The effect of heat transfer mode on heart rate responses and hysteresis during heating and cooling in the estuarine crocodile Crocodylus porosus
Craig E. Franklin1 and
Frank Seebacher2,*
1 Department of Zoology and Entomology, University of Queensland, St Lucia,
Qld 4072, Australia
2 School of Biological Sciences A08, University of Sydney, Sydney, NSW 2006,
Australia
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Fig. 1. Representative examples of the heart rate (solid line) and body temperature
(broken line) response to the different treatments: (A) heat lamp dry; (B)
heat lamp wet; (C) hot water. The vertical lines indicate the time when the
treatment commenced (i.e. when the heat lamp was switched on or hot water was
introduced; line on the left) and when it was concluded (right-hand line).
Note the almost instantaneous increase and decrease in heart rate when the
heat was applied and removed, respectively, which occurred while body
temperature change was negligible. Note that values on the y-axis
denote both heart rate and body temperature (Tb).
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Fig. 2. Mean heart rate (averaged over 1°C body temperature intervals, ±
S.E.M.) was significantly faster during heating than during cooling at any
given body temperature in all treatments – (A) heat lamp dry, (B) heat
lamp wet, (C) hot water – although the difference was least pronounced
in the heat lamp wet treatment.
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Fig. 3. Representative examples of body temperature (broken lines) and heart rate
(solid lines) responses to the control treatments: (A) cold light and (B) cold
water. The solid vertical lines indicate when the cold light was switched on
and off (in A) or when water flow was commenced (in B). None of the control
treatments elicited a significant heart rate response. Note that values on the
y-axis denote both heart rate and body temperature
(Tb).
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Fig. 4. (A) Q10 values for the change in heart rate during heating and
cooling in the heat lamp dry treatment (means ± S.E.M.). During the
initial `rapid cardiac response' to the application or removal of heat, heart
rate changed dramatically while body temperature (Tb)
remained nearly stable. (B,C) Close-up views of representative examples for
the rapid response periods are shown. Q10 values for the heart rate
change during the `rapid response' periods were extremely high (>4000,
represented by >>20 on the y-axis), indicating the temperature
independence of heart rate during those periods. The times when the heat lamp
was switched on and off are indicated by the solid vertical lines. Note that
values on the y-axis in B and C denote both heart rate and body
temperature.
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Fig. 5. Changes in heart rate during heating and cooling changed in proportion to
the heat load experienced at the animal surface (means ± S.E.M.). (A)
The heart rate response to heat load was sigmoidal in shape, with rapid
decreases and increases below –15 W and above 25 W, respectively. The
range of heat loads experienced by the animals was greatest during the hot
water (HW) treatment and least in the heat lamp wet (HLW) treatment. The
fitted line shows a rational function. (B) Changes in heart rate during the
linear portion of the sigmoidal curve (–16 W to 20 W) increased with
increasing heat load. Over this linear range, there were no differences
between the heat lamp dry (HLD) and HLW treatments, but rates of change were
significantly less in the HW treatment. Linear regression lines are shown for
the HW treatment (broken line) and for the HLD and HLW treatments combined
(solid line).
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© The Company of Biologists Ltd 2003
