In an attempt to evaluate the importance of individual daily habits to a freeliving animal, forag... more In an attempt to evaluate the importance of individual daily habits to a freeliving animal, foraging behaviour of kestrels was observed continuously for days in sequence in open country. Data obtained in 2,942 observation hours were used. Flight-hunting was the prominent foraging technique yielding 76% of all prey obtained. Flight-hunting was impeded by rain, fog and wind speeds below 4 m/s and above 12 m/s (Fig. 3). Flight-hunting tended to be suppressed also in response to recent successful strikes and more generally by a high level of post-dawn accumulated prey (Figs. 4, 5). Flight-hunting had a tendency to be enhanced in response to recent unsuccessful strikes (Fig. 6). Trapping results demonstrated a fine-grained daily pattern of common vole trap entries, with peaks at intervals of ca. 2 h (Figs. 7, 8). The interpretation of some of this pattern as representative of vole surface activity was supported by overall strike frequencies of kestrels hunting for voles (Fig. 9). Detailed analysis of the behaviour of three individuals revealed significant peaks in hunting yield and frequency, coinciding with each other and with peaks in vole trapping (Fig. 11). It is suggested that the kestrels adjusted their flight-hunting sessions to times of high ‘expected’ yield. Vole activity peaks sometimes remained unexploited. Meal frequencies culminated shortly before nightfall except in incubating females. The difference between the daily distributions of hunting and eating was due to some of the prey being cached in daytime and retrieved around dusk (Fig. 13). Caching behaviour is interpreted as a circadian strategy allowing separate optimization of hunting-adjusted to prey availability-and eating-adaptive by retaining minimum body weight in daytime flight and by thermo-regulatory savings at night. Some kestrels showed remarkable constancy from day to day in the temporal distribution of specific behaviours (Fig. 16) and of spatial movements (Figs. 18, 19). In three 1–2 week sequences of observation analysed, flight-hunting frequency peaked 24 h after prey capture (Fig. 17). This is probably based on day to day correlations in flight-hunting frequency as well as on increased motivation for hunting in response to prey capture 24 h ago (Table 5). In one individual with three distinct hunting areas, the tendency to return to an area again was maximal 24 h after prey capture in that area (Fig. 21, Table 6). A field experiment tested the effect of prey capture on the daily distributions of hunting and site choice in this individual (Fig. 22). A significant concentration of flight-hunting activity in the experimental feeding area was observed at the daily time of feeding (Fig. 23). Two alternative hypotheses are compatible with the result. Favoured is the one that the birds use “time memory” for the optimization of their daily patterns of flight-hunting and site choice. By adjusting her daily flight-hunting to times of high yield, one kestrel saved 10–22% on her total time spent flight-hunting. Maximal efficiency, by concentration of all hunting activity in the hour of maximal yield, was not attained, presumably because of information constraints. The generality of the contribution of daily habits to survival is discussed.
We studied the limiting factors for brood size in the kestrel, Falco tinnunculus, by measuring pa... more We studied the limiting factors for brood size in the kestrel, Falco tinnunculus, by measuring parental effort in natural broods of different size and parental response to manipulation of food satiation of the brood. Parental effort was quantified as total daily time spent in flight, and total daily energy expenditure, from all-day observations. During nestling care males with different natural brood sizes (4 to 7 chicks), spent an average of 4.75 h . d-' in flight independent of brood size, and expended an average total daily energy of 382 kJ d-'. Due to a higher flight-hunting yield (mammal-prey caught per hour hunting), males with larger natural broods were able to provision their broods with the same amount of food (mainly Microtus arvalis) per chick (62.6 g . d-'), with the same effort as males with smaller broods. This provisioning rate was close to the mean feeding rate of hand-raised chicks in the laboratory, that were fed ad libitum, (66.8 g . d-' . chick-').
The relationship between body size and basal metabolic rate (BMR) in homeotherms has been treated... more The relationship between body size and basal metabolic rate (BMR) in homeotherms has been treated in the literature primarily by comparison between species of mammals or birds. This paper focuses on the intraindividual changes in BMR when body mass (W) varies with different maintenance regimens. BMR varied in individual kestrels in proportion to W 1.67 , which is considerably steeper than the mass exponents for homomorphic change (0.667; Heusner, 1984) for interspecific comparison among all birds (0.677) or raptors (0.678), for interindividual comparison of kestrels on ad libitum maintenance regimens (0.786), and for mass proportionality (1.00). The circadian range of telemetered core temperature also varied more strongly with intraindividual than with interspecific (Aschoff, 1981a) variation in mass. This was due to reduced nocturnal core temperature at low-maintenance regimens, which was, however, insufficient to account for the excessive reduction in BMR. Carcass analysis of eight birds sacrificed revealed a disproportionate reduction in heart and kidney lean mass at low-maintenance regimens. We surmise that variation in BMR primarily reflects variation in these metabolically highly active tissues. This may account for positive correlations found between heart, kidney, and BMR residuals relative to interspecific allometric prediction, and between α and p residuals, as expected on the basis of the constant excess of BMR during α above BMR during p .
Proceedings of The Royal Society B: Biological Sciences, 1996
Each copy of any part of a JSTOR transmission must contain the same copyright notice that appears... more Each copy of any part of a JSTOR transmission must contain the same copyright notice that appears on the screen or printed page of such transmission.
Costs of reproduction have been assessed experimentally by measuring subsequent survival and repr... more Costs of reproduction have been assessed experimentally by measuring subsequent survival and reproduction of parent animals raising enlarged and reduced numbers of offspring. Reported effects on survival have so far always referred to local survival of marked individuals in the study population. They do not provide definitive proof of a cost of reproduction, since reduced local survival may be due either to reduced survival or to an increased tendency to emigrate from the study area. Therefore, it is important to assess mortality rates in connection with brood size experiments. We report here an analysis of time of death in 63 cases where Kestrels, Falco tinnunculus L. had raised broods of manipulated size and were subsequently reported freshly dead. 60 % of the parents raising two extra nestlings were reported dead before the end of the first winter, compared to 29 % of those raising control or reduced broods. This result confirms our interpretation of the manipulation effects on local survival as due to mortality rather than emigration. The extra mortality occurred in the winter following the brood enlargements. Kestrel parents in these experiments have been shown to adjust their daily energy expenditure to the modified brood size. Increased parental effort in this species thus entails an increased risk of death half a year later.
