Extremely low threshold for photic entrainment of circadian activity rhythms in molossid bats (Molossus molossus; Chiroptera – Molossidae)

Circadian rhythms usually deviate from 24 h and must be synchronized (entrained) to the outer 24 h day by certain environmental periodicities called Zeitgebers. For almost all organisms the most efficient Zeitgeber is the light-dark cycle (LD). In mammals the photic Zeitgeber cues are exclusively received via retinal photoreceptors. It is still in debate whether this circadian photoreception is mediated by rods, cones, and/or other retinal cells. From recent results in mouse mutants a circadian photoreception via non-rod/non-cone retinal receptors was deduced. However, earlier observations in bats indicating a very low threshold for photic entrainment imply that circadian photoreception may be mediated by rod-like receptors. In the present study the threshold for photic entrainment was determined in the neotropical mastiff bat Molossus molossus. Six test animals (3 m, 3 f) were kept isolated in recording cages situated in light-tight and sound-attenuating wooden boxes with a special lighting device on top. Under constant ambient temperature of 25 ± 1°C, relative humidity of 60 ± 5%, and an irregular ad libitum feeding schedule, the bats were exposed intermittantly for longer times to constant physiological darkness (LD-X) or 12:12 h light dark cycles with physiological darkness during the dark time (D) and varying low light-time illuminances (L). Half of the bats had an extremely low threshold for photic entrainment, about 10^?5 lux, while the other individuals’ free-running activity rhythm was entrained by LD cycles with 10^?4, 10^?2 and 10^?1 lux in L. The illuminance of only 10^?5 lux is the lowest threshold value for photic entrainment found thus far in vertebrates. Plausibility considerations suggest circadian photoreception via rod-like retinal receptors to be most probably involved in this case.