Daily and seasonal rhythms of locomotor activity and adrenal function in male hedgehogs(Erinaceus europaeus L.)

Abstract

Male hedgehogs were maintained in outdoor parks, under natural climatic conditions. The general locomotor activity was recorded over one year in five hedgehogs, using an infra‐red producer‐receptor system registering entering or leaving the nest box in relation to time. Simultaneously, thecortico‐adrenal activity was studied monthly by sampling without disturbing, at 4 h intervals over a period of 24 h, in groups of 6 animals with a catheter inserted in the left carotid artery. The corticosteroids were assayed using a competitive protein binding technique.

In our study, hedgehogs living in Western Central France showed:

(1) from February‐March to August‐September a daily nocturnal locomotor activity well synchronized with sunset and sunrise,

(2) between August‐September and February‐March an alternation of resting and activity periods which constitute the hibernating period. The activity periods occur still during the night, but there are no relationships with sunset or sunrise,

(3) from February to October, a nycthemeral rhythm of plasma corticosteroids wi th an evening maximum and a morning minimum, as in laboratory animals with nocturnal locomotor activity,

(4) after October, during hibernation, significant variations of plasma corticos teroids with an irregular.pattern (shifting of the maximum or biphasic profile),

(5) a clearly annual cycle of the daily plasma corticosteroids levels, with a considerable increase in autumn (maximum in December), a significant decrease in January and a minimum from January to May.

These daily and seasonal variations of locomotor activity and cortico‐adrenal activity are related to the seasonal variations of the environment.     

Circadian activity rhythms and sensitivity to noise in the Mongolian gerbil (Meriones unguiculatus)

Since consistent data on endogenous circadian rhythms of Mongolian gerbils are not available, the main aim of our study was to identify suitable conditions to receive stable and reproducible free-running rhythms of activity under different light intensities. Another objective was to determine the role of social cues as an exogenous zeitgeber in the absence of a light-dark (LD) cycle. We performed two long-term sets of experiments with adult male gerbils kept in climatic chambers under various photoperiods of at least 30 days each. In all cases, the time of lights on in the chambers differed from the daily starting hour of work in the animal house. Always, two animals per chamber were kept separately in cages with a running wheel while their activity was monitored continuously. During the first set, only three of eight animals developed intra- and interindividual variable free-running rhythms. The activity patterns seemed to be influenced by human activities outside, indicating high sensitivity to external factors. Subsequently, we damped the chambers and the room and restricted access to the room. In the following noise-reduced set, all gerbils developed comparable free-running rhythms of activity. We determined the mean of the free-running period tau, the activity-rest relationship alpha/theta and the amount of running wheel activity per day: tau = 23.7h +/- 0.08h under low light (5 lux) and 25.5h +/- 0.19h under high light intensities (450 lux); alpha/theta = 0.53 +/- 0.08 under 5 lux and 0.34 +/- 0.04 under 450 lux. The amount of daily activity was 12 times as high under 5 lux as under 450 lux. There was no indication that the two animals in one chamber socially synchronized each other. In conclusion, the pronounced rhythm changes in accordance with Aschoff's theory support the view that gerbils are mainly nocturnal animals.     

Influence of light intensity on plasma melatonin and locomotor activity rhythms in tench

Melatonin production by the pineal organ is influenced by light intensity, as has been described in most vertebrate species, in which melatonin is considered a synchronizer of circadian rhythms. In tench, strict nocturnal activity rhythms have been described, although the role of melatonin has not been clarified. In this study we investigated daily activity and melatonin rhythms under 12:12 light-dark (LD) conditions with two different light intensities (58.6 and 1091 microW/cm2), and the effect of I h broad spectrum white light pulses of different intensities (3.3, 5.3, 10.5, 1091.4 microW/cm2) applied at middarkness (MD) on nocturnal circulating melatonin. The results showed that plasma melatonin in tench under LD 12:12 and high light conditions displayed rhythmic variation, where values at MD (255.8 +/- 65.9 pg/ml) were higher than at midlight (ML) (70.7 +/- 31.9 pg/ml). Such a difference between MD and ML values was reduced in animals exposed to LD 12: 12 and low light intensity. The application of 1 h light pulses at MD lowered plasma melatonin to 111.6 +/- 3.2 pg/ml (in the 3.3-10.5 microW/cm2 range) and to 61.8 +/- 18.3 pg/ml (with the 1091.4 microW/cm2 light pulse) and totally suppressed nocturnal locomotor activity. These results show that melatonin rhythms persisted in tench exposed to low light intensity although the amplitude of the rhythm is affected. In addition, it was observed that light pulses applied at MD affected plasma melatonin content and locomotor activity. Such a low threshold suggests that the melatonin system is capable of transducing light even under dim conditions, which may be used by this nocturnal fish to synchronize to weak night light signals (e.g., moonlight cycles).     

