Atmospheric Oxygen, Giant Paleozoic Insects and the Evolution of Aerial Locomotor Performance The University of Arizona, Honors Biology, Group 6, Fall 2006

The Journal of Experimental Biology 201, 1043–1050 (1998)
Printed in Great Britain © The Company of Biologists Limited 1998
JEB1378

ATMOSPHERIC OXYGEN, GIANT PALEOZOIC INSECTS AND THE EVOLUTION OF
AERIAL LOCOMOTOR PERFORMANCE

ROBERT DUDLEY*
Department of Zoology, University of Texas, Austin, TX 78712, USA and Smithsonian Tropical Research Institute,
PO Box 2072, Balboa, Republic of Panama

*e-mail: r dudley@utxvms.cc.utexas.edu

Accepted 28 October 1997; published on WWW 24 March 1998

Summary
Uniformitarian approaches to the evolution of terrestrial and were subsequently eliminated by a late Permian
locomotor physiology and animal flight performance have transition to hypoxia. For extant organisms, the transient,
generally presupposed the constancy of atmospheric chronic and ontogenetic effects of exposure to hyperoxic
composition. Recent geophysical data as well as theoretical gas mixtures are poorly understood relative to
models suggest that, to the contrary, both oxygen and contemporary understanding of the physiology of oxygen
carbon dioxide concentrations have changed dramatically deprivation. Experimentally, the biomechanical and
during defining periods of metazoan evolution. Hyperoxia physiological effects of hyperoxia on animal flight
in the late Paleozoic atmosphere may have physiologically performance can be decoupled through the use of gas
enhanced the initial evolution of tetrapod locomotor mixtures that vary in density and oxygen concentration.
energetics; a concurrently hyperdense atmosphere would Such manipulations permit both paleophysiological
have augmented aerodynamic force production in early simulation of ancestral locomotor performance and an
flying insects. Multiple historical origins of vertebrate flight analysis of maximal flight capacity in extant forms.
also correlate temporally with geological periods of
increased oxygen concentration and atmospheric density.
Arthropod as well as amphibian gigantism appear to have Key words: atmosphere, density, flight, gigantism, hyperoxia, insect,
been facilitated by a hyperoxic Carboniferous atmosphere oxygen, Paleozoic.

Introduction

Changes in atmospheric gas composition impinge globally
on animal and plant physiology. Much attention has recently
been focused on anthropogenic forcing of atmospheric carbon
dioxide, but the potential for historical fluctuations in both
carbon dioxide and oxygen concentrations has rarely been
contemplated by students of metazoan evolution. However,
recent geophysical analyses indicate that Phanerozoic variation
in both respiratory gases has been substantial; the implications
of such changes are profound for the evolution of both
arthropod and tetrapod physiology (Graham et al. 1995, 1997).
This review discusses the geophysical evidence suggesting
large-scale historical fluctuations in the earth’s atmosphere and
qualitatively outlines the underlying biotic and abiotic
mechanisms. In addition to the potential role of hyperoxia in
the evolution of animal flight and of gigantism in diffusion-
limited forms, enhanced oxidative metabolism contributes to
the accumulation of deleterious superoxide radicals and
ultimately to animal senescence. Evolutionary responses to
such senescent damage are poorly understood, but
experimental hyperoxia provides a convenient means to select
for increased metabolic expenditure as well as for adaptive
responses to oxidant exposure. The study of Phanerozoic

changes in atmospheric oxygen levels thus provides an
important historical context for the evaluation of diverse
physiological adaptations in present-day organisms.

Geological records of atmospheric oxygen and carbon
dioxide

The fixation of carbon dioxide and release of oxygen by plants
have long been recognized as potential influences on the earth’s
atmosphere, but the potential impact of plant terrestrialization on
atmospheric composition has only recently been realized
(Berner, 1997). Mechanisms that have driven large-scale
Phanerozoic fluctuations in gas content of the atmosphere derive
from post-Silurian terrestrialization by plants together with
changes in continental weathering, organic carbon deposition
and biotic decomposition (Berner, 1990; Robinson, 1991;
Visscher et al. 1996). In the late Paleozoic, carbon fixation in
terrestrial plant ecosystems became decoupled from
decomposition and release by carbon-reducing organisms,
resulting in extensive deposition of coal and other carbonates.
This disequilibrium between free and stored carbon production,
together with extensive oxygen release by a diversifying


