Seasonal patterns of body temperature in response to experimental photoperiod variation in a non-hibernating ground squirrel.

Unlike numerous other members of the holarctic Tribe Marmotini of the squirrel family (Sciuridae) that typically exhibit spontaneous bouts of torpor that progress into an annual season of hibernation, members of the genus Ammospermophilus (antelope ground squirrels) do not enter torpor, and they remain active throughout the year in nature. We have experimentally evaluated seasonal patterns of variation in the circadian rhythm of body temperature in captive A. leucurus over a two-and-a-half-year period by exposing groups to either a constant daily photoperiod of 12 h light or a seasonally changing photoperiod that cycled between a summer maximum of 16 h per day and a winter minimum of 8 h; ambient air temperature was maintained at 26 °C. All squirrels showed continuous, year-round diurnal locomotor activity, and the group exposed to seasonally changing photoperiod adjusted onset and end of activity to changes in duration of the photoperiod. Animals in both groups showed a marked circadian rhythm of core body temperature with a typical daytime level of about 38 °C and nighttime level of about 35 °C for most of each year, but the group exposed to naturally changing daylength surprisingly reduced the level of its circadian oscillation by about 2 °C at the winter seasonal extreme of shortest daily illumination to a daytime level about 36 °C and a nocturnal level of about 33 °C. Despite this modest experimentally induced reduction in the level of the circadian rhythm of body temperature, we conclude that A. leucurus shows an overall stable annual pattern of circadian rhythmicity of its core body temperature that is consistent with a lack of any other evidence that the species engages in torpor or hibernation.

Circadian rhythmicity of body temperature and metabolism.

This article reviews the literature on the circadian rhythms of body temperature and whole-organism metabolism. The two rhythms are first described separately, each description preceded by a review of research methods. Both rhythms are generated endogenously but can be affected by exogenous factors. The relationship between the two rhythms is discussed next. In endothermic animals, modulation of metabolic activity can affect body temperature, but the rhythm of body temperature is not a mere side effect of the rhythm of metabolic thermogenesis associated with general activity. The circadian system modulates metabolic heat production to generate the body temperature rhythm, which challenges homeothermy but does not abolish it. Individual cells do not regulate their own temperature, but the relationship between circadian rhythms and metabolism at the cellular level is also discussed. Metabolism is both an output of and an input to the circadian clock, meaning that circadian rhythmicity and metabolism are intertwined in the cell.

Diverse styles of running-wheel behavior in antelope ground squirrels.

The running-wheel behavior of white-tailed antelope squirrels was studied in the laboratory by quantitative analysis of wheel revolutions and by visual inspection of video recordings with the assistance of Google Nest's detection algorithm. There was great interindividual diversity of running styles, although no systematic difference was found between male and female squirrels. Some animals ran on the outside of the wheel instead of inside, some ran consistently inside the wheel but alternating directions every few seconds, some ran on a virtual wheel while avoiding the actual wheel and doing back flips in the air instead, and some ran around the cage and occasionally hit the wheel thus generating a stable record of wheel revolutions. On average, the squirrels woke up an hour after lights-on, started activity on the running wheel about 40 minutes later, ran for 10 hours covering a distance of 2 km, and fell asleep about an hour after lights-off. This pattern of running-wheel behavior partially resembles that of laboratory mice, but its extreme diversity is unique to this species.

Persistent homeothermy in large domestic mammals maintained under standard farming conditions.

Background Mammals, particularly large domestic ones, are considered to be year-round homeotherms. Surprisingly, however, very few long-term studies of the stability of core temperature of large domestic mammals have been conducted. Methods We monitored the auricular temperature and rectal temperature of goats, sheep, cows, and donkeys monthly for 2 years and compared their annual variation with their daily and day-to-day variations. Results Although ambient temperature varied from 8 °C in the winter to 30 °C in the summer, auricular temperature varied less than 0.5 °C, and rectal temperature varied less than 0.3 °C. We found that the daily oscillation in body temperature was up to three times as large as the day-to-day and month-to-month variations and that month-to-month variation was negligibly larger than day-to-day variation. Conclusion Our results confirm persistent homeothermy in large domestic mammals with unrestricted access to food and water and provide a quantitative measure of the relationship between long-term homeothermy and the wider range of daily/circadian oscillation.

