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Thermoregulation in lizards
Dirk Bauwens
Thermoregulation
What? Why? How should we study it? Examples studies Mechanisms? Interactions other activities
Thermoregulation
what?
definition
target range
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
Body Temperature (°C)
Rel
ativ
e P
erfo
rman
ce (
%)
20
40
60
80
100
20 25 30 35 40 45
Topt
TPB80
Tb (°C)
24 26 28 30 32 34 36 38 40
sprint speed
catching rate
handling rate
gut-passage rate
energy- intake
body mass change
Topt
TPB80
Thermal optima in Lacerta vivipara
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"
Qa + M - Eb = Tb - + 273) 4 + H(Tb-Ta- )
M-Eb
K
M-Eb
K
Qa = asAsS + asAss + asAgr (S + s) + at(AgRg + AsRa)
H = 3.49 (V/D)O.5
M-Eb = 0.096 eTb/10 - 0.298 e0.0586Tb
Energy Balance Equation
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 (thermo-regulation, social, foraging, …) Continuous observations (1 lizard – 10 min) “Scan sampling” (n lizards – 1 sec)
Thermoregulation in lizards
– Sunny days
– Variation during course of day
Examples studies
Podarcis sicula
Podarcis muralis
European lizards (Lacertidae)
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
5
10
15
20
25
Per
cen
tag
e o
f O
bse
rvat
ion
s
Temperature (°C)
Tb
Podarcis atrata - Body temperatures
10 20 30 40 50 60
Tsel
Podarcis atrata - Operative temperatures
10 20 30 40 50 600
2
4
6
8
Te
Temperature (°C)
Per
cen
tag
e o
f O
bse
rvat
ion
s
10 20 30 40 50 600
2
4
6
8
Tsel
Te
Temperature (°C)
Perc
en
tag
e o
f O
bserv
ati
on
s
5
10
15
20
25
Tb
730 900 1030 1200 1330 1500 1630 1800
20
30
40
50
60
Hour
Tsel
Te
mp
era
ture
(°C
)
Podarcis atrata - Columbretes
Podarcis sicula - Palagruža
6 8 10 12 14 16
15
20
25
30
35
40
45
50
T
empe
ratu
re (
°C)
Hour
Agama atra – Jonkershoek (ZA)
08:00 10:00 12:00 14:00 16:00 18:0015
20
25
30
35
40
45
50
55
T
emp
erat
ure
(°C
)
Hour
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
6 8 10 12 14 16
15
20
25
30
35
40
45
50
T
empe
ratu
re (
°C)
Hour
6 8 10 12 14 16
15
20
25
30
35
40
45
50
T
empe
ratu
re (
°C)
Hour
too cold
too warm
“ideal”
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 timesPostures & 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
06:00 08:00 10:00 12:00 14:00 16:00 18:00 20:0010
15
20
25
30
35
40
45
50
55
60
65
O
pera
tive
Tem
pera
ture
(°C
)
Vaalputs, Karoo Desert (South-Africa)
Cordylus polyzonus
Cordylus polyzonus – Vaalputs (mid summer)
06:00 08:00 10:00 12:00 14:00 16:00 18:00 20:000
1
2
3
4
5
6
7
Num
ber
of Liz
ard
s
Hour
10
15
20
25
30
35
40
45
50
55
60
65
Ope
rativ
e T
empe
ratu
re (
°C)
06:00 08:00 10:00 12:00 14:00 16:00 18:00 20:0010
15
20
25
30
35
40
45
50
55
60
65
Ope
rativ
e T
empe
ratu
re (
°C)
Cordylus polyzonus - Vaalputs (mid-summer)
06:00 08:00 10:00 12:00 14:00 16:00 18:00 20:000
1
2
3
4
5
6
7
Predicted Observed
N
umbe
r of
Liz
ards
Time
Prediction activity times
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
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)
0 20 40 60 80 100
0
10
20
30
40
50 r = 0.840, P < 0.001
%
Tim
e B
aski
ng
% Tes below T
pref
730 900 1030 1200 1330 1500 16300
20
40
60
80 NE E SE S
Bas
kin
g F
req
uen
cy (
%)
Hour
SW W NW
Cordylus polyzonus – orientations & postures
“perpendicular”
Maximize bodysurface exposed
to sun
“transverse” “parallel”
Minimize bodysurface exposed
to sun
Flanks exposed
to sun
Cordylus polyzonus - Orientation to Sun
07:30 08:30 09:30 17:00 18:00 19:00 --0.0
0.2
0.4
0.6
0.8
1.0
Pre
dict
ed P
ropo
rtio
n
Perpendicular Transverse Parallel
0.0
0.2
0.4
0.6
0.8
1.0
Obs
erve
d P
ropo
rtio
n
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
7 9 11 13 15 17
15
20
25
30
35
40
45
50
Sun O
pera
tive
Tem
pera
ture
(°C
)
7 9 11 13 15 17
Partial Shade
7 9 11 13 15 17
15
20
25
30
35
40
45
50
Shade
Ope
rativ
e T
empe
ratu
re (
°C)
Hour
P. sicula - predicted and observed microhabitat use
8 10 12 14 16
0.0
0.2
0.4
0.6
0.8
1.0
Obs
erve
d P
refe
renc
es
Hour
0.0
0.2
0.4
0.6
0.8
1.0
Sun Partial Shade Shade
Pre
dict
ed P
refe
renc
es
8 10 12 14 16
0.0
0.2
0.4
0.6
0.8
1.0
Obs
erve
d P
refe
renc
es
Hour
0.0
0.2
0.4
0.6
0.8
1.0
Sun Partial Shade Shade
Pre
dict
ed P
refe
renc
es
8 10 12 14 16
0.0
0.2
0.4
0.6
0.8
1.0
Obs
erve
d P
refe
renc
es
Hour
0.0
0.2
0.4
0.6
0.8
1.0
Sun Partial Shade Shade
Pre
dict
ed P
refe
renc
es
r = 0.78P < 0.001
Average residence times:
Sun 7'37"Partial Shade 3'02"Shade 27"
Number of foraging strikes / 10 min:
Sun 0.09Partial Shade 0.55Shade 1.09
P. sicula - Diel variation foraging strikes
0 5 10 15 20 25 30 35
0.0
0.1
0.2
0.3
0.4
0.5
0.6
r = 0.781, P < 0.01
F
ora
gin
g s
trik
es /
10 m
in
% Te within T
sel
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)