, 25oC and 30oC. 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 nine 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 20oC, with the lowest average activity occurring at 30oC. The mole-rats maintain 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 influences the level of overall activity, indicating that these animals may use behavioral thermoregulation to maintain a constant body temperature.