Melatonin concentrations in the two jugular veins, and relationship with the seasonal reproductive activity in goats

The authors investigated whether melatonin concentrations vary between the two jugular veins and whether absolute (nocturnal) or relative (nocturnal/diurnal ratio) plasma melatonin concentrations are associated with seasonal reproductive activity measured by oestrus or ovulatory activity in Payoya goats. Thirty-two adult Payoya goats were penned under natural photoperiod. Oestrus activity was tested daily using aproned males—twice a week plasma was sampled for progesterone. Melatonin plasma concentrations were studied at each equinox and solstice of the year in jugular samples taken simultaneously by venipuncture. Nocturnal and diurnal plasma melatonin concentrations from each jugular vein were assessed in 3 and 2 plasma samples per goat, respectively, taken at hourly intervals in each period. No differences in melatonin concentrations between the two veins were observed, but there was a significant interaction (P < 0.001) between jugular vein and animal in nocturnal melatonin concentrations. There was no effect of sampling period on melatonin concentrations and the coefficient of correlation between sampling periods was very high. The analyses performed indicated that neither absolute nor relative melatonin concentrations were related with the dates of onset or end of ovulatory/oestrus activity. Therefore, we concluded that in goats (1) melatonin concentrations are highly variable between jugular veins in the same individual but not in the general population, (2) melatonin concentrations are highly repeatable for each individual, and (3) absolute and relative amplitudes of melatonin concentrations are not linked to the seasonal breeding activity in Mediterranean goats.     

Shortened seasonal photoperiodic cycles accelerate aging of the diurnal and circadian locomotor activity rhythms in a primate

The gray mouse lemur (Microcebus murinus), a prosimian primate, exhibits seasonal rhythms strictly controlled by photoperiodic variations. Previous studies indicated that longevity can be altered by long-term acceleration of seasonal rhythms, providing a model for assessing various aspects of aging. To assess the effect of aging and accelerated aging on the circadian system of this primate, we compared the circadian rhythm of the locomotor activity in adult mouse lemurs (2-4.5 years, n = 9), aged mouse lemurs (5-9 years, n = 10), and adult mouse lemurs that had been exposed from birth to a shortened seasonal photoperiodic cycle (2-4.5 years, n = 7). Compared to adult animals, aged mouse lemurs showed a significant increase in intradaily variability and an advanced activity onset. Aging was characterized by a decrease in amplitude, with both a decrease in nocturnal activity and an increase in daytime activity. When maintained in constant dim red light, aged animals exhibited a shortening of the free-running period (22.8 +/- 0.1 h) compared to adult animals (23.5 +/- 0.1 h). A 3- to 5-year exposure to an accelerated seasonal photoperiodic rhythm ("annual" duration of 5 months) in accelerated mouse lemurs produced disturbances of the locomotor activity rhythm that resembled those of aged mouse lemurs, whether animals were studied in entrained or in free-running conditions. The present study demonstrated a weakened and fragmented locomotor activity rhythm during normal aging in this primate. Increasing the number of expressed seasonal cycles accelerated aging of parameters related to circadian rhythmicity in adult animals.     

Entrainment of circadian locomotor activity rhythm of the nocturnal field mouse Mus booduga using daily injections of melatonin