We present an extensive set of data for five species of raptorial birds to demonstrate that some ... more We present an extensive set of data for five species of raptorial birds to demonstrate that some raptor species produce an excess of daughters early in the season and an excess of sons in late nests, while others show the reverse. By means of a simulation model we investigate an evolutionary explanation for this phenomenon in terms of sex-specific differences in the relation between age at first breeding and date of birth. The model predicts diat that gender should be produced first in the season whose age of first breeding is more strongly accelerated by an early birth date. We argue that this tends to be the male gender in raptor species, such as the common kestrel (Falco tinnunculus), which tend to breed early in life, while it is the female gender in larger species with later onset of breeding, such as the marsh harrier (Circus aeruginosus). The empirical evidence is qualitatively consistent with this hypothesis. Our model is quite general in that it makes no assumptions about the mechanism (primary sex-ratio bias at egg laying or secondary sex-differential mortality before fledging) by which the bias is generated. Yet it is able to create quantitative predictions for species where sufficient demographic and life-history data are available. From die available data set in the common kestrel we derive a quantitative prediction for the seasonal trend in brood sex ratio. The observed trend is in good agreement with this prediction.
Effects of hypothalamic lesions on the ultradian and circadian organization of wheel running and ... more Effects of hypothalamic lesions on the ultradian and circadian organization of wheel running and feeding were studied in the common vole, Microtus arvalis. Circadian organization broke down within 30 days in continuous darkness in 24% of intact voles (n = 135). Ultradian rhythmicity of feeding (period 2-3 hr) persisted in constant conditions in all intact voles. Following lesions of the suprachiasmatic nuclei (SCN), circadian rhythmicity disappeared when lesions were complete (n = 8) or more extensive than 25% of the total SCN volume (n = 5). Absence of circadian rhythmicity was also found in animals with substantial lesions in the diencephalic paraventricular area (PVA) and in the retrochiasmatic area (RCA) and/or adjacent arcuate nucleus (Arc). Complete loss of ultradian and circadian organization occurred in eight voles with damage to the RCA and/or Arc. In three of these, the SCN was intact. The SCN is a likely candidate for a circadian pacemaker in voles (as in other rodents), while the loss of circadian rhythmicity following PVA and RCA/Arc lesions may be due to destruction of efferent pathways from the SCN. The RCA/Arc area is apparently necessary for the expression of ultradian rhythms. The intact SCN is neither necessary nor sufficient for the generation of ultradian rhythmicity.
The accuracy with which a circadian pacemaker can entrain to an environmental 24-h zeitgeber sign... more The accuracy with which a circadian pacemaker can entrain to an environmental 24-h zeitgeber signal depends on (a) characteristics of the entraining signal and (b) response characteristics and intrinsic stability of the pacemaker itself. Position of the sun, weather conditions, shades, and behavioral variations (eye closure, burrowing) all modulate the light signal reaching the pacemaker. A simple model of a circadian pacemaker allows researchers to explore the impact of these factors on pacemaker accuracy. Accuracy is operationally defined as the reciprocal value of the day-to-day standard deviation of the clock times at which a reference phase (0) is reached. For the purpose of this exploration, the authors used a model pacemaker characterized solely by its momentary phase and momentary velocity. The average velocity determines the time needed to complete one pacemaker cycle and, therefore, is inversely proportional to pacemaker period. The model pacemaker responds to light by shifting phase and/or changing its velocity. The authors assumed further that phase and velocity show small random fluctuations and that the velocity is subject to aftereffects. Aftereffects were incorporated mathematically in a term allowing period to contract exponentially to a stable steady-state value, with a time constant of 69 d in the absence of light. The simulations demonstrate that a pacemaker reaches highest accuracy when it responds to light by simultaneous phase shifts and changes of its velocity. Phase delays need to coincide with slowing down and advances with speeding up; otherwise, no synchronization to the zeitgeber occurs. At maximal accuracy, the changes in velocity are such that the average period of the pacemaker under entrained conditions equals 24 h. The results suggest that during entrainment, the pacemaker adjusts its period to 24 h, after which daily phase shifts to compensate for differences between the periods of the zeitgeber and the clock are no longer necessary. On average, phase shifts compensate for maladjustments of phase and velocity changes compensate for maladjustments of period.
Proceedings of The National Academy of Sciences, 1975
Castration of mice in freerunning conditions (total darkness, DD) causes a reduction of running w... more Castration of mice in freerunning conditions (total darkness, DD) causes a reduction of running wheel activity in the beginning of the active period (a) and stimulates activity at the end of a. Simultaneously, the period (r) of the freerunning rhythm is increased. Both effects are abolished by implantation of a Silastic capsule from which a physiological dose of testosterone is released at a constant rate. The results are tentatively explained by differential endocrine influences on two oscillating components in the pacemaker of the circadian activity rhythm. Early work on endocrine involvement in circadian rhythms was mainly aimed at the localization of the clock within the various endocrine glands or within a cycle of nervous-endocrine-metabolic events (1). Much of the research was done in animals exposed to daily light-dark cycles, which precludes evidence on the endogenous nature of the rhythmic phenomena observed. From a series of experiments on blinded rats, Richter (2) stated that freerunning circadian activity rhythms were not impaired by any of the following interferences with the endocrine system: gonadectomy, adrenalectomy, hypophysectomy, hyperor hypothyroidism, and pinealectomy. Apparently, the circadian clock driving the activity-rest cycle was not located in any of the glands involved. The demonstration by Andrews (3) that rat adrenals cultured in vitro continue to produce corticosteroids in a circadian rhythmic fashion considerably modified existing views in the field concerning the regulating function of a localized clock (4). Evidently, since at least one organ, and probably more, is capable of self-sustained oscillations, a centralized "clock" or "pacemaker" would serve to synchronize a number of peripheral oscillators rather than impose its rhythm on them. For instance, a circadian pacemak-
Journal of Comparative Physiology A-neuroethology Sensory Neural and Behavioral Physiology, 1976
Phase response curves for 15′ bright light pulses of four species of nocturnal rodents are descri... more Phase response curves for 15′ bright light pulses of four species of nocturnal rodents are described. All show delay phase shifts early in the subjective night, advance shifts in the late subjective night, and relative insensitivity during the subjective day. The broad scatter in measured phase-shifts is largely due to error of measurement: the response of the pacemakers to light stimuli is more accurate than we observe. Indications are found that the response to a resetting stimulus at a given phase of the rhythm is correlated with the individual \(\bar \tau \) (freerunning period). Fast pacemakers (short \(\bar \tau \) ) tend to be more delayed or less advanced by the light than slow pacemakers (long \(\bar \tau \) ). Within individual mice (Mus musculus) the circadian pacemaker adjusts its resetting response to variations in its frequency: when τ is long (induced as after-effect of prior light treatment) light pulses at a defined phase of the oscillation (ct 15) produce smaller delay phase shifts than when τ is short. Among species there are conspicuous differences in the shape of the phase response curve: where \(\bar \tau \) is long, advance phase shifts are large and delay phase shifts small (Mesocricetus auratus); where \(\bar \tau \) is short, advance shifts are small, and delay shifts are large (Mus musculus;Peromyscus maniculatus). The functional meaning of the interrelationships of τ and PRC is briefly discussed.
