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Circadian rhythms in body temperature of the pigtailed macaque (Macaca nemestrina) exposed to different ambient temperatures.

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American Journal of Primatology 9:l-13 (1985)
Circadian Rhythms in Body Temperature of the Pigtailed
Macaque (Macaca nernestrina) Exposed to Different
Ambient Temperatures
Department of Biological Sciences, University Center at Binghamtoq State University of
New York, Binghamton
The body and leg skin temperatures of five pigtailed macaques (Macaca
nemestrina) were measured at various ambient air temperatures ranging
from 22 to 32°C over 24-h periods. The rhythm in core body temperature
persisted in all ambient air temperatures, and the rhythm in leg skin
temperature was suppressed a t the higher and lower ambient temperatures.
This suggests that at the upper and lower regions of the thermoneutral
zone, either a rhythm in heat production or a heat loss rhythm other than
leg skin temperature is regulating the core body temperature rhythm. It
was also found that the mean colonic temperature increased linearly with
a n increase in ambient temperature and that a phase relationship existed
between ambient temperature and colonic temperature. In addition, as the
ambient temperature increased, additional sites along the distal portion of
the leg were recruited for vasomotor heat loss.
Key words: thermoregulation, Mucucu nemestrina, skin temperature, pigtailed macaque
Homeotherms maintain a relatively constant core body temperature despite
wide changes in the environmental temperature. This thermal balance is achieved
by adjusting heat loss and heat production. Vasomotor heat loss is the primary
avenue for regulating body temperature within a certain range of ambient air
temperatures known as the thermoneutral zone. Below this range, metabolic heat
production increases to offset increased heat loss. Above the thermoneutral zone,
heat loss is achieved by evaporative means.
While core body temperature is carefully regulated, it is not constant. Instead it
exhibits a prominent daily variation. Aschoff et a1 [1974] found that within the
thermoneutral zone the 24-h rhythm in deep body temperature of humans was
mainly due to a rhythm in heat loss. These authors showed that the distal regions
of the extremities were actively involved in heat loss and showed a rhythm in skin
Received August 18, 1984; revision accepted March 26,1985.
Address reprint requests to F.M. Sulzman, Department of Biological Sciences, University Center at
Binghamton, SUNY, Binghamton, NY 13901.
0 1985 Alan R. Liss, Inc.
2 I Wurster, Murrish, and Sulzman
temperature and conductance that were the inverse of the body temperature rhythm.
Heat loss through the skin of the hands and feet was minimal during the day but
began increasing several hours before subjective night and decreasing shortly after
the onset of sleep. At ambient air temperatures above and below the thermoneutral
zone, the rhythm in heat loss was suppressed owing to vasodilation or vasconstriction, but the body temperature rhythm persisted [Aschoff et al, 19741. Presumably,
the rhythm in metabolism plays the major role in generating the body temperature
rhythm below the thermoneutral zone.
Nonhuman primates also exhibit a day-night rhythm in body temperature. The
squirrel monkey Saimiri sciureus shows a persisting colonic temperature rhythm in
various lighting conditions and intensities [Sulzman et al, 19791. The tail is the
major site for regulating heat loss in the ambient temperature range of 25-30°C
[Stitt & Hardy, 19711, and there is a prominent rhythm in tail skin temperature
[Fuller et al, 19791 a t these ambient temperatures. These authors showed that the
tail skin temperature was low during the day, increased several hours before lights
off, and returned to daytime values during the night. Core body temperature and
tail temperature rhythms in a light-dark cycle of 12 h of light and 12 h of dark (LD
12:12) exhibited a tight coupling, and there existed a stable phase relationship
between the two rhythms. In constant light, the average period was similar for the
two temperatures, but there was a tendency for the rhythms to become uncoupled,
suggesting the presence of more than one oscillator in the thermoregulatory timing
Fuller [ 19841 measured the rhythms in colonic and hypothalamic temperatures
of the squirrel monkey at four different ambient temperatures. Although the range
of the waveforms was altered, the rhythms persisted at all ambient temperatures.
The timing of the waveform was not altered by the changing ambient temperature,
suggesting that in these monkeys the environmental temperature does not play a
major role in regulating the body temperature rhythm.
