Non‐Parametric Entrainment by Natural Twilight in the Microchiropteran Bat,Hipposideros Speoris Inside a Cave

The study aimed to determine the influence of repeated natural dawn and dusk twilight pulses in entraining the circadian flight activity rhythm of the microchiropteran bat, Hipposideros speoris, free‐running in constant darkness in a natural cave. The bats were exposed to repeated dawn or dusk twilight pulses at eight circadian phases. All bats exposed to dawn twilight pulses were entrained by advancing transients, and the stable entrainment was reached when the onset of activity occurred about 12 h before the lights‐on of the pulses, irrespective of the initial phase at which the bats were exposed to twilight. All bats exposed to dusk twilight pulses, however, were entrained by delaying transients, and the stable entrainment was reached when the onset of activity occurred about 1.6 h after the lights‐on of the pulses. The entrainment caused by dawn and dusk twilight pulses is discussed in the context of the postulated two photoreceptors: the short wavelength sensitive (S) photoreceptors mediating entrainment via dusk twilight, and the medium wavelength sensitive (M) photoreceptors mediating entrainment via dawn twilight.     

Natural Twilight Phase‐Response Curves for the Cave‐Dwelling Bat,Hipposideros Speoris

Phase‐response curves (PRCs) for the circadian rhythm of flight activity of the microchiropteran bat (Hipposideros speoris) were determined in a cave, employing discrete natural dawn and dusk twilight pulses. These PRCs are reported for the first time for any circadian system and they are unlike other PRCs constructed for nocturnal mammals. Dawn and dusk twilight pulses evoked advance and delay phase shifts, respectively. Advance phase shifts were followed by 3 to 4 advancing transients and a subsequent shortening of free‐running period (τ); whereas, the delay phase shifts were instantaneous without any transients but with a subsequent lengthening of τ.     

Natural twilight phase-response curves for the cave-dwelling bat, Hipposideros speoris

Phase-response curves (PRCs) for the circadian rhythm of flight activity of the microchiropteran bat (Hipposideros speoris) were determined in a cave, employing discrete natural dawn and dusk twilight pulses. These PRCs are reported for the first time for any circadian system and they are unlike other PRCs constructed for nocturnal mammals. Dawn and dusk twilight pulses evoked advance and delay phase shifts, respectively. Advance phase shifts were followed by 3 to 4 advancing transients and a subsequent shortening of free-running period (τ); whereas, the delay phase shifts were instantaneous without any transients but with a subsequent lengthening of τ.     

Natural entrainment without dawn and dusk: the case of the European ground squirrel (Spermophilus citellus).

Observational data collected in the field and in enclosures show that diurnal, burrow-dwelling European ground squirrels (Spermophilus citellus) never were above ground during twilight at dawn or at dusk. The animals emerged on average 4.02 h (SD = 0.45) after civil twilight at dawn and retreated in their burrows on average 2.87 h (SD = 0.47) before civil twilight at dusk. Daily patterns of light perceived by these burrowing mammals were measured with light-sensitive radio collar transmitters in an enclosure (the Netherlands) and in the field (Hungary). The observational data are corroborated by the telemetry data, which show clear daily patterns of timing of light perception including light perceived from the burrow entrances. The first light was observed by the animals on average 3.54 h (enclosure, SD = 0.45) and 3.60 h (field, SD = 0.31) after civil twilight at dawn, whereas the final observed light was on average 3.04 h (enclosure, SD = 0.64) and 2.02 h (field, SD = 0.72) before civil twilight at dusk. Thus, the animals do not perceive the rapid natural light-dark (LD) transitions that occur at civil twilight. Instead, they generate their own pattern of exposure to light within the natural LD cycle. The classical phase response model for entrainment by light or dark pulses cannot explain how the circadian system of this species remains entrained to the external, natural LD cycle while the major LD transitions are created by its own behavior.     