Journal of Comparative Physiology A-neuroethology Sensory Neural and Behavioral Physiology, 1976
The circadian pacemakers controlling activity rhythms in four species of rodents are compared, as... more The circadian pacemakers controlling activity rhythms in four species of rodents are compared, as freerunning systems in constant darkness. In analyzing their stability the distinction is made between (1) spontaneous day-to-day instability of frequency, and (2) a longer-term lability, some of which is traceable to identified causes. Serial correlation analysis indicates that the precision (day-to-day stability) of the pacemaker's period is ca. twice as good (estimated s.d.=0.6% of \(\bar \tau \) inMus musculus) as the already remarkable precision of the activity rhythm it drives (average s.d.=1.2% of \(\bar \tau \) ). Identifiable causes of long-term lability include age and several features of prior entrainment by light. The period and photoperiod of a light cycle have a predictable influence on the subsequent freerunning period (τ) of the pacemaker; they cause “after-effects”. So do single light pulses causing a phase-shift in the freerunning system. Constant light also has an after-effect opposite in sign from the after-effect of long photoperiods. After-effects of “skeleton” photoperiods support the hypothesis that the transitions of light to darkness vv. are involved in the entrainment process which leads to changes in τ. Both day-to-day instability and long term lability are most pronounced in species (Peromyscus maniculatus, Mus musculus) whose \(\mathop \tau \limits^ = \) is considerably shorter than 24 h; they are least pronounced in hamsters whose \(\mathop \tau \limits^ = \) is indistinguishably close to 24 h. The differences between the species in τ and its lability are paralleled by differences in pacemaker lability as measured in light-induced after-effects and in the extent of changes with age. The species evidently differ in the “tightness” with which τ is homeostatically conserved.
Journal of Comparative Physiology A-neuroethology Sensory Neural and Behavioral Physiology, 1976
In thefirst part of the paper, the model of non-parametric entrainment of circadian pacemakers is... more In thefirst part of the paper, the model of non-parametric entrainment of circadian pacemakers is tested for the case of nocturnal rodents. The model makes use of the available data on freerunning period (τ) in constant darkness (Pittendrigh and Daan, 1976a) and on phase response curves (PRC) for short light pulses (Daan and Pittendrigh, 1976a). It is tested in experiments using 1 or 2 light pulses per cycle. Mesocricetus auratus andPeromyscus leucopus entrain to Zeitgebers involving 1 pulse (15′ or 60′) per cycle. The phase angle differences between rhythm and light cycle depends on the periods (τ andT) as predicted by the model. Entrainment ofP. leucopus is unstable due to the after effects on τ created by the light pulse. The limiting values of zeitgeber period to which the animals entrain are much closer to 24 h than inDrosophila pseudoobscura, as the model predicts. However, frequent failures to entrain toT=23 andT=25 h are only explained if we take considerable interindividual variation in both τ and PRC into account. With 2 pulses per cycle, the model predicts that entrainment will be more stable when activity is in the longer interval between the pulses than when it is in the shorter interval. This is true in the experimental data, where the phase relationships match predictions for skeleton photoperiods up to ca. 14 h. Increasing asymmetry forces animals into a “phase jump”, so that activity shifts from the shorter to the longer interval. Theseψ-jumps are accurately predicted in the hamster, but they occur at much longer photoperiods than predicted inP. leucopus. Thus, the unqualified model, using a rigidly fixed species τ and PRC, is surely inadequate to explain entrainment. The extent to which variations in τ and PRC-shape, both “spontaneous” and induced by the entrainment process, can be known or inferred restricts the validity of the predictions. Yet we conclude, from a good deal of agreement between experiment and prediction (i), from the close correspondence between complete and skeleton photoperiods (ii), and on behavioural grounds (iii), that non-parametric entrainment by short light signals has a major share in the entrainment of nocturnal rodent rhythms in nature. With these restrictions in mind, we analyse in thesecond part of the paper how the empirical regularities concerning τ and PRC, and reported earlier (Pittendrigh and Daan, 1976; Daan and Pittendrigh, 1976a, b), contribute to the stabilization of the phase angle difference (ψ) between the pacemaker and the external world. Use is made of computer simulations of artificial pacemakers with variable τ and PRC. ψ is most sensitive to instabilities in τ when \(\bar \tau \) is close to 24 h. Thus the verycircadian nature of these pacemakers helps to conserveψ. Selection pressure for homeostasis ofτ has been large in a species (M. auratus) where \(\bar \tau \) =24 h. The effect ofψ-instability is further reduced by entrainment with 2 pulses (dawn and dusk), made possible by the PRC's having both an advance and a delay section. To analyze the contributions toψ-conservation with seasonally changing photoperiod, we have assumed that it is of functional significance to conserve the phase of activity with respect to dusk (nocturnal animals) or to dawn (diurnal animals). We distinguish three contributions of nocturnal pacemaker behaviour to this type ofψ-conservation: increased amplitude of the PRC (i), asymmetry in the PRC, such that the slope of the delay-part is steeper than the slope of the advance-part (ii), and a short freerunning period in DD (iii). A further contribution must derive from parametric effects of light, which are not traceable by the model, but certainly effective in preventing in complete photoperiods theψ-jump which is seen in skeleton photoperiods. The existence of parametric effects is further demonstrated by the change of τ with light intensity in LL, described by Aschoff's Rule, which presumably reflects differences in PRC-shape between nocturnal and diurnal animals (Daan and Pittendrigh, 1976b). The paper concludes with an attempt to distinguish the features of circadian clocks that are analytically necessary for entrainment to occur (i), or have functional meaning, either in the measurement of the lapse of time (ii) or in the identification of local time (iii).