Chronotype Variability and Patterns of Light Exposure of a Large Cohort of United States Residents.

Chronotype (i.e., disposition for activity early or late in the day) has traditionally been measured with questionnaires. A few studies with small sample sizes have also been conducted using actigraphy devices. In the present study, analysis was conducted of the daily pattern of activity of 1887 United States residents who wore actigraphy devices for a whole week. The devices also recorded the participants' exposure to light. As determined by cosinor analysis, the mean pattern of ambulatory activity exhibited robust 24-hour oscillation with a peak at 14:48. On average, participants went to sleep 2 minutes before midnight and woke up at 07:43. The distribution of chronotypes (defined as the midpoint of sleep) had a mean of 03:50, and 95% of all chronotypes were between 01:00 and 07:00. The mean duration of exposure to bright daylight was 3.57 hours per day. Duration of daily exposure to bright light was moderately but significantly correlated with chronotype (r = -0.18). The acrophase of the rhythm of exposure to bright light was significantly correlated with chronotype (r = 0.27) and with the acrophase of the activity rhythm (r = 0.36). Chronotype did not vary with the seasons, but exposure to bright light was longer in summer and spring than in winter and fall. These results confirmed chronotype findings from actigraphic studies with smaller sample sizes, endorsed their equivalence to the results of questionnaire studies, and confirmed and extended previous observations that urban dwellers have limited daily exposure to sunlight.

Exploring determinants of behavioral chronotype in a diurnal-rodent model of human physiology.

Numerous studies conducted with human participants have shown that differences in chronotype, defined as individual patterns of early or late beginning of daily activity, have implications for many biobehavioral processes, such as cognitive performance, mood, impulsivity, academic achievement of college students, and mental health. However, the determinants of individual variation in chronotype have not been investigated. Basic research on circadian rhythms has provided a basis for investigating the causes of chronotype variation, but experimental tests of pertinent hypotheses are difficult to conduct with human subjects. This limitation can be overcome by use of animal models. This study was conducted with a rodent species, the antelope ground squirrel (Ammospermophilus leucurus), that, like humans, is active during the daytime, exhibits a spread of chronotypes, and has a similar average free-running circadian period. We found chronotype to be a stable trait within individuals based on strong consistency of separate determinations made six months apart (correlation r = 0.91). We also found a moderate correlation of chronotype with the duration of the active phase (r = -0.51) and with free running period (r = 0.34), but weak correlation with rhythm robustness (r = 0.16), and no correlation with photic responsiveness or with masking responses. The best multiple regression model, incorporating the duration of the active phase, free-running period, and rhythm robustness, explained 38% of the variance in chronotype. Although 62% of the variance in chronotype remained unaccounted for, the results are encouraging because they document the possibility of using a diurnal rodent as a model for the investigation of the determinants of chronotype variation in humans.

Focal Epilepsy and the Clock Gene.

Investigators from Children's National Medical Center, Wenzhou Medical University, Virginia Tech, University of Colorado, University of Virginia, Georgetown University, University of Maryland, and Brown University performed transcriptome analysis on human epileptogenic tissue and extended the investigation by creating and testing mouse lines with targeted genetic deletions of the Clock gene.

Diurnally active rodents for laboratory research.

Although inbred domesticated strains of rats and mice serve as traditional mammalian animal models in biomedical research, the nocturnal habits of these rodents make them inappropriate for research that requires a model with human-like diurnal activity rhythms. We conducted a literature review and recorded locomotor activity data from four rodent species that are generally considered to be diurnally active, the Mongolian gerbil ( Meriones unguiculatus), the degu ( Octodon degus), the African (Nile) grass rat ( Arvicanthis niloticus), and the antelope ground squirrel ( Ammospermophilus leucurus). Our data collected under 12-hour light/dark cycles confirmed and expanded the existing literature in showing that the activity rhythms of antelope ground squirrels and African grass rats are stronger and more concentrated in the light phase of the light/dark cycle than the activity rhythms of Mongolian gerbils and degus, making the former two species preferable and more reliable as models of consistent diurnal activity in the laboratory. Among the two more strongly diurnal species, antelope ground squirrels are more exclusively diurnal and have more robust activity rhythms than African grass rats. Although animals of these two species are not currently available from commercial suppliers, African grass rats are indigenous to a wide area across the north of Africa and thus available to researchers in the eastern hemisphere, whereas antelope ground squirrels can be found throughout much of western North America's desert country and, therefore, are more easily accessible to North American researchers.

