The subterranean ecotope, particularly in tropical regions, is almost free of daily fluctuations in environmental factors that may serve as zeitgebers. The question arises as to whether there is circadian periodicity in the activity of its permanent inhabitants and, if so, how it is induced and maintained. We used radiotelemetry to follow the activity of the silvery mole-rat, Heliophobius argenteocinereus (Bathyergidae), in its natural environment in Malawi during the dry season. Silvery mole-rats were found inside their nests during 63% ± 8% SD of radiofixes; the animals were inactive for 72% ± 8% of the within-nest fixes recorded. Activity tended to decrease over the course of the dry season. Although activity was detected at any time of day, most individuals showed a predictable (mostly unimodal, occasionally bimodal) pattern of activity that was associated with slight daily belowground temperature fluctuations (mostly less than 3°C). In the coldest part of the dry season, overall activity rose with daily temperature. At the beginning of the hot and dry season, the period of enhanced daily activity was shifted to earlier (colder) hours. Mole-rats in the field apparently respond to even slight temperature fluctuations and adjusted their activity accordingly. We propose that burrow temperature can work as zeitgeber of circadian activity in this species.
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