Journal of Comparative Physiology A-neuroethology Sensory Neural and Behavioral Physiology, 1976
In a preceding paper (Pittendrigh and Daan, 1976a) differences in the lability of the freerunning... more In a preceding paper (Pittendrigh and Daan, 1976a) differences in the lability of the freerunning circadian period (τ) in constant darkness (DD) were described among four species of rodents. This lability (i) is strongly correlated with the responses of τ to (ii) D2O-administration and to (iii) constant light (LL) of various intensities. The question is raised whether these are three reflections of the action of the same mechanism conserving circadian frequency. A number of qualitative differences exist in the responses to D2O and LL: D2O always decelerates, while LL may decelerate (as in nocturnal rodents) or accelerate circadian rhythms. D2O does not affect the pattern of activity or cause aperiodicity or “splitting” as sometimes observed in LL. The magnitude of the response to D2O is independent of τ in DD (Mus musculus); the response to LL is negatively correlated with τ in DD (Peromyscus maniculatus). The response to D2O appears subject only to the time constants of the processes of deuteriation and dedeuteriation of body tissues; the response to LL involves long time constants, gradual approach to equilibrium frequency, and “after-effects” upon return to DD. Phase response curves for 15′ light pulses are virtually identical in mice drinking D2O (25%) and in mice drinking tap water, although their τ's differ by as much as 1.8 h. This is seen as evidence that D2O-action is not restricted to a specific phase of the circadian cycle. Serum concentrations of D2O, 12 days after onset of deuteriation of the drinking water are 8.6% in hamsters and 13.9% in C57 mice. The difference accounts for the difference in pacemaker response (change in τ by 20% D2O: 3.8% and 6.6%, respectively). Thus the response to D2O is related to the characteristics of water metabolism, and species differences do not reflect differences in the homeostatic mechanism conserving frequency. Concerning the action of constant light, no firm conclusion can be made. The long time constants in the response to LL suggest that τis homeostatically protected in the face of alterations in LL-intensity. On the other hand, the strong correlation both among and within species between LL-response and shape of the phase response curve (PRC; linearly transformed in the analysis to a “velocity response curve”, VRC) suggests that the change of τ with LL is best explained as an artifact caused by the daily curve of light sensitivity, which itself is necessary for entrainment. PRC-shape and the lability of homeostatic conservation of frequency are believed to be functionally interrelated. “Aschoff's Rule”, concerning the differences in response of τ to LL between nocturnal and diurnal animals, is given new support by a literature survey of pertinent data (Table 2). It is again most readily understood as an artifact reflecting different light response curves involved in different strategies of entrainment (Pittendrigh and Daan, 1976b) in nocturnal and diurnal animals.
... In each of these fields, a creased during recovery sleep after sleep deprivation (ZO), ... Ea... more ... In each of these fields, a creased during recovery sleep after sleep deprivation (ZO), ... Early approaches used physical and mathematical oscillators to elucidate the self-maintainance of circadian rhythms under con-stant conditions and the principles of entrainment by ...
The circadian activity rhythms of golden hamsters and two species of deermouse, when released fro... more The circadian activity rhythms of golden hamsters and two species of deermouse, when released from a light-dark cycle of 12 hours light and 12 hours of darkness into constant darkness, had progressively shorter periods as the animals became older. A possible bearing of this fact on the aging process is briefly outlined.
Journal of Comparative Physiology A-neuroethology Sensory Neural and Behavioral Physiology, 1976
This paper is an attempt to integrate in a general model the major findings reported earlier in t... more This paper is an attempt to integrate in a general model the major findings reported earlier in this series on: lability and history dependence of circadian period, τ (Pittendrigh and Daan, 1976 a); dependence of τ and α on light intensity as described in Aschoff's Rule (Daan and Pittendrigh, 1976b); the interrelationships between τ and phase response curves (Daan and Pittendrigh, 1976a); and those inconsistencies between experimental facts on entrainment and theoretical predictions based on a single oscillator with fixed parameters τ and PRC, which pointed to a more complex system (Pittendrigh and Daan, 1976b). The qualitative model developed consists of two oscillators. The evidence that two separate oscillators are involved in circadian activity rhythms rests largely on the “splitting” phenomenon, known to occur in several species of mammals and birds. The empirical regularities of “splitting” in hamsters exposed to constant illumination (LL) are described: Splitting, i.e. the dissociation of a single activity band into two components which become stably coupled in circa 180° antiphase, occurs in about 50% of the animals in 100–200 lux, and has not been observed in lower light intensities. Splitting never occurred before 40 days after the transition to LL, unless the pretreatment had been LL of low intensity. In some animals the unsplit condition returned spontaneously. The attainment of antiphase is usually accompanied by a decrease in τ, and refusion of the two components by an increase in τ. These data show that both the split and the unsplit condition are metastable states, characterized by different phase relationships (ψ EM ) of two constituent oscillators. ψEM is history-dependent and determines τ of the coupled system. Observations in Peromyscus leucopus transferred from LL to DD to LD 12∶12 show that the two components of the bimodal activity peak (in LD) can for some time run at different frequencies (in DD), suggesting that bimodality of activity peaks and splitting are based on the same two-oscillator system. The model developed assumes the existence of two oscillators or principal groups of oscillators E and M, with opposite dependence of spontaneous frequency on light intensity. The dependence of the phase relationship (ψ EM ) between the two on light intensity and the dependence of τ onψ EM account for all the history-dependent characteristics of circadian pacemakers, and for the interdependence of τ, PRC, and τ-lability. The model qualitatively accommodates the interdependence of τ and α summarized in Aschoffs Rule. It is noted that the major intuitive elements in the model have been found to characterize an explicit version of it in computer simulations. The relevance of the model to seasonal change in photoperiod is discussed. A pacemaker comprising two oscillators mutually interacting but coupled separately to sunrise and sunset enhances its competence to accommodate to seasonal change in the daily pattern of external conditions; and it could well be involved in the pacemaker's known ability to discriminate between daylengths in the phenomena of photoperiodic induction.
Nocturnal rodents show diurnal food anticipatory activity when food access is restricted to a few... more Nocturnal rodents show diurnal food anticipatory activity when food access is restricted to a few hours in daytime. Timed food access also results in reduced food intake, but the role of food intake in circadian organization per se has not been described. By simulating natural food shortage in mice that work for food we show that reduced food intake alone shifts the activity phase from the night into the day and eventually causes nocturnal torpor (natural hypothermia). Release into continuous darkness with ad libitum food, elicits immediate reversal of activity to the previous nocturnal phase, indicating that the classical circadian pacemaker maintained its phase to the light-dark cycle. This flexibility in behavioral timing would allow mice to exploit the diurnal temporal niche while minimizing energy expenditure under poor feeding conditions in nature. This study reveals an intimate link between metabolism and mammalian circadian organization.
