Clocks Ticking in the Dark: A Review of Biological Rhythms in Subterranean African Mole-Rats (original) (raw)
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Circadian rhythms of locomotor activity in Ansell's mole‐rat: are mole‐rat's clocks ticking?
Journal of …, 2008
Circadian rhythms of locomotor activity have been investigated in several African mole-rat species. Even though mole-rats spend most of their lives in underground burrows devoid of light, studies have shown that they do possess circadian rhythms to some extent. We investigated the circadian rhythms of locomotor activity in 11 male Ansell's mole-rats Fukomys anselli from Zambia. In order to determine whether these animals can entrain to light and have endogenous rhythms, they were subjected to different light regimes: first, 12 h light/12 h dark, followed by constant darkness, then returned to 12 h light/12 h dark, which was later inversed to 12 h dark/12 h light. Only two individuals displayed arrhythmic activity patterns whereas the other nine (81.8%) exhibited entrainment of their activity to the light regimes. Locomotory activity of Ansell's mole-rat was predominantly during the dark phase in all light regimes. During constant darkness (DD), only five individuals (45.5%) displayed very weak circadian rhythms that free ran but became more indistinct towards the end of the cycle. Under the second LD light cycle, 90.1% of the animals were active during the night phase of the cycle and when placed under an inverse light cycle, seven individuals still displayed activity predominantly during the dark phase. In conclusion, these results suggest that Ansell's mole-rat does have a weak circadian clock and is able to perceive light and entrain to light cycles.
Journal of biological …, 2003
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.
Physiology & …, 2006
The Lesotho mole-rat is a social subterranean rodent that occurs at altitude in the Drakensberg mountain range. As a consequence of living permanently underground these animals rarely if ever are exposed to light. The visual system of African mole-rats is particularly regressed whereas the circadian system is proportionately conserved. This study investigated the locomotor activity patterns of 12 Lesotho mole-rats maintained under a range of different lighting regimes. The majority (91.7%) of mole-rats entrained their activity patterns to a LD photoperiod of 12L/12D. The mole-rats displayed a monophasic nocturnal activity preference. Under constant dark (DD) most of the molerats (83.3%) showed a free running circadian activity pattern with a tau of 23.8 h to 24.4 h (mean ± S.E.M.: 24.07 h ± 0.07 h; n = 10). The phase of the activity rhythms each mole-rat exerted during the previous LD-cycle did not change when the animals started free-running after being placed in constant conditions. The duration of re-entrainment to a second bout of LD 12:12 amounted to 9.4 ± 2.03 days (mean ± S.E.M., n = 10). Eleven mole-rats (91.7%) adjusted their locomotor activity rhythms to an inversed light regime DL 12:12 and displayed significant nocturnal activity preference. The animals required 9.73 ± 2.01 days (mean ± S.E.M., n = 11) to adjust to the DL-photoperiod. The Lesotho mole-rat thus possesses a functional circadian clock that responds to a photic zeitgeber.
Circadian rhythms of locomotor activity in the subterranean Mashona mole rat, Cryptomys darlingi
Physiology & …, 2005
The Mashona mole rat, Cryptomys darlingi, is a social, subterranean African rodent that is rarely, if ever, exposed to light, and that exhibits a regressed visual system. This study investigated locomotor activity patterns of Mashona mole rats (n=12) under different light cycles. Activity was measured using either infrared captors (n=8) or running wheels (n=4). The mole rats entrained their activity to a standard (LD 12:12) photoperiod. They displayed either a nocturnal or diurnal activity preference with one bout of activity and one bout of rest. Therefore, as a species, the Mashona mole rat did not show a clear nocturnal or diurnal activity preference. When the LD (12:12) light cycle was inversed, the animals switched their activity, too. Under constant dark (DD), most mole rats (73%) showed a free-running circadian activity rhythm, but under constant light (LL), only some (36%) did. The free-run period of the rhythm (s) ranged from 23.83 to 24.10 h. The remaining animals were arrhythmic. There was large interindividual and intraindividual variations in the rate and extent of entrainment, time of activity preference, and activity patterns. Possible reasons for the observed variations are discussed. It is concluded that the Mashona mole rat has an endogenous activity rhythm which approximates 24 h, that the mole rat can distinguish between light and dark, and that the endogenous clock utilises this photic information as a zeitgeber. D
Proceedings of the National Academy of Sciences, 2001
Blind subterranean mole rats retain a degenerated, subcutaneous, visually blind but functionally circadian eye involved in photoperiodic perception. Here we describe the cloning, sequence, and expression of the circadian Clock and MOP3 cDNAs of the Spalax ehrenbergi superspecies in Israel. Both genes are relatively conserved, although characterized by a significant number of amino acid substitutions. The glutamine-rich area of Clock, which is assumed to function in circadian rhythmicity, is expanded in Spalax compared with that of humans and mice, and is different in amino acid composition from that of rats. We also show that MOP3 is a bona fide partner of Spalax Clock and that the Spalax Clock͞MOP3 dimer is less potent than its human counterpart in driving transcription. We suggest that this reduction in transcriptional activity may be attributed to the Spalax Clock glutamine-rich domain, which is unique in its amino acid composition compared with other studied mammalian species. Understanding Clock͞MOP3 function could highlight circadian mechanisms in blind mammals and their unique pattern as a result of adapting to life underground.
