Circadian rhythms of locomotor activity in solitary and social species of African mole-rats (Family : Bathyergidae)

Mole-rats are strictly subterranean and hardly, if ever, come into contact with external light. As a result, their classical visual system is severely regressed and the circadian system proportionally expanded. The family Bathyergidae presents a unique opportunity to study the circadian system in the absence of the classical visual system in a range of species. Daily patterns of activity were studied in the laboratory under constant temperature but variable lighting regimes in individually housed animals from 3 species of mole-rat exhibiting markedly different degrees of sociality. All 3 species possessed individuals that exhibited endogenous circadian rhythms under constant darkness that entrained to a light-dark cycle. In the solitary species, Georychus capensis, 9 animals exhibited greater activity during the dark phase of the light cycle, while 2 individuals expressed more activity in the light phase of the light cycle. In the social, Cryptomys hottentotus pretoriae, 5 animals displayed the majority of their activity during the dark phase of the light cycle and the remaining 2 exhibited more activity during the light phase of the light cycle. Finally in the eusocial Cryptomys damarensis, 6 animals displayed more activity during the light phase of the light cycle, and the other 2 animals displayed more activity during the dark phase of the light cycle. Since all three mole-rat species are able to entrain their locomotor activity to an external light source, light must reach the SCN, suggesting a functional circadian clock. In comparison to the solitary species, the 2 social species display a markedly poorer response to light in all aspects. Thus, in parallel with the sociality continuum, there exists a continuum of sensitivity of the circadian clock to light.