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Measuring Thermal Tolerance
Measuring Thermal Tolerance
• CTmax – upper lethal temperature reached while
raising temperature 1 C per minute
– Various endpoints – muscular spasms most
common
– Why is the rate of heat increase important?
• Ctmin – lower lethal temperature
– More difficult to measure due to lack of definitive
endpoint (often a gradual reduction in activity)
– Difficult to quantify in freeze tolerant species
• Ecological end points
• Thermal stress has a strong temporal component
– Thermal stress → disrup on of enzyma c pathways
– Heat hardening (HSP) and acclimation responses adjust
individual physiology
– Extended exposure to tolerable but sub-optimal
temperatures can reduce fitness and eventually be fatal
– Ctmax is not a measure of these sub-optimal but tolerable
effects, it may be correlated
Heat hardening, acclimation and measures of tolerance
Temperature
30
35
34 Co 33 Co 32 Co
Mortality
100
• Often used in tolerance studies (e.g. drug toxicity LD50)
• Temperature (LT50 or UT50) at which lethal effects (50%
mortality) is independent of exposure time.
31 Co 30 Co
50%
Incipient Lethal
Temperature
25
LD50 and UD50
Zone of Resistance
Zone of Tolerance
0
Exposure Time to 50% mortality
Time
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Measuring Thermal Tolerance
• Variability within taxonomic
groups implies strong selective
pressure for tolerance
• Variety of evolutionary
responses
– Behavioral changes
– Modifications or new
enzymes to regulate reaction
rates
– Etc…
Measuring Thermal Tolerance
• Tolerance polygon – a measure
(in units of degrees C2) of upper
and lower thermal tolerance
over a range of acclimation
temperatures
• Captures the thermal niche
• Theoretically centered on the
thermal optima for a species
• Stenotherm vs. eurytherm
Thermal Tolerance
Zone of Thermal Neutrality
• Species thermal
tolerance correlates
with climate
• Why aren’t all
species highly
tolerant?
• Metabolic rate-temperature patterns differ in endotherms
• Endotherm optima within thermoneutral zone
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Moses, M. R., Frey, J. K. & Roemer, G. W. 2012 Elevated surface temperature depresses survival of banner-tailed kangaroo rats: will climate
change cook a desert icon? Oecologia 168, 257–268. (doi:10.1007/s00442-011-2073-2)
Optima and Thermal Preference
• Preferred temperature
• Selected temperature
• Eccritic or Field Temperature
• Thermal preferences as a measure
of optima
• Dynamics and ecology of
preference/optima
– Energetics and thermal shuttling
– Behavioral fever
– Ecdysis
– SDA/Postprandial thermophile
– Torpor
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• Recall that the
setpoint is
dynamic, will
change with
– Acclimation
– Local climate
– Disease
– Etc.
Thermal Optima and Reaction Norms
• Species fitness maximized at thermal optima
• Shape of thermal reaction norms correlated with
tolerance polygon area
Stenotherm
Hatching Success
Measuring Thermal Preference
Eurytherm
Temperature
Performance: Hatching Success
• More broadly distributed
species has a wider
reaction norm (higher
hatch success at extremes)
• Species optima expected
to match local
environments. Northern
populations have lower
optima.
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