Body temperatures of leatherback turtles (Dermochelys coriacea) in temperate waters off Nova Scotia, Canada

The leatherback sea turtle, Dermochelys coriacea (Vandelli, 1761), has the most extensive range of any reptile, migrating from tropical and subtropical nesting areas to distant foraging habitats, including those in temperate and even boreal waters. This implies flexible thermal functioning. It has been inferred that leatherbacks support active foraging by keeping warm in cold water, rather than becoming lethargic as other marine turtles do. However, data consistent with this view have come from captive turtles in unnatural and stressful conditions. In the present case, foraging leatherbacks were captured at sea off Nova Scotia and their body temperature recorded within 10 min, before such large animals could change their body temperatures appreciably. Mean excess temperature over that of the sea surface (15.0–16.7 °C) averaged 8.2 °C. These results attest to, but underestimate, the capacity of free-swimming leatherbacks to keep warm in northern waters, as data from another turtle that was instrumented to record ocean temperature while diving revealed that leatherbacks foraging in this area at the same time of year may spend 40% of their time diving to waters cooler than the surface.

Behavioral arousal blocks light-induced phase advances in locomotor rhythmicity but not light-induced Per1 and Fos expression in the hamster suprachiasmatic nucleus

Both photic and nonphotic stimuli entrain circadian rhythms. Although the adaptive significance of nonphotic clock resetting is unknown, one possibility is that nonphotic cues modulate circadian responses to light. Results of studies on the interaction between photic and nonphotic stimuli support this idea. During the day, light blocks the effects of nonphotic stimuli on the phase of locomotor rhythms and on expression of clock genes in suprachiasmatic nucleus (SCN) neurons. At night, novelty-induced activity prior to and during exposure to light attenuates the phase-shifting response to that light, but the effects of this manipulation on clock gene expression are unknown. The present experiments explore the interaction between behavioral state and response to light at the molecular level. We show that confining hamsters to novel wheels immediately after a light pulse during the late subjective night attenuates light-induced phase advances of wheel-running rhythms and the transient effects on circadian period. In contrast to the striking effect of novelty-induced activity on behavioral responses to light, Fos protein and Per1 mRNA were robustly expressed in the SCN of all light-pulsed animals, regardless of behavioral treatment. Our results are inconsistent with the idea that light and nonphotic stimuli block each other's effects on phase shifts by inducing or attenuating transcription of Per1. Photic regulation of clock genes and spontaneous rhythmic expression of clock genes are probably mediated by different mechanisms.

Melanopsin and rod-cone photoreceptive systems account for all major accessory visual functions in mice

In the mammalian retina, besides the conventional rod-cone system, a melanopsin-associated photoreceptive system exists that conveys photic information for accessory visual functions such as pupillary light reflex and circadian photo-entrainment. On ablation of the melanopsin gene, retinal ganglion cells that normally express melanopsin are no longer intrinsically photosensitive. Furthermore, pupil reflex, light-induced phase delays of the circadian clock and period lengthening of the circadian rhythm in constant light are all partially impaired. Here, we investigated whether additional photoreceptive systems participate in these responses. Using mice lacking rods and cones, we measured the action spectrum for phase-shifting the circadian rhythm of locomotor behaviour. This spectrum matches that for the pupillary light reflex in mice of the same genotype, and that for the intrinsic photosensitivity of the melanopsin-expressing retinal ganglion cells. We have also generated mice lacking melanopsin coupled with disabled rod and cone phototransduction mechanisms. These animals have an intact retina but fail to show any significant pupil reflex, to entrain to light/dark cycles, and to show any masking response to light. Thus, the rod-cone and melanopsin systems together seem to provide all of the photic input for these accessory visual functions.

Period gene expression in the suprachiasmatic nucleus of behaviorally decoupled hamsters

Circadian rhythms in locomotor activity in nocturnal animals typically show activity during the night followed by quiescence during the day. In hamsters, this activity pattern can be altered by confining them to novel running wheels for a few hours during the day, thereby inducing them to be active. After several days of induced activity, two bouts of activity occur spontaneously, one during the day, and the other at night. This phenomenon is known as behavioral decoupling. In the present study we examined the effects of behavioral decoupling on the pattern of SCN Per expression. Our results show robust expression of Period genes in the SCN of animals killed during either of the two inactive phases within a 24-h period, and weak expression of those genes in the SCN of animals killed during the two active phases. This contrasts with the circadian rhythm of Per expression typically seen in the SCN of entrained or free running animals. Our data support the idea that behavioral decoupling reorganizes the circadian system in a manner different from that produced by constant light-induced splitting and constant dark splitting.