In an attempt to evaluate the importance of individual daily habits to a freeliving animal, forag... more In an attempt to evaluate the importance of individual daily habits to a freeliving animal, foraging behaviour of kestrels was observed continuously for days in sequence in open country. Data obtained in 2,942 observation hours were used. Flight-hunting was the prominent foraging technique yielding 76% of all prey obtained. Flight-hunting was impeded by rain, fog and wind speeds below 4 m/s and above 12 m/s (Fig. 3). Flight-hunting tended to be suppressed also in response to recent successful strikes and more generally by a high level of post-dawn accumulated prey (Figs. 4, 5). Flight-hunting had a tendency to be enhanced in response to recent unsuccessful strikes (Fig. 6). Trapping results demonstrated a fine-grained daily pattern of common vole trap entries, with peaks at intervals of ca. 2 h (Figs. 7, 8). The interpretation of some of this pattern as representative of vole surface activity was supported by overall strike frequencies of kestrels hunting for voles (Fig. 9). Detailed analysis of the behaviour of three individuals revealed significant peaks in hunting yield and frequency, coinciding with each other and with peaks in vole trapping (Fig. 11). It is suggested that the kestrels adjusted their flight-hunting sessions to times of high ‘expected’ yield. Vole activity peaks sometimes remained unexploited. Meal frequencies culminated shortly before nightfall except in incubating females. The difference between the daily distributions of hunting and eating was due to some of the prey being cached in daytime and retrieved around dusk (Fig. 13). Caching behaviour is interpreted as a circadian strategy allowing separate optimization of hunting-adjusted to prey availability-and eating-adaptive by retaining minimum body weight in daytime flight and by thermo-regulatory savings at night. Some kestrels showed remarkable constancy from day to day in the temporal distribution of specific behaviours (Fig. 16) and of spatial movements (Figs. 18, 19). In three 1–2 week sequences of observation analysed, flight-hunting frequency peaked 24 h after prey capture (Fig. 17). This is probably based on day to day correlations in flight-hunting frequency as well as on increased motivation for hunting in response to prey capture 24 h ago (Table 5). In one individual with three distinct hunting areas, the tendency to return to an area again was maximal 24 h after prey capture in that area (Fig. 21, Table 6). A field experiment tested the effect of prey capture on the daily distributions of hunting and site choice in this individual (Fig. 22). A significant concentration of flight-hunting activity in the experimental feeding area was observed at the daily time of feeding (Fig. 23). Two alternative hypotheses are compatible with the result. Favoured is the one that the birds use “time memory” for the optimization of their daily patterns of flight-hunting and site choice. By adjusting her daily flight-hunting to times of high yield, one kestrel saved 10–22% on her total time spent flight-hunting. Maximal efficiency, by concentration of all hunting activity in the hour of maximal yield, was not attained, presumably because of information constraints. The generality of the contribution of daily habits to survival is discussed.
We studied the limiting factors for brood size in the kestrel, Falco tinnunculus, by measuring pa... more We studied the limiting factors for brood size in the kestrel, Falco tinnunculus, by measuring parental effort in natural broods of different size and parental response to manipulation of food satiation of the brood. Parental effort was quantified as total daily time spent in flight, and total daily energy expenditure, from all-day observations. During nestling care males with different natural brood sizes (4 to 7 chicks), spent an average of 4.75 h . d-' in flight independent of brood size, and expended an average total daily energy of 382 kJ d-'. Due to a higher flight-hunting yield (mammal-prey caught per hour hunting), males with larger natural broods were able to provision their broods with the same amount of food (mainly Microtus arvalis) per chick (62.6 g . d-'), with the same effort as males with smaller broods. This provisioning rate was close to the mean feeding rate of hand-raised chicks in the laboratory, that were fed ad libitum, (66.8 g . d-' . chick-').
The relationship between body size and basal metabolic rate (BMR) in homeotherms has been treated... more The relationship between body size and basal metabolic rate (BMR) in homeotherms has been treated in the literature primarily by comparison between species of mammals or birds. This paper focuses on the intraindividual changes in BMR when body mass (W) varies with different maintenance regimens. BMR varied in individual kestrels in proportion to W 1.67 , which is considerably steeper than the mass exponents for homomorphic change (0.667; Heusner, 1984) for interspecific comparison among all birds (0.677) or raptors (0.678), for interindividual comparison of kestrels on ad libitum maintenance regimens (0.786), and for mass proportionality (1.00). The circadian range of telemetered core temperature also varied more strongly with intraindividual than with interspecific (Aschoff, 1981a) variation in mass. This was due to reduced nocturnal core temperature at low-maintenance regimens, which was, however, insufficient to account for the excessive reduction in BMR. Carcass analysis of eight birds sacrificed revealed a disproportionate reduction in heart and kidney lean mass at low-maintenance regimens. We surmise that variation in BMR primarily reflects variation in these metabolically highly active tissues. This may account for positive correlations found between heart, kidney, and BMR residuals relative to interspecific allometric prediction, and between α and p residuals, as expected on the basis of the constant excess of BMR during α above BMR during p .
Proceedings of The Royal Society B: Biological Sciences, 1996
Each copy of any part of a JSTOR transmission must contain the same copyright notice that appears... more Each copy of any part of a JSTOR transmission must contain the same copyright notice that appears on the screen or printed page of such transmission.
Costs of reproduction have been assessed experimentally by measuring subsequent survival and repr... more Costs of reproduction have been assessed experimentally by measuring subsequent survival and reproduction of parent animals raising enlarged and reduced numbers of offspring. Reported effects on survival have so far always referred to local survival of marked individuals in the study population. They do not provide definitive proof of a cost of reproduction, since reduced local survival may be due either to reduced survival or to an increased tendency to emigrate from the study area. Therefore, it is important to assess mortality rates in connection with brood size experiments. We report here an analysis of time of death in 63 cases where Kestrels, Falco tinnunculus L. had raised broods of manipulated size and were subsequently reported freshly dead. 60 % of the parents raising two extra nestlings were reported dead before the end of the first winter, compared to 29 % of those raising control or reduced broods. This result confirms our interpretation of the manipulation effects on local survival as due to mortality rather than emigration. The extra mortality occurred in the winter following the brood enlargements. Kestrel parents in these experiments have been shown to adjust their daily energy expenditure to the modified brood size. Increased parental effort in this species thus entails an increased risk of death half a year later.
