Light sensitivity of the circadian system in the social Highveld mole-rat <i>Cryptomys hottentotus pretoriae</i>

Highveld mole-rats (Cryptomys hottentotus pretoriae) are social rodents that inhabit networks of subterranean tunnels. In their natural environment, they are rarely exposed to light, and consequently their visual systems have regressed over evolutionary time. However, in the laboratory they display nocturnal activity, suggesting that they are sensitive to changes in ambient illumination. We examined the robustness of the Highveld mole-rat circadian system by assessing its locomotor activity under decreasing light intensities. Mole-rats were subjected to seven consecutive light cycles commencing with a control cycle (overhead fluorescent lighting at 150 lx), followed by decreasing LED lighting (500, 300, 100, 10 and 1 lx) on a 12 h light:12 h dark (L:D) photoperiod and finally a constant darkness (DD) cycle. Mole-rats displayed nocturnal activity under the whole range of experimental lighting conditions, with a distinct spike in activity at the end of the dark phase in all cycles. The mole-rats were least active during the control cycle under fluorescent light, locomotor activity increased steadily with decreasing LED light intensities, and the highest activity was exhibited when the light was completely removed. In constant darkness, mole-rats displayed free-running rhythms with periods (τ) ranging from 23.77 to 24.38 h, but was overall very close to 24 h at 24.07 h. Our findings confirm that the Highveld mole-rat has a higher threshold for light compared with aboveground dwelling rodents, which is congruent with previous neurological findings, and has implications for behavioural rhythms.