Nocturnal Body Temperature Research Articles

Social status does not affect resting metabolic rate in wintering dark-eyed junco (Junco hyemalis).

Studies of wintering birds have demonstrated a correlation between social rank and energy expenditures. It is assumed that dominance is energetically costly because of increased activity, possibly caused by elevated androgen levels. As winter acclimatization leads to an increase in metabolic rate, maintaining dominance status in a cold climate can be a substantial challenge. We measured resting metabolic rates in dominant and subordinate dark-eyed juncos (Junco hyemalis) living in small groups in a controlled winter environment. We found no significant effect of social rank when controlling for body size. It has been shown previously that high testosterone levels during the nonbreeding season can lead to higher body conductance, fat loss, and higher nocturnal body temperature. A hypothesis explaining our result is that for juncos it is preferable to maintain low androgen levels during winter and to maintain social rank using a mechanism other than higher agonistic activity.

High nocturnal body temperatures and disturbed sleep in women with primary dysmenorrhea.

Primary dysmenorrhea is characterized by painful uterine cramps, near and during menstruation, that have an impact on personal life and productivity. The effect on sleep of this recurring pain has not been established. We compared sleep, nocturnal body temperatures, and hormone profiles during the menstrual cycle of 10 young women who suffered from primary dysmenorrhea, without any menstrual-associated mood disturbances, and 8 women who had normal menstrual cycles. Dysmenorrheic pain significantly decreased subjective sleep quality, sleep efficiency, and rapid eye movement (REM) sleep but not slow wave sleep (SWS), compared with pain-free phases of the menstrual cycle and compared with the controls. Even before menstruation, in the absence of pain, the women with dysmenorrhea had different sleep patterns, nocturnal body temperatures, and hormone levels compared with the controls. In the mid-follicular, mid-luteal, and menstrual phases, the dysmenorrheics had elevated morning estrogen concentrations, higher mean in-bed temperatures, and less REM sleep compared with the controls, as well as higher luteal phase prolactin levels. Both groups of women had less REM sleep when their body temperatures were high during the luteal and menstrual phases, implying that REM sleep is sensitive to elevated body temperatures. We have shown that dysmenorrhea is not only a disorder of menstruation but is manifest throughout the menstrual cycle. Furthermore, dysmenorrheic pain disturbs sleep, which may exacerbate the effect of the pain on daytime functioning.

Energy content of the evening meal alters nocturnal body temperature but not sleep

Meals of varying energy content and episodes of sleep influence body temperature. We compared the effect of an evening meal, varying from high-energy (11.91 ± 0.86 MJ) to average (5.74 ± 0.88 MJ) and a 10-h fast (no evening meal), on nocturnal body temperature and sleep. Seven healthy men (20–24 years, mean body mass index of 23.4 ± 2.6 kg/m 2) reported to the laboratory for an evening meal at 2000 h having consumed similar amounts of food before 1300 h. After completing the meal, subjective hunger ratings were assessed, and a venous blood sample taken. The subjects spent 4 nonconsecutive nights (an adaptation night, followed by either of the two meal conditions or the fast in random order) in the sleep laboratory when polysomnographic recordings were made from 2300 to 0700 h. Meal energy content and serum concentrations of insulin, triglyceride, and low-density lipoproteins (LDL) varied significantly. Lower rectal temperatures were measured during the fast than following the meals. Over the 8-h recording period, thermal response indices (TRI) varied with higher body temperatures following the higher energy meal. Similar rectal temperatures were attained by the end of the sleep periods. There were no significant differences in any of the subjective or objective sleep measures. The physiological responses associated with the transient dietary changes of an evening meal or a 10-h fast altered nocturnal body temperature but did not significantly affect sleep of good sleepers when sleep was initiated 2 to 3 h after finishing the meal.

Nocturnal Variation in Body Temperature of Griffon Vultures

The variation in body temperature over 24 hr was measured by implanting temperature measuring radio transmitters in the abdominal cavity of six captive Griffon Vultures (Gyps fulvus). Body temperature was measured when the birds were under three different feeding regimes: within 24 hr of feeding (while digesting), 2-3 days after feeding (post-absorptive state), and 9.5-10.5 days after feeding (food deprived). For all birds, nocturnal body temperature was significantly lower than diurnal body temperature. The decline in body temperature at night was significantly greater in food deprived birds, which at 10.5 days post-feeding had dropped by 4-6°C. We suggest that this is an adaptation to reduce energy requirements of Griffon Vultures during periods of food shortage.

