The relationship between heat flow and vasculature in the dorsal fin of wild bottlenose dolphins Tursiops truncatus
1 University of North Carolina at Wilmington, Department of Biological
Sciences and Center for Marine Science Research, 601 South College Road,
Wilmington, NC 28403, USA
2 Duke University Marine Laboratory, Duke Marine Lab Road, Beaufort, NC
28516, USA
3 Chicago Zoological Society, c/o Mote Marine Laboratory, 1600 Ken Thompson
Parkway, Sarasota, FL 34236, USA
4 University of North Carolina at Wilmington, Department of Mathematics and
Statistics, 601 South College Road, Wilmington, NC 28403, USA
* Author for correspondence (e-mail: emm3005{at}uncwil.edu)
Accepted 8 August 2002
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Summary |
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Key words: heat flux, vasculature, dorsal fin, bottlenose dolphin, Tursiops truncatus, thermoregulation, heart rate, respiration, temperature
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Introduction |
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To dissipate excess body heat, cetaceans bypass their thermal insulation
and countercurrent heat exchangers. Under these circumstances, heat is
transferred from the core by blood flow, through the blubber and to the skin's
surface (Kvadsheim and Folkow,
1997). Cetaceans also bypass the venous return of the
peri-arterial venous retia and return blood through superficial veins
(Fig. 1) (Scholander and Schevill,
1955
). In the dorsal fin and flukes, blood returning through the
superficial veins functions to cool the reproductive tract
(Rommel et al., 1994
;
Pabst et al., 1995
). Thus, the
blubber and the appendages function as dynamic thermal surfaces, allowing a
cetacean either to conserve or to dissipate body heat, depending on its
thermal requirements.
Several studies have examined the conditions under which cetaceans conserve
or dissipate heat. These studies have reported heat losses as heat flux, a
rate of energy transfer per unit area. McGinnis et al.
(1972) found that a spinner
dolphin (Stenella longirostris hawaiiensis) decreased heat flux
across the dorsal fin as it was cooled in a water bath. Hampton et al.
(1971
) and Noren et al.
(1999
) found that bottlenose
dolphins (Tursiops truncatus) more than doubled heat flux from their
thermal windows (the pectoral flipper and dorsal fin) after exercise. A
bottlenose dolphin, actively conserving body heat during cooling in a water
bath, showed a decline in heat flow from the proximal to distal tip of each
appendage, while a spinner dolphin actively dissipating body heat displayed
heat flux values highest at the distal tip of the appendages (Hampton et al.,
1971
,
1976
). These gradients in heat
flow may be explained by the vascular anatomy of the appendage. Scholander and
Schevill (1955
) suggest that
the countercurrent heat exchange system would result in a steep
proximal-to-distal temperature drop from the body into the appendage.
Conversely, bypassing the countercurrent heat exchangers to dissipate excess
body heat would result in the opposite temperature gradient as warm core blood
reaches the distal periphery and is cooled as it returns along the surface of
the appendage.
Noren et al. (1999) and
Williams et al. (1999
) have
demonstrated that changes in respiration and heart rate also affect heat flow
across the thermal windows. Marine mammals display sinus arrhythmia, a cycling
of heart rate, which increases (tachycardia) during inspiration and decreases
(bradycardia) during the interbreath interval (for a review, see
Elsner, 1999
). This sinus
arrhythmia is most pronounced during diving (e.g.
Elsner et al., 1966
). Heat flux
measurements across the dorsal fins of diving bottlenose dolphins were lower
than values measured at the surface (Noren
et al., 1999
; Williams et al.,
1999
). The decrease in heat flux at depth was attributed to a
suite of physiological responses that occur during diving, including
bradycardia, decreased cardiac output and widespread peripheral
vasoconstriction. In contrast, tachycardia experienced during ventilation
events was associated with an increase in heat flux across the dorsal fin,
suggesting an increase in blood flow resulting from peripheral vasodilation
(Williams et al., 1999
).
