The role of ambient temperature and light as cues in the control of circadian rhythms of Damaraland mole-rat

ABSTRACT

Light is considered the primary entrainer for mammalian biological rhythms, including locomotor activity (LA). However, mammals experience different environmental and light conditions, which include those predominantly devoid of light stimuli, such as those experienced in subterranean environments. In this study, we investigated what environmental cue (light or ambient temperature (T_a)) is the strongest modulator of circadian rhythms, by using LA as a proxy, in mammals that experience a lifestyle devoid of light stimuli. To address this question, this study exposed a subterranean African mole-rat species, the Damaraland mole-rat (Fukomys damarensis), to six light and T_a cycles in different combinations. Contrary to previous literature, when provided with a reliable light cue, Damaraland mole rats exhibited nocturnal, diurnal, or arrhythmic LA patterns under constant T_a. While under constant darkness and a 24-hour T_a cycle mimicking the burrow environment, all mole-rats were most active during the coolest 12-hour period. This finding suggests that in a subterranean environment, which receives no reliable photic cue, the limited heat dissipation and energy constraints during digging activity experienced by Damaraland mole-rats make T_a a reliable and consistent “time-keeping” variable. More so, when providing a reliable light cue (12 light: 12 dark) to Damaraland mole-rats under a 24-hour T_a cycle, this study presents the first evidence that cycles of T_a affect the LA rhythm of a subterranean mammal more strongly than cycles of light and darkness. Once again, Damaraland mole-rats were more active during the coolest 12-hour period regardless of whether this fell during the light or dark phase. However, conclusive differentiation of entrainment to T_a from that of masking was not achieved in this study, and as such, we have recommended future research avenues to do so.

KEYWORDS:

• Entrainment
• masking
• locomotor activity
• subterranean, mole-rat

Acknowledgments

The authors would like to thank Barry van Jaarsveld for helping to conceptualise this project. The authors would like to thank the two anonymous reviewers and Prof Roberto Refinetti for their expert opinions and helpful suggestions.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Author contributions

D.W.H. and N.C.B. conceptualised the project. D.W.H. and K.L.G. collected data and performed animal maintenance. D.W.H., P.J.J and K.L.G. analysed the data. N.C.B. provided funding, equipment, and animals for the project. All authors contributed toward the writing of the manuscript and agreed to be held accountable for the content therein.

Data accessibility

All raw data is available upon request.

Ethical statement

All procedures involving live animals and sample collection described in this manuscript were conducted in accordance with the appropriate national and provincial guidelines, permits and regulations. This study was approved by the animal ethics committee of the University of Pretoria EC038–18.

Additional information

Funding

This study was funded by the Department of Science and Innovation and National Research Foundation (NRF) to the Mammal Behavioural Ecology and Physiology SARChI Chair awarded to NCB [grant no. 64756].