Electroconvulsive therapy increases circadian amplitude and lowers core body temperature in depressed subjects

Reduced amplitude of the circadian temperature rhythm and elevated nocturnal body temperature normalize after successful pharmacotherapy of major depression. Core body temperature was continually monitored in three groups: a) 6 depressed patients before an electroconvulsive therapy (ECT) course and b) after an ECT course; and c) 6 healthy, sex-matched controls of similar age. The 24-hour profile of temperature was significantly different in patients pre-ECT than in patients post-ECT or in controls. Post-ECT subjects and controls manifested 24-hour profiles similar to one another. Circadian temperature rhythm amplitude increased after ECT. The mean asleep and mean 24-hour temperatures were significantly higher in patients pre-ECT than post-ECT and controls. We find that ECT restores a disrupted circadian temperature rhythm in depressed patients.

Participation of Gastrointestinal-Load Volume in ?Setting? the Pigeon's Nocturnal Body Temperature

Participation of Gastrointestinal-Load Volume in ?Setting? the Pigeon's Nocturnal Body Temperature

Nocturnal digestion, cloacal excretion, and digestion-related thermogenesis in pigeons ( Columba livia)

Two indicators of nocturnal digestive activity were identified in pigeons. Experiment 1 showed that a sizable amount of food empties from the crop while pigeons are inactive during the night. Experiment 2 showed that the number and volume of cloacal droppings during the night were directly related to the volume of food consumed during the day. The temporal pattern of cloacal droppings in the night was systematically related to features of the nocturnal body temperature (Tb) curves, suggesting that excretory activity in thermogenic in its own right and/or that it is a marker for a thermogenic process in the upper digestive tract. Questions about the relationship between digestion-related thermogenesis and shivering thermogenesis during the night in birds are highlighted by these findings.

Nonsteroidal anti-inflammatory drugs alter body temperature and suppress melatonin in humans

Nonsteroidal anti-inflammatory drugs (NSAIDs) inhibit prostaglandin synthesis in humans. Prostaglandins are involved in thermoregulation, melatonin synthesis, and sleep. To determine effects of NSAIDs on body temperature (BT) and melatonin synthesis (MT) in humans, and to elucidate mechanisms by which NSAIDs may alter sleep patterns, a series of experiments using the NSAIDs aspirin and ibuprofen was conducted. Seventy-five subjects were tested under several experimental protocols. BT after NSAID or placebo was assessed in both between- and within-subjects designs at night and during the day. MT levels were assessed after NSAID or placebo at night in a within-subjects design. The normal nocturnal BT decrease was attenuated and MT was suppressed after NSAID relative to after placebo administration. Lower MT levels were associated with a relative flattening of BT. Daytime BT was not affected by NSAIDs. These results are compatible with the hypothesis that some of the behavioral changes associated with NSAIDs, including changes in sleep, are due to changes in BT and MT. We speculate that NSAID effects on sleep and BT are related to prostaglandin synthesis inhibition and/or suppression of MT.

Regularity of daily life in relation to personality, age, gender, sleep quality and circadian rhythms.

A diary-like instrument to measure lifestyle regularity (the 'Social Rhythm Metric'-SRM) was given to 96 subjects (48 women, 48 men), 39 of whom repeated the study after at least one year, with additional objective measures of rest/activity. Lifestyle regularity as measured by the SRM related to age, morningness, subjective sleep quality and time-of-day variations in alertness, but not to gender, extroversion or neuroticism. Statistically significant test-retest correlations of about 0.4 emerged for SRM scores over the 12-30 month delay. Diary-based estimates of bedtime and waketime appeared fairly reliable. In a further study of healthy young men, 4 high SRM scorers ('regular') had a deeper nocturnal body temperature trough than 5 low SRM scorers ('irregular'), suggesting a better functioning circadian system in the 'regular' group.