We present an extensive set of data for five species of raptorial birds to demonstrate that some ... more We present an extensive set of data for five species of raptorial birds to demonstrate that some raptor species produce an excess of daughters early in the season and an excess of sons in late nests, while others show the reverse. By means of a simulation model we investigate an evolutionary explanation for this phenomenon in terms of sex-specific differences in the relation between age at first breeding and date of birth. The model predicts diat that gender should be produced first in the season whose age of first breeding is more strongly accelerated by an early birth date. We argue that this tends to be the male gender in raptor species, such as the common kestrel (Falco tinnunculus), which tend to breed early in life, while it is the female gender in larger species with later onset of breeding, such as the marsh harrier (Circus aeruginosus). The empirical evidence is qualitatively consistent with this hypothesis. Our model is quite general in that it makes no assumptions about the mechanism (primary sex-ratio bias at egg laying or secondary sex-differential mortality before fledging) by which the bias is generated. Yet it is able to create quantitative predictions for species where sufficient demographic and life-history data are available. From die available data set in the common kestrel we derive a quantitative prediction for the seasonal trend in brood sex ratio. The observed trend is in good agreement with this prediction.
Effects of hypothalamic lesions on the ultradian and circadian organization of wheel running and ... more Effects of hypothalamic lesions on the ultradian and circadian organization of wheel running and feeding were studied in the common vole, Microtus arvalis. Circadian organization broke down within 30 days in continuous darkness in 24% of intact voles (n = 135). Ultradian rhythmicity of feeding (period 2-3 hr) persisted in constant conditions in all intact voles. Following lesions of the suprachiasmatic nuclei (SCN), circadian rhythmicity disappeared when lesions were complete (n = 8) or more extensive than 25% of the total SCN volume (n = 5). Absence of circadian rhythmicity was also found in animals with substantial lesions in the diencephalic paraventricular area (PVA) and in the retrochiasmatic area (RCA) and/or adjacent arcuate nucleus (Arc). Complete loss of ultradian and circadian organization occurred in eight voles with damage to the RCA and/or Arc. In three of these, the SCN was intact. The SCN is a likely candidate for a circadian pacemaker in voles (as in other rodents), while the loss of circadian rhythmicity following PVA and RCA/Arc lesions may be due to destruction of efferent pathways from the SCN. The RCA/Arc area is apparently necessary for the expression of ultradian rhythms. The intact SCN is neither necessary nor sufficient for the generation of ultradian rhythmicity.
The accuracy with which a circadian pacemaker can entrain to an environmental 24-h zeitgeber sign... more The accuracy with which a circadian pacemaker can entrain to an environmental 24-h zeitgeber signal depends on (a) characteristics of the entraining signal and (b) response characteristics and intrinsic stability of the pacemaker itself. Position of the sun, weather conditions, shades, and behavioral variations (eye closure, burrowing) all modulate the light signal reaching the pacemaker. A simple model of a circadian pacemaker allows researchers to explore the impact of these factors on pacemaker accuracy. Accuracy is operationally defined as the reciprocal value of the day-to-day standard deviation of the clock times at which a reference phase (0) is reached. For the purpose of this exploration, the authors used a model pacemaker characterized solely by its momentary phase and momentary velocity. The average velocity determines the time needed to complete one pacemaker cycle and, therefore, is inversely proportional to pacemaker period. The model pacemaker responds to light by shifting phase and/or changing its velocity. The authors assumed further that phase and velocity show small random fluctuations and that the velocity is subject to aftereffects. Aftereffects were incorporated mathematically in a term allowing period to contract exponentially to a stable steady-state value, with a time constant of 69 d in the absence of light. The simulations demonstrate that a pacemaker reaches highest accuracy when it responds to light by simultaneous phase shifts and changes of its velocity. Phase delays need to coincide with slowing down and advances with speeding up; otherwise, no synchronization to the zeitgeber occurs. At maximal accuracy, the changes in velocity are such that the average period of the pacemaker under entrained conditions equals 24 h. The results suggest that during entrainment, the pacemaker adjusts its period to 24 h, after which daily phase shifts to compensate for differences between the periods of the zeitgeber and the clock are no longer necessary. On average, phase shifts compensate for maladjustments of phase and velocity changes compensate for maladjustments of period.
Proceedings of The National Academy of Sciences, 1975
Castration of mice in freerunning conditions (total darkness, DD) causes a reduction of running w... more Castration of mice in freerunning conditions (total darkness, DD) causes a reduction of running wheel activity in the beginning of the active period (a) and stimulates activity at the end of a. Simultaneously, the period (r) of the freerunning rhythm is increased. Both effects are abolished by implantation of a Silastic capsule from which a physiological dose of testosterone is released at a constant rate. The results are tentatively explained by differential endocrine influences on two oscillating components in the pacemaker of the circadian activity rhythm. Early work on endocrine involvement in circadian rhythms was mainly aimed at the localization of the clock within the various endocrine glands or within a cycle of nervous-endocrine-metabolic events (1). Much of the research was done in animals exposed to daily light-dark cycles, which precludes evidence on the endogenous nature of the rhythmic phenomena observed. From a series of experiments on blinded rats, Richter (2) stated that freerunning circadian activity rhythms were not impaired by any of the following interferences with the endocrine system: gonadectomy, adrenalectomy, hypophysectomy, hyperor hypothyroidism, and pinealectomy. Apparently, the circadian clock driving the activity-rest cycle was not located in any of the glands involved. The demonstration by Andrews (3) that rat adrenals cultured in vitro continue to produce corticosteroids in a circadian rhythmic fashion considerably modified existing views in the field concerning the regulating function of a localized clock (4). Evidently, since at least one organ, and probably more, is capable of self-sustained oscillations, a centralized "clock" or "pacemaker" would serve to synchronize a number of peripheral oscillators rather than impose its rhythm on them. For instance, a circadian pacemak-
Journal of Comparative Physiology A-neuroethology Sensory Neural and Behavioral Physiology, 1976
Phase response curves for 15′ bright light pulses of four species of nocturnal rodents are descri... more Phase response curves for 15′ bright light pulses of four species of nocturnal rodents are described. All show delay phase shifts early in the subjective night, advance shifts in the late subjective night, and relative insensitivity during the subjective day. The broad scatter in measured phase-shifts is largely due to error of measurement: the response of the pacemakers to light stimuli is more accurate than we observe. Indications are found that the response to a resetting stimulus at a given phase of the rhythm is correlated with the individual \(\bar \tau \) (freerunning period). Fast pacemakers (short \(\bar \tau \) ) tend to be more delayed or less advanced by the light than slow pacemakers (long \(\bar \tau \) ). Within individual mice (Mus musculus) the circadian pacemaker adjusts its resetting response to variations in its frequency: when τ is long (induced as after-effect of prior light treatment) light pulses at a defined phase of the oscillation (ct 15) produce smaller delay phase shifts than when τ is short. Among species there are conspicuous differences in the shape of the phase response curve: where \(\bar \tau \) is long, advance phase shifts are large and delay phase shifts small (Mesocricetus auratus); where \(\bar \tau \) is short, advance shifts are small, and delay shifts are large (Mus musculus;Peromyscus maniculatus). The functional meaning of the interrelationships of τ and PRC is briefly discussed.
