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Homeostasis in Animals
Temperature
Processes of Heat Transfer
 Radiation = heat transfer through waves of
energy
 Conduction = direct transfer of heat from one
object to another
 Convection = heat transfer by circulation of a
fluid (gas or liquid)
 Evaporation = heat loss as water changes from
liquid to vapor
Radiation
 Animals gain radiant heat directly from the
sun and reflected from the atmosphere
and objects in the environment
 Animals also lose heat as they radiate it to
their environment
 Net gain during daytime, net loss at night
Conduction
 Animals can either gain or lose heat to the
ground or other objects depending on their
relative temperatures
lizard
rock
30°C
20°C
Animal loses heat when it’s
warmer than contacted object
30°C
40°C
Animal gains heat when it’s
cooler than contacted object
Convection
 Animals can gain or lose heat depending
on relative temperature of animal and fluid
 Typically animals lose heat through
convection because their body
temperature is higher than air temperature
 Insulation of fur or feathers reduces
convective heat loss
brr!
Evaporation
 Always results in heat loss from the animal
 Sweating, panting, and bathing are
adaptations to increase evaporative heat
loss to prevent overheating
Bergmann’s Rule
Individuals of a given species are larger in
cold climates than in warm climates
(generally speaking)
White-tailed deer skulls
Source: http://www.mun.ca/biology/scarr/
Bergmann's_rule_in_Odocoileus.htm
Allen’s Rule
Individuals of a given species have shorter
extremities in cold climates than in warm climates
(generally speaking)
Arctic hare Snowshoe hare Black-tailed Antelope
jackrabbit
- boreal forest jackrabbit
- Arctic
- deserts
- prairies
tundra
Source: http://www.mun.ca/biology/scarr/Lepus_variation.htm
HOW SIZE MATTERS IN
REGULATING BODY
TEMPERATURE
Concept
 Small-bodied animals or plant parts (e.g.,
leaves) heat up and cool down faster;
bigger and/or thicker bodies heat up and
cool down slower.
Explanation
 Smaller/thinner bodies have a larger
surface area to volume ratio.
 Bodies gain and lose heat out of the surface
of their body; more surface area means
greater gains and losses.
 Bodies retain heat within their bodies; more
volume means more heat retention. When the
surface area is large compared to the volume
(small/thin things), heat is gained and lost
quickly.
Example: compare 1 cm x 1 cm x 1 cm
cube with 10 cm x 10 cm x 10 cm cube
10
Large
animal
10
1
Small animal
1
1
10
Surface area = 1x1x6 = 6 cm2
Volume = 1x1x1 = 1 cm3
SA:V ratio = 6:1 = 6.0
Surface area = 10x10x6 = 600 cm2
Volume = 10x10x10 = 1000 cm3
SA:V ratio = 600:1000 = 0.6
SIZE MATTERS
 Small cookies cool down faster than larger
cookies after coming out of the oven. Also small
cookies burn faster.
 Your hand has the same volume whether it is
balled up (fat) or spread out (thin). On a cold
day, your hand will get cold faster when spread
out because balling up you hand into a fist
effectively reduces surface area because now
the part of your hand within your fist is no longer
"surface".
Advantages and Disadvantages to
Being Large:
 Heat is gained and lost more slowly so, for
example, on a hot summer day, the a large
animal may never reach lethal temperatures by
the time the sun sets.
 Because heat is lost more slowly, the animal
doesn't have to replace lost heat as quickly
 Therefore, the animal doesn't have to eat as much
compared to its body weight (e.g., only has to eat 1/4
its body weight).
 However, it usually does have to eat more total food
than a smaller animal.
Advantages and Disadvantages to
Being Small:
 Heat is gained and lost faster so, for example,
on a hot summer day, a small leaf (less than
about 1 square centimeter) will shed heat as fast
as it acquires heat
 Therefore the leaf will never reach temperatures
higher than air temperature (compared to a large,
thick leaf that acquires a heat load and can reach
very high, lethal temperatures).
 Because heat is lost faster, the animal has to eat
faster to replace the lost energy (e.g., very small
mammals may eat up to 4 times their body
weight each day).
Summary Example
 A plant can either have very small leaves
that never heat up higher than air
temperature or very large leaves that heat
up so slowly that they never reach lethal
temperatures, but a plant doesn't want to
have leaves in the middle sizes if the
temperatures are going to be extremely
hot or cold.