In this paper, we report the effects of daily injections of melatonin on the locomotor activity rhythm of the nocturnal field mouse Mus booduga. The locomotor activity rhythm of 45 animals was first monitored in constant darkness (DD) of the laboratory for about 15 days. The animals were then divided into three groups (experimental, vehicle-treated control, and the nontreated control groups) and subjected to three different treatments. The animals from the experimental group (n=19) were administered daily a single subcutaneous (s.c.) injection of melatonin (1 mg/kg) for about 45 days. The vehicle treated controls (n=13) were administered daily injections of 50% dimethyl sulfoxide (DMSO) for about 45 days, and the nontreated controls (n=13) were handled similar to the other two groups without being administered injections. Following the treatments, the animals were maintained in DD for about 20 days, after which the experiments were terminated. A significantly larger percentage of animals from the experimental group either entrained or showed phase control to daily treatments, compared to the animals from the two control groups. These results suggest that externally administered melatonin can influence the phase of the circadian locomotor activity rhythm of M. booduga. The fact that none of the nontreated controls showed any sign of phase control to daily handling, clearly demonstrates that the entrainment or phase control in the melatonin treated group of animals is caused by melatonin alone and not due to handling.     

Interaction between photoperiod and an endogenous seasonal factor in influencing the diel locomotor activity of the benthic polychaete Nereis virens Sars

The locomotor activity of Nereis virens Sars associated with food prospecting was investigated in response to photoperiod and season using an actograph. Experimental animals which had been reared under natural photoperiods were exposed to two constant photoperiodic treatments, LD 16:8 and LD 8:16, in both the autumn and winter and in the absence of tidal entrainment. Autocorrelation analysis of rhythmicity showed that during the autumn, animals under the LD 16:8 photoperiod displayed a strong nocturnal rhythm of activity, whereas animals under the LD 8:16 photoperiod showed only a weak nocturnal activity rhythm. This is believed to represent an autumn feeding cessation that is triggered when the animals pass through a critical photoperiod LD(crit) <12:>12. Later in the winter, however, animals exposed to both photoperiodic treatments showed strong rhythms of foraging activity irrespective of the imposed photoperiod. It is suggested that the autumn cessation may maximize the fitness of N. virens, a spring-breeding semelparous organism, by reducing risk during gamete maturation, while spontaneous resurgence of activity after the winter solstice permits animals that are not physiologically competent to spawn to accrue further metabolic reserves. This response is believed to be initiated by a seasonal (possibly circannual) endogenous oscillator or interval timer.     

Analysis of the Mechanism underlying the Rhythm Reversal from Diurnal to Nocturnal in the Cricket Gryllus bimaculatus, with Special Reference to the Role of Serotonin

The cricket, Gryllus bimaculatus, shows a rhythm reversal from diurnal to nocturnal in about a week after the imaginal molt. In the present study, we investigated the role of serotonin (5-HT) in the rhythm reversal. The 5-HT content in the brain measured by HPLC equipped with an electrochemical detector gradually increased after the imaginal molt, and in fully nocturnal adults it was about 2 times of nymphal level. We then examined the effects of 5,7-dihydroxytryptamine (5,7-DHT), a selective neurotoxine to serotonergic neurons, on the locomotor rhythm. In most animals with 5,7-DHT (25 muM or 250 muM, 32.2 nl) injected into the brain, daytime activity significantly increased even after the rhythm reversal, while nighttime activity was not significantly affected, forming rather diurnal pattern. The serotonin content in the brain of animals injected with 250 muM 5,7-DHT was reduced by about 30%. On the basis of these results, possible involvement of 5-HT in the neural mechanism controlling the locomotor rhythm is discussed.     

Melatonin effects on food intake and activity rhythms in two fish species with different activity patterns: Diurnal (goldfish) and nocturnal (tench)

Melatonin has several known physiological functions, the main one being synchronization of daily and seasonal rhythms. In addition, melatonin has been reported to influence food intake and behavioral rhythms with varying results depending on the species. The aim of this research was to evaluate the effects of intraperitoneal melatonin injection on food intake and locomotor activity in two different fish species: goldfish (diurnal) and tench (nocturnal), under different light regimes: constant light (LL) conditions or LD 12:12, with melatonin administration at mid-light (ML), mid-dark (MD), and after a 1-h light pulse at MD. In addition to these acute tests, in the case of goldfish we also investigated the effects of daily melatonin administration for 1 week. Our results indicated that acute melatonin administration significantly decreased goldfish food intake (16-52% inhibition, depending on the light regime) and locomotor activity (55-100%), with the chronic treatment inducing a similar total food intake inhibition that persisted for 7 days. In tench, a nocturnal fish species, acute melatonin administration at MD and ML reduced food intake (37% and 29%, respectively), while locomotor activity was not affected at MD and slightly increased at ML. Taken together, these results indicated that melatonin reduced food intake in both species, while its effects on locomotor activity depended on the time of administration (light or dark phase) and the activity patterns of the species.     