1044 R. DUDLEY

Concentration (%)

35

30

25

20

15

10

5

0.5

0.4

0.3

0.2

0.1

PAL
O2
C O S D C P Tr J K T
OS D C P Tr J K T
PAL
C
CO2
600 500 400 300 200 100 0
Time (MYBP)

Fig. 1. Estimates of Phanerozoic oxygen and carbon dioxide
atmospheric concentrations from Berner (1990, 1994) and Berner and
Canfield (1989); see Berner (1997) for a summary of independent
geochemical estimates of Proterozoic carbon dioxide concentrations.
PAL, present atmospheric level (20.9 % O2; 0.036 % CO2); MYBP,
million years before present; -C, Cambrian; O, Ordivician ; S, Silurian;
D, Devonian; C, Carboniferous; P, Permian ; Tr, Triassic; J, Jurassic;
K, Cretaceous; T, Tertiary.

arborescent flora, dramatically altered atmospheric carbon
dioxide and oxygen concentrations (Fig. 1). Carbon dioxide
levels through much of the Devonian and Carboniferous were
probably much higher than the contemporary 0.036 %, with an
approximately tenfold reduction evident from the middle to late
Paleozoic (Berner, 1990, 1994; Mora et al. 1996; Retallack,
1997). The late Paleozoic drawdown of this greenhouse gas was
also probably associated with a substantial reduction in earth
surface temperatures (Berner, 1994).

Concomitant with this reduction in carbon dioxide
concentration, the oxygen concentration of the late Paleozoic
atmosphere may have risen to as high as 35 % (Berner and
Canfield, 1989; see Fig. 1), a remarkable value compared with
the 20.9 % of the contemporary atmosphere. This elevation of
oxygen partial pressure occurred against the background of a
constant nitrogen partial pressure (Hart, 1978; Holland, 1984),
yielding an increased total pressure of the atmosphere.
Atmospheric oxygen concentrations are unlikely to have
exceeded 35 %, as this value represents an approximate
threshold for spontaneous combustion of the biosphere (Watson
et al. 1978; Kump, 1989). Both modelling (Berner and Canfield,
1989) and empirical (Makowski et al. 1989; Wignall and
Twitchett, 1996; Isozaki, 1997) results demonstrate an oxygen
crash from the high Upper Carboniferous levels to an early
Triassic value as low as 15 % (Fig. 1). Berner and Canfield
(1989) indicate a subsequent, but apparently smaller, pulse in
atmospheric oxygen concentrations starting in the mid Jurassic
and continuing through the Cretaceous and much of the Tertiary
(Fig. 1). Although not as pronounced as the late Paleozic pulse

in oxygen, this latter increase represents a secondary but still
significant Cretaceous and Tertiary elevation in atmospheric
oxygen relative to the present-day level. Furthermore, aquatic
levels of carbon dioxide and oxygen are coupled to the
atmospheric partial pressures of these gases (Richards, 1965),
suggesting widespread effects of the trends depicted in Fig. 1
across virtually all ecosystems. Biologists may well view with
skepticism the accuracy of large-scale geophysical models in
predicting climatic features of ancient ecosystems, but the
qualitative mechanisms of carbon dioxide depletion and oxygen
enhancement must apply in any model of plant terrestrialization
during the Paleozoic. The precise magnitudes of these dramatic
changes remain only broadly constrained, but a combination of
biotic and abiotic factors was clearly sufficient to have driven
major fluctuations in both carbon dioxide and oxygen
concentrations of the atmosphere (Berner, 1997).