Sickles et a1 [unpublished data] found an inverse relationship between core
temperature and ankle skin temperature rhythms of pigtailed macaques (Macaca
nemestrina) in cycles of LD 12:12, LD 1623, and constant light (LL). This suggests
that, as Aschoff found in humans, these animals may use their limbs as a regulatory
mechanism for the deep body temperature rhythm.
To examine the control of the body temperature rhythm in M. nemestrina, we
exposed these monkeys to ambient air temperatures ranging from 22 to 32°C and
recorded the rhythm in deep body temperature. Skin temperatures at sites along
the leg were also measured to determine how the lower extremities are used in
regulating the body temperature rhythm in response to varying heat loads placed
on the body.
Five male pigtailed macaques (Macaca nemestrina) were used. They were juveniles (2-4 yr old) weighing between 4 and 7 kg. The macaques were housed individually in cages in a common room with a n air temperature between 25 and 26°C.
Lighting was maintained on a light-dark cycle of 16 h of light and 8 h of dark (LD
16231, with lights on a t 0800 h EST, circadian time (CT) 0, and lights off at 2400 h
EST (CT 16). The light intensity was kept a t 600 lux during the light period and 0
lux during the dark period. Monkey chow biscuits, fruit, and water were provided
twice daily, and animals were able to feed and drink ad libitum.
Body and skin temperatures were measured at ambient temperatures of 22,24,
26, 28, 30, and 32°C. Only four animals were measured at 22 and 32°C. The
experiments took place in a walk-in environmental chamber with the ambient
Circadian Rhythm of Body Temperature / 3
temperature maintained a t 0.1"C. The light-dark cycle and the illumination were
the same a s in the colony housing room.
Typically, a n experiment lasted 3 days. A monkey was instrumented in the
afternoon of the first day. Data collection began at CT 0 with lights on on the second
day and stopped a t CT 24 on the third day, whereupon the monkey was returned to
the colony housing room. The monkey was not involved in experiments for a t least
a week or more.
Skin temperatures were measured at five sites on the leg: toe, foot, ankle, calf,
and thigh, using 26-gauge copper-constantan thermocouples. Each thermocouple
was attached with surgical tape. The foot thermocouple was attached on the back of
the foot between the digit bones. The colonic temperature was measured using a
thermocouple inserted approximately 8 cm into the monkey's rectum. The end of
the probe was taped to the monkey's tail to prevent expulsion during defecation.
The animal also wore a short-sleeved mesh jacket.
Handling of monkeys was facilitated by a n injection of ketamine (20 mgkg).
The effects lasted between 3 and 4 h. The drug was used only on the first day for
instrumenting the monkey and a t the end of the experiment. The monkey was
placed in a specially designed restraint chair after attachment of the thermocouples.
All monkeys were previously trained to tolerate the chair for prolonged periods.
While in the restraint chair, food and water were available ad libitum and fruit was
given three times daily. We do not feel that these procedures produce artifacts in the
temperature patterns because 1) our animals were extensively trained to tolerate
chair restraint before experiments were begun, 2) ketamine is rapidly cleared from
the body, and 3) we do not use data until the beginning of the second day in the
chair. Further, in experiments with monkeys in restraint chairs [Sulzman e t al,
unpublished data], we have observed that after the first 4-6 h in the restraint chair,
the rhythms of rest-activity, body temperature, and various skin temperatures are
stable for more than a week. Finally, we have done experiments in which the
temperatures of monkeys were measured in restraint chairs when ketamine was not
used during animal handling. The temperature patterns were the same in both
The calibrated thermocouple leads were connected to a ten-channel multiplexer
(Omega). Each channel recorded on a strip chart recorder for 20 sec at 3-min
intervals. Ambient temperature and light, measured by a light sensor, were also
indicated on the recorder.
The temperature data were transcribed from the strip chart paper using an
Apple computer digitizer. Data points were recorded every 30 min from the 24 h of
data, resulting in a total of 48 points per measurement site. Editing and plotting of
data was done using a n Apple II+ microcomputer. Standard statistical methods
[Snedecor & Cochran, 19671 were used in analyzing the data.
An average waveform using temperature data from each monkey was determined. An average waveform consisting of 48 points was obtained by calculating
the mean of all temperatures for each 30 min. For example, using the thigh skin
temperature of each monkey a t 0800 h, a n average thigh skin temperature for 0800
h was determined. This was done for each 30-min reading, producing a 24 h average
waveform for the thigh skin temperature.