Increased masking response to light after ablation of the visual cortex in mice

Mice are known to suppress their wheel running when given a pulse of light in the night (masking response). The amount of suppression can be quantified; the response varies with the level of irradiance used during the light pulse. After ablation of the visual cortex, mice suppressed their activity more than sham-operated controls. In addition, the lesioned animals responded to lower levels of irradiance than controls. It is suggested that the visual cortex is not needed for the suppression of locomotor activity after a light pulse. Nevertheless it exerts an inhibitory influence on the masking response to light mediated by an irradiance detection system. When this inhibition is removed, even though pattern vision is lost, masking responses to ambient level of light are enhanced.

Afferent projections to the hamster intergeniculate leaflet demonstrated by retrograde and anterograde tracing

The intergeniculate leaflet (IGL) is considered involved in nonphotic shifting of the circadian clock through a direct connection, the geniculo-hypothalamic tract. The brain areas mediating nonphotic arousal to the hamster IGL have not been thoroughly investigated by both retrograde and anterograde tracing. We, therefore, reinvestigated the IGL afferent connections with the retrograde tracer Cholera toxin B and subsequently verified the results with the anterograde tracer Phaseolus vulgaris-leucoagglutinin. We also defined a subset of neurons projecting to the IGL that were activated by arousal using c-Fos immunocytochemistry. Apart from a dense afferent projection from the retina- and the contralateral leaflet, there were ipsilateral projections from other structures: layer V and VI of the prefrontal cortex, the zona incerta, the magnocellular part of the subparafascicular nucleus, the dorsal raphe nucleus, the locus coeruleus, and the cuneiform nucleus. Dense bilateral projections to the leaflet from the pretectal nuclei were found. Hypothalamic afferents were observed dorsal to the suprachiasmatic nuclei, in the retrochiasmatic area (RCh) and in the ventromedial hypothalamic nuclei. All of these projections were confirmed by anterograde tracing. Furthermore, arousal (wheel-running) induced c-Fos in neurons projecting to the IGL (prefrontal cortex, RCh, pretectum). Taken together, the data strengthen the view that the IGL integrates photic and nonphotic information.

Contribution of classic photoreceptors to entrainment

The ability to phase shift and entrain in response to light is spared in retinally degenerate mice (rd/rd). In the present work, fewer retinally degenerate C57BL/6 mice than wildtypes entrained in dim lights, suggesting that rods and/or cones contribute toward entrainment even though they are not necessary. Thresholds for entrainment appear to be a more sensitive test of deficits in entrainment than phase shifts in response to light pulses.

Aschoff's rule in retinally degenerate mice

Both retinally degenerate and wildtype mice lengthened the period of their free-running circadian rhythms and reduced the amount of wheel running when exposed to increasing levels of constant illumination, in accordance with Aschoff's rule. Decreased locomotor activity may contribute toward lengthening of period in bright light. However, the known effects of activity on free-running period are small compared to those obtained by changing illumination. This suggests that Aschoff's rule in mice is not dependent on changes in nonphotic input, but results from a direct effect of light on the circadian system. The sparing of Aschoff's rule in retinally degenerate mice is further evidence that circadian photoreception depends on mechanisms other than rods and cones.

Beyond the suprachiasmatic nucleus

The suprachiasmatic nucleus is the master oscillator controlling circadian rhythms in mammals. Yet extensive temporal restructuring of behavior can occur without participation of the suprachiasmatic nucleus. This raises questions about current thinking about how to cope with jet lag and shift work.