Variable oxygen partial pressures coupled with constant
nitrogen partial pressure through the Phanerozoic further
indicate that such physical features as air density, viscosity,
diffusivity and heat conductivity varied substantially through
the late Paleozoic and to a lesser extent during the Cretaceous
and Tertiary (see Graham et al. 1995). Because Paleozoic
hyperoxia involved an increase in oxygen concentration as well
as in atmospheric density, this condition differs from that
obtained experimentally in a hyperbaric chamber, within which
oxygen concentration remains constant but total pressure
increases. The use of the terms hypo- and hyperdensity
therefore supplements the classical terminology of hypo- and
hyperbaria, and is warranted to emphasize the potential
decoupling of total pressure and air density. This approach also
retains the use of hypo- and hyperoxia to describe oxygen
partial pressure relative to normobaric values at the present
atmospheric concentration of 20.9 %.

Late Paleozoic gigantism and the end-Permian extinctions

A well-known yet poorly quantified feature of late Paleozoic
terrestrial faunas is the widespread taxonomic distribution of
animal gigantism (Briggs, 1985; Graham et al. 1995).
Carboniferous gigantism was most evident among diverse
lineages of flying insects (Carpenter, 1992), but was also
present in additional arthropod taxa, such as millipedes and
arthropleurids, and among the terrestrial labyrinthodont
amphibians. The wingspans of the extinct dragonfly order
Protodonata exceeded 70 cm in one species, whereas the
wingspans of late Paleozoic Paleodictyoptera ranged from 0.9
to 43 cm (Shear and Kukalová-Peck, 1990; Carpenter, 1992).
Some Carboniferous mayflies were characterized by wingspans
of 8.5–45 cm (Kukalová-Peck, 1985). Giant Carboniferous
hexapods are also found in the phylogenetically basal orders of
wingless insects (Diplura and Thysanura; see Kukalová-Peck,
1987). Millipedes 1 m long, giant arthropleurids (an extinct
arthropod class) and even giant arachnids round out the late
Paleozoic cast of terrestrial arthropod giants (Briggs, 1985;
Graham et al. 1995). Among terrestrial vertebrates, large
amphibians reached body lengths of up to 2 m (Carroll, 1988).


These large amphibians were probably limited by the capacity
for cutaneous respiration which, at least in contempory
urodeles, is known to restrict maximum body size through
diffusive limitations (see Ultsch, 1974).

Few causal hypotheses have been advanced for Paleozoic
gigantism, although predatory defense (Vermeij, 1987; Shear
and Kukalová-Peck, 1990) as well as enhanced flight
performance (Kukalová-Peck, 1978) are plausible ecological
explanations. The suggestion that variable oxygen
concentrations characterized Paleozoic times, however, implies
a direct mechanism underlying increased body size in diffusion-
limited forms (Graham et al. 1995). The most immediate
physiological effect of an increased oxygen partial pressure is
to increase diffusive flux in the tracheal system. Limits to insect
body size imposed by tracheal diffusion (see Weis-Fogh, 1964)
can therefore shift upwards as oxygen partial pressure increases
(Graham et al. 1995); oxidative metabolism will be similarly
enhanced. This straightforward hypothesis for late Paleozoic
gigantism was suggested by Rutten (1966), Schidlowski (1971)
and Tappan (1974), all of whom argued that the very large
Carboniferous Protodonata would have required a hyperoxic
atmosphere. However, Paleozoic gigantism through mitigation
of aerobic diffusional constraints would be expected in a
diversity of taxa supplemental to insects (Graham et al. 1995).
Although hyperoxia does not exclude other potential
mechanisms promoting gigantism (e.g. predator–prey
interactions), a causal mechanism of diffusive enhancement
would be imposed globally and would apply irrespective of
taxonomic association. Not only hexapod insects but a diversity
of arthropod classes as well as various amphibians displayed
gigantism, consistent with hyperoxic relaxation of diffusion-
limited respiration in these taxa. Although an increase in
atmospheric pressure will decrease oxygen diffusivity within
tracheal systems, given the dependence of gas diffusion
coefficients on total pressure (Paganelli et al. 1975), the net
effect of elevated oxygen levels on diffusive flux is nonetheless
substantial. The elevation of oxygen partial pressure to an
atmospheric concentration of 35 %, when coupled with a
constant nitrogen partial pressure, increases rates of oxygen
diffusion by approximately 67 %. This value probably
represents the maximum increase during the Paleozoic oxygen
pulse and would clearly have had a substantial impact on the
function of respiratory systems.