Season- and latitude-dependent effects of simulated twilights on circadian entrainment

Groups of Syrian hamsters were exposed to LD cycles with twilight transitions and photoperiods simulating natural lighting conditions at the summer solstice (SS), equinox, and winter solstice (WS) at 41 degrees N and at the winter solstice at the Arctic Circle (WS 66 degrees N) but with daytime illuminance truncated at 10 lux (LD-twilight). Separate groups were kept under matching rectangular cycles (LD-rectangular). The inclusion of twilights affected several circadian parameters in a season-and latitude-dependent manner. The most striking difference was in the timing of activity onsets, which followed dusk in the presence of twilights but were more closely related to dawn (lights-on) in their absence. Activity offsets and midpoints were also earlier in LD-twilight than in LD-rectangular, with the differences being most pronounced under WS 66 degrees N. In LD-twilight, longer nights resulted in earlier offsets and midpoints, but in LD-rectangular, midpoints were later under long than under short nights while offsets did not vary significantly. In LD-twilight, activity duration (alpha) increased monotonically with increasing nighttime duration, but in LD-rectangular, alpha was shorter under WS 66 degrees N than under WS conditions. These effects of season and latitude observed in LD-twilight were similar to those reported in animals exposed to natural illumination, while those observed in LD-rectangular differed in several respects. The presence of twilights also resulted in lower day-to-day variability in activity onset times (greater precision), supporting the earlier conclusion that twilights increase the strength of the LD zeitgeber. Free-running periods in constant darkness (DD) were shorter in LD-twilight than in LD-rectangular, especially under WS 66 degrees N, raising the possibility that the effects of twilights on the timing of the entrained activity rhythm reflect their effects on the period of that rhythm. Increasing daytime illuminance to 100 lux (WS conditions only) resulted in earlier activity offsets and midpoints and a shorter alpha but had no effect on activity onsets or on subsequent period in DD. These results indicate that exposure to low twilight illuminances alone can account for several of the documented differences between the effects of natural and rectangular light cycles on circadian entrainment.     

Varying the length of dim nocturnal illumination differentially affects the pacemaker controlling the locomotor activity rhythm of Drosophila jambulina

Photic entrainment of animals in the field is basically attributed to their exposure to the dimly lit nights flanked by the dawn and dusk twilight transitions. This implicates the functional significance of the dimly lit nights as that of the twilight transitions. Recently, the authors have demonstrated that the dimly lit night at 0.0006 lux altered the attributes of the circadian rhythm of locomotor activity of Drosophila jambulina. The present study examined whether the durations of such dimly lit nights affect the entrainment and free-running rhythmicity of D. jambulina. Flies were subjected for 10 days to two types of 24-h lighting regimes in which the photophase (L) was at 10 lux for all flies but the scotophase, which varied in duration from 9 to 15 h, was either at 0 lux (D phase) for control flies or 0.0006 lux (the artificial starlight or S phase) for experimental flies. Thereafter, they were transferred to constant darkness (DD) to compare the after-effects of the dimly lit nights on the period (τ) of free-running rhythm in DD with that of the completely dark nights. Control flies were entrained by all LD cycles, but the experimental flies were entrained only by five LS cycles in which the duration of the S phases ranged from 10 to 14 h. The two LS cycles with very short (9 h) and long (15 h) S phases rendered the flies completely arrhythmic. Control flies started activity shortly before lights-on and continued well after lights-off. The experimental flies, however, commenced activity several hours prior to lights-on but ended activity abruptly at lights-off as the result of a negative masking effect of nocturnal illumination. Length of the midday rest was considerably shorter in the control than in the experimental flies in each lighting regime. The active phase in the control flies was predictably shortened; nonetheless, it was invariable in the experimental flies as the nights lengthened. Transfer from lighting regimes to DD initiated robust free-running rhythmicity in all flies including the arrhythmic ones subjected to LS cycles with 9 and 15 h of scotophases. The τ was profoundly affected by the nocturnal irradiance of the prior entraining lighting regime, as it was always shorter in the experimental than in the control flies. Thus, these results indisputably demonstrate the changes in fundamental properties of the circadian pacemaker of D. jambulina were solely attributed to the extremely dim nocturnal irradiance. This strain of D. jambulina is entrained essentially by the dimly lit natural nights, since it is never exposed to the prevailing photic cues such as the twilight transitions or bright photoperiod, owing to the dense vegetation of its habitat.     

