Circadian Patterns of Locomotor Activity and Body Temperature in Blind Mole-Rats, Spalax ehrenbergi

Abstract

A wide variety of organisms exhibit circadian rhythms, regulated by internal clocks that are entrained primarily by the alternating cycle of light and darkness. There have been few studies of circadian rhythms in fossorial species that inhabit a microenvironment where day-night variations in most environmental parameters are minimized and where exposure to light occurs only infrequently. In this study, daily patterns of locomotor activity and body temperature (Tb) were examined in adult blind mole-rats (Spalax ehrenbergi). These fossorial rodents lack external eyes but possess rudimentary ocular structures that are embedded in the Harderian glands and covered by skin and fur. Most individual mole-rats exhibited circadian rhythms of locomotor activity, but some animals were arrhythmic. Individuals that did exhibit robust rhythms of locomotor activity also showed rhythms of Tb. In most cases, Tb was highest during the phase of intense locomotor activity. Locomotor activity rhythms could be entrained to light:dark cycles, and several mole-rats exhibited entrainment to non-24-h light cycles (T-cycles) with period lengths ranging from T = 23 h to T = 25 h. Some individuals also showed entrainment to daily cycles of ambient temperature. There was considerable interindividual variation in the daily patterns of locomotor activity among mole-rats in virtually all the conditions of environmental lighting and temperature employed in this study. Thus, whereas it appears likely that photic cues have a significant role in the entrainment of circadian rhythms in mole-rats, the amount of variability in rhythm patterns among individuals appears to be much greater than for most species that have been studied.

Phototransduction and clock gene expression in the troglobiont beetlePtomaphagus hirtusof Mammoth cave

“…Thus, clock gene transcript level oscillations need to be studied in relation to diurnal and seasonal light and temperature variations to elucidate the zeitgeber mechanisms in P. hirtus. Examples of behavorial or metabolic rhythmicity in subterranean species include blind mole rats, the cavefish genera Astyanax and Nemacheilus, myriapods and crustaceans (Cooper et al, 1993;De La O-Martinez et al, 2004;Erckens and Martin, 1982;Goldman et al, 1997;Jegla and Poulson, 1968;Koilraj et al, 2000;Kumar Pati, 2001) (for a review, see Lamprecht and Weber, 1992). Curiously, while our data represent the first demonstration of the expression of the complete circadian gene network in a troglobiont, no evidence of oscillating behavior or metabolic activities has so far been reported for P. hirtus.…”

Spectral and duration sensitivity to light-at-night in ‘blind’ and sighted rodent species

“…For M. socialis, photoperiodic changes are an important environmental cue for seasonal acclimatization (Banin et al, 1994). Similarly, various photoperiodic responses in physiology (Haim et al, 1983), gene expression (Avivi et al, 2004) and behavior (Goldman et al, 1997) were reported for S. ehrenbergi. Photoperiodadjusted daily rhythms in urine production rates, 6-sulfatoxymelatonin [6-SMT; the major metabolite of MLT in urine (Arendt, 2006)], urinary metabolites of adrenalin and cortisol, and oxygen consumption are well established for M. socialis and S. Light-at-night sensitivity in rodents ehrenbergi as well (Zubidat et al, 2010a;Zubidat et al, 2010b).…”

Photosensitivity to different light intensities in blind and sighted rodents

“…Social voles are semifossorial rodents spread throughout the Mediterranean grasslands and cultivated fields in Israel (Harrison and Bates, 1991). Both M. socialis and S. ehrenbergi use ambient photic information for entrainment of several biological rhythms (Haim et al, 1983;Rado et al, 1992;Goldman et al, 1997;Haim et al, 2005;. Whilst the retina of social voles has normal neural projections to the circadian clock (both VPR and NVPR pathways), the eyes of mole rats are severely degenerated and the vestigial retina only displays NVPR projections.…”