MARINE MAMMAL SCIENCE, 26(1): 53–66 (January 2010)
C 2009 by the Society for Marine Mammalogy
DOI: 10.1111/j.1748-7692.2009.00319.x
Using infrared thermography to assess seasonal trends
in dorsal fin surface temperatures of free-swimming
bottlenose dolphins (Tursiops truncatus) in Sarasota
Bay, Florida
M. M. BARBIERI
W. A. MCLELLAN
Department of Biology and Marine Biology,
University of North Carolina Wilmington,
601 South College Road,
Wilmington, North Carolina 28403, U.S.A.
E-mail: michelle_barbieri@yahoo.com
R. S. WELLS
Chicago Zoological Society,
Mote Marine Laboratory,
1600 Ken Thompson Parkway,
Sarasota, Florida 34236, U.S.A.
J. E. BLUM
Department of Mathematics and Statistics,
University of North Carolina Wilmington,
601 South College Road,
Wilmington, North Carolina 28403, U.S.A.
S. HOFMANN
J. GANNON
Mote Marine Laboratory,
1600 Ken Thompson Parkway,
Sarasota, Florida 34236, U.S.A.
D. A. PABST
Department of Biology and Marine Biology,
University of North Carolina Wilmington,
601 South College Road,
Wilmington, North Carolina 28403, U.S.A.
ABSTRACT
The temperature differential (T) between a body surface and the environment
influences an organism’s heat balance. In Sarasota Bay, FL, where ambient water
temperature (T w ) ranges annually from 11◦ to 33◦ C, T was investigated in a
resident community of bottlenose dolphins (Tursiops truncatus). Dorsal fin surface
temperatures (T dfin ) were measured on wild, free-swimming dolphins using infrared
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thermography. Field and laboratory calibration studies were also undertaken to
assess the efficacy of this non-invasive technology in the marine environment. The
portability of infrared thermography permitted measurements of T dfin across the
entire range of environmental temperatures experienced by animals in this region.
Results indicated a positive, linear relationship between T dfin and T w (r2 = 0.978,
P < 0.001). On average, T dfin was 0.9◦ C warmer than T w across seasons, despite
the 22◦ C annual range in T w . Changes in integumentary and vascular insulation
likely account for the stability of T dfin−w and the protection of core temperature
(T core ) across seasons. The high thermal conductivity of water may also influence
this T. The use of infrared thermography is an effective, non-invasive method of
assessing dorsal fin skin surface temperatures (±1◦ C) across large numbers of wild,
free-swimming dolphins throughout their thermally dynamic aquatic environment.
Key words: thermoregulation, bottlenose dolphins, Tursiops truncatus, dorsal fin,
surface temperature, infrared thermography.
A community of approximately 150 bottlenose dolphins (Tursiops truncatus) resides
year-round throughout the waters of Sarasota Bay, FL, and up to several kilometers
offshore in the adjacent Gulf of Mexico (Scott et al. 1990, Wells 2003). These nonmigratory dolphins experience considerable seasonal variation in water temperature,
which ranges from 11◦ to 33◦ C annually (Irvine et al. 1981, Wells et al. 1987). This
temperature range exceeds that which has been reported for conspecifics along the
U.S. Atlantic coast (Barco et al. 1999, Zolman 2002), in the Moray Firth, Scotland
(Wilson et al. 1997), and in Shark Bay, Australia (Heithaus and Dill 2002). Thus,
Sarasota Bay offers a unique opportunity to assess the physiological response of
bottlenose dolphins to relatively large seasonal changes in water temperature. This
study utilizes infrared thermography to non-invasively measure the thermal responses
of free-swimming dolphins to changing environmental conditions, and thus presents
a tool that requires no animal handling or restraint.