Twilights widen the range of photic entrainment in hamsters

The range of entrainment of the circadian rhythm of locomotor activity was compared in four groups of Syrian hamsters (eight animals per group) initially exposed to daily light-dark (LD) cycles with either abrupt transitions between light and darkness (LD-rectangular) or simulated twilights (LD-twilight). Lighting was provided by arrays of white light-emitting diodes; daytime illuminance (10 lux) and the total amount of light emitted per day were the same in the two conditions. The period (T) of the LD cycles was then gradually increased to 26.5 h or gradually decreased to 21.5 h, at the rate of 5 min/day. Under LD-rectangular, the upper and lower limits of entrainment were 25.0 to 25.5 h and 22.0 to 22.5 h, respectively, whereas under LD-twilight, 50% of the animals exposed to the lengthening cycles were still entrained at T = 26.5 h and 50% of those exposed to the shortening cycles were still entrained at T = 21.5 h. In a second experiment, two groups of hamsters were exposed to fixed T = 25 h LD-rectangular (n = 15) or LD-twilight cycles (n = 7). Only 33% of the animals entrained in LD-rectangular, whereas 86% of the animals entrained in LD-twilight. Free-running periods in constant darkness were longer following successful entrainment to T = 25 h but did not differ between the animals that entrained to LD-rectangular and those that entrained to LD-twilight. The widening of the range of entrainment observed in LD-twilight indicates that twilight transitions increase the strength of the LD zeitgeber. In LD-twilight, successful entrainment to T = 26.5 h was accompanied by an expansion of activity time to 16.52+/-1.22 h, with activity onsets preceding mid-dusk by 12.56+/-2.15 h. Together with earlier data showing similar phase response curves for hour-long dawn, dusk, and rectangular light pulses, these results suggest that the effect of twilights on the range of entrainment may involve parametric rather than nonparametric mechanisms.     

Effects of twilights on circadian entrainment patterns and reentrainment rates in squirrel monkeys

Entrainment patterns of the circadian rhythms of body temperature and locomotor activity were compared in 6 squirrel monkeys (Saimiri sciureus) exposed to daily illumination cycles with abrupt transitions between light and darkness (LD-rectangular) or with gradual dawn and dusk transitions simulating natural twilights at the equator (LD-twilight). Daytime light intensity was 500 lux, and the total amount of light emitted per day was the same in the two conditions. Mean daytime body temperature levels were stable in LD-rectangular but increased gradually in LD-twilight, reaching peak levels during the dusk twilight. Locomotor activity showed a similar pattern, but with an additional, secondary peak near the end of dawn. Activity duration was about 0.5 h longer in LD-twilight than in LD-rectangular, but the time of activity midpoint was similar in the two conditions. Reentrainment of the body temperature rhythm was faster following an 8-h advance of the LD cycle than following an 8-h delay, but did not differ significantly between the two LD conditions. These results provide no evidence that the inclusion of twilight transitions affected the strength of the LD Zeitgeber, and suggest that the observed differences in the daily patterns reflected direct effects of light intensity on locomotor activity and body temperature rather than an effect of twilights on circadian entrainment mechanisms.Abbreviation LD light-dark     

Varying the Length of Dim Nocturnal Illumination Differentially Affects the Pacemaker Controlling the Locomotor Activity Rhythm of Drosophila Jambulina