Journal of Comparative Physiology A-neuroethology Sensory Neural and Behavioral Physiology, 1976
The circadian pacemakers controlling activity rhythms in four species of rodents are compared, as... more The circadian pacemakers controlling activity rhythms in four species of rodents are compared, as freerunning systems in constant darkness. In analyzing their stability the distinction is made between (1) spontaneous day-to-day instability of frequency, and (2) a longer-term lability, some of which is traceable to identified causes. Serial correlation analysis indicates that the precision (day-to-day stability) of the pacemaker's period is ca. twice as good (estimated s.d.=0.6% of \(\bar \tau \) inMus musculus) as the already remarkable precision of the activity rhythm it drives (average s.d.=1.2% of \(\bar \tau \) ). Identifiable causes of long-term lability include age and several features of prior entrainment by light. The period and photoperiod of a light cycle have a predictable influence on the subsequent freerunning period (τ) of the pacemaker; they cause “after-effects”. So do single light pulses causing a phase-shift in the freerunning system. Constant light also has an after-effect opposite in sign from the after-effect of long photoperiods. After-effects of “skeleton” photoperiods support the hypothesis that the transitions of light to darkness vv. are involved in the entrainment process which leads to changes in τ. Both day-to-day instability and long term lability are most pronounced in species (Peromyscus maniculatus, Mus musculus) whose \(\mathop \tau \limits^ = \) is considerably shorter than 24 h; they are least pronounced in hamsters whose \(\mathop \tau \limits^ = \) is indistinguishably close to 24 h. The differences between the species in τ and its lability are paralleled by differences in pacemaker lability as measured in light-induced after-effects and in the extent of changes with age. The species evidently differ in the “tightness” with which τ is homeostatically conserved.
Journal of Comparative Physiology A-neuroethology Sensory Neural and Behavioral Physiology, 1976
In thefirst part of the paper, the model of non-parametric entrainment of circadian pacemakers is... more In thefirst part of the paper, the model of non-parametric entrainment of circadian pacemakers is tested for the case of nocturnal rodents. The model makes use of the available data on freerunning period (τ) in constant darkness (Pittendrigh and Daan, 1976a) and on phase response curves (PRC) for short light pulses (Daan and Pittendrigh, 1976a). It is tested in experiments using 1 or 2 light pulses per cycle. Mesocricetus auratus andPeromyscus leucopus entrain to Zeitgebers involving 1 pulse (15′ or 60′) per cycle. The phase angle differences between rhythm and light cycle depends on the periods (τ andT) as predicted by the model. Entrainment ofP. leucopus is unstable due to the after effects on τ created by the light pulse. The limiting values of zeitgeber period to which the animals entrain are much closer to 24 h than inDrosophila pseudoobscura, as the model predicts. However, frequent failures to entrain toT=23 andT=25 h are only explained if we take considerable interindividual variation in both τ and PRC into account. With 2 pulses per cycle, the model predicts that entrainment will be more stable when activity is in the longer interval between the pulses than when it is in the shorter interval. This is true in the experimental data, where the phase relationships match predictions for skeleton photoperiods up to ca. 14 h. Increasing asymmetry forces animals into a “phase jump”, so that activity shifts from the shorter to the longer interval. Theseψ-jumps are accurately predicted in the hamster, but they occur at much longer photoperiods than predicted inP. leucopus. Thus, the unqualified model, using a rigidly fixed species τ and PRC, is surely inadequate to explain entrainment. The extent to which variations in τ and PRC-shape, both “spontaneous” and induced by the entrainment process, can be known or inferred restricts the validity of the predictions. Yet we conclude, from a good deal of agreement between experiment and prediction (i), from the close correspondence between complete and skeleton photoperiods (ii), and on behavioural grounds (iii), that non-parametric entrainment by short light signals has a major share in the entrainment of nocturnal rodent rhythms in nature. With these restrictions in mind, we analyse in thesecond part of the paper how the empirical regularities concerning τ and PRC, and reported earlier (Pittendrigh and Daan, 1976; Daan and Pittendrigh, 1976a, b), contribute to the stabilization of the phase angle difference (ψ) between the pacemaker and the external world. Use is made of computer simulations of artificial pacemakers with variable τ and PRC. ψ is most sensitive to instabilities in τ when \(\bar \tau \) is close to 24 h. Thus the verycircadian nature of these pacemakers helps to conserveψ. Selection pressure for homeostasis ofτ has been large in a species (M. auratus) where \(\bar \tau \) =24 h. The effect ofψ-instability is further reduced by entrainment with 2 pulses (dawn and dusk), made possible by the PRC's having both an advance and a delay section. To analyze the contributions toψ-conservation with seasonally changing photoperiod, we have assumed that it is of functional significance to conserve the phase of activity with respect to dusk (nocturnal animals) or to dawn (diurnal animals). We distinguish three contributions of nocturnal pacemaker behaviour to this type ofψ-conservation: increased amplitude of the PRC (i), asymmetry in the PRC, such that the slope of the delay-part is steeper than the slope of the advance-part (ii), and a short freerunning period in DD (iii). A further contribution must derive from parametric effects of light, which are not traceable by the model, but certainly effective in preventing in complete photoperiods theψ-jump which is seen in skeleton photoperiods. The existence of parametric effects is further demonstrated by the change of τ with light intensity in LL, described by Aschoff's Rule, which presumably reflects differences in PRC-shape between nocturnal and diurnal animals (Daan and Pittendrigh, 1976b). The paper concludes with an attempt to distinguish the features of circadian clocks that are analytically necessary for entrainment to occur (i), or have functional meaning, either in the measurement of the lapse of time (ii) or in the identification of local time (iii).
