The distribution of the urban mosquitoes, Culex pipiens pipiens f. molestus (Diptera: Culicidae) in Petrozavodsk City

The distribution of the urban mosquitoes, Culex pipiens pipiens f. molestus in Petrozavodsk city was studied for the first time. Immature mosquito stages were found in 21 of 68 basements flooded with polluted waters, the occurrence of the mosquitoes varied in different districts of the sity. The mean siphonal index of the mosquito larvae from 6 underground populations varied from 3.8 up to 4.2, i. e. within the range typical for C. p. p. f. molestus.     

Comparative study on the relationship between photoperiodic host-seeking behavioral patterns and the eye parameters of mosquitoes

Relationships between the ommatidial structure and photoperiodic behavior of several mosquito species were investigated. Host-seeking behavioral patterns of mosquitoes were classified into four main groups based on previously compiled reports on field or laboratory biting activity. These groups were pattern I and I' (nocturnal), pattern II (crepuscular and nocturnal), pattern III (crepuscular and diurnal), and pattern IV (diurnal). Eye parameters (product of facet diameter and interommatidial angle) of mosquitoes that belong to the pattern I and I' group were higher (2.7-4.2) than those of mosquitoes that belong to the pattern IV group (0.8-2.3). Eye parameters of the mosquitoes categorized in the pattern II and III groups were intermediate (2.3-2.6). These results suggest that the crepuscular behavior of mosquitoes undergoes a transition in the course of evolution from nocturnal behavior to diurnal behavior. Large variations in the eye parameters were observed even within the same genus depending on their photoperiodic behavior. Therefore, the ommatidial structure of mosquitoes appears to be determined, not taxonomically, but evolutionarily by the photoenvironment in which the mosquitoes are most active.     

Two Steady‐Entrainment Phases and Graded Masking Effects by Light Generate Different Circadian Chronotypes in Octodon degus

Processes involved in the operation of the circadian pacemaker are well characterized; however; little is known about what mechanisms drive the overt diurnal, nocturnal, or crepuscular behavior in a species. In this context, dual‐phasing rodents, such as Octodon degus, emerge as a useful model to decipher these keys. Two main chronotypes, nocturnal and diurnal, have been traditionally described in laboratory‐housed degus based on the percentage of activity displayed by the animals during the scotophase or photophase. However, if one considers also the entrainment phase angle during the first days following a change from LD to DD conditions, a third chronotype (intermediate)—or more properly, a continuous grading of circadian expressions between diurnal and nocturnal chronotype—can be observed. Our experiments suggest the pacemaker of the diurnal animal is entrained to the photophase, and light does not exert a negative masking effect. The pacemaker of the nocturnal degus, on the other hand, is entrained to the scotophase, and light exerts a strong negative masking effect. Finally, the intermediate chronotype is characterized by variable negative masking effect of light overlapping a pacemaker entrained to the photophase. The phase shift inversion from diurnal to nocturnal chronotype is related to the availability of a wheel in the cage, and the effect may be located downstream from the clock. However, body temperature rhythm recordings, less affected by masking effects, point to an involvement of the circadian pacemaker in chronotype differentiation, as transient entrainment cycles, and not an abrupt phase shift, were detected after providing access to the wheel. The diurnality of degus seems to be the result of a variety of mechanisms, which may explain how different processes can lead to similar chronotypes.     

Entomophthorosis of Culex pipiens (Culicidae) mosquitoes breeding in basements

Foci of mycosis of C. p. pipiens and C. p. molestus caused by the parasitic fungus Entomophthora conglomerata Sorok. were found out in basements flooded with water. The conditions prevailing in the foci, sites of infection of mosquitoes and seasonal changes in the epizootic process are described. Developmental periods of conidiospores in mosquitoes were established experimentally. Entomophthorosis epizootia was found to limit the possibilities of mosquitoes to scatter from the hatching sites and to reduce the number of biting females in the population.     