Also consistent with the hypothesis of diffusive constraints
on oxidative metabolism is the winnowing of giant terrestrial
arthropods parallel with the increasingly hypoxic conditions of
the late Permian (Graham et al. 1995; see Fig. 1). Most
characteristically, the various insect taxa that attained
exceptionally large body sizes during the Carboniferous do not
persist after the Permian (see Carpenter, 1992). The severe
end-Permian extinctions of both terrestrial and marine taxa
have been, in part, attributed to anoxic conditions (e.g. Berner,
1989; Gruszczy´nski et al. 1989; Hallam, 1991; Wignall and
Hallam, 1992, 1993; Knoll et al. 1996), although a diversity of
biotic and abiotic factors may have contributed synergistically
to this effect (Erwin, 1992, 1994). The disappearance of giant

Oxygen and animal flight performance 1045

terrestrial arthropods with diffusion-limited respiratory
systems is, however, consistent with the causal mechanism of
atmospheric hypoxia restricting such taxa to progressively
smaller body sizes (Graham et al. 1995). Similar conclusions
apply to the giant semiaquatic and terrestrial amphibians of the
late Paleozoic that became extinct by the end of the Permian.
Further analysis of this hypothesis must rely on quantitative
analyses of phyletic size change in relation to atmospheric
levels of oxygen during the late Paleozoic, a task that will be
substantially complicated by the paucity of complete fossil
specimens from this period (see Carpenter, 1992). However, a
secondary peak of insect gigantism (e.g. among the
Ephemeroptera; R. Dudley, in preparation) appears to occur in
the Cretaceous, when the atmosphere was also hyperoxic.
Further paleontological description and analysis of arthropod
gigantism are thus likely to be rewarding.

Physically variable atmospheres and the origin of animal
flight

In addition to the direct physiological effects of variable
oxygen concentrations, the physical consequences of differing
atmospheric composition are both substantial in magnitude and
diverse in character (Dudley and Chai, 1996). The physical
effects of an increased air density would be most profound for
the evolution of aerial locomotion; a greater density will result
in increased force production by aerodynamic structures
(Vogel, 1994) and possibly advantageous shifts of the
Reynolds number. Harlé and Harlé (1911), in fact, proposed
that giant Paleozoic insects would have required hyperdense
air in order to fly. The end-Devonian to early Carboniferous
origin of flight in insects (Wootton, 1990) correlates well with
an increasing air density in the late Paleozoic (see Fig. 1).
Enhanced aerodynamic force production by bodies and
protowinglets of ancestral insects, together with higher
Reynolds numbers favorable for lift generation (Vogel, 1994),
would have facilitated evolution of both gliding and flapping
flight in protopterygotes. The initial evolution of the intensely
oxidative flight metabolism of insects would also have been
facilitated by enhanced oxygen diffusion within the tracheal
respiratory system. A hyperoxic atmosphere in the late
Paleozoic would thus have contributed both biomechanically
and physiologically to the evolution of aerial locomotor
performance in animals. Secondary effects associated with
changing atmospheric composition may also have contributed
to insect flight evolution. For example, a reduction in earth
surface temperatures associated with declining Paleozoic
carbon dioxide concentrations might have increased the
selective advantages of incipient thermoregulation in
protopterygote winglets prior to fully developed aerodynamic
functions (see Kingsolver and Koehl, 1985, 1994).

The three independent origins of vertebrate flapping flight
may also have been facilitated by hyperdense and hyperoxic
atmospheres (Table 1). The precise timing of flight evolution
in birds, bats and pterosaurs remains indeterminate, but a likely
late Jurassic appearance and Cretaceous diversification of birds


1046 R. DUDLEY
Table 1. Evolution of animal flight and insect gigantism in relation to atmospheric oxygen and density

Atmospheric Atmospheric Evolution of
Geological range oxygen density flapping flight Insect gigantism

Pre-Carboniferous Hypoxic Hypodense (No taxa) (None)
Carboniferous to end-Permian Hyperoxic Hyperdense Insects Present
Pterosaurs (?)
Triassic to early Jurassic Hypoxic Hypodense (No taxa) Absent
Mid-Jurassic to Hyperoxic Hyperdense Birds Present (e.g.
Cretaceous/Tertiary Bats (?) Ephemeroptera)
End-Tertiary Normoxic Normodense (No taxa) Absent

