Seasonal variation of the daily activity patterns in a subterranean rodent in response to underground temperature fluctuations

Daily activity patterns of free-living subterranean rodents have often been associated with temperature fluctuations in shallow soil layers, but their seasonal variation has been understudied. We analysed activity data from free-living silvery mole-rats Heliophobius argenteocinereus radio-tracked over six periods, including the coldest and hottest periods of the year, to investigate whether their activity is concentrated during parts of the day when thermoregulation costs are presumed to be lowest. During the coldest period, mole-rat activity correlated most strongly and positively with the temperature at depths of 5–10 cm, corresponding to shallow burrow branches leading to mounds or food resources. In contrast, during the two hottest periods, activity was more closely and negatively related to temperatures at a depth of 25 cm. While the activity pattern detected during the coldest period aligns with the expected greater daily variation in thermoregulatory costs associated with burrowing closer to the soil surface, the patterns from the hottest periods are more difficult to interpret. We hypothesise that during the hottest periods of the year, mole-rats either (i) preferentially construct and use deeper burrow sections, adjusting their daily activity patterns to the small temperature fluctuations found there, or (ii) concentrate their activity to a part of the day when temperatures across a range of soil depths converge, provided these remain below their thermal neutral zone (TNZ).