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

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

Daily patterns of metabolic rate among New Zealand lizards (Reptilia: Lacertilia: Diplodactylidae and Scincidae)

In addition to the effects of temperature fluctuations on metabolic rate, entrained endogenous rhythms in metabolism, which are independent of temperature fluctuations, may be important in overall energy metabolism in ectotherms. Daily entrained endogenous rhythms may serve as energy-conserving mechanisms during an animal's active or inactive phase. However, because nocturnal lizards often take advantage of thermal opportunities during the photophase (light), their daily metabolic rhythms may be less pronounced than those of diurnal species. We measured the rate of oxygen consumption (VO(2)) as an index of metabolic rate of eight temperate lizard species (four nocturnal, three diurnal, and one crepuscular/diurnal; n = 7-14) over 24 h at 13 degrees C and in constant darkness to test whether daily patterns (including amplitude, magnitude, and time of peak VO(2)) of metabolic rate in lizards differ with activity period. We also tested for phylogenetic differences in metabolic rate between skinks and geckos. Three daily patterns were evident: 24-h cycle, 12-h cycle, or no daily cycle. The skink Cyclodina aenea has a 12-h crepuscular pattern of oxygen consumption. In four other species, VO(2) increased with, or in anticipation of, the active part of the day, but three species had rhythms offset from their active phase. Although not correlated with activity period or phylogeny, amplitude of VO(2) may be correlated with whether a species is temperate or tropical. In conclusion, the metabolic rate of many species does not always correlate with the recorded activity period. The dichotomy of ecology and physiology may be clarified by more in-depth studies of species behaviors and activity periods.     

Temperature Cycles as Zeitgeber for the Circadian Clock of Two Burrowing Rodents, the Normothermic Antelope Ground Squirrel and the Heterothermic Syrian Hamster

The circadian systems of two burrowing rodents, the normothermic diurnal antelope ground squirrel (Ammospermophilus leucurus) and the heterothermic nocturnal Syrian hamster (Mesocricetus auratus) were compared with respect to entrainment by temperature cycles. Both species were subjected to the same ambient temperature (Ta) cycles with amplitudes between 4 and 12ºC at constant illuminations (100 and 0.05 lux in squirrels; 1.0 lux in hamsters). Wheel running activity was continuously measured. There was considerable interindividual variation in the daily pattern of wheel-running activity and in the ability to entrain to Ta cycles of the same amplitude in both species. The activity rhythms of about 33 to 67% of the animals of the two species entrained to Ta cycles with amplitudes of 6 to 12ºC. One of six squirrels and one of nine hamsters even entrained to Ta cycles of 4ºC. In the antelope ground squirrels, activity occurred predominantly in the cooler phase of the Ta cycle, whereas hamsters were mainly active during the warmer phase. In some squirrels, the activity rhythms were split in two main components which were both entrained to the cooler fraction of the Ta cycle, sometimes with additional (masking) activity during the warmer fraction (above 30ºC). The results do not support the earlier view that temperature cycles affect the circadian systems of heterothermic mammals, including hibernators, more strongly than those of normothermic species. It is suggested that behavioral and physiological adjustments to the environmental conditions play an important role for mammalian circadian systems to respond to temperature changes as a zeitgeber.     

Temperature Cycles as Zeitgeber for the Circadian Clock of Two Burrowing Rodents,the Normothermic Antelope Ground Squirrel and the Heterothermic Syrian Hamster

The circadian systems of two burrowing rodents, the normothermic diurnal antelope ground squirrel (Ammospermophilus leucurus) and the heterothermic nocturnal Syrian hamster (Mesocricetus auratus) were compared with respect to entrainment by temperature cycles. Both species were subjected to the same ambient temperature (Ta) cycles with amplitudes between 4 and 12ºC at constant illuminations (100 and 0.05 lux in squirrels; 1.0 lux in hamsters). Wheel running activity was continuously measured. There was considerable interindividual variation in the daily pattern of wheel-running activity and in the ability to entrain to Ta cycles of the same amplitude in both species. The activity rhythms of about 33 to 67% of the animals of the two species entrained to Ta cycles with amplitudes of 6 to 12ºC. One of six squirrels and one of nine hamsters even entrained to Ta cycles of 4ºC. In the antelope ground squirrels, activity occurred predominantly in the cooler phase of the Ta cycle, whereas hamsters were mainly active during the warmer phase. In some squirrels, the activity rhythms were split in two main components which were both entrained to the cooler fraction of the Ta cycle, sometimes with additional (masking) activity during the warmer fraction (above 30ºC). The results do not support the earlier view that temperature cycles affect the circadian systems of heterothermic mammals, including hibernators, more strongly than those of normothermic species. It is suggested that behavioral and physiological adjustments to the environmental conditions play an important role for mammalian circadian systems to respond to temperature changes as a zeitgeber.     