These studies have demonstrated that changes in heat flux across the
thermal windows of dolphins are associated with changes in the pattern of
blood flow. No study to date, however, has directly assessed the influence of
the underlying vasculature on heat flux. Prior studies have measured heat flux
at single, or in a few cases multiple, sites on the dorsal fin. None have
noted underlying vascular structures in relation to their sites of measurement
nor taken measurements at multiple sites simultaneously. Infrared thermal
imaging suggests that the pattern of surface temperatures across the dorsal
fin is influenced by the underlying vasculature
(Fig. 2)
(Pabst et al., 2002). Because
blood flow is the force driving heat flux across the appendages, heat flux
should vary across the fin depending upon the location from which it is
measured, i.e. whether directly over or away from superficial veins.
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Thus, the primary goal of this study was to investigate the role of vascular structures on heat flow by simultaneously and continuously measuring heat flux and skin temperature at three positions on the dorsal fins of bottlenose dolphins. These positions were chosen both to compare with prior studies and to assess the influence of underlying venous structures on heat flow across the height of the fin. Experiments were conducted in early summer in Sarasota Bay, FL, USA, a warm water environment; thus the predictions were that (1) heat flux would be higher when measured directly over a superficial vein and (2) heat flux would be highest at the distal tip of the fin. The second goal was to investigate the role of respiratory arrhythmia on the temporal pattern of heat flux across the dorsal fin in dolphins held stationary at the water's surface. Continuous, simultaneous measurements of heart rate and respiration were used to test whether heat flux across the dorsal fin increased during periods of tachycardia at inspiration and decreased during periods of bradycardia at interbreath intervals.
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Materials and methods |
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Experimental design
Heat flux and skin temperature were measured simultaneously and
continuously at three positions on the dorsal fin of each dolphin: (1) the
distal tip of the dorsal fin directly over a superficial vein, (2) the center
of the fin directly over a superficial vein and (3) the center of the fin
avoiding a superficial vein. All measurements were collected for periods of
approximately 15 min under two experimental conditions: (1) the animal held
stationary, with its dorsal fin above the surface of the water, for the first
7-8 min and (2) the animal held stationary with the head above water and the
dorsal fin submerged just below the surface of the water for an additional 7-8
min.
To determine whether temporal changes in heat flux were associated with respiratory events and/or a change in heart rate, these measurements were collected simultaneously and continuously with heat flux and skin temperature.
Heat flux
Heat flow across the skin was measured with three square (2.54 cm length)
heat flux transducers (B-episensor, Vatell Corporation, Christiansburg, VA,
USA) waterproofed with a rubberized coating (Plastidip, PDI Inc., Circle
Pines, MN, USA). Each heat flux disk had a unique calibration coefficient,
supplied by the manufacturer, which was used to convert transducer output from
mV to W m-2. In addition, to ensure that the outputs of the three
rubberized coated heat flux disks, as configured for the experiments, were
consistent with each other, a series of calibration tests were run. The disks
were immersed in a controlled water bath (RE-120 Lauda Ecoline, Brinkmann
Instruments, Inc., Westbury, NY, USA) at six different water temperatures (20,
25, 30, 35, 40 and 45°C). The disks were allowed to stabilize at each
water temperature, for approximately 45 s to 2 min. The output of each disk
was then recorded at each of the six water temperatures to yield the disk's
offset value (in W m-2). These offsets were averaged and used to
make final calibration corrections in the data presented. To be conservative,
heat flux measurements were considered different from each other if they
differed by more than 10 W m-2.
Heat flux measurements were collected from the left side of the dorsal
fins. The positions of large-diameter, superficial veins were determined by
palpation and/or visual inspection of the fin. The heat flux transducers were
then mounted in a plastic harness that held them on the dorsal fin in their
correct position relative to the underlying vasculature
(Fig. 3). The internal surface
of the transducers pressed against the skin of the animal. To ensure that
ambient air or water flowed freely on the external side of the transducers,
they were mounted on thin springs, leaving a space of approximately 1 cm
between skin and harness. The harness and heat flux disks added extra
insulation to the dorsal fin, which introduced a negative bias in heat flux
measurements (e.g. Kvadsheim and Folkow,
1997), but one that was expected to affect all measurements
equivalently. Heat flux transducer outputs (in mV d.c.) were amplified (to V
d.c.), downloaded to a Fluke Hydra data logger (Fluke Corporation, Everett,
WA, USA) at 3 s intervals and logged onto a laptop computer.