The origin of flapping flight in insects is unclear but probably occurred in either the late Devonian or Lower Carboniferous (Wootton, 1990).
Similarly, the origins of flight in bats and pterosaurs remain only broadly constrained in the fossil record (see text).

is temporally correlated with a time of increasing air density
and oxygen availability (see Fig. 1). The widely debated flight
abilities of Archaeopteryx (e.g. Ruben, 1991, 1993) may thus
be placed in the broader context of increased oxygen
concentration and air density in the early Cretaceous. The
earliest known pterosaurs date from the Triassic and are
consistent with Permian origins of flight, whereas pterosaur
gigantism (e.g. Quetzalcoatlus) is confined to the Cretaceous
(Wild, 1984; Wellnhofer, 1991). Most recently, the earliest
fossils of microbats have been dated at 50 million years before
the present (MYBP) and demonstrate seemingly modern
morphology, whereas bat origins may be placed either within
the early Paleocene or late Cretaceous (Jepsen, 1970;
Altringham, 1996). One potential commonality among these
three unrelated vertebrate taxa is origination and diversification
during periods of elevated atmospheric oxygen partial
pressure. Only a detailed fossil record for these taxa will
unequivocally test this correlational hypothesis, but a non-
uniformitarian perspective of atmospheric composition is
minimally required as one (of numerous) alternative
interpretations of vertebrate flight evolution. The aerodynamic
effects of variable atmospheric composition cannot be
determined for the extinct transitional forms of flying animals,
but one approach to evaluating historical scenarios is to
investigate the performance of extant organisms under
postulated ancestral atmospheric conditions.

Neontological perspectives

The hyperdense conditions of the late Paleozoic were causally
linked with hyperoxia, but the density and oxygen concentrations
of experimental gas mixtures can be varied independently
through combinations of oxygen and nitrogen with selected
noble gases (Dudley and Chai, 1996; see Fig. 2). Unfortunately,
hyperoxic manipulations have received little experimental
attention relative to biomedically oriented studies of hypoxia.
This deficiency is unfortunate because hyperoxic conditions are
likely to have prevailed during much of early tetrapod
diversification as well as during the multiple evolutionary origins
of animal flight. The use of hyperoxia provides a classic test of

hypotheses for diffusive limitations in respiratory pathways; if
enhanced oxygen availability does not yield increased rates of
oxygen consumption, non-diffusive (i.e. convective and/or
perfusive) limits to aerobic capacity are indicated. Much of the
early literature in this area is apparently confounded
methodologically (Welch, 1982), although whole-animal studies
of exercising humans show that hyperoxia facilitates an increase
in the maximal rate of oxygen consumption (Plet et al. 1992;
Knight et al. 1993). Curiously, no experimental tests have been
made to evaluate the effects of hyperoxia on the exercise
metabolism of amphibians and reptiles.

An analysis of hyperoxic effects for a wide size range of
amphibian taxa would also be informative to evaluate
allometries of maximum rates of cutaneous and pulmonary
respiration. However, neontological simulations of amphibian
paleophysiology must necessarily consider carbon dioxide
levels as well as oxygen availability. The wide range of carbon
dioxide concentrations through the Paleozoic may have
significantly influenced the evolution of vertebrate activity
metabolism and of tetrapod endothermy (Graham et al. 1997).
For example, hypercapnia can interact via respiratory acidosis
with the metabolic effects of hypoxia (e.g. Kuhnen et al. 1987)
and hyperoxia (e.g. Graham and Wilson, 1983). The induction
of hypothermia by hypoxia in various vertebrates (Wood,
1991) suggests that a comparable exploration of the effects of
hyperoxia on thermoregulation will be informative. Potential
interactions between oxygen availability and hypercapnia
during thermoregulatory responses are also relevant in the light
of the covariance between these two gases during the Permian
elaboration of tetrapod endothermy (Graham et al. 1997).