Circadian Rhythms of Locomotor Activity in Lycosa tarentula (Araneae, Lycosidae) and the Pathways of Ocular Entrainment

Lycosa tarentula is a ground-living spider that inhabits a burrow where it awaits the appearance of prey or conspecifics. In this study, circadian rhythms of locomotor activity were examined as well as the ocular pathway of entrainment. Thirty-three adult virgin females were examined under constant darkness (DD); all of them exhibited robust circadian rhythms of locomotor activity with a period averaging 24.1h. Fourteen of these spiders were studied afterwards under an LD 12:12 cycle; they usually entrained to in the first or second day, even when the light intensity was as low as 1 lx. During the LD cycle, locomotor activity was generally restrained to the darkness phase, although several animals showed a small amount of diurnal activity. Ten males were also examined under LD; they were also nocturnal, but were much more active than the females. Seven females were examined under constant light (LL); under this they became arrhythmic. Except for the anterior median eyes (OMAs), all the eyes were capable of entraining the locomotor activity to an LD cycle. These results demonstrate that under laboratory conditions and low light intensities locomotor activity of Lycosa tarentula is circadian and in accordance with Aschoff's 'rule'. Only OMAs are unable to entrain the rhythm; the possible localization of circadian clock is therefore discussed.     

Circadian and acamprosate modulation of elevated ethanol drinking in mPer2 clock gene mutant mice

The PER2 clock gene modulates ethanol consumption, such that mutant mice not expressing functional mPer2 have altered circadian behavior that promotes higher ethanol intake and preference. Experiments were undertaken to characterize circadian-related behavioral effects of mPer2 deletion on ethanol intake and to explore how acamprosate (used to reduce alcohol dependence) alters diurnal patterns of ethanol intake. Male mPer2 mutant and WT (wild-type) mice were entrained to a 12:12?h light-dark (12L:12D) photocycle, and their locomotor and drinking activities were recorded. Circadian locomotor measurements confirmed that mPer2 mutants had an advanced onset of nocturnal activity of about 2?h relative to WTs, and an increased duration of nocturnal activity (p < .01). Also, mPer2 mutants preferred and consumed more ethanol and had more daily ethanol drinking episodes vs. WTs. Measurements of systemic ethanol using subcutaneous microdialysis confirmed the advanced rise in ethanol intake in the mPer2 mutants, with 24-h averages being ～60 vs. ～25?mM for WTs (p < .01). A 6-day regimen of single intraperitoneal (i.p.) acamprosate injections (300?mg/kg) at zeitgeber time (ZT) 10 did not alter the earlier onset of nocturnal ethanol drinking in the mPer2 mutants, but reduced the overall amplitude of drinking and preference (both p < .01). Acamprosate also reduced these parameters in WTs. These results suggest that elevated ethanol intake in mPer2 mutants may be a partial consequence of an earlier nighttime activity onset and increase in nocturnal drinking activity. The suppressive action of acamprosate on ethanol intake is not due to an altered diurnal pattern of drinking, but rather a decrease in the number of daily drinking bouts and amount of drinking per bout.     