Water temperature is an important environmental feature to which marine mammals, as endothermic homeotherms, must respond, as this highly conductive aquatic
medium is capable of removing body heat 25 times faster than air at the same temperature (Schmidt-Nielsen 1998). Bottlenose dolphins appear to be able to control
heat loss across their surfaces by changing the quality and quantity of their insulative
blubber layer (Wells 1993, Dunkin et al. 2005, Samuel and Worthy 2005) and by
changing blood flow to their poorly insulated dorsal fin, pectoral flippers, and flukes
(e.g., Noren et al. 1999; Williams et al. 1999; Meagher et al. 2002, 2008). These
appendages are dynamic thermoregulatory control surfaces, called thermal windows,
which possess deep arterio-venous countercurrent heat exchangers that function to
conserve heat, and superficial veins that facilitate heat loss at the skin-water interface
(Scholander and Schevill 1955, Elsner et al. 1974). These integumentary and vascular adjustments affect the temperature differential between the body surface and
the ambient water (T b − T w ◦ C), which, in turn, influences heat loss (reviewed in
McNab 2002).
Previous studies have shown that mean surface temperatures at one thermal window, the dorsal fin, tend to remain within 1◦ C of water temperature. Noren et al.
(1999) demonstrated this result on captive bottlenose dolphins at rest (T w : 28.5◦ C–
31.5◦ C), as did Meagher et al. (2002) on wild, temporarily restrained bottlenose
dolphins in the Sarasota Bay region in summer (T w : 27.8◦ C–31.9◦ C). More recently,
Meagher et al. (2008) demonstrated this approximate 1◦ C temperature differential
BARBIERI ET AL.: INFRARED THERMOGRAPHY
55
at the dorsal fin, as well as the pectoral flipper and flukes of temporarily restrained
Sarasota dolphins, across seasons (T w : 15.0◦ C–32.9◦ C). Deployments of thermal data
logging Trac-Pac tags (Trac-Pac Inc., Fort Walton Beach, FL) on the dorsal fins of
individual dolphins in the Sarasota Bay region support these results, as the skin
surface temperatures measured were consistently warmer than that of the ambient
water, by about 1◦ C (Westgate et al. 2007).
While the techniques used in these studies permit detailed insight into the thermal status of an individual, each requires animal handling, which may limit the
possible number of animals sampled. With the exception of the Trac-Pac studies, the
measurements described above have also been made on stationary dolphins.
The goal of this study was to investigate the efficacy of non-invasive infrared
(IR) thermography to compare the surface temperatures of wild, free-swimming
dolphins to those of their environment across the wide range of T w found seasonally
in the Sarasota Bay region. Although it is a well-established method of temperature
measurement in terrestrial applications (Williams 1990, Klir and Heath 1992,
McCafferty et al. 1998, Phillips and Heath 2001), only two studies to date have
utilized this technology on wild cetaceans (Cuyler et al. 1992, Perryman et al. 1999).
These studies were primarily aimed at testing IR thermography as a tool to detect
free-swimming baleen whales. The amount of IR radiation that is emitted from a
surface is proportional to its temperature (Watmough et al. 1970, Cena and Clark
1973, Clark 1976), but because of the high reflectivity of water, care must be taken
when using this technology in a marine environment (reviewed in Cuyler et al. 1992).
Therefore, field studies of free-swimming Sarasota dolphins were coupled with
both field and laboratory calibration studies to determine whether this approach
would yield accurate thermal data. The dorsal fin was chosen as the measurement site
because it is a known thermal window and because it is the only body surface that
is routinely exposed above the water surface. If effective, measurements of this kind
would permit comparisons to previous studies to determine whether the relationships
observed using smaller sample sizes of captive or temporarily restrained wild dolphins
were representative of free-swimming animals across a wide range of environmental
temperatures.
MATERIALS AND METHODS
Infrared Thermal Imaging of Free-Swimming Dolphins
Dorsal fin surface temperatures of free-swimming bottlenose dolphins were assessed using IR thermography during routine photo-identification surveys in the
Sarasota Bay region. Surveys were conducted from approximately 0900–1700 aboard
a 6-m-long powerboat with at least three observers.