Journal of Comparative Physiology A-neuroethology Sensory Neural and Behavioral Physiology, 1976
In a preceding paper (Pittendrigh and Daan, 1976a) differences in the lability of the freerunning... more In a preceding paper (Pittendrigh and Daan, 1976a) differences in the lability of the freerunning circadian period (τ) in constant darkness (DD) were described among four species of rodents. This lability (i) is strongly correlated with the responses of τ to (ii) D2O-administration and to (iii) constant light (LL) of various intensities. The question is raised whether these are three reflections of the action of the same mechanism conserving circadian frequency. A number of qualitative differences exist in the responses to D2O and LL: D2O always decelerates, while LL may decelerate (as in nocturnal rodents) or accelerate circadian rhythms. D2O does not affect the pattern of activity or cause aperiodicity or “splitting” as sometimes observed in LL. The magnitude of the response to D2O is independent of τ in DD (Mus musculus); the response to LL is negatively correlated with τ in DD (Peromyscus maniculatus). The response to D2O appears subject only to the time constants of the processes of deuteriation and dedeuteriation of body tissues; the response to LL involves long time constants, gradual approach to equilibrium frequency, and “after-effects” upon return to DD. Phase response curves for 15′ light pulses are virtually identical in mice drinking D2O (25%) and in mice drinking tap water, although their τ's differ by as much as 1.8 h. This is seen as evidence that D2O-action is not restricted to a specific phase of the circadian cycle. Serum concentrations of D2O, 12 days after onset of deuteriation of the drinking water are 8.6% in hamsters and 13.9% in C57 mice. The difference accounts for the difference in pacemaker response (change in τ by 20% D2O: 3.8% and 6.6%, respectively). Thus the response to D2O is related to the characteristics of water metabolism, and species differences do not reflect differences in the homeostatic mechanism conserving frequency. Concerning the action of constant light, no firm conclusion can be made. The long time constants in the response to LL suggest that τis homeostatically protected in the face of alterations in LL-intensity. On the other hand, the strong correlation both among and within species between LL-response and shape of the phase response curve (PRC; linearly transformed in the analysis to a “velocity response curve”, VRC) suggests that the change of τ with LL is best explained as an artifact caused by the daily curve of light sensitivity, which itself is necessary for entrainment. PRC-shape and the lability of homeostatic conservation of frequency are believed to be functionally interrelated. “Aschoff's Rule”, concerning the differences in response of τ to LL between nocturnal and diurnal animals, is given new support by a literature survey of pertinent data (Table 2). It is again most readily understood as an artifact reflecting different light response curves involved in different strategies of entrainment (Pittendrigh and Daan, 1976b) in nocturnal and diurnal animals.
... In each of these fields, a creased during recovery sleep after sleep deprivation (ZO), ... Ea... more ... In each of these fields, a creased during recovery sleep after sleep deprivation (ZO), ... Early approaches used physical and mathematical oscillators to elucidate the self-maintainance of circadian rhythms under con-stant conditions and the principles of entrainment by ...
The circadian activity rhythms of golden hamsters and two species of deermouse, when released fro... more The circadian activity rhythms of golden hamsters and two species of deermouse, when released from a light-dark cycle of 12 hours light and 12 hours of darkness into constant darkness, had progressively shorter periods as the animals became older. A possible bearing of this fact on the aging process is briefly outlined.
Journal of Comparative Physiology A-neuroethology Sensory Neural and Behavioral Physiology, 1976
This paper is an attempt to integrate in a general model the major findings reported earlier in t... more This paper is an attempt to integrate in a general model the major findings reported earlier in this series on: lability and history dependence of circadian period, τ (Pittendrigh and Daan, 1976 a); dependence of τ and α on light intensity as described in Aschoff's Rule (Daan and Pittendrigh, 1976b); the interrelationships between τ and phase response curves (Daan and Pittendrigh, 1976a); and those inconsistencies between experimental facts on entrainment and theoretical predictions based on a single oscillator with fixed parameters τ and PRC, which pointed to a more complex system (Pittendrigh and Daan, 1976b). The qualitative model developed consists of two oscillators. The evidence that two separate oscillators are involved in circadian activity rhythms rests largely on the “splitting” phenomenon, known to occur in several species of mammals and birds. The empirical regularities of “splitting” in hamsters exposed to constant illumination (LL) are described: Splitting, i.e. the dissociation of a single activity band into two components which become stably coupled in circa 180° antiphase, occurs in about 50% of the animals in 100–200 lux, and has not been observed in lower light intensities. Splitting never occurred before 40 days after the transition to LL, unless the pretreatment had been LL of low intensity. In some animals the unsplit condition returned spontaneously. The attainment of antiphase is usually accompanied by a decrease in τ, and refusion of the two components by an increase in τ. These data show that both the split and the unsplit condition are metastable states, characterized by different phase relationships (ψ EM ) of two constituent oscillators. ψEM is history-dependent and determines τ of the coupled system. Observations in Peromyscus leucopus transferred from LL to DD to LD 12∶12 show that the two components of the bimodal activity peak (in LD) can for some time run at different frequencies (in DD), suggesting that bimodality of activity peaks and splitting are based on the same two-oscillator system. The model developed assumes the existence of two oscillators or principal groups of oscillators E and M, with opposite dependence of spontaneous frequency on light intensity. The dependence of the phase relationship (ψ EM ) between the two on light intensity and the dependence of τ onψ EM account for all the history-dependent characteristics of circadian pacemakers, and for the interdependence of τ, PRC, and τ-lability. The model qualitatively accommodates the interdependence of τ and α summarized in Aschoffs Rule. It is noted that the major intuitive elements in the model have been found to characterize an explicit version of it in computer simulations. The relevance of the model to seasonal change in photoperiod is discussed. A pacemaker comprising two oscillators mutually interacting but coupled separately to sunrise and sunset enhances its competence to accommodate to seasonal change in the daily pattern of external conditions; and it could well be involved in the pacemaker's known ability to discriminate between daylengths in the phenomena of photoperiodic induction.
Nocturnal rodents show diurnal food anticipatory activity when food access is restricted to a few... more Nocturnal rodents show diurnal food anticipatory activity when food access is restricted to a few hours in daytime. Timed food access also results in reduced food intake, but the role of food intake in circadian organization per se has not been described. By simulating natural food shortage in mice that work for food we show that reduced food intake alone shifts the activity phase from the night into the day and eventually causes nocturnal torpor (natural hypothermia). Release into continuous darkness with ad libitum food, elicits immediate reversal of activity to the previous nocturnal phase, indicating that the classical circadian pacemaker maintained its phase to the light-dark cycle. This flexibility in behavioral timing would allow mice to exploit the diurnal temporal niche while minimizing energy expenditure under poor feeding conditions in nature. This study reveals an intimate link between metabolism and mammalian circadian organization.
Uploads
Papers by Serge Daan