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Skin temperature
Skin surface temperature Tskin was determined using a
copper-constantan (Type T) thermocouple implanted on the surface of each heat
flux disk (Omega Engineering, Inc., Stamford, CT, USA). Thermocouples were
connected to the Fluke Hydra data logger and outputs (in °C) were
downloaded at 3 s intervals and logged onto a laptop computer. The
thermocouples were calibrated in a water bath (RE-120 Lauda Ecoline) at the
same six water temperatures as the heat flux disks and found to be within
0.1°C of each other.
Heart rate and respiration
Heart rate was recorded during experimental sessions by a Polar Vantage NV
heart rate monitor and standard watch receiver (Polar Electro, Inc., Woodbury,
NY, USA). The monitor was fixed around the thoracic cavity of the dolphin by
an elastic belt and transmitted information to the receiver attached directly
to the belt. The Polar Vantage NV recorded RR intervals (intervals
between depolarization of the heart's ventricles), thus, effectively measuring
each heart beat. After experimental sessions, heart rate data were downloaded
from the receiver to a laptop computer for analysis via the Polar
Vantage Interface system. Heart rate data were cleared from the receiver
between each experimental session.
In the Polar Vantage Interface software, each RR interval was converted to an instantaneous heart rate (beats min-1). These data were then saved as text files and exported to spreadsheet software for analysis.
The time of each respiratory event, collected to the nearest second, was recorded on a datasheet by an observer. Each event was also recorded by another observer with an electrical trigger, connected to the data logger, which placed an event marker in the data file.
Analyses
Heat flux, skin temperature and heart rate data were examined using
spreadsheet software (Excel, Microsoft Corporation, Redmond, WA, USA). To
analyze the heat flux and skin temperature data from a full 15 min experiment,
data for the fin in air and the fin submerged were separated. For each
segment, a 2 min period was subsampled and analyzed, 2 min after recording for
that segment began (Fig. 4).
Because it took up to 2 min for the disks to stabilize during calibration
experiments, this method of analysis allowed time for the transducers to
equilibrate after the harness was put on the fin and after the fin was
submerged. The data contained within each 2 min segment were averaged to yield
a mean heat flux and skin temperature value at each of the three positions for
each animal. These mean heat flux and skin temperature values could then be
compared across different animals and between experimental conditions (fin in
air or fin submerged).
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Although a heat flux transducer could, with confidence, be placed directly over a superficial vein, it was more difficult to ensure that a transducer was definitely not over a vein, given the close spacing of large superficial veins on some fins (see Fig. 2). Thus, although experiments were conducted on 19 dolphins, in only 13 of these animals could the heat flux transducers be confidently placed away from a large, superficial vein in the center of the fin. For analyses relating to vascular structures in the dorsal fin only these 13 animals were examined (Table 1).
To analyze heart rate data, the instantaneous heart rates were first edited. Heart rate values below 20 beats min-1 and above 200 beats min-1 were deleted from the record, because these values appeared to be a result of the receiver missing heart beats or an inability of the receiver to record sudden increases. As heart rates were instantaneous values converted from RR intervals, some seconds of real time contained multiple values of heart rate. In these cases, representing periods of tachycardia, peak heart rate values were selected. The heart rate data, after editing, had a value for each second of real time during the experiment and, thus, required alignment with the heat flux and skin temperature data, which were recorded at 3 s intervals. Heat flux and skin temperature values were averaged between 3 s intervals, yielding a value for each second of real time during the experiment.
Thus, eight simultaneous measurements (heart rate, respiration, heat flux and skin temperature at each of three positions) were recorded during each experimental session. As the Fluke Hydra data logger recorded heat flux and skin temperature every 3 s, this time interval was assumed to be the range of error in each synchronous time series.
The relationship between tachycardia and a respiratory event was determined by analyzing a time series of heart rate and respiration data. This examination revealed that the majority of tachycardic peaks occurred within 6 s before or after a respiratory event. Thus, the percentage of tachycardia events that fell within this time period was determined.