A prominent day-night difference in body temperature was evident in the colonic
temperature of monkeys at a n ambient temperature of 26°C (Fig. 1). The decline in
body temperature began approximately 4 h before lights off (CT 16 h), and body
temperature began to increase 1 h before lights on (CT 24 or 0 h). Colonic tempera-
4 I Wurster, Murrish, and Sulzman
g 39
0 37
Fig. 1. Colonic temperature of one pigtailed macaque for a 24-h period at an ambient air temperature of
26°C. The light-dark cycle is indicated at the top of the figure. Lights on at circadian time (CT) 0 h and
lights off at CT 16 h.
ture leveled off at a stable daytime level around CT 1.5 h. The difference between
the average day and average night body temperatures was about 1.3"C.
The average body temperature pattern of the five monkeys at each experimental
air temperature is shown in Figure 2. Clear day-night differences in colonic temperatures can be seen a t all ambient temperatures. The average range of the waveform
a t each temperature was approximately 1.9"C (see Table I). Although there was
little change in the range, as the ambient temperature increased, the daily mean
colonic temperature increased. Figure 3 represents the overall 24-h mean for colonic
temperature a t each ambient temperature. The correlation coefficient (r) for the
regression line of the colonic temperatures in Figure 3 was 0.89, and the r value for
the line from just 24°C to 30°C was 0.99. Since ? indicates the proportion of the
variance in the sample that is due to correlation between the variables, this value
was calculated as a further measure of the influence of ambient temperature on
body temperature. Estimates of r2 indicated that 95% of the variance was due to a
linear increase in mean body temperature a t air temperatures from 24°C to 30°C
and that 75% of the variance was due to a linear increase for all the ambient
The phase of colonic temperatures appeared to shift with the LD cycle a t the
different environmental temperatures (Fig. 2). The estimated time of day a t which
body temperature began to decrease to nighttime levels differed with changes in the
ambient temperature (Fig. 4).This time was estimated by the intersection of one
line drawn through the plateau of the stable daytime temperature and another line
through the decreasing temperature region. The decrease to night body temperature
began earliest at 32°C and latest 22"C, with the time of the initial decrease beginning later in the day with each 2°C drop in the ambient temperature. An estimate
of r2 indicated that in Figure 4,83% of the variance was due to a linear decrease,
and a t-test showed that the slope of the line was significantly different from zero (P
< 0.01). There also appeared to be a phase delay at 32°C and a phase advance a t
22°C in the time of day that body temperature began to increase.
Circadian Rhythm of Body Temperature / 5
E 380
Fig. 2. The average waveform of colonic temperature at six ambient temperatures. Each point represents
the average standard error of the body temperature of the monkeys at each half-hour measurement.
TABLE I. Mean Maximum and Mean Minimum Colonic Temperatures
+_ SE and the Range of the Waveform at Six Ambient Temperatures
38.9 k 0.6
39.0 0.3
38.4 +_ 0.4
38.4 t 0.4
38.0 k 0.4
38.3 k 0.2
37.0 k 0.7
37.0 0.5
36.5 t 0.3
36.4 k 0.3
36.3 k 0.3
36.2 k 0.1
Fig. 4. The time of day when the core body temperature of the pigtailed macaques began to decline to
nighttime levels at each of the six ambient temperatures.
Skin temperatures were measured at various sites on the leg to evaluate the
role of the extremities in the alteration of the body temperature rhythm at different
ambient temperatures. The colonic, thigh, foot, and ambient temperatures of a
representative monkey at 26°C are shown in Figure 5. Although the means differ,
the waveforms of the core and thigh are very similar. Both showed an increase in
temperature about lights on, a high temperature during the day, a decrease in
Circadian Rhythm of Body Temperature / 7
Fig. 5. The waveforms of the colonic (Co), thigh (TH), and foot (Fo) temperatures of a representative
pigtailed monkey at an ambient (AM)temperature of 26°C.
TABLE 11. A Comparison of the Mean Skin Temperaturesof Different Sites Along the
Legs of Monkeys at an Ambient Temperature of 26°C
0-8 h
f 0.2
f 0.3
f 0.3
f 0.2
* 0.1
8-16 h
27.3 f 0.3
31.0 + 0.3
32.7 f 0.1
33.7 f 0.1
34.8 0.1
16-24 h
f 0.2
f 0.1
f 0.1
f 0.1
& 0.1
N>M; A>M
A>M"; A > N
A>M; A > N
M>A; A > N
The 24-h day, beginning at lights on at circadian time 0, was divided into three 8-h sections: morning,
afternoon, and night. The last column indicates those values found to be significantly greater (P<O.Ol)by
Student's t-test.