IR thermal images were collected during these surveys for 5–10 d each in fall
(November, n = 144), winter (February, n = 179) and summer (June, n = 232) from
2002 to 2004. Here, n is the total number of images collected. The resolution of
the IR camera was insufficient to discriminate distinctive markings of individuals,
thus, it is likely that more than one image of the same individual was taken during
a sighting. However, it is equally likely that each individual within a sighting was
imaged at least once.
Water temperature (within 0.5 m of surface) was also measured throughout
each sighting (digital AquaCal ClineFinder, Catalina Technologies, Tucson, AZ,
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MARINE MAMMAL SCIENCE, VOL. 26, NO. 1, 2010
or mercury thermometer). To investigate whether these measurements were representative of the temperature through the water column, water temperatures were
recorded at 0.5 m intervals from the bottom to the surface at multiple locations
throughout the study area (ClineFinder). Temperatures measured through the water column were usually within 1◦ C of surface water temperature; thus, the water
temperatures collected during each sighting (within 0.5 m of surface) were used in
analyses as the independently measured ambient water temperature (T w ).
IR thermal images were taken during brief (2–3 s) surfacing intervals typically
associated with respiration. These images provided an instantaneous visualization of
temperature distribution across the entire surface of the dolphin dorsal fin and the
associated boundary layer of water. IR thermal images were collected throughout
each sighting using a FLIR Agema 570 IR camera (emissivity = 1.0), mounted
side-by-side with a video camera (Sony Digital Handycam DCR-TRV 103) on a
monopod. Real-time video and a second, IR video (Sony Digital Handycam DCRTRV 340 (Sony Corporation, Tokyo, Japan) connected to the IR thermal camera)
were recorded simultaneously during all periods of thermal imaging. IR thermal
images were downloaded to a laptop computer daily and analyzed using ThermaCam
Researcher 2001 software (FLIR Systems AB, Danderyd, Sweden). Image quality
was evaluated and only those images that were in sharp focus, where the angle of
the dorsal fin was estimated to be less than 30◦ to the plane of the camera, and
where the dorsal fin occupied at least 15% of the image, were used. Dorsal fin surface
temperatures were measured at three sites in each image: the distal tip and at the
cranial and caudal regions of the fin base (Fig. 1). Care was taken to avoid fin margins,
where edge effects can distort IR temperature measurements (Watmough et al. 1970,
Cena and Clark 1973).
In each image the difference between the dorsal fin surface temperature measurement (T dfin ) and the independently measured ambient water temperature (T w )
was calculated and reported as the temperature differential (T dfin−w ). T dfin was
compared across each of the three measurement sites using a mixed-model analysis of variance (ANOVA), in which each IR image was treated as a set of three
Figure 1. IR thermal image of bottlenose dolphin dorsal fin and body illustrating sites of
dorsal fin temperature measurements. Dorsal fin surface temperatures (T dfin ) were measured
at the distal tip and the cranial and caudal regions of the fin base (circled) in each IR thermal
image. Circles were drawn to encompass the maximum possible area available in each image,
while avoiding the extreme edges of the fin.
BARBIERI ET AL.: INFRARED THERMOGRAPHY
57
repeated measures (SAS Version 8, SAS Institute, Inc., Cary, NC). T dfin−w was
also compared across each of the three measurement sites, using a one-way analysis of variance (ANOVA) (JMPIN version 5, SAS Institute, Inc., Cary, NC). On
average, the caudal region of the fin was significantly warmer (0.08◦ C) than the
distal tip and cranial region (P < 0.0001) across all seasons. Meagher et al. (2002)
also found that the trailing edge of the dorsal fin tended to be warmer than other
regions. Though significant, this difference was less than 0.1◦ C, and was consistent
across seasons; thus, T dfin−w was averaged across these three sites for all subsequent
analyses.
