Evolution of

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Lecture 7 Evolution of Endothermy
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Endothermy has evolved multiple times in both
animals and plants birds and mammals, or their
ancestors some reptiles (brooding pythons, large
sea turtles, maybe dinosaurs) scombrid fishes
(e.g., tunas and billfishes) lamnid
sharks insects (e.g., moths, bees) at least eight
genera of plants within the family Araceae
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Endothermy is often considered to represent a
"key innovation,i.e., a characteristic that
allows a fundamentally new way of life and may
lead to an adaptive radiation.
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Early comparative physiologists assumed that
ectothermy was "primitive" and, in general,
inferior to endothermy, e.g., "lower" vs.
"higher" vertebrates. Now, it is recognized that
many characteristics of "lower" vertebrates are
actually specializations that promote a
low-energy lifestyle. A general separation in
average body sizes also exists. Low energy
requirements go along with small body size
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Body Mass Distributions
K. S. Kilburn,Old Dominion University
and small endotherms have severe problems with
heat loss because of unfavorable surface/volume
ratios.
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Ectotherm Benefits Do not spend direct metabolic
energy on thermoregulation Can spend more on
growth and reproduction Require less food overall
than do endotherms
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Allometry of field metabolic rate based on doubly
labeled water studies. Note that the lowest
mammals and the highest reptiles overlap! Lines
are least-squares linear regressions plus 95
prediction intervals.
Nagy, K. A., I. A. Girard, and T. K. Brown. 1999.
Energetics of free-ranging mammals, reptiles, and
birds. Annual Review of Nutrition 19247-277.
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Nagy, K. A. 2005. Field metabolic rate and body
size. Journal of Experimental Biology
2081621-1625.
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Legend for previous slide
Nagy, K. A. 2005. Field metabolic rate and body
size. Journal of Experimental Biology
2081621-1625.
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Nagy, K. A. 2005. Field metabolic rate and body
size. Journal of Experimental Biology
2081621-1625.
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Ectotherm Benefits Do not spend direct metabolic
energy on thermoregulation Can spend more on
growth and reproduction Require less food overall
than do endotherms
Can spend less time in risky foraging
activities Lose less water by evaporation because
their metabolic rate is lower and because their
Tb is generally lower Generally "preadapted" for
life in hot deserts
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Ectotherm Costs Activity times more constrained
on a daily and seasonal basis Cannot sustain high
work rates aerobically (lower maximal aerobic
speed)
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Endotherm Benefits Are more independent of
ambient temperature on a daily and seasonal
basis, as well as latitudinally and
altitudinally Can grow more rapidly
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Case, T. J. 1978. On the evolution and adaptive
significance of postnatal growth rates in the
terrestrial vertebrates. Q. Rev. Biol. 53243-282.
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Case, T. J. 1978. On the evolution and adaptive
significance of postnatal growth rates in the
terrestrial vertebrates. Q. Rev. Biol. 53243-282.
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Endotherm Benefits Are more independent of
ambient temperature on a daily and seasonal
basis, as well as latitudinally and
altitudinally Can grow more rapidly
Can sustain high rates of work aerobically (highe
r maximal aerobic speed) Long-distance
migrations Powered flight in relatively larger
animals Intense feeding of offspring Enzymes can
function more efficiently over a narrower
temperature range
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Endotherm Costs Need a lot more food Need larger
home ranges and spend more time foraging,
conspicuously, and may be more apparent to
predators Note that many ectotherms (e.g.,
lizards) combine activities, alternating between
basking, foraging, signaling, defending their
territories, looking for mates, watching for
predators
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Where did endothermy evolve in the lineage
leading to modern mammals? What characterizes a
modern mammal? Endothermy High Metabolic
Rate Homeothermy Response to Lowered
Ambient Temperature (shivering
non-shivering thermogenesis) Fur or Hair Live
Birth Parental Care Lactation Large
Brain 4-chambered Heart
Most of these features are absent or unclear in
the fossil record.
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In the fossil record, mammals are defined mainly
by the presence of a single bone (dentary) in the
lower jaw. Other features include Multi-cuspid
Teeth, Differentiated Along the
Jaw Secondary Palate Upright Posture Large
Brains These changes occur gradually in the
fossil record.