Circadian rhythms of body temperature and locomotor activity in the antelope ground squirrel, Ammospermophilus leucurus.

We studied circadian rhythms of body temperature and locomotor activity in antelope ground squirrels (Ammospermophilus leucurus) under laboratory conditions of a 12L:12D light-dark cycle and in constant darkness. Antelope ground squirrels are diurnally active and, exceptionally among ground squirrels and other closely related members of the squirrel family in general, they do not hibernate. Daily oscillations in body temperature consisted of a rise in temperature during the daytime activity phase of the circadian cycle and a decrease in temperature during the nighttime rest phase. The body temperature rhythms were robust (71% of maximal strength) with a daily range of oscillation of 4.6°C, a daytime mean of 38.7°C, and a nighttime mean of 34.1°C (24-h overall mean 36.4°C). The body temperature rhythm persisted in continuous darkness with a free-running period of 24.2h. This pattern is similar to that of hibernating species of ground squirrels but with a wave form more similar to that of non-hibernating rodents. Daily oscillations in body temperature were correlated with individual bouts of activity, but daytime temperatures were higher than nighttime temperatures even when comparing short episodes of nocturnal activity that were as intense as diurnal activity. This suggests that although muscular thermogenesis associated with locomotor activity can modify the level of body temperature, the circadian rhythm of body temperature is not simply a consequence of the circadian rhythm of activity.

Daily rhythmicity of behavior of nine species of South American feral felids in captivity.

The authors analyzed the daily activity rhythms of the domestic cat and of eight of the ten feral felid species that are indigenous to South America. All species showed daily rhythmicity of activity in captivity under a natural light-dark cycle. The robustness of the rhythmicity varied from species to species, but the grand mean of 34% was within the range of robustness previously described for mammalian species ranging in size from mice to cattle. There was not a sharp division between diurnal and nocturnal felids. Instead, what was found was a gradient of diurnality going from the predominantly nocturnal margay (72% of activity counts during the night) to the predominantly diurnal jaguarundi (87% of activity counts during the day) with the remaining species lying in between these two extremes. The ecological implications of temporal niche variations are discussed.

Chronotype, class times, and academic achievement of university students.

Numerous studies over the years have documented an effect of human chronotypes on physiological and psychological processes. Studies evaluating the impact of an individual's chronotype on his/her academic achievement have indicated that morning chronotypes have an academic advantage over evening chronotypes. However, these studies did not account for the time of day in which the participants were being evaluated. The goal of the present study was to examine whether morning chronotypes do have an academic advantage over evening chronotypes when the time of day of classes and exams is taken into consideration. We obtained morningness-eveningness scores and course grades from 207 university students who took classes (and exams) at different times of the day. We confirmed that morning chronotypes attain better grades than evening chronotypes, although the association is weak (r2 = 0.02). The difference persisted even after the time of day of classes and exams was taken into consideration. This is probably due to the fact that evening chronotypes are generally more sleep deprived than morning chronotypes as a result of the early schedule of most schools, which can impair their performance both early and late in the day.

Daily rhythms of physiological parameters in the dromedary camel under natural and laboratory conditions.

Camels are well adapted to hot arid environments and can contribute significantly to the economy of developing countries in arid regions of the world. Full understanding of the physiology of camels requires understanding of the internal temporal order of the body, as reflected in daily or circadian rhythms. In the current study, we investigated the daily rhythmicity of 20 physiological variables in camels exposed to natural oscillations of ambient temperature in a desert environment and compared the daily temporal courses of the variables. We also studied the rhythm of core body temperature under experimental conditions with constant ambient temperature in the presence and absence of a light-dark cycle. The obtained results indicated that different physiological variables exhibit different degrees of daily rhythmicity and reach their daily peaks at different times of the day, starting with plasma cholesterol, which peaks 24min after midnight, and ending with plasma calcium, which peaks 3h before midnight. Furthermore, the rhythm of core body temperature persisted in the absence of environmental rhythmicity, thus confirming its endogenous nature. The observed delay in the acrophase of core body temperature rhythm under constant conditions suggests that the circadian period is longer than 24h. Further studies with more refined experimental manipulation of different variables are needed to fully elucidate the causal network of circadian rhythms in dromedary camels.