Mean T dfin−w was compared across seasons using an ANOVA (JMPIN). For all
comparisons, an alpha value of 0.05 was used to determine statistical significance. The
Tukey Honestly Significant Difference Test (JMPIN) was used to identify significant
differences in dorsal fin surface temperatures and temperature differentials across
seasons. Linear regression analysis (JMPIN) was used to investigate the relationship
between water temperature and both dorsal fin surface temperature and T dfin−w
across seasons.
IR Calibration Experiments
The use of IR thermography as both a diagnostic and field-portable investigative
tool is well-documented; however, some precautions regarding its quantitative accuracy should be considered (e.g., Watmough et al. 1970, Clark 1976, Cuyler et al.
1992). For example, accurately measuring water surface temperature using the IR
thermal camera is difficult given the high reflectivity of water. Calibration experiments were conducted to determine the accuracy of the IR camera. Its accuracy is
high when the imaged surface is parallel to the camera. IR thermal images taken
parallel to the surface of a temperature-controlled water bath (RE-120 Lauda Ecoline,
Brinkmann Instruments, Inc., Westbury, NY) were measured at T water bath = 10◦ C,
15◦ C, 20◦ C, 25◦ C, 30◦ C, 35◦ C. Two IR thermal images were taken of the water bath
surface at each temperature and the mean for that T water bath was calculated. The mean
difference between the temperature of the water bath and the IR camera-reported
temperature (T water bath − T IR camera ) was 0.15◦ C (range 0.1◦ C–0.2◦ C).
In an additional set of calibrations, the effects of the dorsal fin’s distance from and
angle to the camera were tested using a dorsal fin model. Three copper-constantan,
T-type thermocouples (Omega Engineering, Inc., Stamford, CT) were embedded
between two Plexiglas sheets that were carved into the size and shape of a bottlenose
dolphin dorsal fin and painted with flat gray, epoxy paint. Three holes were drilled
through the surface of one Plexiglas sheet at dorsal fin sites similar to those measured in field experiments. Thermocouple tips were pressed through the holes flush
with the outside surface of this sheet and secured using epoxy for waterproofing.
In free-swimming dolphins, the IR camera measured the temperature of the thin
layer of water coating the dorsal fin surface. To hold a similar layer of water over
the fin model, an elastic, matte gray, nylon sock was stretched tightly around the
Plexiglas.
To simulate a wet dorsal fin, the fin model was submerged and thermocouples were
allowed to equilibrate until they were within 0.5◦ C of water temperature. Water
temperature was approximately 35◦ C and was measured continuously throughout the
experiment using a fourth thermocouple. IR thermal images were taken immediately
after emergence of the fin model from the water (to simulate a brief surfacing event)
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Table 1. Infrared camera calibration data measured on a dorsal fin model at the distal tip,
and cranial and caudal portions of the fin base. Values are the difference (T TC−IR ) between
the thermocouple measurement (TC) and the IR thermal image measurement (IR). Negative
values indicate that the temperature of the thermocouple was lower than that of the IR
thermal image. All IR thermal images were taken of the dorsal fin immediately (within 2 s)
of emergence from a water bath (T w : 35◦ C) to mimic a surfacing dolphin in the wild. All
images were taken on a sunny day.
Data averaged over
5, 10, 15, 20 m distances
5, 10, 15, 20 m distances
5, 10, 15, 20 m distances
5, 10, 15, 20 m distances
0, 15, 30◦
0, 15, 30◦
0, 15, 30◦
0, 15, 30◦
0, 15, 30◦
angles
angles
angles
angles
angles
Experimental
variable
0◦ angle
15◦ angle
30◦ angle
Mean, all
angles
5 m distance
10 m distance
15 m distance
20 m distance
Mean, all
distances
TC – IR:
Distal
(◦ C)
TC – IR:
Cranial
(◦ C)
TC – IR:
Caudal
(◦ C)
Mean all
three sites
(◦ C)
0.32
−0.12
0.15
0.12
0.47
0
0.19
0.22
0.49
0.14
0.38
0.33
0.43
0.01
0.24
0.22
−0.28
0
0.12
0.63
0.25
−0.27
0.23
0.44
0.48
0.38
−0.09
0.28
0.41
0.73
0.48
−0.21
0.17
0.33
0.61
0.37
for each combination of four distances (5, 10, 15, 20 m) and three angles (0◦ ,
15◦ , 30◦ ) of the model to the camera. All IR thermal images in this calibration
were taken in full sun to include any potential effects of solar radiation on our
measurements.
