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 poorly studied. 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 into parts of the day when thermoregulation costs are supposed to be lowest. During the coldest period, mole-rat activity correlated most strongly with the temperature at depths of 5–10 cm (positively), corresponding to the superficially situated burrow branches leading to mounds or food resources located in the shallow soil layers. In contrast, during two hottest periods, activity was more closely related to temperatures at a depth of 25 cm (negatively). 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 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 they remain below their thermal neutral zone (TNZ).