Variability of behavioral chronotypes of 16 mammalian species under controlled conditions.

Human chronotypes (differences in preference for early or late rising each day) have been extensively studied in recent years, but no attempt has been made to compare human chronotypes with the chronotypes of other animal species. We evaluated behavioral chronotypes in 16 mammalian species along a body size gradient of five orders of magnitude (from mice to cattle). Individuals of all species were studied under a 12L:12D photoperiod in a thermoneutral environment with food and water available at all times. Rhythms of locomotor activity were analyzed for onset time, acrophase, and robustness. Neither of these rhythmic parameters was significantly related to body size, but onset time and acrophase varied considerably from species to species, thus characterizing diurnal and nocturnal species. Chronotype spreads ranged from less than an hour in sheep to almost 24h in cats, thus extending both below and above the human chronotype spread of 6h. The variability of chronotype (as quantified by the standard deviation of group means) was much larger between species than within species and also larger between individuals of a species than within individuals on consecutive days. These results help situate the matter of human chronotypes within the broader context of variability in the phase angle of entrainment of circadian rhythms in animals.

Daily Activity and Nest Occupation Patterns of Fox Squirrels (Sciurus niger) throughout the Year.

The authors investigated the general activity and nest occupation patterns of fox squirrels in a natural setting using temperature-sensitive data loggers that measure activity as changes in the microenvironment of the animal. Data were obtained from 25 distinct preparations, upon 14 unique squirrels, totaling 1385 recording days. The animals were clearly diurnal, with a predominantly unimodal activity pattern, although individual squirrels occasionally exhibited bimodal patterns, particularly in the spring and summer. Even during the short days of winter (9 hours of light), the squirrels typically left the nest after dawn and returned before dusk, spending only about 7 hours out of the nest each day. Although the duration of the daily active phase did not change with the seasons, the squirrels exited the nest earlier in the day when the days became longer in the summer and exited the nest later in the day when the days became shorter in the winter, thus tracking dawn along the seasons. During the few hours spent outside the nest each day, fox squirrels seemed to spend most of the time sitting or lying. These findings suggest that fox squirrels may have adopted a slow life history strategy that involves long periods of rest on trees and short periods of ground activity each day.

Effects of sodium nitroprusside on mouse erythrocyte catalase activity and malondialdehyde status.

There is controversy about the anti- or pro-oxidative effects of the nitric oxide (NO)-donor sodium nitroprusside (SNP). Hence, the activity of the antioxidant enzyme catalase (CAT) and the status of malondialdehyde (MDA) were investigated after a 2.5 mg/kg dose of SNP had been i.p. administered to different and comparable groups of mice (n = 48). The drug was administered at two different circadian times (1 and 13 h after light onset [HALO]). There were, irrespectively of sampling time, no significant differences in the means of CAT activity and MDA status between control and SNP-treated groups, no matter the treatment time. However, CAT activity was significantly (Student's t-test, p < 0.001) increased 1 h following SNP administration at 1 HALO, whereas the significant (p < 0.001) increase in the enzyme activity was found only 3 h after injection at 13 HALO. The drug dosing either at 1 or 13 HALO resulted in no significant differences of MDA status between control and treated groups regardless to the sampling time. Two-way analysis of variance (ANOVA) detected a significant (F0.05(7,88)= 5.3; p < 0.0006) interaction between sampling time and treatment in mice injected at 1 HALO, suggesting the influence of treatment on sampling-time-related changes in CAT activity. However, ANOVA validated no interaction between the two factors in mice treated at 13 HALO, illustrating that the sampling-time differences in enzyme activity were greater. Furthermore, two-way ANOVA revealed no interaction in the variation of MDA status in animals treated either at 1 or 13 HALO. This study indicates that SNP significantly affected the anti-oxidant system.

Evidence for daily and weekly rhythmicity but not lunar or seasonal rhythmicity of physical activity in a large cohort of individuals from five different countries.