Population of the mosquitoes Culex pipiens molestus (Diptera: Culicidae) in dwellings of Lensk city

Culex pipiens molestus has been distributed in Siberia and the Russian Far East since 80s of XX century. In early 90s, C. p. molestus reached South Yakutia (Nerungri city), and nowadays it occurs in South-West Yakutia (Lensk city). The number of breeding places with larvae and pupae of mosquitoes was not numerous in Lensk; among 56 examined houses, the places with larvae and pupae mosquitoes were registered in seven houses, while the adult mosquitoes occurred in the majority of the houses. The occurrence of larvae varied from 12.5 to 33.3%, occurrence of adults-from 12.5 to 100%. The coefficient of variation of larvae in the samples was equal to 0.92, and adults had the following coefficients: 0.84 in the houses without basement; 0.92 in the houses with dry basements; 1.05 in the houses with pools in basements. This data are referred to the casual type, it means that the distribution of mosquitoes in space is dispersed. Imago spread actively in stone house of Lensk that provides mosquitoes with new places of breeding, and supports the stability of population in new areas.     

Circadian Rhythms of Locomotor Activity in Lycosa tarentula (Araneae,Lycosidae) and the Pathways of Ocular Entrainment

Lycosa tarentula is a ground-living spider that inhabits a burrow where it awaits the appearance of prey or conspecifics. In this study, circadian rhythms of locomotor activity were examined as well as the ocular pathway of entrainment. Thirty-three adult virgin females were examined under constant darkness (DD); all of them exhibited robust circadian rhythms of locomotor activity with a period averaging 24.1h. Fourteen of these spiders were studied afterwards under an LD 12:12 cycle; they usually entrained to in the first or second day, even when the light intensity was as low as 1 lx. During the LD cycle, locomotor activity was generally restrained to the darkness phase, although several animals showed a small amount of diurnal activity. Ten males were also examined under LD; they were also nocturnal, but were much more active than the females. Seven females were examined under constant light (LL); under this they became arrhythmic. Except for the anterior median eyes (OMAs), all the eyes were capable of entraining the locomotor activity to an LD cycle. These results demonstrate that under laboratory conditions and low light intensities locomotor activity of Lycosa tarentula is circadian and in accordance with Aschoff's 'rule'. Only OMAs are unable to entrain the rhythm; the possible localization of circadian clock is therefore discussed.     

Low cost of locomotion in lizards that are active at low temperatures

The nocturnality hypothesis of K. Autumn and coworkers states that nocturnal geckos have evolved a low energetic cost of locomotion (C(min)). A low C(min) increases maximum aerobic speed and partially offsets the decrease in maximum oxygen consumption caused by activity at low nocturnal temperatures. We tested whether a low C(min) is unique to nocturnal geckos or represents a more general pattern of convergent evolution among lizards that enables nocturnality and/or cold-temperature activity. We measured C(min) in four carefully selected lizard species from New Zealand (two nocturnal and two diurnal; n=5-9 individuals per species), including a nocturnal and diurnal gecko (a low C(min) is a gecko trait and is not related to nocturnality), a nocturnal skink (a low C(min) is related to being nocturnal), and a diurnal skink active at low temperatures (a low C(min) is related to being active at low body temperatures). The C(min) values of the four species measured in this study (range=0.21-2.00 mL O(2) g(-1) km(-1)) are lower than those of diurnal lizards from elsewhere, and the values are within or below the 95% confidence limits previously published for nocturnal geckos. A low C(min) increases the range of locomotor speeds possible at low temperatures and provides an advantage for lizards active at these temperatures. We accepted the hypothesis that nocturnal lizards in general have a low C(min) and provide evidence for a low C(min) in lizards from cool-temperate environments. The low C(min) in lizards living at high latitudes may enable extension of their latitudinal range into otherwise thermally suboptimal habitats.     

Polymorphism of mitochondrial DNA and infection with symbiotic cytoplasmic bacterium Wolbachia pipientis in mosquitoes of the Culex pipiens complex from Russia

A total of 208 mosquitoes of the Culex pipiens complex from 15 basement and terrestrial populations collected in different regions of the European part of Russia and Siberia were examined by genetic methods. Among these, two major mitotypes, M and P, were identified. These mitotypes differed by six substitutions in the 246-bp mitochondrial DNA cytochrome oxidase I gene fragment examined. Urban basement mosquito ecotype C. pipiens (form molestus) were characterized by the presence of mitotype M and infection with the endosymbiotic bacteria of the genus Wolbachia. Mosquitoes of the C. pipiens complex inhabiting opened biotopes harbored mitotype P, or its variety, mitotype P1, and were not infected with Wolbachia. Thus, in natural conditions marked linkage disequilibrium between cytoplasmic elements, mitochondrial DNA and Wolbachia, can be observed. Similarity of mitotypes in form molestus mosquito from different geographical localities favors the hypothesis on the common ancestry of urban mosquitoes.     