Investigating the relationship between tachycardia and increases in heat flux across the dorsal fin required additional data manipulation. Heat flux values tended to change throughout the length of the experiment, even after the 2 min stabilization period (see Fig. 4). Thus, overall increasing or decreasing trends in the heat flux time series were eliminated by fitting linear (for the dorsal fin in air) and quadratic models (for the dorsal fin submerged) to each heat flux record using statistical software (JMP IN, SAS Institute, Inc., Cary, NC, USA). Residuals were then calculated from these models. The residuals represented the changes in heat flux values that could potentially be temporally related to changes in heart rate. Because tachycardia was temporally related to respiratory events, respiration records were compared to those of heat flux to determine whether periods of tachycardia were related to increases in heat flux across the dorsal fin. The residuals were plotted with the respiration data in a time series to determine how often the highest heat flux residual value for the interbreath interval fell within 9 s, before or after, a respiratory event. The time window around each respiratory event was increased from 6 s to 9 s, in an attempt to account for delays that might exist between tachycardia and a change in peripheral vasodilation at the dorsal fin. The time window was limited to 9 s to prevent overlapping time periods between two separate respiratory events. Only changes in heat flux equal to or greater than 10 W m-2 were included in this analysis.
To determine whether mean heat flux and skin temperature differentials (temperature differential = Tskin-Tamb) varied by position and whether respiration rate differed when the fin was in air rather than submerged, the ShapiroWilk W-test was used to first test for normality. Once normal distributions were confirmed, one-way analyses of variance (ANOVAs) were performed to test for significant differences in these data sets. P=0.05 was used for all tests. All statistical tests were performed using JMP IN statistical software.
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Results |
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Heat flux over superficial veins
Although there was spatial variation in heat flux between the three dorsal
fin positions within individuals, for all dolphins pooled together, the mean
heat flux values measured at the three fin positions were not significantly
different (d.f.=38, F=1.69, P=0.20) when the dorsal fin was
submerged. The spatial pattern in heat flux within individuals was, however,
related to patterns of superficial vasculature when the fin was submerged. Of
the 13 dolphins where the placement of the heat flux transducers relative to
superficial vessels had been confidently established, 12 (92%) showed heat
flux values that were higher when measured over superficial veins (either
distal or center) when the fin was submerged
(Table 3). Eight of these 13
animals (62%) had the highest heat flux values at the distal tip of the fin.
For the remaining four animals, measurements at the distal tip and the center
of the fin over a superficial vein were within 10 W m-2 of each
other.
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For the dorsal fin in air, there were no significant differences in mean heat flux values measured between the three fin positions when all dolphins were pooled together (d.f.=38, F=0.60, P=0.55). There was also no predominant spatial pattern in heat flux values observed within individual dolphins. Three dolphins had heat flux values that were highest at the distal tip of the fin, four had heat flux values highest at the center of the fin when measured over a superficial vein and four had heat flux values highest at the center of the fin measured away from superficial veins (Table 3). The remaining two animals had highest heat flux values, within 10 W m-2 of each other, at two fin positions.
Skin temperature
For the dorsal fin submerged, the mean temperature differentials between
the skin and the ambient environment did not vary between the three fin
positions when all dolphins were pooled together (d.f.=35, F=0.23,
P=0.79) (Table 4).
There were, though, discernable patterns within individuals. For 11 out of 12
animals (92%) (no water temperature was determined for the 13th animal, FB101,
thus only 12 were used for this analysis), the highest temperature
differentials corresponded to the highest heat flux values. A single animal,
FB25, had the highest heat flux value at the distal tip of the fin, but the
highest temperature differential at the center of the fin when measured away
from a superficial vein. Of these 12 animals, 10 (83%) had both the highest
temperature differential and highest heat flux values when measured over
superficial veins in either the distal tip or the center of the fin. In
addition, despite non-significant differences between the temperature
differentials, heat flux at those three positions could vary greatly. Heat
flux values measured directly over superficial veins could be 60 W
m-2 higher than values measured away from superficial veins, with
only a 0.2°C temperature difference between the three positions (see
Fig. 4).
