Ultradian rhythms of activity in a wild subterranean rodent

Many animals adapt their activity patterns to the best environmental conditions using diel rhythms. African mole-rats (family Bathyergidae) are among the mammals that have become models for studying how these rhythms can be entrained by different cues (light or temperature) in experimental laboratory studies. However, it is unclear whether they exhibit similar circadian rhythms in their natural lightless, subterranean environment. In this study, we used biologging to investigate the activity rhythms of wild, highveld mole-rats. We show that their activity cycle exhibited an ultradian rhythm with a length between 4 - 8 hours. On an individual level, mole-rats displayed about 5 activity bouts per day, occurring at various times during the day and night. On a population level, activity peaked in the afternoon, coinciding with the peak in ambient temperature. Our research suggests that wild subterranean mammals, which experience constant darkness and reduced environmental variation, are unlikely to show clear circadian rhythmicity. Instead, activity periods are distributed over several bouts throughout the day and night and is likely synchronized to daily temperature cycles. We propose that ultradian rhythms may be more common than previously thought and discuss how physiological processes may generate differences in periodicity between laboratory and wild populations.

Methods
Data was collected using accelerometer devices attached to highveld mole-rats (Cryptomys pretoriae). Accelerometer devices were set to record at 25hz and were deployed for up to 14 days (mean duration 12.95 +/- 2.2 days). Data was downloaded from the accelerometer devices using the program X Manager provided by Technosmart  (https://www.technosmart.eu/). As a result of the high frequency, accelerometer data files for each individual were over 1GB in size, and even reducing the data to mean values every 2 seconds results in a 1.7GB file with over 11.5 million rows. To allow public sharing of data we have shared the mean overall dynamic body acceleration (ODBA) values in 10 minute windows. Temperature data (soiltemp.csv) was recorded at the study site every 10 minutes with ibuttons buried in the soil and in full sun at 2m high.