For all experiments, a Fluke Hydra data logger (Fluke Corporation, Everett, WA)
recorded temperature measurements from each thermocouple every three seconds.
These data were downloaded continuously to a laptop computer. In each IR image,
the temperature of the model was calculated using the mean of measurements taken
from the distal tip and cranial and caudal regions of the fin model base, similar to
the methods used on free-swimming dolphins. Surface temperatures reported by the
thermocouples (TC) were subtracted from those reported by the IR thermal camera
(IR) to calculate T TC−IR . T TC−IR was averaged over three replicates for each set
of experimental conditions. Across all fin angles, the mean T TC−IR was 0.22◦ C
(range 0.01◦ C–0.43◦ C); across all distances, the mean T TC−IR was 0.37◦ C (range
0.21◦ C–0.61◦ C) (Table 1).
A third set of calibrations was carried out on wild, temporarily restrained bottlenose
dolphins in Sarasota Bay, FL. This calibration experiment was done as part of a
health monitoring program conducted by the Chicago Zoological Society, under
a National Marine Fisheries Service research permit (permit number 552–1569).
These experiments, which were conducted in the winter and summer of 2004 (n =
9 dolphins), compared temperature measurements of the IR camera and those taken
with thermocouples held against the dorsal fin surface.
Infrared thermal images were taken of the dorsal fin immediately following its
submergence for 1 min, to simulate a free-swimming dolphin. Within 15 min, the
dorsal fin surface temperature of the same individual was measured using thermocouples placed at the distal tip and base of the fin (Meagher et al. 2008). Stable
temperature measurements from the thermocouples were compared to IR camera
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Table 2. Comparisons of dorsal fin temperature data collected on temporarily restrained
dolphins using thermocouples (TC) and the IR thermal camera (IR) at the base and distal tip
of the dorsal fin. TC and IR data were collected within 15 min of each other. TC values were
subtracted from IR values; thus, negative values indicate that TC measurements were warmer
than IR measurements. IR temperatures were measured similarly to those on free-swimming
dolphins; cranial fin base measurements were used for comparison to TC base measurements.
Elapsed time
between IR cranial TC Base: IR distal TC distal Distal tip:
Dolphin
TC and IR
base
base IR – TC
tip
tip
IR – TC
(◦ C) (◦ C)
(◦ C)
(◦ C)
(◦ C)
ID
Season
(min)
(◦ C)
188
99
159
20
185
148
228
118
179
Summer
Summer
Summer
Summer
Summer
Summer
Winter
Winter
Winter
1101
1402
1222
1307
1152
1358
1105
1123
1448
32.57
31
31.17
31.08
31.91
32.55
16.34
18.15
18.26
33
−0.43
31.2 −0.20
30.9
0.27
30.5
0.58
31.7
0.21
31.8
0.75
15.5
0.84
17.7
0.45
18.3 −0.04
32.63
31.44
31.42
31.26
32.25
32.12
17.12
18.15
18.44
33.1
31.4
31.3
30.9
31.8
31.7
16.2
17.9
18.1
−0.47
0.04
0.12
0.36
0.45
0.42
0.92
0.25
0.34
measurements at the distal tip and cranial region of the dorsal fin (Table 2). The IR
camera tended to report warmer temperatures than the thermocouples by 0.27◦ C,
on average ( IR−TC range: −0.47◦ C–0.92◦ C).