Pivotal temperature for loggerhead turtles (Caretta caretta) from Kyparissia Bay, Greece

Pivotal temperature (the constant temperature giving 50% of each sex) for two clutches of loggerhead sea turtles (Caretta caretta) from Kyparissia Bay, Greece, was 29.3°C. Pivotal incubation duration (the time from laying to hatching giving 50% of each sex) was 52.6 days. These values are close to those obtained for this species in Brazil and the United States, providing further evidence that these characteristics are relatively conservative in different populations. Methodological differences between different experiments and limitations on accuracy of equipment make the detection of small differences problematic. Comparison of incubation durations in the field with the pivotal durations obtained here suggest that hatchling sex ratio on some Mediterranean beaches is female biased but probably varies considerably within this region.

Learned arbitrary responses to light in mice without rods or cones

The aim of this investigation was to discover whether mice lacking classical photoreceptors (rods and cones) can nevertheless be trained to respond to light. Mice with the coneless ( cl) transgene have an attenuated diphtheria toxin fused to a cone opsin promotor. Mutant mice homozygous for the retinal degeneration ( rd) gene undergo loss of their rods. By mating these two strains, mice lacking both cones and rods can be generated (Lucas et al. 1999). Such coneless-rodless mice were able to use light as a signal to make a behavioural response to avoid impending shock. Nevertheless, especially initially, they used the light as a cue less often than wildtype controls, indicating that normally the rods and cones are used for such responses. However, other photoreceptors are able to take over this role to some extent. When the lights were covered with opaque material, the performance of rodless-coneless mice dropped to chance level, indicating that they had been using the light as a cue for avoidance.

Behavioral responses to light in mice with dorsal lateral geniculate lesions

Light has rapid direct effects on behavior and physiology that may be distinguished from its indirect effects that occur via synchronization of the biological clock. In nocturnal animals, light at night acutely suppresses the wheel running activity usually observed at that time of day. This is known as masking because light masks the overt expression of the circadian activity rhythm. In the present study, we compared the effects of light on wheel running in mice with bilateral electrolytic lesions of the dorsal lateral geniculate nucleus (DLG) to those in sham-operated animals. DLG-lesioned animals exhibited greater suppression of wheel running in response to bright light than did the controls, but failed to exhibit the increased activity in response to dim light observed in intact animals. These findings support the view that masking effects of light on behavior comprise two opposing processes, one that increases activity and is mediated by the classical visual system, and another that suppresses activity and is mediated by a non image-forming irradiance detection system.

Cycle of period gene expression in a diurnal mammal (Spermophilus tridecemlineatus): implications for nonphotic phase shifting

Ground squirrels, Spermophilus tridecemlineatus, were kept in a 12:12 h light-dark cycle. As expected for a diurnal species, their locomotor activity occurred almost entirely in the daytime. Expression of mPer1 and mPer2 in the suprachiasmatic nucleus was studied at six time points by in situ hybridization. For both these genes, mRNA was highest in the first part of the subjective day (about zeitgeber time 5). This is close to the time when mPer1 and mPer2 expression is maximal in nocturnal rodents. These results have implications for understanding nonphotic phase response curves in diurnal species and thereby for guiding research on nonphotic phase shifting in people.

Further characterization of the phenotype of mCry1/mCry2-deficient mice

Mice lacking cryptochromes (mCry1/mCry2-/-) were kept in a 16h light, 8h dark, light-dark (16:8 LD) cycle and were given additional pulses of light of different brightness, starting 2h after dark onset and lasting for 1h. The suppression of wheel running during these light pulses (i.e., masking) was compared to that of wild types. No evidence of any decrement in the masking response to light was detected. As well as studying masking, minor bouts of activity occurring in the main light portion of a light-dark cycle were quantified. One possible explanation of such predark activity is that some damped endogenous process is spared in mCry1/mCry2 double-knockout mice.

Photic and nonphotic circadian phase resetting in a diurnal primate, the common marmoset

Despite the considerable literature on circadian entrainment, there is little information on this subject in diurnal mammals. Contributing to this lack of understanding is the problem of separating photic from nonphotic (behavioral) phase-resetting events in diurnal species. In the present study, photic phase resetting was obtained in diurnal common marmosets held under constant dim light (DimDim; <0.5 lx) by using a 20-s pulse of bright light to minimize time available for behavioral arousal. This stimulus elicited phase advances at circadian time (CT) 18-22 and phase delays at CT9-12. Daily presentation of these 20-s pulses produced entrainment with a phase angle of approximately 11 h (0 h = activity onset). Nonphotic phase resetting was obtained under DimDim with the use of a 1-h-induced activity pulse, consisting of intermittent cage agitation and water sprinkling, delivered in total darkness to minimize photic effects. This stimulus caused phase delays at CT20-24, and entrainment to a scheduled daily regimen of these pulses occurred with a phase angle of approximately 0 h. These results indicate that photic and nonphotic phase-response curves (PRCs) of marmosets are similar to those of nocturnal rodents and that nonphotic PRCs are keyed to the phase of the suprachiasmatic nucleus pacemaker, not to the phase of the activity-rest cycle.