Photic entrainment of animals in the field is basically attributed to their exposure to the dimly lit nights flanked by the dawn and dusk twilight transitions. This implicates the functional significance of the dimly lit nights as that of the twilight transitions. Recently, the authors have demonstrated that the dimly lit night at 0.0006 lux altered the attributes of the circadian rhythm of locomotor activity of Drosophila jambulina. The present study examined whether the durations of such dimly lit nights affect the entrainment and free-running rhythmicity of D. jambulina. Flies were subjected for 10 days to two types of 24-h lighting regimes in which the photophase (L) was at 10 lux for all flies but the scotophase, which varied in duration from 9 to 15?h, was either at 0 lux (D phase) for control flies or 0.0006 lux (the artificial starlight or S phase) for experimental flies. Thereafter, they were transferred to constant darkness (DD) to compare the after-effects of the dimly lit nights on the period (τ) of free-running rhythm in DD with that of the completely dark nights. Control flies were entrained by all LD cycles, but the experimental flies were entrained only by five LS cycles in which the duration of the S phases ranged from 10 to 14?h. The two LS cycles with very short (9?h) and long (15?h) S phases rendered the flies completely arrhythmic. Control flies started activity shortly before lights-on and continued well after lights-off. The experimental flies, however, commenced activity several hours prior to lights-on but ended activity abruptly at lights-off as the result of a negative masking effect of nocturnal illumination. Length of the midday rest was considerably shorter in the control than in the experimental flies in each lighting regime. The active phase in the control flies was predictably shortened; nonetheless, it was invariable in the experimental flies as the nights lengthened. Transfer from lighting regimes to DD initiated robust free-running rhythmicity in all flies including the arrhythmic ones subjected to LS cycles with 9 and 15?h of scotophases. The τ was profoundly affected by the nocturnal irradiance of the prior entraining lighting regime, as it was always shorter in the experimental than in the control flies. Thus, these results indisputably demonstrate the changes in fundamental properties of the circadian pacemaker of D. jambulina were solely attributed to the extremely dim nocturnal irradiance. This strain of D. jambulina is entrained essentially by the dimly lit natural nights, since it is never exposed to the prevailing photic cues such as the twilight transitions or bright photoperiod, owing to the dense vegetation of its habitat. (Author correspondence: drdsjoshi08@gmail.com)     

Entrainment by different environmental stimuli in the frugivorous bats from the Lonar crater

The timings of emergence at dusk and return at dawn of a colony of frugivorous bats Rousettus leschenaulti, roosting in a temple ruin of the Lonar crater (19.97°N, 76.52°E), were investigated at 10-day intervals for one year. The onset of emergence occurred about 18 min after sunset throughout the year irrespective of the prevailing light intensity or temperature but the end of returning activity occurred at a fixed light intensity of about 4 lux irrespective of the time of sunrise or prevailing temperature. Apparently, the evening oscillator controlling the onset of emergence was set by the sunset, i.e. the lights-off stimulus of the natural light - dark cycles, while the morning oscillator controlling the end of activity was set by a certain invariant threshold intensity of the dawn twilight.     

Social entrainment in the old frugivorous bats, Rousettusleschenaulti from the Lonar crater

Onset and end of activity of the old frugivorous bats Rousettus leschenaulti, roosting in a temple ruin of the Lonar crater were observed at 10-day intervals for one year. The old bats emerged about 4 h after and returned about 4 h before the young bats. Onset of activity of the old bats was entrained by the loud vocalization of the early emerging young conspecifics at dusk. The old bats free-ran when the young conspecifics were evicted from the roost but re-entrained when they were re-introduced. Socially isolated old bats in the laboratory also free-ran in the scotophase of LD 12:12 cycles until the onset of activity coincided with lights-off which resulted in a delayed phase shift. After the exposure to light, the sequence of events repeated all over again. Ecological implications of this social entrainment are discussed.     

Colour As a Signal for Entraining the Mammalian Circadian Clock

Twilight is characterised by changes in both quantity (“irradiance”) and quality (“colour”) of light. Animals use the variation in irradiance to adjust their internal circadian clocks, aligning their behaviour and physiology with the solar cycle. However, it is currently unknown whether changes in colour also contribute to this entrainment process. Using environmental measurements, we show here that mammalian blue–yellow colour discrimination provides a more reliable method of tracking twilight progression than simply measuring irradiance. We next use electrophysiological recordings to demonstrate that neurons in the mouse suprachiasmatic circadian clock display the cone-dependent spectral opponency required to make use of this information. Thus, our data show that some clock neurons are highly sensitive to changes in spectral composition occurring over twilight and that this input dictates their response to changes in irradiance. Finally, using mice housed under photoperiods with simulated dawn/dusk transitions, we confirm that spectral changes occurring during twilight are required for appropriate circadian alignment under natural conditions. Together, these data reveal a new sensory mechanism for telling time of day that would be available to any mammalian species capable of chromatic vision.     

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.     

