Light sensitivity and diel activity rhythms in the Angoni vlei rat (Otomys angoniensis) under natural and artificial conditions

Abstract

Artificial light at night (ALAN) can disrupt daily rhythms of wildlife, yet little is known about its effects on African rodents. We investigated the diel activity rhythms of the Angoni vlei rat (Otomys angoniensis), a species with an inconclusive temporal niche. We exposed wild-caught vlei rats to three treatments: (1) standard laboratory light–dark cycle; (2) laboratory light–dark cycle with low-intensity ALAN (2 Lux); and (3) natural ambient light and temperature fluctuations. Activity was recorded via infrared sensors, and the activity rhythm was quantified using Cosinor analysis. Activity was predominantly nocturnal across all treatments. However, rhythm amplitude, MESOR and robustness were nearly three-times higher under natural conditions than in both laboratory treatments. ALAN did not significantly suppress activity, suggesting minimal masking effects (unlike strictly nocturnal animals) and possible flexibility in the temporal niche. In nature, O. angoniensis occupies dense grass habitats, and therefore, its predominantly nocturnal activity in captivity could be due to the lack of adequate cover during experiments. Moreover, the greater activity under natural conditions could be a consequence of brighter daytime light and natural temperature cycles. We suggest that although O. angoniensis displays nocturnal activity in the laboratory, it may be more diurnal in its natural habitat. Our findings demonstrate that the temporal activity of O. angoniensis is shaped by environmental context, with implications for contextualising circadian flexibility in species inhabiting human-altered landscapes.