The above experiments demonstrate that the accuracy and precision of the IR
measurements are dependent upon the complexity of the calibration technique. IR
thermography yields more accurate and precise temperature measurements (mean difference 0.15◦ C; range 0.1◦ C–0.2◦ C) of parallel-oriented water surfaces than it does of
either dorsal fin models at different angles and distances from the camera (mean difference 0.37◦ C, range 0.21◦ C–0.61◦ C) or of wild, temporarily restrained bottlenose
dolphins across different time frames (11–15 min—mean difference 0.27◦ C; range:
−0.47◦ C–0.92◦ C). These calibration studies do, though, demonstrate that under all
experimental conditions, IR-measured temperatures are within 1◦ C of independently
measured temperatures. These results suggest that the IR camera can be used to effectively assess the dorsal fin surface temperature, within ±1◦ C, of free-swimming
bottlenose dolphins.
RESULTS
Across the 2-yr-study period, there was a significant positive relationship between
mean dorsal fin surface temperature and water temperature (r2 = 0.978, P < 0.001)
(Fig. 2A). Thus, the temperature differential (T dfin−w ) was relatively constant and
the mean dorsal fin surface temperature across all seasons was 0.9◦ C warmer than
water temperature (Fig. 2B).
Although on average, most dorsal fin temperatures remained within approximately
1◦ C of water temperature, they could reach temperature differentials as high as 4◦ C.
One such occasion was documented in November 2002, when rain and cold air
temperatures dramatically reduced water temperature by 10◦ C across a 3-d-period.
The highest temperature differentials recorded in this study were measured during
the 2 d following this event (Fig. 2B).
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Mean dorsal fin surface temperature ( oC)
34
32
30
28
26
24
22
20
18
16
14
12
10
10
12
14
16
18
20
22
24
26
)
24
26
o
)
Water temperature ( C)
B
28
o
30
32
34
36
5
Temperature differential ( Tdfin-w) (oC)
4
3
2
1
0
-1
-2
-3
10
12
14
16
18
20
22
Water temperature ( C)
28
30
32
34
36
Figure 2. (A) Mean dorsal fin surface temperatures (T dfin ) plotted against water temperature
(T w ). There was a significant, positive relationship between T dfin and T w (r2 = 0.978; y =
0.587 + 1.01x; P < 0.0001). (B) The temperature differential between the dorsal fin and
the water (T dfin−w ) (◦ C) plotted against water temperature. T dfin−w values were relatively
consistent across winter, fall and summer seasons, although they tended to be slightly higher
at warmer temperatures. Mean temperature differential across all seasons was 0.9◦ C (r2 =
0.008; y = 0.589 + 0.014x; P = 0.0333). The highest temperature differentials measured in
this study (circled) were observed after a 10◦ C drop in water temperature in November 2002.
Symbols represent each field season (closed circle, November 2002; open circle, November
2003; closed square, June 2003; open square, June 2004; closed triangle, February 2003;
open triangle, February 2004).
BARBIERI ET AL.: INFRARED THERMOGRAPHY
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Figure 3. Comparison of temperature differentials across field seasons. There was no consistent trend in temperature differentials (T dfin−w ) across seasons. Field seasons labeled with
the same letter are not significantly different from each other. Horizontal line represents
overall mean for entire data set (0.9◦ C).
Mean values of T dfin−w were calculated for each season and year, which ranged
from 0.12◦ C to 1.35◦ C (Fig. 3). There was no consistent pattern in mean T dfin−w
across seasons; for example, values measured in winter were not always smaller than
those measured in summer. Mean values of T dfin−w measured in winter displayed
the greatest range: the largest was measured in winter 2003 and the smallest was
measured in winter 2004.
