thermoregulation in lizards dirk bauwens. thermoregulation n what? n why? n how should we study it?...
TRANSCRIPT
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Thermoregulation in lizards
Dirk Bauwens
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Thermoregulation
What? Why? How should we study it? Examples studies Mechanisms? Interactions other activities
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Thermoregulation
what?
definition
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target range
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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
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Thermoregulation
why?
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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
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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)
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Thermoregulation
How to study it?
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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
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Study thermoregulation
Requires information on:
• Tbs active lizards
• Target range (Tsel)
• Environmental temperatures
• Mechanisms: behaviour / physiology
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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!
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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
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Study thermoregulation
Environmental temperatures Quantification of heat exchange
between organisms and their environment
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Heat exchange with environment
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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"
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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
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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
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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
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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)
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Thermoregulation in lizards
– Sunny days
– Variation during course of day
Examples studies
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Podarcis sicula
Podarcis muralis
European lizards (Lacertidae)
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Islas Columbretes(Spain)
Palagruža (Croatia)
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Agama atra (South-Africa)
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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
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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
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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
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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
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730 900 1030 1200 1330 1500 1630 1800
20
30
40
50
60
Hour
Tsel
Te
mp
era
ture
(°C
)
Podarcis atrata - Columbretes
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Podarcis sicula - Palagruža
6 8 10 12 14 16
15
20
25
30
35
40
45
50
T
empe
ratu
re (
°C)
Hour
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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
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How “well” do lizards regulate Tb?
– Tbs almost always within Tsel : high “accuracy” of thermoregulation
– Tbs deviate considerably from Tes: high “effectiveness” of thermoregulation
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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
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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
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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”
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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)
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Behavioural thermoregulation
Main “mechanisms”:Restriction of activity timesPostures & orientation Selection thermally “suitable” microhabitats
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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
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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
)
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Vaalputs, Karoo Desert (South-Africa)
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Cordylus polyzonus
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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)
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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
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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)
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Postures & orientation
Usage of postures & adjustment of orientation
Modify the rate of heat exchange with environment
Increase when Te < Tsel
Decrease when Te >Tsel
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Basking posture
Increase heating rate
Prediction: more often when Te < Tsel
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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
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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
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Cordylus polyzonus – orientations & postures
“perpendicular”
Maximize bodysurface exposed
to sun
“transverse” “parallel”
Minimize bodysurface exposed
to sun
Flanks exposed
to sun
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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
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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
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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
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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
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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
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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
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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)