*P < 0.05.
temperature several hours before lights off, and low values at night. In contrast, the
waveform of foot skin temperature of this monkey is the inverse of that of the thigh
and core. Foot temperature was low throughout the day and began to rise before
lights off and approximately a t the time colonic and thigh temperatures began to
fall. Higher foot temperatures were measured at night, which then slowly fell to
daytime values.
The skin temperatures of the thigh and foot of all five monkeys a t three ambient
air temperatures are shown in Figure 6. The mean skin temperature of the foot
increased markedly as ambient temperature was raised. At 32 and 22"C, there was
little day-night difference in the skin temperatures were not significantly different
(P < 0.05). At 26°C there was a day-night difference with lower daytime values
during the first few hours of light. The mean skin temperatures of each site along
the leg at a n ambient temperature of 26°C for three time periods during the
circadian day were calculated (Table 11).The toe skin temperature during the night
was significantly higher than in the morning. The temperature of the foot a t night
and in the afternoon was significantly higher than in the morning. The afternoon
8 I Wurster, Murrish, and Sulzman
Fig. 6. The average waveforms of the foot skin temperature (upper panel) and the thigh skin temperature
(lower panel) at an ambient air temperature of 32,26, and 22°C.
ankle and calf temperatures were each significantly greater than both the morning
and night temperatures. Both the morning and afternoon thigh temperatures were
significantly greater than the night temperature. In Figure 6, the thigh showed a n
elevation in skin temperature as the ambient temperature increased. There was a
day-night difference with high daytime values and lower nighttime values a t all
Skin temperatures of the leg showed a direct correlation with ambient temperature. Because of the large individual variation in skin temperatures at the different
sites, it is difficult to compare the contribution of each site to the heat budget of the
monkeys. However, if one calculates the ratio of core temperature minus mean skin
temperature to core temperature minus ambient temperature, one derives a number
between 0 and 1which reflects cutaneous vasoconstriction. As the ratio approaches
1, the site is more vasoconstricted, and as it approaches 0, the site is more vasodilated. Figure 7 shows the vasoconstriction ratio for each site a t a given ambient
temperature. At all temperatures the ratio for the thigh and calf remained between
0.2 and 0.4,indicating little participation in temperature regulation. For other sites
along the leg, as the ambient temperature increased, the temperature became closer
to 0, showing that these sites were being recruited for heat dissipation. At 30°C, the
upper end of the thermoneutral zone of the pigtailed macaque, the thigh and calf
were the warmest, the ankle and foot were almost as warm, and the toe was cooler.
Circadian Rhythm of Body Temperature / 9
Fig. 7. The vasoconstriction ratio (TB-TOB-TA) of various sites on the legs of five pigtailed macaques
maintained at different ambient temperatures. The vasomotor state is represented by this ratio, with the
value 1 indicating maximal vasoconstriction and 0 indicating maximal vasodilation.
This indicates that the foot and ankle were vasodilated and actively participating in
heat dissipation. At 32"C, the vasoconstriction ratios for all sites on the leg were
low and approximately the same, indicating that the leg was maximally involved in
vasomotor heat loss. At the lower end of the thermoneutral zone, heat must be
conserved by minimizing the gradient between ambient temperature and skin temperature. The high ratios for the toe, foot, and ankle at 22°C indicated that these
sites were vasoconstricted and being used for heat conservation.
The core body temperature of pigtailed macaques showed a clear day-night
difference a t six different ambient temperatures. The range of this rhythm was
similar a t the different ambient temperatures, but the maximum and minimum
body temperatures and the 24-h mean colonic temperatures increased as the environmental temperature increased. It is possible that as the heat load placed on the
animal changed, the set point for the maximum and minimum values also changed,
but the range of oscillation of the body temperature rhythm was regulated at a
relatively constant value. The observation of a relatively constant range of pigtailed
macaque body temperature rhythm differs from the results of two other studies with
primates, in which the body temperature rhythm was monitored at different ambient temperatures. Fuller [1984]found that the nighttime minimum body temperature of squirrel monkeys was altered significantly by changes in the ambient
temperature. However, in contrast to the macaques in the present study, the daytime
maximum did not change as much, and the range was inversely related to ambient
10 I Wurster, Murrish, and Sulzman
temperature. The range of oscillation of the rectal temperature rhythm of humans
has also been shown to depend on the ambient temperature [Aschoff & Pohl, 19701.