Biological Rhythmicity in Normal infants During the First 3 Months of Life

The mammalian "biological clock," which resides in the hypothalamic suprachiasmatic nucleus, has an important role in both the timing and organization of sleep and in the coordination of sleep with other physiological rhythms such as temperature regulation and respiratory control. We wished to describe the development of the circadian system in normal infants during the first 3 months of life. Ten healthy full term infants were studied in the infant's home for three consecutive days at 1 month and 3 months postnatal age. Rectal temperature, abdominal skin temperature, heart rate, and activity were recorded at 2-minute intervals during each study using a small microcomputer. Circadian periodicity for most variables was seen at 1 month of age and significantly increased at 3 months. Differences in the pattern of rhythmicity during these two developmental periods were highlighted by an increase in activity during the subjective day and a decrease in Trec during the subjective night at 3 months compared to 1 month. Correlational analysis revealed that all pairs of variables, exclusive of Tsk, showed a significantly higher association at 3 months relative to 1 month. The lengthening of the interfeeding interval at 3 months of age corresponded with an increased consolidation of sleep during the night and a relatively lower nocturnal body temperature minima compared to 1 month of age. The results of this study underscore the subtle changes in the nature and interaction of several infant variables during this critical developmental period, which may reflect maturation of the circadian system and its coupling with homeostatic effector systems that are developing in parallel.

Nocturnal and circadian body temperatures of depressed outpatients during symptomatic and recovered states

Forty-one recurrent unipolar depressed outpatients were studied at baseline (symptomatic period) and at recovery. Rectal temperatures were measured continously between 1800h and 0600h on 2 successive nights at each time point. In those 24 patients who recovered after receiving interpersonal psychotherapy (IPT) alone (“psychotherapy responders”) and who thus remained drug-free throughout, there was no difference in nocturnal body temperatures between baseline and recovery time points. Moreover, noctrunal temperatures patterns appeared to be very similar to those of a healthy contrast group ( n=17). The contrast group was not matched for age and gender with the patient group, though, so the comparison was only suggestive. At baseline, those recovering after psychotherapy alone ( n=24) did not differ from those eventually failing to respond to IPT and requiring medications to achieve recovery (“medication responders”) ( n=17). Medication responders did show some baseline versus recovery differences in nocturnal temperatures, but these may possibly have been a function of the medications used. At recovery, a subset of 19 psychotherapy responders and 13 medication responders underwent an “unmasking” experiment involving 36 hours of wakeful bedrest. In the resulting endogenous temperature rhythms, there was no evidence of any reliable differences between the psychotherapy responder and medication responder patient groups. Both groups showed rhythms that were very similar to those of a healthy contrast group ( n=17), although, again, the contrast group was not matched for age or gender with the patient group. In conclusion, in measures of body temperature, at least, there appeared to be little evidence of circadian dysfunction in this group of recurrent outpatient depressives.

Restricted energy intake affects nocturnal body temperature and sleep patterns

Changes in nocturnal body temperature, sleep patterns, and blood variables with energy restriction (3347 kJ/d) were studied in nine overweight (body mass index 26.1 +/- 2.8) premenopausal women aged 20-36 y. Variables were measured both 2 wk before and in the final 2 wk of 4-wk dieting. Data collected 28 d apart were compared to attenuate menstrual cycle differences. Subjects lost 8 +/- 4% of their initial body mass after 4 wk of energy restriction and plasma triiodothyronine (T3) was significantly reduced from 5.9 +/- 0.7 to 5.1 +/- 0.6 pmol/L (P < 0.05). The implied suppression of heat production (metabolic rate) with reduced T3 may account for the observed decrease in minimum nocturnal rectal temperature (from 36.5 +/- 0.3 to 36.3 +/- 0.3 degree C, P < 0.05). Furthermore, dieting significantly altered sleep patterns; sleep onset latency was lengthened and slow-wave sleep decreased (P < 0.05). These changes may be indicative of reduced restorative/biosynthetic requirements. It thus appears that energy restriction results in a hypometabolic state that affects nocturnal body temperature and sleep patterns.

