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)