Endothermy: Opportunities and Cost
Endothermy: the character
Derived character of mammals (synapsids) and birds (sauropsids) – 2 lineages
evolved endothermy independently
- Costs and benefits are the same for both – way to become relatively
independent of many of the challenges in the physical environment
- High and relatively constant internal body temperature allows a fine tuning of
metabolism, high muscular power output, fast growth, significant degree of
independence from environmental temperature
- BUT it is very energetically expensive, costly to evolve, but allows evolution of
other behavioural activites (ie. remains constraint) and to invade other
environments
- Metabolic rates of birds and animals are nearly an order magnitude greater than
amphibians and reptiles (Energy to sustain those high metabolic rates come from
food, endotherms need more food than do ectoderms)
Endothermy generally used term, heated from within in a controlled
manner by high basal metabolic rate
Ectothermy –heated from outside, usually by the sun. Basking can lead to
high internal temperatures
Poikilothermy variable body temperature, true of most reptiles but also
small mammals that undergo
hibernation or daily torpor
Homeothermy maintaining more or less steady body temperature, so true of
endothermy but also probably
large ectotherms such as dinosaurs – inertial homeothermy
Heterothermy relatively high body temperatures but restricted to certain
organs eg muscles and brain
when hunting
Bradymetabolism low metabolic rate, as in reptiles and amphibians
Pachymetabolism high metabolic rate, as in mammals and birds
Basal metabolic rate oxygen consumption at rest and not digesting food
(BMR)
Field metabolic rate normal daily activities, including digestion and foraging or
(FMR) hunting
Maximal metabolic rate maximal oxygen consumption
(MMR)
Aerobic scope the absolute difference between basal and maximal or field
, metabolic rate in units of power (If scope = 10, MMR is 10x
higher than BMR)
Factorial aerobic scope the ratio of BMR to FMR or MMR, unitless.
Importance of resting metabolic rate
Many animals are heterothermic: tuna, sharks, some insects
and plants
Heterothermy typically seen during activity so qualitatively
and (quantitatively) distinct from true endothermy in
mammals and birds
BMR increases with temperature ectotherms, decreases with
temperature in endotherms
- BMR scales with exponenet between 0.67 and 0.75
- So larger animals have a lower metabolic rate per gram
and an opportunity
Standard metabolic rate = basal metabolic rate
~70% BMR derives from visceral organs (eg. oxygen use 17% goes to human liver,
body mass 2%)
- Actual proportion varies with size and phylogeny: humans have lower contribution
from liver than rats but higher contribution from brain
- Skeletal muscles and cardiovascular system do not contribute much to BMR
(contribute more in birds than in mammals)
Effects of HEAT – some advantages to increasing body temp.?
1. Heat increases metabolic rate
- Both resting and maximum rate increased by rising temperatures, up to limits
substrate supply and enzyme stability
- Actual increases depend on adaptations and tissue morphology, but similar factorial
increments seen in both endotherms and ectotherms
2. Heat increases mitochondrial respiration
- State 3 respiration is maximal coupled respiratory rate in absence of ADP,
phosphate, oxygen and substrates (pyruvate and succinate)
- Increasing temp. from 10-20oC nearly doubles metabolic power from respiratory
enzymes per gram (Clarke and Portner, 2010)
3. Heat increases muscle power
- Muscle power depends on temperature also nearly doubles for each 10 oC rise
- Major advantages to increasing body temperature…BUT
Increasing rate of feeding does not significantly increase temperature
, - A large meal increases metabolic rate 4-5 fold but only increases post-prandial
temperature by at most 0.5oC
- Though metabolic rate is high and endotherms do need to eat more
- Benefits of endothermy is NOT just raised metabolic rate leading to increased
body temperature
Cost of Endothermy
High rate of metabolism needed to sustain endothermy requires a great deal of
food
A mammal or bird requires 5-10 times more energy for maintenance than a reptile
or other vertebrate ectotherm of similar size and body temperature (38- to 40°C).
As ambient temp decreases, body temperature of endotherm remains constant and
that of the ectotherm decreases, increasing metabolic differential between
animals.