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The endothermy of pythons, scombrid fishes,
sharks, and sea turtles results primarily, but
perhaps not exclusively, from myogenic heat
production coupled with relatively large body
size The endothermy of insects is also
myogenically based, but given their small size,
endothermy in insects also requires a highly
effective insulation. The endothermy of birds and
mammals is distinctive, because at rest it
results primarily from metabolic heat production
by visceral organs (and the brain) rather than by
muscles.
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Only birds and mammals have metabolic rates high
enough and insulation effective enough (or body
size large enough) that they can maintain body
temperatures elevated above ambient while at rest
and in the absence of contractions by skeletal
muscles. (If the slow swimming needed to
ventilate the gills of obligate ram ventilators
is equated with the effort needed to ventilate
the lung of a bird or mammal, then it can be
argued that some fishes also maintain elevated
body temperatures while at rest, i.e., during
slow swimming.)
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Increased resting metabolic rate has been
hypothesized to be associated with selection
for 1) thermal niche expansion (Bakken and Gates
1975 Crompton et al. 1978 Block et al.
1993) 2) homeothermy (stable body temperature)
and increased metabolic efficiency (Heinrich
1977 Avery 1979) 3) homeothermy followed by
decreasing body size (McNab 1978) 4) postural
changes that enhance exercise performance (Heath
1968, cf. Carrier 1987) 5) increased brain size
(cf. Hulbert 1980)
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6) increased aerobic capacity during
exercise (Regal 1978 Bennett and Ruben 1979) 7)
parental care (Farmer 1998, 2000 Koteja 2000) 8)
resting metabolic rate set to optimize cardiovasc
ular O2 transport (Krosniunas and Gerstner 2003)
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Recent attempts to elucidate the selective
regime(s) responsible for the evolution of avian
and mammalian endothermy (i.e., ultimate causes)
have focused principally on 6, which is now
termed the aerobic capacity model. Most recently,
parental care has been discussed, as in two of
your readingsFarmer, C. G. 2000. Parental care
the key to understanding endothermy and
other convergent features in birds and mammals.
Am. Naturalist 155326-334.Angilletta, M. J. and
M. W. Sears. 2003. Parental care as a selective
factor for the evolution of endothermy? American
Naturalist 162821-825.Farmer, C. G. 2003.
Reproduction The adaptive significance of
endothermy. American Naturalist 162826-840.
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Bennett and Ruben (1979) argued that selection
for higher capacity for sustainable activity,
supported by aerobic metabolic rate, was
important during the evolution of
endothermy. They noted that the energy (food)
cost of increasing resting metabolic rate was
high compared to the thermoregulatory benefits,
especially for small increases in resting
metabolism, for which thermoregulatory
improvements would be insignificant. In contrast,
any increase in maximal aerobic capacity will be
reflected in higher capacity for sustainable
activity.
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The aerobic capacity model has two major
parts. 1. directional selection related to
activity capacity resulted in the evolution of a
higher maximal aerobic metabolic rate. Increase
in VO2max would increase maximal aerobic speed
and thus enable animals to exercise longer at
higher levels. This would be advantageous for
many reasons, e.g., better at capturing prey or
defending territories, able to traverse and hence
forage over greater areas. However, higher
aerobic capacity by itself would not result in
endothermy of resting animals.
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2. maximal and resting metabolism are somehow
linked in a causal, mechanistic sense thus,
evolutionary changes in the two traits cannot
occur independently. This idea was based on the
empirical observation that in vertebrates maximal
oxygen consumption during exercise (VO2max) is
typically 5-10 times resting oxygen consumption.
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"Thus there appears to be a consistent linkage
between resting and maximal levels of oxygen
consumption in the vertebrates. When an animal
is in any given physiological state, oxygen
consumption may increase an average of only five-
to tenfold." (Bennett and Ruben 1979, p.
651) They saw what they interpreted as a
relatively constant factorial aerobic scope, and
viewed this as indicative of a fundamental
property of vertebrate physiology. But they
concluded that the mechanisms underlying such a
relationship between activity and resting
metabolic rate were unclear.
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One simple verbal model is the Volkswagen -
Ferrari analogy.
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In any case, the aerobic capacity model assumes
that selection for increased maximum aerobic
capacity will necessarily result in increased
resting metabolic rate and, eventually,
endothermy. Note that this model is an
interesting mix of ultimate and proximate
explanations for why and how mammalian and avian
endothermy evolved!
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Possible exam question Discuss two different
models for the evolution of mammalian and avian
endothermy. Who developed these models? How do
the components of the models fall into the
categories of ultimate and proximate explanation?
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Stopped here 1 Feb. 2007