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For the dorsal fin in air, the mean temperature differentials between the skin and the ambient environment also did not vary significantly across the fin when all dolphins were pooled together (d.f.=38, F=2.32, P=0.11) (Table 4). The highest temperature differentials, however, corresponded to the highest heat flux values in only one of 13 animals (8%) for the dorsal fin in air. Additionally, as the temperature differential between the skin and the ambient environment increased over time, heat flux across the surface of the dorsal fin decreased (see Fig. 4).
Heart rate and respiration
Heart rate data were successfully collected on six of the 19 dolphins. Mean
heart rates were calculated from the longest periods of data for each animal.
These periods were all longer than 4 min. Mean heart rates ranged from 72 to
101 beats min-1 (Table
5). Heart rates were arrhythmic, with periods of tachycardia
associated with respiratory events and bradycardia during intervening
interbreath intervals (Fig. 5).
The highest heart rate value occurred within 6 s before or after a respiratory
event, 89±7.5% (mean ± S.D.) of the time (range 75-97%).
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Respiration rates were calculated under both experimental conditions (fin in air and fin submerged) for these six dolphins (Table 5), and there were no significant differences in the respiration rates under either condition (d.f.=11, F=3.04, P=0.11).
Respiration and heat flux
Because tachycardia was temporally related to respiratory events, the
respiration records were compared to those of heat flux to determine whether
periods of tachycardia were related to increases in heat flux across the
dorsal fin. No clear relationship between a respiratory event and an increase
in heat flux across the dorsal fin, in either air or water, was observed
(Fig. 6). A subsample of eight
animals, representing a range of heat flux values, was selected for initial
analyses of respiration and heat flux across the dorsal fin at the distal tip
in air (Table 1). Heat flux
data were first examined for the dorsal fin in air, as these records showed
greater fluctuations than the submerged records. These fluctuations may have
represented changes in heat flux across the surface of the fin or,
alternatively, changes in the microenvironment surrounding the fin. The distal
tip of the fin was selected for initial analyses, because it was expected that
blood from the core would be delivered via the central arteries to
this site first and, thus, any relationship between a respiratory event,
tachycardia and a change in heat flux would be most obvious here. At the
distal tip of the fin in air, the highest heat flux values were temporally
related to a respiratory event only 37±21% (mean ± S.D.) of the
time.
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A sub-sample of ten animals, selected from those with the highest heat flux values, was examined to determine whether there was a relationship between respiratory events, tachycardia and changes in heat flux across the dorsal fin when submerged (Table 1). At the distal tip of the fin measured over a superficial vein, 42±21% (mean ± S.D.) of the highest heat flux values were temporally related to a respiratory event. At the center of the fin measured over a superficial vein, this relationship reduced to 17±17% and at the center of the fin measured away from a superficial vein to 18±11%.
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Discussion |
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When the fins were submerged, spatial differences in heat flux could be predicted based upon their position relative to underlying vascular structures the highest heat flux values were recorded either at the distal tip of the fin directly over superficial veins or at the center of the fin over superficial veins. Thus, these results support the predictions that heat flux is higher over superficial veins and highest at the distal tip. The single individual (FB 178) that had the highest heat flux at the center of the fin measured away from a superficial vein also had the highest combined total heat flux at all three positions (all >200 W m-2) of any of the 13 animals, which suggest that this animal may have been using a greater proportion of the surface area of the fin to dissipate heat (see below).
Although the highest heat flux values during submergence were recorded over
superficial veins at the distal tip of the fin, there was not as clear a
pattern at the center of the fin. At the fin's center, heat flux measured over
a superficial vein was not always higher than that measured away from a vein.
This result may partially be explained by the vascular anatomy of the fin.
Large superficial veins are the only ones that could have been assessed
visually and/or by palpation during the experiments and, thus, avoided. Visual
inspection of cross-sectioned dorsal fins suggests that there are many smaller
superficial veins at the dorsal fin's surface
(Elsner et al., 1974; E. M.