In situ studies on heart rate and locomotor activity in the freshwater crayfish, Astacus astacus (L.) in relation to natural fluctuations in temperature and light intensity

1. By the aid of a computer technique, the in situ variations in heart rate and locomotor activity were recorded continuously for 7 days in sixteen individuals of the crayfish, Astacus astacus (Linnaeus 1758), in association with natural changes in water temperature and light intensity.
2. All sixteen crayfish investigated were found to express circadian rhythmicity in heart rate with a periodicity of ≈ 24 h. For 88% of the animals investigated, rhythmicity was significant ( P < 0.05). Ninety‐three per cent of the crayfish showed circadian rhythmicity in locomotor activity with a 24 ± 2 h periodicity and, for 67% of the animals, this rhythmicity was significant ( P < 0.05). In general, the expression of circadian rhythms was associated with nocturnal behaviour, heart rates and locomotor activity levels being higher at night than by day.
3. A positive correlation was observed between heart rate and locomotor activity. Temperature and light intensity exerted positive and negative influences on heart rate in A. astacus , respectively.
4. In the first experiment, 84% of the variation in heart rate could be explained by the changes in locomotor activity, temperature and light intensity. In the second experiment 35% of the variation in the heart rate was explained using these parameters.     

Seasonality of the endogenous activity rhythm in Talitrus saltator (Montagu) from a sandy beach in northeastern Tunisia

The endogenous activity of the sandhopper Talitrus saltator (Montagu) has been investigated in a population from a relatively exposed beach in northeast Tunisia. The animals were found to exhibit a nocturnal circadian rhythm of locomotor activity, with no evidence of a circatidal component. The rhythm was monitored in individual animals over the four seasons, under constant laboratory conditions and seasonal variation in free-running period; the signal-to-noise ratio and waveform of the rhythm have been investigated, as well as the incidence of rhythmic animals in the population. The variation in these rhythm parameters is considered in relation to changes in environmental conditions prevailing at the site of collection.     

Entrainment to light of circadian activity rhythms in tench (Tinca tinca)

The present article analyzes locomotor activity rhythms in Tinca tinca. To that end, three different experiments were conducted on 24 animals (20 g body weight) kept in pairs in 60-liter aquaria fitted with infrared sensors connected to a computer to continuously record fish movements. The first experiment was designed to study the endogenous circadian clock under free-running conditions [ultradian 40:40 min LD pulses and constant dark (DD)] and after shifting the LD cycle. Our results demonstrate that tench has a strictly nocturnal activity pattern, an endogenous rhythm being evident in 45.8% of the fish analyzed. The second experiment was conducted to test the influence of different photoperiods (LD 6:18, 12:12, 18:6, and 22:2) on locomotor activity, the results showing that even under an extremely long photoperiod, tench activity is restricted to dark hours. The third experiment examined the effect of light intensity on locomotor activity rhythms. When fish were exposed to decreasing light intensities (from 300:0 lux to 30:0, 3:0, and 0.3:0 lux) while maintaining a constant photoperiod (LD 12:12), the highest percentage of locomotor activity was in all cases associated with the hours of complete darkness (0 lux). In short, our results clearly show that (a) tench is a species with a strictly nocturnal behavior, and (b) daily activity rhythms gradually entrain after shifting the LD cycle and persist under free-running conditions, pointing to their circadian nature. However, light strongly influences activity rhythms, since (c) the length of the active phase is directly controlled by the photophase, and (d) strictly nocturnal behavior persists even under very dim light conditions (0.3 lux). The above findings deepen our knowledge of tench behavior, which may help to optimize the aquacultural management of this species, for example, by adjusting feeding strategies to their nocturnal behavior.     