Thyroid hormones, body temperature, and antidepressant treatment

Depression is frequently observed in patients with primary thyroid disorders (Whybrow and Felrell 1974). Moreover, the importance of thyroid function in the regulation of mood and affective illness has been noted by several investigators in recent years (Joffe et al 1984; Loosen 1986; Bauer and Whybrow 1988). In particular, several abnormalities of thyroid function have been reported in patients with primary affective disorder (Gold et al 1981; Joffe et al 1985). For instance, Loosen and Prange (1982) found that approximately one-third of depressed patients exhibit a blunted thyrotropin (TSH) response to thyrotropin releasing hormone (TRH). Furthermore, it has been shown that there is a decrease of '13 and T4 and an increase of TSH after antidepressive treatment with carbamazepine (Roy-Byrne et al 1984), lithium (Benaim and Page 1984), the combination of carbamazepine and lithium (Kramlinger and Post 1990), and electroconvuisive treatment (Kirkegaard and Faber 1981). These results are consistent with animal studies indicating that tricyclic antidepressants decrease thyroid function (Fischetti 1962; Morley et al 1981). Nevertheless, there are reports that did not find any alterations in thyroid hormones with tricyclic antidepressant treatment (Gregoire et al 1977; Linnoila et al 1979; Nordgren and Sheele 1981). Recent studies also indicete that a significant reductio~ in total thyroxine (tT4) and free thyroxine (f14) might be associated with the antidepressant response to clomipramine and maprotiline (Baumgartner e, al 1988) and to desipramine (Joffe and Singer 1990). Recently, Wehr (1990) hypothesized that thermoregulation may provide a framework for understanding the pathophysiology of affective disorders. Higher nocturnal body temperature has been reported to be associated with lower levels of TSH in acutely depressed patients when compared with controls or with remitted cases (Souetre et al 1988). Based on existing knowledge, we attempted to further elicidate the effects of antidepressive medication on the hypothalamic-pituitary-thyroid axis (HPTA) and body temperature. We choose fluvoxamine, which is considered to be a selective serotonine reuptake inhibitor (Claasen 1983) and maprotiline, which is believed to exert its antidepressant effect predominantly via the noradrenergic (NA) system (Maitre et al 1971), to find out if these drugs induced chaages in HPTA or in body temperature.

High nocturnal body temperature in premenstrual syndrome and late luteal phase dysphoric disorder

Because women with late luteal phase dysphoric disorder (LLPDD) experience symptomatic affective states predictably, they can be studied to determine whether there are biological findings related solely to the clinically symptomatic state. The authors sought to answer the question, Does body temperature change with affective state? The core body temperature and motor activity patterns of 10 women with premenstrual syndrome (PMS), six of whom also met criteria for LLPDD, and no other psychological or medical illness were compared to those of six women with chronic, noncyclic dysphoria and six asymptomatic comparison women at four phases of the menstrual cycle. The nocturnal temperatures of the women with PMS/LLPDD were significantly higher than those of the comparison subjects across the entire menstrual cycle, but there were no differences in nocturnal activity levels. The women with noncyclic dysphoria had a mean nocturnal temperature in the follicular phase as high as that of the women with PMS/LLPDD. The temperatures of all women were higher in the luteal phase than in the follicular phase. These findings suggest that in the future investigators should document menstrual cycle phase in all female subjects and, when studying body temperature, should carefully monitor symptomatic state in comparison subjects.

Regulation of body temperature, metabolic rate, and sleep in fasting pigeons diurnally infused with glucose or saline

To test the hypothesis that nocturnal body temperature (Tb) and metabolic rate (MR) in the pigeon are regulated during sleep at levels proportional to energy reserves, continuous recordings of Tb, oxygen consumption (VO2), carbon dioxide production, and electrophysiological measures were taken from five pigeons subjected to two separate 4-day fasts. Energy reserves were depleted differentially during the fasts by 12-h diurnal infusions of either saline or isosmotic glucose solutions. Although Tb and VO2 were closely correlated, VO2 declined throughout the fast during diurnal and nocturnal phases of the 12:12 light-dark cycle whereas significant declines in Tb were restricted to the night. Diurnal thermal conductance declined over days of fasting, especially during saline infusions, and was reduced to minimal levels each night. The durations and distributions of arousal states did not change during the fast or differ between conditions. The results were consistent with the hypothesis of a nocturnal regulation of Tb and metabolic rate proportional to energy reserves.

Twenty-four-hour profiles of body temperature and plasma TSH in bipolar patients during depression and during remission and in normal control subjects.

The 24-hour patterns of body temperature and plasma thyrotropin (TSH) were measured in eight bipolar patients in both depressed and recovered (after 3 weeks of treatment) states and in eight normal control subjects. Clear circadian patterns were detected for both temperature and TSH. Nocturnal body temperature was increased and the nocturnal surge of TSH was blunted during depression; these abnormalities were corrected after recovery. The inverse relationship between changes in body temperature and TSH levels at night suggests that changes in thermoregulation may be responsible for the neuroendocrine disturbance and may play a role in the pathophysiology of depression.

Spread-Winged Posture of Turkey Vultures: Single or Multiple Function?