Preferences of mice, Mus musculus, for different types of running wheel

Mice are increasingly used in research. In particular, their wheel running is often used as a measure of activity, and as a marker of phase of circadian rhythms. Learning about the preferences of mice for different types of wheel may improve their welfare and suggest ways of increasing activity levels. Mice, Mus musculus, were given a choice between different types of running wheel by putting them in cages equipped with two wheels. Strong preferences were shown for wheels with a plastic mesh flooring, rather than the standard metal rods only. The mesh was even preferred over a solid base, although this effect was not seen in mice that had been given access only to wheels with the solid base immediately prior to the choice test. Small diameter wheels, sometimes sold as mouse wheels, were preferred less than standard-sized wheels with rods. The results suggest that types of running wheel often used in laboratories can be improved by considering the animals' preferences. The types of wheel tested here are easy to maintain and entail little additional cost, while increasing the mouse's interest in running and exercise.

Responses to light after retinal degeneration

Transgenic rodless mice were given 1-h pulses of light of varying brightness at times of the night when they were normally active. The rodless mice showed decreases in locomotor activity during light pulses brighter than 2 lux; these decreases were significantly greater than those in wildtypes (ANOVA, P < 0.01). However, with very dim light, rodless mice showed no changes in activity, whereas wildtype mice actually increased their activity. It is suggested that irradiance detection could be enhanced by absence of image-forming vision. Enhanced inhibition of activity around twilight may be adaptive for mice in some circumstances and so help maintain genes for retinal degeneration in natural populations.

Rapid down-regulation of mammalian period genes during behavioral resetting of the circadian clock

The pervasive role of circadian clocks in regulating physiology and behavior is widely recognized. Their adaptive value is their ability to be entrained by environmental cues such that the internal circadian phase is a reliable predictor of solar time. In mammals, both light and nonphotic behavioral cues can entrain the principal oscillator of the hypothalamic suprachiasmatic nuclei (SCN). However, although light can advance or delay the clock during circadian night, behavioral events trigger phase advances during the subjective day, when the clock is insensitive to light. The recent identification of Period (Per) genes in mammals, homologues of dperiod, which encodes a core element of the circadian clockwork in Drosophila, now provides the opportunity to explain circadian timing and entrainment at a molecular level. In mice, expression of mPer1 and mPer2 in the SCN is rhythmic and acutely up-regulated by light. Moreover, the temporal relations between mRNA and protein cycles are consistent with a clock based on a transcriptional/translational feedback loop. Here we describe circadian oscillations of Per1 and Per2 in the SCN of the Syrian hamster, showing that PER1 protein and mRNA cycles again behave in a manner consistent with a negative-feedback oscillator. Furthermore, we demonstrate that nonphotic resetting has the opposite effect to light: acutely down-regulating these genes. Their sensitivity to nonphotic resetting cues supports their proposed role as core elements of the circadian oscillator. Moreover, this study provides an explanation at the molecular level for the contrasting but convergent effects of photic and nonphotic cues on the clock.

Pivotal temperature and predicted sex ratios for hatchling hawksbill turtles from Brazil

Like all other species of sea turtle, the hawksbill turtle (Eretmochelys imbricata) exhibits temperature-dependent sexual differentiation, with high incubation temperatures producing females and low temperatures producing males. Relatively little is known about the sex ratios of hatchlings produced by nesting populations of hawksbill turtles. Here we estimate the overall seasonal sex ratios of hatchling hawksbill turtles produced in Bahia, Brazil, during 6 nesting seasons, based on incubation durations, pivotal temperature, and pivotal incubation duration. The overall sex ratio of hatchlings produced in Bahia from 1991-1992 through 1996-1997 was estimated to be >90% female, which is more female-biased than estimated sex ratios of hatchling loggerhead turtles from Bahia and Florida, U.S.A. The biological and conservation implications of skewed sex ratios are discussed.