Testosterone and photoperiod interact to regulate locomotor activity in male hamsters

Testicular size, plasma testosterone levels, copulatory behavior, and daily locomotor activity are reduced in male hamsters after 10 weeks of exposure to short days. The role of testosterone in the short day-induced decline in locomotor activity was investigated, determining whether or not photoperiod could alter the effect of testosterone on activity. Castrated adult hamsters were allowed to acclimate to running wheels (wired to digital counters) and then were kept on either long (L:D 14:10) or short (L:D 6:18) days for 60 days. On Day 60, half of the animals on each light cycle were implanted with 12-mm-long testosterone-filled Silastic capsules; half received empty capsules. Digital counting of wheel-running activity continued for another 140 days. Blood samples taken on Day 200 confirmed L:D 14:10 and L:D 6:18 testosterone-treated hamsters had equivalent plasma testosterone levels. After an initial decline in activity, L:D 14:10 animals exhibited a progressive rise in mean running activity (from ~2000 to ~5000 wheel revolutions per day) through 100 days after the initiation of testosterone treatment. In contrast, activity levels in testosterone-treated L:D 6:18 animals remained uniform (~2000 wheel revolutions per day) during this time, indicating exposure to short days rendered the hamsters less sensitive to the stimulatory effect of testosterone on activity. Of further interest was a marked increase in activity after 160–200 short days in animals treated with either testosterone-filled or empty capsules. It appears the total amount of daily locomotor activity in the hamster is modulated by circulating testosterone levels in a manner which is dependent upon the environmental photoperiod.     

Analysis of the locomotor activity of a nocturnal desert lizard (Reptilia: Gekkonidae: Teratoscincus scincus) under varying moonlight

1. This project seeks to identify determinants of the variation observed in the foraging behavior of predatory animals, especially in moonlight, using a lizard as a model. 2. Moonlight generally enhances the foraging efficiency of nocturnal visual predators and often depresses the locomotor activity of prey animals. Previous evidence has indicated for three different nocturnal species of smallish gecko lizards that they respond to moonlight by increasing their activity. 3. In this study some aspects of the foraging activity of the somewhat larger nocturnal psammophilous Teratoscincus scincus, observed near Repetek and Ashgabat, Turkmenistan, were significantly depressed by moonlight, while several confounding factors (sex, maturity, size, sand temperature, hour, prior handling and observer effect) were taken into account. 4. This behavioral difference may relate to the eye size of the various species. 5. Additionally, a novel method of analyzing foraging behavior shows that in this species the duration of moves increases the duration of subsequent stationary pauses. Measurement of locomotor speed, yielding an average speed of 220% of the maximum aerobic speed, indicates a need for these pauses. Secondarily, pause duration decreases the duration of subsequent moves, precluding escalation of move duration. 6. The results of this and related projects advocate the taking into account of physiological and environmental factors that may affect an animal's foraging behavior.     

Altered feeding response as a cause for the altered heartbeat rate and locomotor activity of Schistosoma mansoni-infected biomphalaria glabrata

Biomphalaria glabrata infected with Schistosoma mansoni for 33 days fed more often than uninfected snails. Whereas uninfected snails had nocturnal increases in feeding, snails with a 33-day-old infection of S. mansoni fed as often during the day as in the night. Using direct observation and film analysis, we found that feeding increased the heartbeat rate and locomotor activity of B. glabrata. When snails were allowed to feed ad lib., infected snails had higher heartbeat rates than uninfected snails both during the day (P less than 0.01) and the night (P less than 0.001). However, when the snails were deprived of food for 24 hr, infected snails had slightly higher heartbeat rates than uninfected snails only during the day (P less than 0.05). There was no difference between the heartbeat rates of feeding, infected snails and the heartbeat rates of uninfected snails that were starved for 8 hr, and then allowed to feed. Uninfected snails had nocturnal increases in heartbeat rate regardless of feeding schedule, but infected snails had greater nighttime heartbeat rate than daytime heartbeat rate only when they were not allowed to feed. Infected snails had less nocturnal locomotor activity than uninfected snails when feeding, but there was no difference between the locomotor activity of infected and uninfected snails when the snails were deprived of food for 24 hr. Absence of food also resulted in an increased nighttime to daytime ratio of locomotor activity of infected snails. These results suggest that the increased heartbeat rate and altered rhythms of heartbeat rate and locomotor activity in B. glabrata infected with S. mansoni for 33 days were caused by the altered feeding response of these snails.     