The behavior and spread-winged postures of Turkey Vultures (Cathartes aura) were examined at a communal roost near Superior, Arizona, from April-June, 1979. The temporal occurrence of wing-spreading during the morning pre-departure period was positively correlated with the intensity of the sun's rays, but was independent of the ambient low overnight temperature. Wing-spreading was more common when vultures were wet than when they were dry. Turkey Vultures appear to spread their wings for at least two reasons: (1) to dry feathers, and (2) to ameliorate the thermal gradient between themselves and their environment, although the two functions are not mutually exclusive. Spreading the wings to realign feathers or to increase the mobility ofectoparasites seems unlikely. Nocturnal body temperature (Tb) of captive Turkey Vultures (Cathartes aura) varies by as much as 4?C (Heath 1962), and from 0.4 to 2.6?C (Hatch 1970), below normothermic levels (390C) for periods of many days. Diurnal fluctuations in Tb are apparently unrelated to food consumption, night-time low temperature, size or weight of the bird, age, or photoperiod (Hatch 1970). Because these vultures exhibit shallow regulated hypothermia, it has been proposed that their wing-spreading postures are energy-conserving behaviors; the birds may warm themselves or modify their thermal gradients by absorbing solar radiation (Heath 1962, Curry-Lindahl 1970, Ohmart and Lasiewski 1971, Kushlan 1973). Owing to their dark coloration, vultures would be expected to absorb much solar radiation (Hamilton and Heppner 1967, Lustick 1969, Marder 1973) at low wind speeds (Walsberg et al. 1978), even without spreading their wings. By adopting this posture, however, they may absorb much more sunlight, as suggested from the work of Lustick et al. (1980) with Herring Gulls (Larus argentatus). In Little Cormorants (Phalacrocorax niger), spreading the wings dries them efficiently; the wetter the bird, the longer it spreads its wings after swimming (Winkler 1983). Anhingas (Anhinga anhinga) also dry quickly when spreading their wings (Hennemann 1982, 1983). Their wings are often spread even when dry, enabling the birds to increase rates of heat gain by enhancing absorption of sunlight. Anhingas spread their wings most frequently when solar radiation is intense and when ambient temperatures are low (Hennemann 1982). Much is known about the wing-spreading behavior of aquatic birds, but the reasons why vultures spread their wings are far less certain ( .g., Cl rk 1969, Mueller 1972, Grier 1975). We therefore examined the behavior of Turkey Vultures in order to determine the significance of wing-spreading. We asked the following: (1) is wing-spreading independent of overnight low ambient temperature, or is wing-spreading by free-living vultures more frequent during cold w ather, as shown for Anhingas? (2) is wingspreading more common following rain than following dry conditions, owing to the heatdraining effects of wet plumage (Kennedy 1970, Mahoney 1984)? and, (3) is the occurrence of wing-spreading during the morning related to the intensity of solar radiation? Answers to hese questions would allow us to address other hypo heses advanced to explain the occurrence of wing-spreading by various birds (e.g., Kennedy 1969, Mueller 1972, Houston 1980).