Nonphotic entrainment in a diurnal mammal, the European ground squirrel (Spermophilus citellus)

Entrainment by nonphotic, activity-inducing stimuli has been investigated in detail in nocturnal rodents, but little is known about nonphotic entrainment in diurnal animals. Comparative studies would offer the opportunity to distinguish between two possibilities. (1) If nonphotic phase shifts depend on the phase of the activity cycle, the phase response curve (PRC) should be about 180 degrees out of phase in nocturnal and diurnal mammals. (2) If nonphotic phase shifts depend on the phase of the pacemaker, the two PRCs should be in phase. We used the diurnal European ground squirrel (Spermophilus citellus) in a nonphotic entrainment experiment to distinguish between the two possibilities. Ten European ground squirrels were kept under dim red light (<1 lux) and 20 +/- 1 degrees C. During the entrainment phase of the experiment, the animals were confined every 23.5 h (T) to a running wheel for 3 h. The circadian rhythms of 6 squirrels entrained, 2 continued to free run, and 2 possibly entrained but displayed arrhythmicity during the experiment. In a second experiment, a photic pulse was used in a similar protocol. Five out of 9 squirrels entrained, 1 did not entrain, and 3 yielded ambiguous results. During stable entrainment, the phase-advancing nonphotic pulses coincided with the end of the subjective day, while phase-advancing light pulses coincided with the start of the subjective day: mean psi(nonphotic) = 11.4 h; mean psi(photic) = 0.9 h (psi defined as the difference between the onset of activity and the start of the pulse). The data for nonphotic entrainment correspond well with those from similar experiments with nocturnal Syrian hamsters where psi(nonphotic) varied from 8.09 to 11.34 h. This indicates that the circadian phase response to a nonphotic activity-inducing stimulus depends on the phase of the pacemaker rather than on the phase of the activity cycle.

Validation of incubation duration as an index of the sex ratio of hatchling sea turtles

One method of estimating the sex ratio of hatchling sea turtles is to use the incubation duration. Long and short durations imply low and high temperatures, respectively. In turtle species whose sex is determined by temperature, males are produced at low temperatures and females at high temperatures. This study assesses the validity of using incubation duration to estimate the sex ratio. Samples of hatchling loggerhead turtles (Caretta caretta) were collected from nests with known incubation durations, and sex was ascertained by means of histology. The sex ratio of groups of nests determined by histology was compared with that predicted from previous relationships between incubation duration and sex ratio. For conditions causing relatively long or relatively short incubation durations, the sex ratio could be predicted with considerable accuracy. For conditions causing durations nearer to the pivotal duration (that which gives 50% of each sex), predictions could be off by 10%, depending on the distribution of incubation durations, but it was still possible to determine whether ratios were highly skewed or approximately balanced. Estimating sex ratios of hatchling sea turtles from incubation durations is simple, cheap, and can be used retrospectively.

Masking: history, definitions, and measurement

Masking and entrainment are two different processes that result in rhythms in physiology and behavior. Considering its functional importance, lamentably little study has been devoted to masking compared to that lavished on entrainment. This paper discusses the origin and definition of terminology (positive and negative masking, paradoxical masking) and methods of quantifying masking. How masking is scored can greatly influence the interpretation of results.

Further experiments on the relationship between the period of circadian rhythms and locomotor activity levels in hamsters

A number of experiments in the past have demonstrated that rats and mice have shorter free-running circadian rhythms when they have access to a running wheel in their cage. Moreover, within groups of rats and hamsters, individuals making most use of their running wheels tend to have shorter circadian rhythms. However, these effects are not always evident. This article analyzes the results of four additional experiments on hamsters, some showing correlations between high activity and fast rhythms, and others not. It is suggested that failure to find this relationship occurs when there is an insufficient range of activity levels within a group. When present, correlations between locomotor activity and periodicity reflect causal links because shorter rhythms can be produced by providing a type of running wheel on which hamsters run more. The effects of possible changes in activity on circadian period should be considered when interpreting experiments on physiological manipulations of the circadian period.