At temp of 20°C, an ectothermic vertebrate uses only 2 to 3 percent of the energy
required by an endotherm and, at 10°C, only 1 percent. (Bennett and Ruben, 1979)
At 20°C an endotherm e.g. mouse or rat must consume as much food in one
day as a lizard would consume in 1 month
Aerobic Capacity – an alternative hypothesis (Bennett and Ruben, 1979)
Most widely accepted explanation for evolution of endothermy has been for
selection for enhanced aerobic capacity to allow increasingly sustained locomotor
activity
- Evolution of a higher body temperature and endothermy followed as sexondary
events
B and R compared resting and maximal metabolic rates in a range of vertebrate
ectotherms (fish, amphibians, reptiles) with birds and mammals
- Factorial aerobic scope is broadly similar in all these groups but differs between
individual taxa
- Since resting metabolic rate of endotherms exceeds that of ectotherms by an
order of magnitude, absolute (net) aerobic scope) is also much greater in
endotherms
- Typical endotherm thus has an order of magnitude more energy available for
locomotion than does a typical ectotherm of similar mass
- Earlier studies showed net cost of transport is similar in terrestrial lizards and
mammals indeed the maximal power from reptilian muscle can exceed that of
mammals and birds, enabling to out-run a mammal of similar size and escape
endothermic predators/capture endothermic prey
- Reptiles cannot undertake sustained aerobic activity typical of mammals and birds -
Endothermy: the character
Derived character of mammals (synapsids) and birds (sauropsids) – 2 lineages
evolved endothermy independently
- Costs and benefits are the same for both – way to become relatively
independent of many of the challenges in the physical environment
- High and relatively constant internal body temperature allows a fine tuning of
metabolism, high muscular power output, fast growth, significant degree of
independence from environmental temperature
- BUT it is very energetically expensive, costly to evolve, but allows evolution of
other behavioural activites (ie. remains constraint) and to invade other
environments
- Metabolic rates of birds and animals are nearly an order magnitude greater than
amphibians and reptiles (Energy to sustain those high metabolic rates come from
food, endotherms need more food than do ectoderms)
Endothermy generally used term, heated from within in a controlled
manner by high basal metabolic rate
Ectothermy –heated from outside, usually by the sun. Basking can lead to
high internal temperatures
Poikilothermy variable body temperature, true of most reptiles but also
small mammals that undergo
hibernation or daily torpor
Homeothermy maintaining more or less steady body temperature, so true of
endothermy but also probably
large ectotherms such as dinosaurs – inertial homeothermy
Heterothermy relatively high body temperatures but restricted to certain
organs eg muscles and brain
when hunting
Bradymetabolism low metabolic rate, as in reptiles and amphibians
Pachymetabolism high metabolic rate, as in mammals and birds
Basal metabolic rate oxygen consumption at rest and not digesting food
(BMR)
Field metabolic rate normal daily activities, including digestion and foraging or
(FMR) hunting
Maximal metabolic rate maximal oxygen consumption
(MMR)
Aerobic scope the absolute difference between basal and maximal or field
, metabolic rate in units of power (If scope = 10, MMR is 10x
higher than BMR)
Factorial aerobic scope the ratio of BMR to FMR or MMR, unitless.
Importance of resting metabolic rate
Many animals are heterothermic: tuna, sharks, some insects
and plants
Heterothermy typically seen during activity so qualitatively
and (quantitatively) distinct from true endothermy in
mammals and birds
BMR increases with temperature ectotherms, decreases with
temperature in endotherms
- BMR scales with exponenet between 0.67 and 0.75
- So larger animals have a lower metabolic rate per gram
and an opportunity
Standard metabolic rate = basal metabolic rate
~70% BMR derives from visceral organs (eg. oxygen use 17% goes to human liver,
body mass 2%)
- Actual proportion varies with size and phylogeny: humans have lower contribution
from liver than rats but higher contribution from brain
- Skeletal muscles and cardiovascular system do not contribute much to BMR
(contribute more in birds than in mammals)
Effects of HEAT – some advantages to increasing body temp.?
1. Heat increases metabolic rate
- Both resting and maximum rate increased by rising temperatures, up to limits
substrate supply and enzyme stability
- Actual increases depend on adaptations and tissue morphology, but similar factorial
increments seen in both endotherms and ectotherms
2. Heat increases mitochondrial respiration
- State 3 respiration is maximal coupled respiratory rate in absence of ADP,
phosphate, oxygen and substrates (pyruvate and succinate)
- Increasing temp. from 10-20oC nearly doubles metabolic power from respiratory
enzymes per gram (Clarke and Portner, 2010)
3. Heat increases muscle power
- Muscle power depends on temperature also nearly doubles for each 10 oC rise
- Major advantages to increasing body temperature…BUT
Increasing rate of feeding does not significantly increase temperature
, - A large meal increases metabolic rate 4-5 fold but only increases post-prandial
temperature by at most 0.5oC
- Though metabolic rate is high and endotherms do need to eat more
- Benefits of endothermy is NOT just raised metabolic rate leading to increased
body temperature
Cost of Endothermy
High rate of metabolism needed to sustain endothermy requires a great deal of
food
A mammal or bird requires 5-10 times more energy for maintenance than a reptile
or other vertebrate ectotherm of similar size and body temperature (38- to 40°C).
As ambient temp decreases, body temperature of endotherm remains constant and
that of the ectotherm decreases, increasing metabolic differential between
animals.
At temp of 20°C, an ectothermic vertebrate uses only 2 to 3 percent of the energy
required by an endotherm and, at 10°C, only 1 percent. (Bennett and Ruben, 1979)
At 20°C an endotherm e.g. mouse or rat must consume as much food in one
day as a lizard would consume in 1 month
Aerobic Capacity – an alternative hypothesis (Bennett and Ruben, 1979)
Most widely accepted explanation for evolution of endothermy has been for
selection for enhanced aerobic capacity to allow increasingly sustained locomotor
activity
- Evolution of a higher body temperature and endothermy followed as sexondary
events
B and R compared resting and maximal metabolic rates in a range of vertebrate
ectotherms (fish, amphibians, reptiles) with birds and mammals
- Factorial aerobic scope is broadly similar in all these groups but differs between
individual taxa
- Since resting metabolic rate of endotherms exceeds that of ectotherms by an
order of magnitude, absolute (net) aerobic scope) is also much greater in
endotherms
- Typical endotherm thus has an order of magnitude more energy available for
locomotion than does a typical ectotherm of similar mass
- Earlier studies showed net cost of transport is similar in terrestrial lizards and
mammals indeed the maximal power from reptilian muscle can exceed that of
mammals and birds, enabling to out-run a mammal of similar size and escape
endothermic predators/capture endothermic prey
- Reptiles cannot undertake sustained aerobic activity typical of mammals and birds -