Dietary Benefits of Twilight Foraging by the Insectivorous Bat Hipposideros speoris1

Although bats are nocturnal, many species emerge from roosts to forage during twilight, despite a presumed high risk of predation at this time. Here, we describe twilight foraging by a maternity colony of Schneider's leafnosed bat (Hipposideros speoris) in the dry zone of Sri Lanka and determine the dietary benefits of such behavior. Bats usually began foraging during dusk, sometimes before sunset, and also foraged during twilight in the morning. Mean use of available twilight by four radio‐tagged bats was 75 percent. Twilight foraging made up, on average, 47 percent of the total foraging time of these bats (range = 25–96%), although twilight consisted of only 12 percent of the available time between sunset and sunrise the next morning. Eight species of potential predators (7 birds and 1 mammal) were observed within a 1 km radius of the colony, of which 5 species are predicted to regularly capture bats. Bats took a wide diversity of prey (11 insect orders, including at least 27 families, and spiders) that ranged in wing length from 2.0 to 54.0 mm. Major orders in the diet were Coleoptera, Lepidoptera, and Diptera. Prey of secondary importance included Hemiptera, Hymenoptera, Isoptera, and Neuroptera. Bats captured large numbers of insects that were only available or had marked peaks in abundance during twilight. These groups included small, swarming insects (especially flies) that have peaks in flight activity at dusk and dawn, large diurnal species (especially dragonflies) that have crepuscular activity, and winged termites that emerge in swarms at dusk. Access to these insects was a clear benefit of twilight foraging.     

Light-Dark Entrainment of Circadian Activity Rhythms of the Diurnal Indian Palm Squirrel (Funambulus pennanti)

A recent focus of chronobiological studies has been to establish diurnal models as alternatives to the more frequently used nocturnal rodents. In the present study, light-dark (LD) entrainment characteristics were examined in one diurnal species, the Indian palm squirrel (Funambulus pennanti). Palm squirrels showed strongly diurnal locomotor activity rhythms (? 88 percent) under light-dark (LD) cycles, with activity bimodally distributed during the L phase. In comparison to a dim LD cycle, exposure to a bright LD cycle caused a phase advance in the onset of activity, an increase in daily activity levels and an increase in the duration of activity. Percentage diurnality, however, did not vary between bright and dim LD cycles. Activity rhythms reentrained in significantly fewer days after an 8 hour phase delay of the LD cycle compared to an 8 hour phase advance. In both cases, the direction of reentrainment followed the direction of the LD shift. When exposed to single light pulses (1 hour) presented at the same time each day, 6/7 squirrels entrained. Under a skeletal photoperiod cycle (2 x 1 hour light pulses each day), 6/8 squirrels showed stable entrainment. The remaining squirrels exhibited rhythm splitting, with each component synchronising in an unstable manner with one of the light pulses. Under entrainment to single light pulses and to the skeletal photoperiod cycle, the phase angle of entrainment was negatively correlated with t. Finally, when exposed to a skeletal scotoperiod cycle (2 x 1-hour dark pulses each day), only 3/8 squirrels entrained, while the others free-ran. Two of the entrained squirrels showed spontaneous phase reversals during entrainment. As with other species, the activity rhythm of palm squirrels appears to be controlled by two separate self-sustaining oscillators. The strongly diurnal nature of palm squirrels make them a promising diurnal model for studies examining endogenous and exogenous influences on circadian functioning.     

Light-Dark Entrainment of Circadian Activity Rhythms of the Diurnal Indian Palm Squirrel (Funambulus pennanti)

A recent focus of chronobiological studies has been to establish diurnal models as alternatives to the more frequently used nocturnal rodents. In the present study, light-dark (LD) entrainment characteristics were examined in one diurnal species, the Indian palm squirrel ( Funambulus pennanti ). Palm squirrels showed strongly diurnal locomotor activity rhythms (~ 88 percent) under light-dark (LD) cycles, with activity bimodally distributed during the L phase. In comparison to a dim LD cycle, exposure to a bright LD cycle caused a phase advance in the onset of activity, an increase in daily activity levels and an increase in the duration of activity. Percentage diurnality, however, did not vary between bright and dim LD cycles. Activity rhythms reentrained in significantly fewer days after an 8 hour phase delay of the LD cycle compared to an 8 hour phase advance. In both cases, the direction of reentrainment followed the direction of the LD shift. When exposed to single light pulses (1 hour) presented at the same time each day, 6/7 squirrels entrained. Under a skeletal photoperiod cycle (2 x 1 hour light pulses each day), 6/8 squirrels showed stable entrainment. The remaining squirrels exhibited rhythm splitting, with each component synchronising in an unstable manner with one of the light pulses. Under entrainment to single light pulses and to the skeletal photoperiod cycle, the phase angle of entrainment was negatively correlated with t. Finally, when exposed to a skeletal scotoperiod cycle (2 x 1-hour dark pulses each day), only 3/8 squirrels entrained, while the others free-ran. Two of the entrained squirrels showed spontaneous phase reversals during entrainment. As with other species, the activity rhythm of palm squirrels appears to be controlled by two separate self-sustaining oscillators. The strongly diurnal nature of palm squirrels make them a promising diurnal model for studies examining endogenous and exogenous influences on circadian functioning.     