Effects of environmental factors on circadian activity in the flesh fly, Sarcophaga crassipalpis

Abstract.The diel locomotor activity patterns of wandering larvae in the flesh fly, Sarcophaga crassipalpis Macquart (Diptera: Sarcophagidae), were examined using a novel apparatus and shown to be primarily diurnal, but with a minority (37%) showing nocturnal activity. In response to the environmental stress of heat shock, a significantly larger proportion (72%) of the larvae became nocturnal. In comparison, adult circadian activity also was predominantly diurnal, but not correlated with the larval activity patterns. In addition, adult patterns showed age-related changes in entrainment and free running period. Finally, the phase of circadian-gated adult eclosion was shown to be entrained by a 3-day exposure to light–dark cycles delivered prior to pupariation, with the phase maintained throughout pupal–adult metamorphosis under constant dark conditions. These results demonstrate that environmental changes may have profound effects on the expression of 24-h activity patterns and circadian rhythms during different life stages throughout development.     

Spacing behaviour in resting Culex pipiens (Diptera, Culicidae): a computer modelling approach

Abstract. We compared two models based on Monte Carlo simulations with the spacing patterns of resting adults of the mosquito Cx pipiens L. (form molestus ) (Diptera, Culicidae). The comparison with the first model which assumed a distribution of complete spatial randomness showed that there was a tendency for mosquitoes to repel other mosquitoes which were too close, resulting in high local regularity. The second model took this repulsion into account. A final test of global patchiness using Model 2 indicated that the spacing behaviour of Cx pipiens was not sufficiently described by local regularity only, but also required a gregarious element. This supports the hypothesis that the molestus form of Cx pipiens shows spaced-out gregariousness in the sense of Kennedy & Crawley (1967).     

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

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

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

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

Circadian flight schedules in night-migrating birds caught on migration

Many species of migratory birds migrate in a series of solitary nocturnal flights. Between flights, they stop to rest and refuel for the next segment of their journey. The mechanism controlling this behaviour has long remained elusive. Here, we show that wild-caught migratory redstarts (Phoenicurus phoenicurus) are consistent in their flight scheduling. An advanced videographic system enabled us to determine the precise timing of flight activity in redstarts caught at a northern European stopover site during their return trip from Africa. Birds were held captive for three days in the absence of photoperiodic cues (constant dim light) and under permanent food availability. Despite the absence of external temporal cues, birds showed clear bimodal activity patterns: intense nocturnal activity alternating with diurnal foraging and resting periods. The onset of their migratory activity coincided with the time of local sunset and was individually consistent on consecutive nights. The data demonstrate that night-migrating birds are driven by autonomous circadian clocks entrained by sunset cues. This timekeeping system is probably the key factor in the overall control of nocturnal songbird migration.     

Body temperature and behaviour of golden hamsters (Mesocricetus auratus) and ground squirrels (Spermophilus tridecemlineatus) in a thermal gradient

Golden hamsters and thirteen-lined ground squirrels were maintained individually in a thermal gradient (14°C to 33°C) for several weeks under a 14L: 10D light-dark cycle. Animals of both species showed robust daily rhythms of body temperature and locomotor activity with acrophases consistent with the habits of the species (diurnal acrophases in the diurnal squirrels and nocturnal acrophases in the nocturnal hamsters). Hamsters showed a robust daily rhythm of temperature selection 180° out of phase with the rhythms of body temperature and locomotor activity. Squirrels did not show a daily rhythm of temperature selection. These results raise the hypothesis that a daily rhythm of temperature selection is exhibited by nocturnal but not by diurnal endotherms.