Circadian rhythms of locomotor activity in naked mole-rats (Heterocephalus glaber)
Physiology & Behavior, 2000
A wide variety of organisms exhibit various circadian rhythms in their behavior and physiology. Circadian rhythms are regulated by internal clocks that are generally entrained primarily by the environmental light:dark (L:D) cycle. There have been few studies of circadian rhythms in fossorial species that inhabit an environment where day ± night variations are minimal and where exposure to light occurs infrequently. In this study, circadian patterns of wheel-running activity were examined in naked mole-rats (Heterocephalus glaber). Naked mole-rats are fossorial and eusocial, living in colonies of 60 ± 70 animals with only one breeding female. Most individual mole-rats that ran on wheels (65%) exhibited robust circadian rhythms of locomotor activity, entrained to various L:D cycles, and free-ran in constant darkness (DD) with taus averaging 23.5 h. The remainder of the animals either free-ran or were arrhythmic under the various L:D cycles. Mole-rats generally failed to entrain to non-24-h T-cycles with period lengths ranging from T = 23 h to T = 25 h. There was considerable inter-individual variation in the circadian patterns of locomotor activity in naked mole-rats as is observed in other subterranean mammals that have been studied. In contrast to the results obtained when mole-rats were individually housed with access to running wheels, circadian rhythms of general locomotor activity were typically not observed for animals monitored while they were housed in a colony setting. However, clear nocturnal rhythms of general locomotor activity were displayed by four males while residing in their home colonies. Two of these males exhibited the physical appearance of a disperser morph Ð subordinate individuals that are believed to leave their home colonies to achieve reproductive opportunities elsewhere. All four of these males were among the largest males in their respective colonies. These results demonstrate that although naked mole-rats are not frequently exposed to light, the species has retained the capacity to exhibit locomotor patterns of circadian rhythmicity and has the ability to entrain to 24-h L:D cycles. The possible adaptive function of this circadian capacity is discussed.
Current Biology, 2002
eral organs of the body are synchronized to generate a concerted rhythm for the whole organism [11]. Among the genes driving the clock in the SCN are the two Period genes Per1 and Per2 [12-14]. Both homologs show a circadian rhythm of activation within the SCN and can serve as markers for the phase of the circadian clock [15-17]. Switzerland 2 Institute of Evolution Recent data indicate that, at both the behavioral and the molecular levels, the blind mole rat Spalax ehren-University of Haifa Mount Carmel bergi superspecies has a functional circadian clock despite its isolated subterranean ecotope [3, 6, 7]. Haifa 31905 Israel Twenty-five million years of selective adaptation to this environment have resulted in a radical degeneration of its visual system, leading to atrophied (600 m wide), totally fur-covered eyes that lack any image-forming Summary ability [18]. Interestingly, the degenerated retina contains opsins and melanopsin, which might be responsi-The subterranean mole rat Spalax ehrenbergi superspecies represents an extreme example of adaptive ble for light detection [19, 20]. The SCN, however, is well developed and receives clock-related signals from visual and neuronal reorganization [1, 2]. Despite its total visual blindness, its daily activity rhythm is en-the retina via the retinohypothalamic tract [7, 18, 21].
Circadian activity rhythms in colonies of ‘blind’ Molerats,Cryptomys damarensis(Bathyergidae)
South African Journal of Zoology, 1993
Various activity rhythms (general, feeding, and toilet) were measured under controlled laboratory conditions in two colonies of the Damara molerat Cryptomys damarensis, for 140 consecutive days (following a 30 day test period) under various photoperiod regimes (16: 8 LD, 12: 12 LD, and constant dark DD). As a general rule, all activities showed a significant diurnal rhythm with a period of 24 h under LD photoperiods. However, under a 16 : 8 LD photoperiod a very prominent activity component during the first part of the subjective night was observed, especially following a shift in photoperiod. The molerats responded rapidly to LD phase shifts. In constant dark, all activities had free-running periods of T = 24,1-24,2 h, thus indicating that light can synchronize and entrain endogenous circadian rhythms in these mole rats. Verskeie aktiwiteitsritmes (algemeen, voeding en toilet) is onder gekontroleerde laboratoriumtoestande gemeet in twee kolonies van die Damara vaalmol Cryptomys damarensis, vir 140 agtereenvolgende dae (na 'n 30 dae toetsperiode) onder verskeie fotoperiodes (16 : 8 LD, 12 : 12 LD, en ononderbroke danker DD). Oar die algemeen het al die aktiwiteite 'n betekenisvolle daaglikse ritme met 'n periode van 24 h onder LD fotoperiodes getoon. Daar was egter gedurende die 16 : 8 LD fotoperiode 'n baie prominente aktiwiteitskomponent tydens die eerste deel van die subjektiewe nag, veral na 'n verandering in fotoperiode. Vaalmolle het baie vinnig op die LD faseveranderings gereageer. In ononderbroke danker het aile aktiwiteite aaneenlopende periodes van T = 24,1-24,2 h gehad, wat aandui dat lig interne sirkadiese ritmes in hierdie vaalmolle kan meebring en sinkroniseer.