Concepts
 Van't Hoff's Rule: for every temperature rise of 10
degrees C, rate of biochemical reactions (most body
functions) doubles, up to a point (when proteins break
down). What is the other name of this law?
 Energy Allocation: all energy taken in by plants and
animals is portioned out (allocated) to the following areas
 Growth
 Reproduction
 Activity
 Maintenance
 Storage.
 Savings in one area (e.g., maintenance which includes
thermoregulation) means more energy can be diverted to
other areas (e.g., storage for hard times later).
Types of Animals Based on Means
of Thermoregulation:
 Ectotherms: animals whose principal
source of body heat is the environment.
Includes essentially all animals except
birds and mammals.
 Endotherms: animals whose principal
source of body heat is from their own body
generating heat metabolically. Birds and
mammals are endotherms.
Types of Thermoregulation
 Behavioral Thermoregulation: using posture,
orientation, and microclimate selection to
regulate body temperature. For example, a
lizard that wants to heat up will spread eagle
(posture) on the top of a hot rock (microclimate)
and turn its entire back to the sun (orientation).
 Physiological Thermoregulation: altering
metabolic generation of heat to regulate body
temperature.
Poikilotherms (ectotherms)
 No internal physiological mechanism for keeping
body temp constant
 Body temp matches environmental temp
 As temp drops body processes slow down
 Source of body heat is the environment
 Plants
 Reptiles
 Amphibians
 Most fish
 Most invertebrates
Coping with Temperature Variation
Poikilotherms
 Behavior
 When temp is low lizard sits broadside to sun and
presses belly against warm rocks
 When temp is high lizard moves to shade, burrow,
walks on tiptoe
 When temp is too low lizard seeks protected
habitat and becomes inactive; outcome depends
on how cold it gets
Example
 Desert Iguana (Dipsosaurus dorsalis) in the
southwestern US desert.
 In summer temperatures can exceed 45oC, and in
winter temperatures are often below 0o
 During mid-July the thermal environment changes
so rapidly that activity is limited to 45 minutes in
the morning, and 45 minutes in the afternoon.
 Figure describes the diurnal and seasonal
pattern of behavior:
Coping with Temperature Variation
Poikilotherms
 Physiology
 produce chemicals that lower the freezing
point of cytoplasm
 freeze-tolerant species - have ice-nucleating
agents (ECF freezes at lower temp than
cytoplasm; prevents ice crystals)
 freeze-susceptible - produce glycerol etc; facilitate
supercooling; lower freezing point to avoid ice
crystals
Advantages of being an
Ectotherm
 Greater efficiency: allocate 30%-90% of
ingested energy towards growth (<5% in
most endotherms)
 Lower energy demands: can tolerate long
periods with low food availability
 Less energetic cost for small body size:
able to occupy more niches for smallbodied animals
Large Ectotherms?
 As body size increases,
surface area-to-volume ratio
decreases, and large
ectotherms are thus
improbable.
 This had led to speculation
that large dinosaurs may
have had some degree of
endothermy.
Homeotherms (endotherms)
 Physiological mechanisms for controlling body temp
 Keeping warm
 Metabolic heat: increased metabolism, shivering
 Insulation (fat, fluffed fur or feathers) e.g. Birds,
mammals
 Body size and shape
 Keeping cool
 Lower metabolic heat production
 Evaporation: panting, sweat
 Body size and shape
 Birds
 Mammals
Range and limits of homeothermy
 Thermoneutral zone = range of environmental
temperatures over which an animal can maintain
a constant body temperature without raising its
metabolic rate
 Lower and upper critical temperatures =
environmental temperatures at which animal
must raise its metabolic rate to maintain
homeothermy
 Lower and upper lethal temperatures =
environmental temperatures at which animal can
no longer raise its metabolic rate and dies
Coping with Temperature
Variation Homoiotherms
 Behavior
 Burrows, dens,
shade during
hottest part of day
 Sled dog, arctic fox
wrap tail around
face
 Grouse burrows
under snow
 Gular fluttering
 Urohydrosis
 Urohydrosis
 Excretion on the feet or legs
which evaporates and then
cools the organism as the
excretion evaporates
 This is a common
adaptation in birds such as
turkey vultures.
 A similar adaptation is seen
in Kangaroos which lick their
forearms to wet them with
saliva.
Cactus Wren: Daily behavior
reflects the temperature of the
microhabitats used. The
orientation of the nest changes
during the breeding season in
order to maximize cooling in this
desert environment.
