Thermoregulation in lizards
Dirk Bauwens
Thermoregulation
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What?
Why?
How should we study it?
Examples studies
Mechanisms?
Interactions other activities
Thermoregulation
what?
 definition
35
30
Temperatuur (°C)
25
20
15
10
5
0
target range
tijd
35
30
Temperatuur (°C)
25
20
15
10
5
0
10
20
30
tijd
40
50
Definition thermoregulation:
Proces by which organisms attempt to
maintain their body temperature (Tb):
 within a specific target range
 divergent from the environmental
temperatures
 by physiological and/or behavioural
adjustments
Thermoregulation
why?
Thermal sensitivity: The rate of biological
processes is temperature dependent
1
0
0
8
0
RelativePerformance(%)
TPB80
6
0
Topt
4
0
2
0
2
0
2
5
3
0
3
5
B
odyT
em
perature(°C
)
4
0
4
5
Thermal optima in Lacerta vivipara
Topt
TPB80
body mass change
energy- intake
gut-passage rate
handling rate
catching rate
sprint speed
24
26
28
30
32
34
36
38
40
Tb (°C)
source: Van Damme et al. 1991 (Funct. Ecol. 5: 507- 517)
Thermoregulation
How to study it?
Definition thermoregulation:
Proces by which organisms attempt to
maintain their body temperature (Tb):
 within a specific target range
 divergent from the environmental
temperatures
 by physiological and/or behavioural
adjustments
Study thermoregulation
Requires information on:
• Tbs active lizards
• Target range (Tsel)
• Environmental temperatures
• Mechanisms: behaviour / physiology
Study thermoregulation
Body temperatures (Tbs)
maintained during activity
 Tbs = final result of regulatory proces
 How measure?
 “grab and jab”
 telemetry
 Obtain measurements at different times
and places!
Study thermoregulation
Target range for Tbs
 Tbs in “ideal” conditions for regulation
 Reflect Topt
 How measure?
 In thermogradient
 Tsel : upper- and lower limits (80 of 95%)
of Tbs maintained
Study thermoregulation
Environmental temperatures
 Quantification of heat exchange
between organisms and their
environment
Heat exchange with environment
How to measure environmental
temperatures?
 Analytical model:
 Measure relevant traits of lizards
(size, surface area, reflectance skin, ...)
 Micro-meteo measurements
(radiation, wind, T° air, T° substrate, …)
in various microhabitats, at different
times!
 Solve "energy balance equation"
Energy Balance Equation
Qa = asAsS + asAss + asAgr (S + s) + at(AgRg + AsRa)
M-lEb = 0.096 eTb/10 - 0.298 e0.0586Tb
Qa + M - lEb = e s ( Tb -
M-lEb
K
+ 273)
4+
H(Tb-Ta-
M-lEb
K
H = 3.49 (V/D)O.5
)
How to measure environmental
temperatures?
 Analytical model: complex & expensive
 “Physical” models:
 Objects that mimic heat exchange between
organisms and environment
 e.g. dead lizards, copper models, copper
tubes, cans…
 “Tb” of model = Te
(“operative environmental temperature”)
 Tb of non thermoregulating organism
 (Relatively) easy & cheap
How to measure environmental
temperatures?
 “Physical” models:
 “Tb” of model = Te
(“operative environmental temperature”)
 Te  Tb of non-thermoregulating organism
 (Relatively) easy & cheap
 Large numbers can be used to measure in
different microhabitats and times
Study thermoregulation
 Tbs active lizards
 Target range: Tsel
 Environmental temperatures: Te
 Behavioural observations (thermoregulation, social, foraging, …)
 Continuous observations (1 lizard – 10 min)
 “Scan sampling” (n lizards – 1 sec)
Thermoregulation in lizards
Examples studies
– Sunny days
– Variation during course of day
European lizards (Lacertidae)
Podarcis sicula
Podarcis muralis
Islas Columbretes
(Spain)
Palagruža
(Croatia)
Agama atra (South-Africa)
Studies thermoregulation
How “well” do lizards thermoregulate?
– Similarity of Tbs with target range (Tsel):
“accuracy” thermoregulation
– Deviation of Tbs from operative
temperatures (Tes): “effectiveness”
thermoregulation
Podarcis atrata - Body temperatures
Percentage of Observations
25
T
Tsel
20
b
15
10
5
10
20
30
40
Temperature (°C)
50
60
Percentage of Observations
Podarcis atrata - Operative temperatures
8
Te
6
4
2
0
10
20
30
40
Temperature (°C)
50
60
T
sel
25
T
b
20
Percentage of Observations
15
10
5
8
T
e
6
4
2
0
10
20
30
40
Temperature (°C)
50
60
Podarcis atrata - Columbretes
Temperature (°C)
60
50
Tsel
40
30
20
730
900
1030 1200 1330 1500 1630 1800
Hour
Podarcis sicula - Palagruža
50
Temperature (°C)
45
40
35
30
25
20
15
6
8
10
12
Hour
14
16
Agama atra – Jonkershoek (ZA)
55
Temperature (°C)
50
45
40
35
30
25
20
15
08:00
10:00
12:00
14:00
Hour
16:00
18:00
How “well” do lizards regulate Tb?
– Tbs almost always within Tsel :
high “accuracy” of thermoregulation
– Tbs deviate considerably from Tes:
high “effectiveness” of thermoregulation
How do lizards regulate their Tb?
What “mechanisms” are used?