Meagher, personal observation). While the large veins could be avoided in the
majority of the experiments (13 out of 19), these smaller superficial veins
could not. Thus, the disks were most certainly measuring heat flux over small
superficial veins at all three positions. Smaller vessels, particularly
arterioles, capillaries and venules, act as ideal heat exchangers, while
vessels of increasing diameter become less efficient at transferring heat to
the surrounding tissue (Chato,
1980
). Therefore, smaller vessels, including small superficial
veins, could also have influenced the heat flux measurements.
The variables that affect conductive heat transfer are surface area,
thermal conductivity and the temperature differential between the skin and the
ambient environment (see Equation 1). Because a constant surface area was
being assessed in these experiments, measured heat flux at the three dorsal
fin positions was controlled by changes in skin temperature and/or changes in
the conductivity of the skin. Although conductivity is a physical constant,
and thus cannot be increased or decreased, convection, which is the movement
of a fluid relative to an object, can substantially increase the rate of
thermal diffusion (Kvadsheim and Folkow,
1997; Denny, 1993
;
Schmidt-Nielsen, 1997
). For
example, for the 13 animals investigated, 83% had the highest temperature
differentials and highest heat flux values measured at positions over
superficial veins when the fin was submerged. This result suggests that
increases in skin surface temperature and thus in heat flux, at these sites
are due to the convective transfer of core heat via blood. In
addition, when the complete heat flux and skin temperature records were
examined for the dorsal fin submerged, it was apparent that as the temperature
differential between skin and water decreased, heat flux also decreased (see
Fig. 4). Small changes in the
temperature differential between the dorsal fin and the ambient environment
could also be associated with large differences in heat flux, further
suggesting that when the fin was submerged there was active, convective
delivery of heat to the surface of the skin.
The results for the dorsal fin in air were quite different. For example,
there was no relationship between high heat flux values and high temperature
differentials. Although the distal tip of the fin tended to have higher
temperature differentials relative to the other two positions, only 8% of the
dolphins had their highest heat flux values at this position. Additionally, as
the temperature differentials across the dorsal fin increased over time, heat
flux values decreased (see Fig.
4). The unexpected relationship between heat flux values and
temperature differentials for the dorsal fin in air suggests that the
conductivity of the ambient environment was affecting heat dissipation from
the fin. The following equation reflects how the boundary layer, the layer of
air or water with which the dolphin skin is in contact, affects heat
dissipation:
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Heart rate, respiration and heat flux
The second objective of this study was to examine whether changes in heat
flux across the dorsal fin were temporally related to periods of tachycardia
and bradycardia. While respiring at the surface, some cetaceans, such as the
harbor porpoise Phocoena phocoena
(Reed et al., 2000) and
bottlenose dolphin (Elsner et al.,
1966
; Kanwisher and Ridgway,
1983
), display tachycardia at inspiration and bradycardia during
interbreath intervals. The results from this study are similar, as 89% of
respiratory events and the onset of tachycardia occurred within 6 s of each
other.
Although the dolphins in the present study displayed respiratory
arrhythmia, there was no relationship between respiration and increased heat
flux across the dorsal fin, either when the fin was in air or when it was
submerged. There are several potential reasons why this relationship was not
observed. Williams et al.
(1999) demonstrated that in a
dolphin undergoing a dive response, bradycardia was associated with a decrease
in heat flux across the dorsal fin. In those experiments, a bottlenose dolphin
diving freely to 50 m showed a decline in mean heart rate from 102 beats
min-1 to 37 beats min-1
(Williams et al., 1999
). Upon
surfacing and breathing, the dolphin displayed an increase in heart rate and
heat flux. In the present study, the decline in heart rate measured during
bradycardia at the surface was not as pronounced as that reported by Williams
et al. (1999
) during diving.
The lowest heart rate value determined from the six dolphins in the present
study was 50 beats min-1; however, the mean heart rate during the
interbreath interval for all six animals was 66 beats min-1
(Table 5). It appears that
although the dolphins in these experiments were experiencing bradycardia
during the interbreath interval, it was slight compared to that experienced
during a dive response. Thus, the dorsal fin may not have experienced
noticeably reduced blood flow during the interbreath intervals.