DISCUSSION
In this study, the use of IR thermography permitted the non-invasive measurement
of dorsal fin surface temperatures of wild, free-swimming dolphins across the broad
annual range of water temperatures they experience. The temperature differential
values (T dfin−a ) measured in this study were small (mean = 0.9◦ C and within
the range of ±1◦ C accuracy of the IR camera as determined through calibration
experiments) and relatively consistent across seasons. Thus, the temperature of the
dorsal fin surface varies seasonally and is positively correlated with water temperature
(Fig. 2).
This result suggests that the gradient through the dolphin body from the core to the
body-water interface must change dramatically across seasons. Figure 4 illustrates this
gradient by comparing T dfin−w to the calculated temperature differential between
the body core, which remains at approximately 37◦ C across all seasons (Rommel
et al. 1994; Pabst et al. 1995; Meagher et al. 2008; Pabst, unpublished data) and the
dorsal fin surface (T core−dfin ). The temperature gradient through the body can be as
large as 23◦ C in winter, but is constrained to 4◦ C–7◦ C in summer, as T w approaches
T core .
Seasonally dynamic changes in insulation, both vascular and integumentary, may
regulate this core-surface temperature gradient across seasons. The maintenance of a
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Temperature differential (Tcore - Tdfin) (oC)
22
20
18
16
14
12
10
( Tcore-dfin) - ( Tdfin-w)
8
6
4
2
0
-2
-4
12
14
16
18
20
22
24
26
28
30
32
34
o
Water temperature ( C)
Figure 4. Comparison of temperature differentials between body core temperature and
dorsal fin (T core − T dfin , large symbols) and the dorsal fin and the water (T dfin − T w ,
small dots). Core temperatures, which are stable across seasons (Rommel et al. 1994; Pabst
et al. 1995; Pabst, unpublished data), were assumed to be 37◦ C (Rommel et al. 1994, Pabst
et al. 1995). Symbols represent each field season (closed circle, November 2002; open circle,
November 2003; closed square, June 2003; open square, June 2004; closed triangle, February
2003; open triangle, February 2004; small dot, T dfin−w from Fig. 2A).
large gradient between the body core and surface in winter (Fig. 4) may be attributed
to enhanced integumentary insulation, as both blubber lipid content and thickness
increase in winter (Meagher et al. 2008, Wells 1993). Increases in vascular insulation
are affected through countercurrent heat exchangers found in the dorsal fin, flukes
and flippers, which reduce the temperature of the blood exposed to the skin-water
interface (Scholander and Schevill 1955, Elsner et al. 1974). Thus, by seasonally
dynamic changes in insulation, the temperature differential between the dorsal fin
surface and the ambient water is maintained at a steady 1◦ C.
The relatively consistent temperature differential between the dorsal fin surface
and the ambient water that was found in this study is similar to that measured for
both captive (Noren et al. 1999) and temporarily restrained wild dolphins (Meagher
et al. 2002, Meagher et al. 2008). Comparative data from other delphinids suggest
a similarly consistent relationship between skin and water temperatures, although
over much narrower water temperature ranges. Appendage skin surface temperatures
of three captive Hawaiian spinner dolphins (Stenella longirostris) were within approximately 1◦ C of the water, when it was maintained at a constant 26◦ C (Hampton
and Whittow 1976). IR thermography of wild, free-swimming spotted dolphins
(Stenella attenuata) in the Eastern Tropical Pacific (T a : 27.6◦ C–29.8◦ C) demonstrated
that skin surface temperatures were also positively correlated with water temperature
(Pabst et al. 2002).
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The consistency of T dfin−w reported for these aquatic homeotherms may also be
due to the physical properties of the fluid environment in which they live. Water
is a more thermally conductive environment than air; thus, any heat delivered to
the dorsal fin surface will be rapidly lost to the surrounding water. The effect of
water as a heat sink is more likely to be observed at the thermal windows because
these appendages are uninsulated and are primarily composed of non-thermogenic
connective tissue.