Intensity-dependent phase-adjustments in the locomotor activity rhythm of the nocturnal field mouse Mus booduga

The locomotor activity rhythm of the nocturnal field mouse Mus booduga was monitored under constant darkness (DD) and free-running periods (tau) were estimated. Following a free-run of about 15 days in DD, the animals were exposed to periodic light pulses (LPs) of various intensities (1 lux, 10 lux, 50 lux, 100 lux, and 1,000 lux) and 15 minutes duration for 65 days at intervals of 24 hours to investigate the influence of intensity of light on the phase-angle-difference (psi) between the onset of locomotor activity and the time of LP administration. The experimentally observed values of psi and tau for a LP of 1,000 lux intensity used for 15 minutes every 24 hr, showed a sigmoid shaped relationship with tau. This relationship was similar to that predicted based on the nonparametric model of entrainment, which uses the tau and the LP phase response curve (PRC) constructed using LP of similar duration and intensity. The functional nature of the relationship between psi and tau was not found to change significantly with increasing intensities of LP used to entrain the locomotor activity rhythm. However, psi was significantly modulated by the intensity of LP. These results suggest that the periodic sensitivity of the circadian pacemaker underlying the locomotor activity rhythm in the nocturnal field mouse M. booduga to LPs plays an important role in maintaining a characteristic psi with the zeitgeber and the psi changes in a light intensity-dependent manner.     

High-intensity endurance training increases nocturnal heart rate variability in sedentary participants

The effects of endurance training on endurance performance characteristics and cardiac autonomic modulation during night sleep were investigated during two 4-week training periods. After the first 4-week training period (3 x 40 min per week, at 75% of HRR) the subjects were divided into HIGH group (n = 7), who performed three high-intensity endurance training sessions per week; and CONTROL group (n = 8) who did not change their training. An incremental treadmill test was performed before and after the two 4-week training periods. Furthermore, nocturnal RR-intervals were recorded after each training day. In the second 4-week training period HIGH group increased their VO₂max (P = 0.005) more than CONTROL group. At the same time, nocturnal HR decreased (P = 0.039) and high-frequency power (HFP) increased (P = 0.003) in HIGH group while no changes were observed in CONTROL group. Furthermore, a correlation was observed between the changes in nocturnal HFP and changes in VO₂max during the second 4-week training period (r = 0.90, P < 0.001). The present study showed that the increased HFP is related to improved VO₂max in sedentary subjects suggesting that nocturnal HFP can provide a useful method in monitoring individual responses to endurance training.     

Residual circadian rhythmicity after bilateral lamina-medulla removal or optic stalk transection in the cricket, Gryllus bimaculatus

Bilateral optic stalk severance or lamina-medulla region removal were carried out in 47 adult male crickets Gryllus bimaculatus DeGeer. Effects of the operations on circadian locomotor activity were investigated under 12 h light: 12 h dark and at a constant temperature of 26°C. In the pre-operative days, 39 of the animals showed a typical nocturnal activity rhythm (normal rhythm), but the remaining 8 exhibited an atypical rhythm which is diurnal rather than nocturnal (abnormal rhythm). The operations eventually caused an arrhythmicity in all animals, suggesting that the crucial part of the central nervous system controlling the cricket circadian activity is located in the lamina-medulla region. However, in some of the post-operative crickets, the rhythm did not immediately disappear but persisted for a while: the diurnal increase of activity was observed up to 2 weeks in all 8 abnormal- and 4 normal-rhythm animals. In addition, 8 out of 39 normal-rhythm animals showed a single well-defined post-operative peak which occurred approximately in phase with the nocturnal peak prior to surgery. These results are discussed in relation to a possibility of involvement of the oscillatory structure outside the optic lobes.     

Different behavior of body temperature and total locomotor activity daily rhythms during light/dark cycle in stabled Oryctolagus cuniculus

In order to investigate the potential causal link between the rhythm of activity and body temperature, we simultaneously recorded rectal temperature and total locomotor activity in five clinically healthy female rabbits (blue Vienna breed), 12 week old and mean body weight 2.7 ± 0.3. Animals were housed in individual cages (90?×?50?×?35 cm) under natural 12/12 light/dark cycle. Total locomotor activity was monitored for 15 days by an activity data logger. On day 1, 5, 10, and 15 rectal temperature was recorded every 2 h for a 24-h period. Application of single cosinor method showed a nocturnal daily rhythm of rectal temperature with a range of oscillation of about 1 °C, acrophase after dusk and low robustness value. The daily rhythm of locomotor activity showed its acrophase in the middle of the scotophase and a high robustness value. This information improves the knowledge available on the circadian biology of rabbits useful in the evaluation of physiology of this species.