The dolphins in these experiments were also in a highly artificial
behavioral setting. Stress, in any form, stimulates the sympathetic nervous
system, leading to an increase in heart rate
(Bullock, 1996). It is possible
that the capture and restraint process inhibited bradycardia that would
normally have been present during the interbreath interval. Respiration rates
of captive, trained bottlenose dolphins are 2-3 breaths min-1, and
normal heart rates immediately after inspiration are 70-100 beats
min-1 (Ridgway,
1972
). According to Ridgway
(1972
), heart rate during the
interbreath interval in bottlenose dolphins then falls to 30-40 beats
min-1, regardless of whether the animal is swimming or resting on a
mat out of the water. While the average respiration rate of dolphins in the
present study was within this range (see
Table 5), heart rates during
both tachycardia and bradycardia were higher than those reported by Ridgway
(1972
). Heart rates after
inspiration in these experiments were 105-118 beats min-1, while
heart rates during the interbreath interval were 50-88 beats min-1.
Thus, the relatively higher heart rates of the dolphins in these experiments
may have affected the relationship between heart rate and patterns of heat
flux across the dorsal fin.
In addition, these dolphins were in a warm environment and were actively
dissipating body heat, as indicated by their heat flux values. Hampton et al.
(1971) and Noren et al.
(1999
) recorded mean heat flux
values of 27-70 W m-2 from the submerged dorsal fins of bottlenose
dolphins at rest in water, with temperatures ranging from 26.5°C to
29.8°C, respectively. A heat flux value of 70 W m-2 was
exceeded by at least two of the three dorsal fin positions, when the fin was
submerged, in 15 out of the 19 dolphins in the present study. Noren et al.
(1999
) also measured mean heat
flux values of 70-135 W m-2 from the submerged dorsal fins of
bottlenose dolphins after surface swimming for 11-13 min at 4.3 m
s-1. A heat flux value of 135 W m-2 was exceeded by at
least one of the three dorsal fin positions, when the fin was submerged, in 11
out of the 19 dolphins in the present study. Heat flow increases with
increasing total skin blood flow through circulatory channels [e.g. venules,
arteriovenous anastomoses (AVA), arterioles and capillary beds] and is
highest when both capillary and AVA flows are at their highest levels
(Grayson, 1990
;
Hales, 1985
). While we did not
directly measure blood flow through the dorsal fin, it is probable that these
animals were maximizing the use of their circulatory channels to regulate
their body temperature in the warm waters of Sarasota Bay. In this case, an
increase in heat flux associated with tachycardia may not exist, or may be so
small that the signal is lost in the background levels of overall high heat
flow.
Although a relationship between an increase in heat flux and tachycardia
has been demonstrated in diving dolphins
(Williams et al., 1999), no
study has attempted to detect the correlation in animals at rest on a
breath-to-breath time scale. While the circulatory changes associated with
diving result in pronounced vasoconstriction, it is possible that the modest
changes in blood flow associated with non-diving respiratory arrhythmia may
have no effect on heat flux across the dorsal fin. Understanding the large
changes in heat flux that occur over time and between different behavioral
states is important. Further research into the subtle changes in heat flux
that occur within minutes could elucidate the dynamic control mechanisms that
dolphins utilize to thermoregulate in their highly conductive marine
environment.
The combined results of this study suggest that the dorsal fin of the bottlenose dolphin is a spatially heterogeneous thermal surface and that patterns of heat flux across the dorsal fin are strongly influenced by underlying vasculature, particularly at the distal tip of the fin. Heat flux across the dorsal fin can vary over small distances and care should be taken in future studies to record non-continuous measurements from a consistent location, particularly when comparing heat flux between different behavioral states. A single heat flux value recorded from the center of the dorsal fin may not provide an accurate representation of the amount of heat being dissipated by the animal. In addition, this study demonstrates that heat flux across the dorsal fin varies temporally in an animal under static conditions. Measuring heat flux and skin temperatures from dolphins under more natural conditions would provide further insights into the dynamic function of their thermal windows.
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Acknowledgments |
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References |
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Bullock, B. L. (1996). Pathophysiology: Adaptations and Alterations in Function. Philadelphia, PA: Lippincott-Raven Publishers.
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