Phase-shifting the fruit fly clock without cryptochrome

The blue light photopigment cryptochrome (CRY) is thought to be the main circadian photoreceptor of Drosophila melanogaster. Nevertheless, entrainment to light-dark cycles is possible without functional CRY. Here, we monitored phase response curves of cry(01) mutants and control flies to 1-hour 1000-lux light pulses. We found that cry(01) mutants phase-shift their activity rhythm in the subjective early morning and late evening, although with reduced magnitude. This phase-shifting capability is sufficient for the slowed entrainment of the mutants, indicating that the eyes contribute to the clock's light sensitivity around dawn and dusk. With longer light pulses (3 hours and 6 hours), wild-type flies show greatly enhanced magnitude of phase shift, but CRY-less flies seem impaired in the ability to integrate duration of the light pulse in a wild-type manner: Only 6-hour light pulses at circadian time 21 significantly increased the magnitude of phase advances in cry(01) mutants. At circadian time 15, the mutants exhibited phase advances instead of the expected delays. These complex results are discussed.     

Rhythmicity of torpor in a marsupial hibernator, the mountain pygmy-possum (Burramysparvus), under natural and laboratory conditions

Circadian rhythms have been observed in most mammals, but their importance and function remain controversial with respect to daily cycles during hibernation. We investigated the timing of arousals from and entries into hibernation for both free-living and captive mountain pygmy-possums (Burramys parvus). Under both natural and laboratory conditions most arousals and entries were entrained with the light-dark cycle. Entries occurred mainly during the night and arousals preferably around dusk, which coincides with the onset of the normal activity phase for the nocturnal pygmy-possums. This entrainment prevailed throughout the hibernation season although only the laboratory animals were constantly subjected to photoperiodic stimuli, whereas under natural conditions hibernacula are shielded from photic cues and diurnal temperature fluctuations. Nevertheless, possums left their hibernacula frequently throughout winter and were occasionally trapped close to the snow surface suggesting that during the periods of post-arousal normothermia they can be exposed to environmental stimuli. It thus appears that the synchronisation with the photocycle was governed by a temperature-compensated circadian clock which was reset periodically during short activity periods. For the mountain pygmy-possum, entrainment with the photocycle probably has two functions: 1. Entrainment ensures that foraging bouts during the hibernation season remain synchronised with the dark phase. 2. Information about the prevailing climatic conditions sampled during short activity periods enables them to time final spring emergence from hibernation when snow melt begins and ensures that the breeding season can commence as early as possible. Accepted: 26 August 1998     

Activity patterns of the lesser horseshoe bat Rhinolophus hipposideros at summer roosts

Dusk to dawn observations, using a bat detector and occasionally an image intensifier, were made outside two nursery roosts of lesser horseshoe bats Rhinolophus hipposideros from late May to September. Emergence was correlated with sunset but delayed by extended twilight. Light intensity was important in triggering departure and cloud cover advanced it. Light-testing behaviour was invariably undertaken, in the form of brief flights out and back into the roosts. The exit from one roost was shaded by trees and exploratory flights were generally more extended there. Heavy rain inhibited emergence. There was almost always intermittent activity throughout the night, with many individuals returning and departing, and no indication of seasonal or overnight peaks. A bat detector inside a third roost confirmed overnight observations at the other two. Some bats often returned to the roost for the night before dawn. Dawn return was linked to sunrise, prolonged twilight in midsummer hastening it. Colony size varied appreciably over periods of a few days and even overnight. There is some limited evidence that increased colony size, perhaps through social interaction, may have influenced timing of departure at dusk and return at dawn.