Ecological Energetics and Foraging Behavior of Overwintering Bald Eagles

The ecological energetics and foraging behavior of overwintering Bald Eagles (Haliaeetus leucocephalus) were studied in 1978—1979 and 1979—1980 on the Nooksack River in northwestern Washington and in the laboratory. We investigated eagle energy requirements and energy relationships in a winter environment, and adaptations to winter energy and food (chum salmon [Oncorhynchus keta]) stress. We developed a model to predict the energy metabolism of free—living eagles. Information of food consumption and energy requirements of captive Bald Eagles was supplemented with laboratory and field data to determine energy costs of free existence. Basal metabolic rate, as measured by oxygen consumption on four eagles, was 11.595 kJ°g—1°h—1; a lower critical temperature of 10.6°C was recorded. Standard metabolic rate increased linearly with decreasing temperature. Thermal conductance was 0.347 kJ°g—1°h—1°°C—1. The diel range in body temperature was from 38.9° to 41.2°. Energy metabolism was measured in response to artificially induced rainfall. During 40 trials of 4 h of continuous rain, metabolism increased up to 9 and 21% at 6.1 and 22.2 cm/h rain levels, respectively. Effects of rain on heat loss by wild eagles on the Nooksack River were negligible even though precipitation was high. Data on metabolic responses were incorporated in a black—body heat—budget model which predicted energy costs of free existence. Ambient temperature, wind velocity, long—wave radiation, and rainfall data collected at three microhabitats used by eagles also were incorporated into the heat—budget model to determine heat production by wild birds. Activity budgets were assessed by tracking four radio—tagged eagles for 38 d. Flight activity occurred only during 1% of the 24—h day, and energy costs of flight (110 kJ/d) were included in the model. The daily energy budget (total energy metabolized), daily energy consumption (total food energy required), and daily food (salmon) consumption (total mass of food required) were 1703 kJ, 2068 kJ, and 489 g, respectively, for a 4.5—kg Nooksack eagle. Energy and food needs of free—living eagles were °10% more than for captive eagles. Most eagles roosted in coniferous rather than deciduous forest even though they expended more energy to travel there. By roosting in conifers they accrued a net daily energy savings of 61 kJ, or °5% of the daily energy budget after accounting for energy costs of flight to and from the roost. Energy savings in this protected microclimate were afforded by higher ambient temperature and long—wave radiation levels, and lower wind velocity; reduced rainfall had little effect. Social interactions by feeding eagles were quantified during 46 d of observation. Adult eagles were dominant over younger birds and were more successful at kleptoparasitizing (stealing) food. Kleptoparasitism was the primary means by which food was acquired. Interaction frequency averaged 0.27 interactions/min during feeding and was positively correlated with the size of the feeding group. Juveniles and subadults were less effective at feeding than adults. They consumed 410 and 459 g°bird—1°d—1 of salmon, respectively, while the adults ingested 552 g°bird—1°d—1. Young eagles failed to acquire the needed 489 g°bird—1°d—1 of salmon. The effects of this socially mediated food deprivation resulted in a less than optimal use of time and energy by young birds. When food is limited, young eagles probably incur the highest mortality rates. Our data indicate that overwintering Bald Eagles are effective at exploiting and conserving energy. They maximize energy gain by foraging in groups, gorging, and assimilating more energy during cold stress. They minimize energy loss by becoming sedentary, seeking protective microclimates, reducing nocturnal body temperatures, and reducing foraging costs by living in groups. Because Bald Eagles are suspected to be food—limited, protective management policies that reduce energy stress could reduce overwinter morality.

Body Temperature Regulation in Red-Tailed Hawks and Great Horned Owls: Responses to Air Temperature and Food Deprivation

-Core body temperatures of Red-tailed Hawks (Buteojamaicensis) and Great Homed Owls (Bubo virginianus) were monitored by telemetry at intervals throughout the winter and summer. The mean body temperature of the hawks over a 24-h period was 41.1PC, compared to 39.50C for the owls. Body temperature of both species fluctuated daily in association with periods of activity and rest. In the hawks, however, the difference between active and inactive body temperature increased in cold weather and in response to food deprivation because the birds maintained a lower nocturnal (inactive) body temperature. Hawks that were fasted in the winter kept their body temperatures 3.20C lower at night than during the day, which may result in a substantial savings of energy for this species at night. Body temperatures of Great Homed Owls were not influenced by cold, but were lower during inactive periods in response to food deprivation. Thus, owls may also reduce metabolic expenses during periods of food shortage by physiological adjustments. Most birds exhibit cyclical daily changes in body temperature (Tb), with highest temperatures occurring when they are active and lowest when they are inactive (see Dawson and Hudson 1970, Calder and King 1974 for reviews). A 1-30C difference in Tb between the active and inactive phases of the daily cycle may be augmented by cold exposure or food deprivation, and is especially apparent in small species (<50 g), which are incapable of subsisting on their energy reserves for more than two to three days (Calder and King 1974, Chaplin 1976, Biebach 1977, Ketterson and King 1977, Bucher and Worthington 1982). Birds of prey typically feed irregularly and may have to endure long periods of fasting during the winter when low temperatures and snow cover diminish or obscure the activity of their prey. Despite the energetic advantage gained by reducing Tb while inactive or fasting, daily fluctuations in Tb rarely exceed 1-20C in several species of owls (Coulombe 1970, Siegfried et al. 1975, Gessaman 1978), even when they are fasted to 75% of their initial body mass (Ligon 1969). Daily cycles in Tb, specifically in response to cold and/or fasting, have been reported for only a few falconiforms, and the manner in which the birds respond is still unclear. For example, Heath (1962) and Hatch (1970) disagreed as to the stability of Tb of the Turkey Vulture (Carthartes aura) when it is fasted or cold-stressed. However, the Tb of the Black Vulture (Coragyps atratus) is very labile and highly dependent on air temperature (Larochelle et al. 1982). Is such lability characteristic of falconiforms, and are the metabolic adjustments of these birds to winter therefore different from those of owls, their nocturnal counterparts? In this study, we compared the thermoregulatory responses of Red-tailed Hawks (Buteo jamaicensis) and Great Homed Owls (Bubo virginianus) to changes in air temperature and weight loss as a test of their metabolic adaptations to winter conditions. These two species are similar in size and in their utilization of habitat and prey (Craighead and Craighead 1956, Orians and Kuhlman 1956, Hagar 1957, Korshgen and Stuart 1972, McInvaille and Keith 1974, Petersen 1979, and Jaksic 1982). Their geographic ranges overlap throughout most of the United States year-round (Bent 1937, Robbins et al. 1983); even their home ranges overlap greatly in some localities (Craighead and Craighead 1956, Petersen 1979). In fact, the most notable ecological difference between them is the period of the day when they are active. Since these two species, representing different orders, are so similar ecologically, we wondered if their metabolic responses to such typical winter stresses as cold exposure and food limitation were also similar.

