The rabbit: a diurnal or a nocturnal animal?

Although the rabbit is an almost 'classical' laboratory animal, chronobiological research in this species is in its infancy. It appears not even clear, whether the rabbit is a predominantly diurnal, crepuscularly active or nocturnal animal. In an ordinary, non sound-isolated animal room rabbits exhibited elevated locomotor activity, hard faeces excretion, food intake and water intake during elevated external animal-house noise. When the hours of external noise coincided with the 12 h light period 8/15 rabbits displayed a predominantly diurnal pattern, in 3/15 rabbits no preference existed for light or dark time and 4/15 animals had a typically nocturnal pattern. In contrast, in a properly sound-isolated laboratory locomotor activity, hard faeces excretion, food intake, water intake and urine excretion were significantly higher during the dark period of a LD 12:12. In constant light conditions of 7 lux either function free-ran with a circadian period-length greater than 24 h, which, too, is typically in nocturnally active animals. When food access was restricted to 4/12 h of the light period, most events of the five behavioral functions were concentrated around the time of restricted food access (RF), the rabbits now being almost exclusively light active. In the absence of any other zeitgeber RF in fact did entrain the free-running circadian rhythms and, thus, is zeitgeber for the circadian oscillator system of the rabbit. Thus, while the rabbit endogenously is a nocturnally active animal, external noise or scheduled feeding during the light period can turn it to a predominantly diurnal animal.