Coping with Temperature Variation
Homoiotherms
 Morphology
 Coloration is thought to be an important factor in the
reduction of heat absorption.
 A lighter colored coat will reflect more light relative
to a darker coat which will absorb more light. The
result is reduced body temperatures and more
water conservation.
 Insulation in the form of feathers, hair, or even body
fat which protects tissues beneath by insulation.
 Enlarged appendages (desert Jackrabbit’s ears) in
order to increase surface area and hence promote
heat loss.
Desert Jackrabbit
Arabian oryx
Coping with Temperature Variation
Homoiotherms
 Physiology – keeping warm requires lots
of food





Increased body fat for cold weather insulation
Increased metabolic rate
Shivering (heat from muscle activity)
Increase air spaces by fluffing feathers, fur
Counter current mechanisms
During the winter, the
Willow Ptarmigan
produces a denser coat
of feathers and actually
down-regulates its basal
metabolism, which
reduces the gradient
between the internal
body temperature and
the external
temperature. This is
the same as keeping
your house at a cooler
temperature in order to
reduce your heating bill.
Both responses reduce
the amount of energy
necessary to stay warm.
Metabolic Heat Production
 Endotherms produce heat by oxidizing energyrich carbohydrate molecules
C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + energy
 Some of the energy is stored but much is
dissipated as heat
 Metabolic rate = rate at which energy-releasing
chemical reactions occur
 Basal (standard) metabolic rate = lowest rate of
energy expenditure of resting, fasting animal in
its comfortable temperature range
Counter current heat
exchange
Mammalian Counter Current Example
Reduce Heat Input
 Staying out of the sun
 Shading (e.g., spines/fur)
 Posture and orientation (e.g., orienting
leaves vertically to minimize surface area
directly hit by sun)
 Insulation
 Shiny surfaces that reflect sun, etc.
Dissipating Heat
 If heat reduction wasn't
enough, then it is time to
get rid of body heat by
 evaporation (costs water
though),
 long appendages (legs,
ears, etc.), or
 small "bodies" (e.g.,
whole body, leaves, etc.)
that radiate heat.
Tolerate Hyperthermia
 Some plants and animals
can survive body
temperatures that would
be dangerous to humans
(e.g., this antelope
ground squirrel tolerates
body temperatures over
104 degrees F!).
Advantages of being an endotherm
 Tolerate a wider range of
environmental conditions
 Can be active day or night,
year round
 Can sustain high levels of
activity for longer periods
because rely more on
aerobic metabolism
(anaerabic in ectotherms)
Disadvantages of Homeothermy
 Homoiotherms can’t shut down completely if food,
water, or oxygen disappear
 Homeotherms can’t be as small as poikilotherms.
 Lose too much heat as SA/V increases and can’t eat
enough to keep warm.
 Homeotherms can't be wormlike or snakelike in
shape.
 High SA/V - lose too much heat.
 Aquatic env favors poikilotherms because of high
rate of convection.
Other Adaptations
 Partial homeothermy - Swordfish, Tuna, GW Sharks
 Hunts near surface during the day; goes deep at night
 T may be as much as 19oc
 Has brain-eye heater to keep these organs at stable temp
(needed for hunting)
 Tissue rich in mitochondria; countercurrent blood supply;
thick layer of fat around brain and eyes
 Heterotherms - warm-blooded species that
hibernate (temporary poikilothermy)
 Temporary homeothermy during egg incubation by
some snakes
 Heat stable enzymes in thermophilic bacteria
 Endothermy in insects:
sphinx moths
 active moths maintain
constant metabolic rate
 regulate body temp by
modifying cooling rate
 heat produced by flight
muscles in thorax is
transported via
circulatory system to
abdomen
Ecology of Bumblebees
 Bumblebees live in many
cool regions.
 Bernd Heinrich estimated
the energy budget of
individuals foraging on
different flowers and
under different
temperatures.
 Energy Gain / Loss
 Energy Intake - Energy
Expenditure.
How to Keep Warm
More Examples
Bask
Compact Body
Shiver
Hibernate
Hibernate
 The Grizzled Checkerspot butterfly also is
very short lived as an adult. As a
caterpillar, it takes two summers of eating
before it is ready to pupate and
metamorphose into an adult butterfly, so it
has to hibernate twice. Since most of this
species is on the same two-year cycle,
you are only likely to see these butterflies
in odd years!
Burrow Below Ground
"Gopher Eskers"
Burrow Below Ground
Pocket Gopher
Stay Awake and Hoard Food
Migrate