Abstract:
Biological rhythms are rhythmic fluctuations of biological functions that occur in almost
all organisms and on several time scales. These rhythms are generated endogenously
and entail the coordination of physiological and behavioural processes to predictable,
external environmental rhythms. The light-dark cycle is usually the most prominent
environmental cue to which animals synchronise their rhythms. Biological rhythms are
believed to provide an adaptive advantage to organisms. In the present review, we
will examine the occurrence of circadian and seasonal rhythms in African mole-rats
(family Bathyergidae). African mole-rats are strictly subterranean, they very rarely emerge
aboveground and therefore, do not have regular access to environmental light. A key
adaptation to their specialised habitat is a reduction in the visual system. Mole-rats
exhibit both daily and seasonal rhythmicity in a range of behaviours and physiological
variables, albeit to different degrees and with large variability. We review previous
research on the entire circadian system of African mole-rats and discuss output rhythms
in detail. Laboratory experiments imply that light remains the strongest zeitgeber for
entrainment but in the absence of light, animals can entrain to ambient temperature
rhythms. Field studies report that rhythmic daily and seasonal behaviour is displayed in
their natural habitat. We suggest that ambient temperature and rainfall play an important
role in the timing of rhythmic behaviour in mole-rats, and that they likely respond directly
to these zeitgebers in the field rather than exhibit robust endogenous rhythms. In the
light of climate change, these subterranean animals are buffered from the direct and
immediate effects of changes in temperature and rainfall, partly because they do not
have robust circadian rhythms, however, on a longer term they are vulnerable to changes
in their food sources and dispersal abilities.