The range was greatest a t 24°C and smallest a t 20°C and 32°C. The significance of
these three different types of regulation is unclear.
Ambient temperature also had a n effect on the phase angle relationship between
colonic temperature and the light-dark cycle of pigtailed macaques. As the ambient
temperature was increased, the time of day when body temperature began decreasing occurred earlier (Fig. 4). In contrast, the phase relationships of both the hypothalamic and colonic temperatures of squirrel monkeys to the light-dark cycle were not
altered by changing ambient temperatures [Fuller, 19841. The light-dark cycle of the
present study was LD 1623, whereas with squirrel monkeys it was LD 12:12. Although a n LD 12:12 cycle might be more natural for macaques, the timing of the
body temperature decrease could be masked by lights off. Masking is defined as any
alteration of a n observed rhythm by a n environmental factor without affecting its
underlying oscillator. Fuller et a1 [1981] found that in a n LD 2:2 cycle, the body
temperature rhythm increased to daytime values whenever lights came on, suggesting a masking effect of the light-dark cycle.
The waveform of the body temperature rhythm of the pigtailed macaques was
similar in a n LD 1 6 3 cycle, LD 12:12 cycle, and in constant light conditions [Sickles
et al, unpublished data]. Because body temperature began to decrease several hours
before lights off, masking effects of the LD cycle are reduced in a n LD 16:8 cycle,
and the phase effect of other environmental factors can be evaluated. The shift in
the phase of the body temperature rhythm with changing ambient temperature (Fig.
2) indicates that ambient temperature plays a role in the entrainment mechanism
of the circadian timing system. In fact, Aschoff [1979] found that the activity rhythm
of M. nemestrina could be entrained by a n ambient temperature cycle.
Aschoff and Heise [1972] concluded that in humans the core temperature rhythm
is regulated primarily by a rhythm in heat loss. From studies on conductance and
skin temperature of the extremities, they showed that rhythm in heat loss accounts
for about 75% of the regulation of the rhythm in core temperature and that heat
production is involved in only 25% of the regulation. In humans, as well as in other
primates, the extremities are the major site for vasomotor heat loss. The skin
temperature of the distal regions of the arm and legs in humans have a waveform
that is the inverse of that of the colonic temperature waveform, whereas the proximal regions have a waveform similar to the colonic temperature [Aschoff & Heise,
In the present study, the skin temperatures at sites along the leg were measured
to evaluate their role in regulating the deep body temperature rhythm. The leg was
chosen for technical reasons, but since the arms and legs of humans showed similar
temperature patterns [Grant & Pearson, 19381, the results on the leg in this study
can probably be applied to the arm as well. Analysis of the waveforms of the skin
temperature rhythm of pigtailed macaques showed only a minor role for the limb in
regulating the rhythm of deep body temperature at low ambient temperatures. Skin
temperature waveform patterns of the thigh and foot were inversely related a t 26°C.
The thigh followed the rhythm in core temperature (higher in the day than at night;
Table 11), and the foot showed an inverse pattern with morning being the lowest
temperature (Table ID. At the temperature extremes, the range of the core temperature rhythm was relatively unaltered, but no rhythm could be discerned for either
the thigh or the foot skin temperature. At low ambient temperatures the leg is not
involved in generating the rhythm in body temperature. However, a t high ambient
temperatures the absence of a skin temperature rhythm does not necessarily mean
Circadian Rhythm of Body Temperature / 11
that the leg is not involved in regulating the core temperature rhythm. It is known
that in humans in warm temperatures there can be alterations in blood flow [Cooper
et al, 1949; Felder et al, 19541 and heat flow [Hertzman, 19531 without changes in
skin temperature. Although skin temperature patterns suggest that the leg
plays a minor role and because heat and blood flow were not measured in this study,
we cannot completely evaluate the role of the leg a t high ambient temperatures.