BACKGROUND: Biological rhythmicity has been extensively studied in animals for many decades. Although temporal patterns of physical activity have been identified in humans, no large-scale, multi-national study has been published, and no comparison has been attempted of the ubiquity of activity rhythms at different time scales (such as daily, weekly, monthly, and annual). METHODS: Using individually worn actigraphy devices, physical activity of 2,328 individuals from five different countries (adults of African descent from Ghana, South Africa, Jamaica, Seychelles, and the United States) was measured for seven consecutive days at different times of the year. RESULTS: Analysis for rhythmic patterns identified daily rhythmicity of physical activity in all five of the represented nationalities. Weekly rhythmicity was found in some, but not all, of the nationalities. No significant evidence of lunar rhythmicity or seasonal rhythmicity was found in any of the groups. CONCLUSIONS: These findings extend previous small-scale observations of daily rhythmicity to a large cohort of individuals from around the world. The findings also confirm the existence of modest weekly rhythmicity but not lunar or seasonal rhythmicity in human activity. These differences in rhythm strength have implications for the management of health hazards of rhythm misalignment.

Daily activity patterns of 2316 men and women from five countries differing in socioeconomic development.

Daily rhythmicity in the locomotor activity of laboratory animals has been studied in great detail for many decades, but the daily pattern of locomotor activity has not received as much attention in humans. We collected waist-worn accelerometer data from more than 2000 individuals from five countries differing in socioeconomic development and conducted a detailed analysis of human locomotor activity. Body mass index (BMI) was computed from height and weight. Individual activity records lasting 7 days were subjected to cosinor analysis to determine the parameters of the daily activity rhythm: mesor (mean level), amplitude (half the range of excursion), acrophase (time of the peak) and robustness (rhythm strength). The activity records of all individual participants exhibited statistically significant 24-h rhythmicity, with activity increasing noticeably a few hours after sunrise and dropping off around the time of sunset, with a peak at 1:42 pm on average. The acrophase of the daily rhythm was comparable in men and women in each country but varied by as much as 3 h from country to country. Quantification of the socioeconomic stages of the five countries yielded suggestive evidence that more developed countries have more obese residents, who are less active, and who are active later in the day than residents from less developed countries. These results provide a detailed characterization of the daily activity pattern of individual human beings and reveal similarities and differences among people from five countries differing in socioeconomic development.

Dosing-time dependent effects of sodium nitroprusside on cerebral, renal, and hepatic catalase activity in mice.

To investigate the time dependence of sodium nitroprusside- (NPS-) induced oxidative effects, the authors study the variation of the antioxidant enzyme CAT activity in various tissues after the administration of a single 2.5 mg/kg dose of SNP or sodium chloride (NaCl 0.9%). For each of the two dosing times (1 and 13 hours after light onset, HALO, which correspond to the beginning of diurnal rest span and of nocturnal activity span of mice, resp.), brain, kidney, and liver tissues were excised from animals at 0, 1, 3, 6, 9, 12, 24, and 36 h following the drug administration and CAT activity was assayed. The results suggest that SNP-induced stimulation of CAT activity is greater in all three tissues when the drug is administered at 1 HALO than at 13 HALO. Two-way ANOVA revealed that CAT activity significantly (P < 0.004) varied as a function of the sampling time but not of the treatment in all three tissues. Moreover, a statistically significant (P < 0.004) interaction between the organ sampling-time and the SNP treatment was revealed in kidney regardless of the dosing time, whereas a highly significant (P < 0.0002) interaction was validated in liver only in animals injected at 13 HALO.

Comparison of light, food, and temperature as environmental synchronizers of the circadian rhythm of activity in mice.

Although entrainment (and masking) of circadian rhythms by light has been extensively studied, much less attention has been given to other environmental cycles that can modulate circadian rhythms in mammals. In this study in mice, the entraining strength of different environmental cycles was compared. Running-wheel activity was monitored before, after, and while the animals were under one of four environmental cycles: a full light-dark cycle with 12 h of light and 12 h of darkness each day, a cycle of 1 h of light per day, a cycle of food availability consisting of 80% of the baseline free-feeding amount presented once a day, and an ambient temperature cycle consisting of 23 h at 24 °C and 1 h at 12 °C each day. Four measures of zeitgeber strength were used: percentage of animals that entrained, rhythm robustness in the entrained state, stability of activity onsets, and stability of acrophases. The results indicate that, at least in mice, a full light-dark cycle is the most powerful modulator of the circadian rhythm of locomotor activity, as a consequence of both entrainment and masking. When entrainment alone is considered, temperature seems to be as strong a modulator as light, while food restriction is a weaker modulator and affects primarily a food-anticipatory component of the activity rhythm.