Body Temperatures of Incubating versus Non-Incubating Roadrunners

Core temperatures of free-living Roadrunners (Geococcyx californianus) were monitored with temperature-sensitive radio transmitters. Breeding males were found to perform all of the nocturnal incubation and to maintain normothermic body temperatures at night. Body temperatures of females and non-breeding males dropped at night to as low as 330C, depending on the ambient temperature. Facultative, short-term hypothermia appears to be a regular energysaving feature of this species that is conspicuously not employed by the nocturnal incubator. Most birds exhibit a circadian pattern of body temperature, involving a reduction of 1 to 30C during the nighttime hours (Dawson 1954, Aschoffand Pohl 1970). Lower nocturnal body temperatures appear to be due to reduced motor activity and other cyclic phenomena of sleep, since nocturnally active birds show a reversal of the pattern, i.e., lower diurnal body temperatures. Only a handful of species exhibit true hypothermia, defined operationally as a reduction of body temperature below 360C that may or may not involve torpor (Dawson and Hudson 1970). Facultative hypothermia is generally argued to be an energy-saving adaptation in response to periodic food shortages and/or excessively low ambient temperatures (Calder and King 1974). Species that exhibit hypothermia include nectar-feeding hummingbirds, aerial insectivores such as caprimulgids, swallows and swifts, small passerines overwintering in cold climates, and a few tropical species such as anis and colies (Dawson and Hudson 1970, Calder and King 1974). It has been argued that short-term hypothermia should occur only in small animals due to the theoretically prohibitive effects of thermal inertia and high rewarming cost for large animals (Calder and King 1974), yet vultures are known to lower body temperatures to 340C. Hypothermia is a complex phenomenon for which the costs and benefits are still poorly understood. The constraints that hypothermia places on breeding birds have not been explored. In particular, incubation and hypothermia may not be compatible. Incubating hummingbirds have generally been found to maintain normothermic body temperatures at night except during unusual cold (Calder 1971, Vleck 1982). Egg temperatures fall well below normal incubation temperatures during these lapses into hypothermia. It is not clear whether non-incubators under similar conditions would show the same or different responses. If incubators typically maintain high body temperatures under conditions when non-incubators show hypothermia, then incubation may be metabolically expensive. Furthermore, the female of a facultatively hypothermic species may be energetically constrained during incubation if she has laid a large clutch of eggs and must recuperate depleted reserves via hypothermia. Thus facultative hypothermia may have important implications for both clutch size and the sex that performs the incubation. The Roadrunner (Geococcyx californianus) is a large (350 g) cuckoo that has been reported to lower its body temperature under stressful conditions and to warm up by sunning (Ohmart and Lasiewski 1971). Hypothermic birds have been found to conserve a significant amount of energy compared to normothermic birds at the same ambient temperature. The Roadrunner is also among the few species in which the male performs most of the incubation, especially at night (Calder 1967, Ohmart 1973, Skutch 1976). In order to obtain a better understanding of the breeding energetics of this unusual bird, I monitored the body temperatures of free-living Roadrunners using radio-telemetry techniques while simultaneously following breeding activities. I here report significant differences in body temperature for Roadrunners as a function of sex and