– Ectotherms: physiology unimportant
– Behavioural thermoregulation:
• Restriction of activity times
• Postures & orientation
• Selection thermally “suitable” microhabitats
Behavioural thermoregulation
 To what extent does thermoregulation
determine lizard behaviour?
 Recall the copper models:
Te  Tb of non-thermoregulating lizard
 At times / places with Te  Tsel lizards
can easily attain Tb  Tsel
 Let’s look at distributions of Te provided
by the copper models
Suitability of habitat / time: % models
with Te  Tsel
50
Temperature (°C)
45
40
35
too warm
“ideal”
30
25
20
too cold
15
6
8
10
12
Hour
14
16
Behavioural thermoregulation
 To what extent does thermoregulation
determine lizard behaviour?
 At times / places with Te  Tsel lizards
can easily attain Tb  Tsel
 If thermoregulation dictates behaviour,
lizards should restrict actvity to times /
places where a high % of Te  Tsel
(“only thermoregulation” hypothesis)
Behavioural thermoregulation
Main “mechanisms”:
 Restriction of activity times
 Postures & orientation
 Selection thermally “suitable”
microhabitats
Restriction of activity times
 Prediction: active only when heat loads
permit to attain Tbs  Tsel
– Seasonal activity (hibernation; aestivation)
– Diurnal vs. nocturnal activity
– Can we predict daily activity times?
– Activity restricted to times when minimal
fraction of Tes  Tsel
Prediction activity times
65
Operative Temperature (°C)
60
55
50
45
40
35
30
25
20
15
10
06:00
08:00
10:00
12:00
14:00
16:00
18:00
20:00
Vaalputs, Karoo Desert (South-Africa)
Cordylus polyzonus
Cordylus polyzonus – Vaalputs (mid summer)
65
Operative
Operative Temperature
Temperature (°C)
(°C)
60
55
50
45
40
35
30
25
20
15
10
06:00
08:00
10:00
12:00
14:00
16:00
18:00
20:00
Number of Lizards
7
6
5
4
3
2
1
0
06:00 08:00 10:00 12:00 14:00 16:00 18:00 20:00
Hour
Cordylus polyzonus - Vaalputs (mid-summer)
Number of Lizards
7
Predicted
Observed
6
5
4
3
2
1
0
06:00
08:00
10:00
12:00
14:00
Time
16:00
18:00
20:00
Prediction activity times
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Overall good agreement between
observed and predicted
Major discrepancies in early morning
Why differences?
– Predictions assume “only thermoregulation”
and lizards also do other things (forage, social
interactions, …)
– Lizards avoid Tbs > Tsel (overheating)
Postures & orientation
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Usage of postures & adjustment of
orientation
Modify the rate of heat exchange with
environment
Increase when Te < Tsel
Decrease when Te >Tsel
Basking posture
 Increase heating rate
 Prediction: more often when Te < Tsel
Podarcis sicula – Palagruža (1 point = 1 hour period)
50
r = 0.840, P < 0.001
% Time Basking
40
30
20
10
0
0
20
40
60
% Tes below Tpref
80
100
Basking Frequency (%)
80
NE
E
SE
S
60
SW
W
NW
40
20
0
730
900
1030
1200
Hour
1330
1500
1630
Cordylus polyzonus – orientations & postures
“perpendicular”
“transverse”
Maximize body
surface exposed
to sun
Flanks
exposed
to sun
“parallel”
Minimize body
surface exposed
to sun
Cordylus polyzonus - Orientation to Sun
Perpendicular
Transverse
Parallel
Predicted Proportion
1.0
0.8
0.6
0.4
0.2
0.0
07:30
Observed Proportion
1.0
0.8
0.6
0.4
0.2
0.0
08:30
09:30
17:00
18:00
19:00
--
r = 0.84, P < 0.001
Selection microhabitats
 Selection of thermally suitable micro-habitats
 “Only thermoregulation”: hour-to-hour

variation in thermal suitability (and
availability) of microhabitats determines their
usage
Te measurements  predictions about
microclimate usage at different times
Podarcis sicula - microhabitat use
Sun
Partial Shade
45
40
35
30
25
20
15
7
9
11
13
15
17
7
9
11
13
15
50
Shade
Operative Temperature (°C)
Operative Temperature (°C)
50
45
40
35
30
25
20
15
7
9
11
13
Hour
15
17
17
P. sicula - predicted and observed
microhabitat use
Sun
Partial Shade
Shade
Predicted Preferences
1.0
0.8
0.6
0.4
0.2
r = 0.78
P < 0.001
0.0
Observed Preferences
1.0
0.8
0.6
0.4
0.2
0.0
8
10
12
Hour
14
16
Average residence times:
Sun
Partial Shade
Shade
7'37"
3'02"
27"
Number of foraging strikes / 10 min:
Sun
Partial Shade
Shade
0.09
0.55
1.09
P. sicula - Diel variation foraging strikes
0.6
r = 0.781, P < 0.01
Foraging strikes / 10 min
0.5
0.4
0.3
0.2
0.1
0.0
0
5
10
15
20
% Te within Tsel
25
30
35
Conclusions:
 The lizards studied regulate their Tb with


high accuracy and effectiveness
Activity times, diel variation in posturing
and in microhabitat use, are to a large
extent induced by the interaction with the
thermal environment
The needs to thermoregulate may conflict
with, or constrain the time devoted to other
demands (e.g., foraging)