The influence of the physical properties of air and water on body surface temperatures has been noted by other investigators. Meagher et al. (2002) measured higher
temperature differentials on the dorsal fins of temporarily restrained, wild dolphins
in warm air than in water and attributed this result to the different conductivities of
these two fluids. Harbor seals (Phoca vitulina) in 0◦ C water maintained temperature
differentials of 1◦ C–2◦ C (Irving and Hart 1957), but in air (T air = 5◦ C–12◦ C),
temperature differentials of as high as 24◦ C have been reported (Mauck et al. 2003).
Similarly, in muskrats (Ondatra zibethicus), temperature differentials between most
body surfaces and the environment were smaller in water (1◦ C or less) than in air
(approximately 2◦ C–7◦ C) (Fish 1979). Terrestrial homeotherms that have been investigated using IR thermography, such as woodchucks, barn owls, foxes, and elephants,
can achieve body surface temperatures greater than 20◦ C above ambient air temperatures (Williams 1990, Klir and Heath 1992, McCafferty et al. 1998, Phillips and
Heath 2001). Differences in temperature between the body surface and the ambient
environment tend to be consistently small in water, and more highly variable in
air. Thus, it is possible that the conductive properties of the aquatic environment
are responsible for the overall conformity of Sarasota Bay dolphin dorsal fin surface
temperatures to that of the water.
Although there was an overall similarity observed in this study between skin surface and ambient water temperatures across all seasons, T dfin−w was not invariant
(see Fig. 2B). The level of activity, feeding occurrences, and reproductive status of
the animal could all influence its thermal status, which is reflected in T dfin−w (e.g.,
Costa and Williams 1999, Noren et al. 1999, Pabst et al. 2002). The largest observed
temperature differentials of free-swimming dolphins in this study (4◦ C) were measured in November 2002, after a precipitous 10◦ C decrease in water temperature that
occurred over a period of 3 d. The relatively large T dfin−w values observed after this
environmental change suggest that dolphins may increase metabolic heat production
in response to rapidly cooling ambient temperatures. Meagher et al. (2008) found
that heat flux across two other thermal windows, the flipper and tail fluke, was more
variable in winter, when water temperature was cooler. The T dfin−w data collected
in the present study in November 2002 suggest that this pattern may also apply to
the dorsal fin.
In summary, the use of infrared thermography is likely an effective, non-invasive
tool in the measurement of skin surface temperatures on free-swimming dolphins.
The results of this study support those of previous studies, but demonstrate that
these trends persist across a large number of individuals and across a wide range of
environmental temperatures.
The response of bottlenose dolphins to seasonal changes in water temperature in the Sarasota Bay region is characterized by the maintenance of a small
and steady temperature differential between the dorsal fin surface and the water. This relationship is likely driven by seasonal changes in integumentary insulation that are supplemented by shorter-term adjustments in vascular insulation. However, this relationship is also likely to be strongly influenced by the
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MARINE MAMMAL SCIENCE, VOL. 26, NO. 1, 2010
physical properties of the highly conductive medium in which these mammals
reside.
ACKNOWLEDGMENTS
This work was supported by Harbor Branch Oceanographic Institute’s Protect Wild Dolphins license tag program, NOAA Fisheries Service, Dolphin Quest, Earthwatch Institute,
Disney’s Animal Programs, Chicago Zoological Society, Mote Marine Laboratory, and the
Dolphin Biology Research Institute. We thank Dr. Erin Meagher for her thoughtful insight
and support throughout the project. We thank Drs. Robert Roer and Laela Sayigh for their
input, as well as Ari Friedlaender and Andrew Westgate for their assistance in data collection
and analysis. This work would not have been possible without the help of Drs. Michael Scott
and Blair Irvine, and volunteers from the Sarasota Dolphin Research Program and Earthwatch
Institute, especially Gene Stover. The experiments conducted in this study comply with the
current laws of the United States and were completed under NOAA Scientific Permit Number
522-1569 and UNCW IACUC Protocol Number 2002-009.
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Received: 8 July 2008
Accepted: 1 April 2009