The rhythm in body temperature of the macaque could also be regulated by
vasomotor heat loss at other sites of the body. Baker et a1 [1976] found that certain
regions of the head of Macaca nemestrina showed increased skin temperatures upon
arousal and that the distal extremities showed a decrease in skin temperature. It is
not known what role the face and head play in regulating the body temperature
rhythm in these animals. Respiratory heat loss could also be involved, but no studies
have evaluated its role in regulating the body temperature rhythm. Alternatively,
a rhythm in heat production may play a more major role in generating the core
temperature rhythm in the macaque.
Humans in temporal isolation can show internal desynchronization, with the
activity and colonic rhythms free running at different periods [Wever, 1979; Czeisler
et al, 19801. If the rest-activity rhythm is similar to the rhythm in metabolic heat
production, then this observation is consistent with the conclusions of Aschoff and
Heise [1972] that the rhythm in heat loss is the primary cause of the colonic
temperature rhythm in humans. To date, no spontaneous desynchronization between
these two rhythms has been shown to occur in monkeys [Sulzman, et al, 19791.
However, the core body and tail skin temperature rhythms of squirrel monkeys do
have a tendency to uncouple in constant light [Fuller et al, 19791. Fuller et a1 [1981]
found that in squirrel monkeys with the suprachiasmatic nuclei (SCN) removed, the
activity rhythm ceased but the colonic rhythm persisted (although it was not as
regular a s when the SCN were intact). This suggests that although the core temperature rhythm is normally tightly coupled to the activity rhythm, a second oscillator
may be timing the rhythm after the removal of the SCN. Additionally, a recent
study on Macaca nemestrina by Sickles et a1 [unpublished data] found that in
constant light, activity and colonic temperature were tightly synchronized with one
another. The synchrony between activity and core temperature rhythms in nonhuman primates suggests that metabolic rate and therefore heat production (as indicated by the activity pattern) may play a larger role in regulation of body temperature
than they do in humans. To determine whether another major site is involved in
heat loss, or if heat production is regulating the deep body temperature rhythm,
more measurements need to be made, especially those of oxygen consumption, to
evaluate the extent to which metabolism is involved in the body temperature
The use of different portions of the leg for heat dissipation or heat conservation
with changes in the ambient temperature was estimated by a ratio which reflects
vasomotor activity. The thigh and calf temperatures were found to have approximately the same ratio of all ambient temperatures. This indicates that for macaques,
like humans [Aschoff & Pohl, 19701, the thigh and calf follow the core temperature
and are not active heat dissipators. Compared to the lower extremities, the thigh
and calf have a low surface area to volume ratio, suggesting that they are not as
effective heat dissipators as the distal regions of the leg. The ankle, foot, and toe
play the largest role in heat loss in the leg as seen by the decrease in the vasoconstriction ratio. As the ambient temperature increased, each site was progressively
recruited. At 28°C the ankle was recruited for heat dissipation, at 30°C the foot
12 / Wurster, Murrish, and Sulzman
became dilated, and at 32°C the toe was finally recruited. Each of these sites may
be used a t various times of the day for heat loss, but on the average, as the ambient
temperature increased a new site at the distal portion of the leg was being recruited.
It has been shown that in Macaca nemestrina, the timing of the waveform, the
maximum and minimum temperatures, and the 24-h mean colonic temperatures
were altered by changes in the ambient temperature. However, the range of the
oscillation is approximately the same at all ambient temperatures studied. The
rhythm in leg skin temperature was shown to be affected by ambient temperature,
with the rhythm becoming obscured a t the high and low ambient temperatures.
These data suggest that although the core body temperature is primarily regulated
by a change in heat loss in thermoneutrality, perhaps the circadian rhythm of core
temperature of these monkeys (unlike humans) is regulated more by heat production
rather than by vasomotor heat loss.
1.The rhythm in core body temperature persisted a t all ambient temperatures.
2. The rhythm in leg skin temperature was suppressed at the higher and lower
ambient temperatures.
3. The mean colonic temperature increased linearly with a n increase in ambient
4.As the ambient temperature increased, additional sites along the distal portion of the leg were recruited for vasomotor heat loss.
This research was supported by NASA contract NAS2-10621.The authors thank
Narinder Bhalla for his help in conducting this study and Bruce Bailey and Mark
Haley for their help with programming.
Ellen C. Wurster is now with the Physiology Group, School of Biological Sciences, University of Kentucky, Lexington.
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