Animal flight poses a particularly interesting context for the
study of maximal aerobic metabolism given that this locomotor
mode demonstrates the physiological extremes of mass-specific
oxygen uptake rates among vertebrate and invertebrate taxa.
Given interests in high-altitude flight and the efficiency of
oxygen extraction, the effects of hypoxia on avian respiratory
physiology have been widely studied, albeit primarily in the
context of resting metabolism (Faraci, 1991). However, a recent
study found no hyperoxic enhancement of the maximum
metabolic rates of hovering hummingbirds flying in hypodense


Oxygen and animal flight performance 1047

Air density

O2

Hypoxia

Normoxia

(20.9% O2 at STP)

Hyperoxia

1. Helium replacement of
nitrox (decrease in %O2)
2. Hypobaric chamber/
altitudinal ascent
(%O2 constant)

HypodenseNormodensity HyperdenseIncreasing air
mixtures (1.2–1.3 kg m–3) mixtures density

Xenon replacement of nitrox

O2<20.9%;

(decrease in %O2)

balance N2 and Ar

Heliox replacement
of nitrox

O2>20.9%; balance He

Normodense
normoxia

O2>20.9%;
balance N2 and He

Xenox replacement
of nitrox

1. O2>20.9%; balance Xe
2. Hyperbaric chamber
Increasing O2 partial pressure

Fig. 2. Experimental methods for covarying oxygen partial pressure and air density of gas mixtures. Heliox (also known as helox), 20.9 %
O2/79.1 % He; nitrox, 20.9 % O2/79.1 % N2; xenox, 20.9 % O2/79.1 % Xe. See Dudley and Chai (1996) for details.

air (Chai et al. 1996), suggesting non-diffusive limits to
maximal flight performance. Flight is energetically more costly
at lower air densities, and the use of helium to replace nitrogen
is a simple and benign method of eliciting increased power
production from flying animals (Dudley, 1995; Dudley and
Chai, 1996). In hypodense and hypoxic gas mixtures achieved
by replacing nitrox with pure helium (Fig. 2), hummingbird
flight performance is clearly limited by oxygen availability
(Chai and Dudley, 1996). Failure while hovering in such gas
mixtures occurs at oxygen concentrations of 11–14 %,
corresponding to the oxygen partial pressure at an elevation of
approximately 4000 m (Chai and Dudley, 1996).

This remarkable performance demonstrates a lower bound
to oxygen utilization by hummingbird flight muscle, but
potential upper bounds to flight metabolism remain unclear.
The use of hypodense but normoxic gas mixtures suggests,
however, that biomechanical rather than physiological
constraints exist for the maximum flight capacity of hovering
hummingbirds. Ruby-throated hummingbirds flying in heliox
(21 % O2/79 % He) fail to sustain hovering at air densities
slightly less than half of those of normobaric air; the oxygen
concentration is unchanged in this manipulation, although the
density corresponds to an altitude of approximately 6000 m
(Chai and Dudley, 1995). Hummingbirds fail in hovering when
the angular extent of wing motions on either side (the stroke
amplitude) is near 180 °; interference from the opposite wing
appears to limit aerodynamic force production and ultimately
hovering ability. The failure of hyperoxia to enhance hovering
performance in this taxon is therefore not surprising.

Altitudinal gradients involve changes in both oxygen partial
pressure and air density (see Fig. 2). Flight performance at
reduced air density has not been well studied in insects, even

though high-altitude distributions are commonplace in many
insects. Insects demonstrate a remarkable ability to fly at low
air densities under both hypoxic (e.g. Galun and Fraenkel,
1961) and normoxic (e.g. Dudley, 1995) conditions. The
allometric dependence of flight performance at low density has
not yet been completely determined, although limited data
suggest that smaller animals can fly at lower densities and
attain higher levels of muscle power output (Dudley and Chai,
1996). Hummingbirds are an ideal taxon in which to evaluate
further questions of physiological and biomechanical
adaptation to hypoxic and hypodense air. Hummingbird
species diversity is greatest over the altitudinal range
1500–2500 m, and some of the larger species are, somewhat
paradoxically, most common at high altitudes. For example,
the 20–22 g giant hummingbird (Patagona gigas) is resident at
elevations up to 4000 m (Ortiz-Crespo, 1974); this species
would be an ideal candidate for focal studies of flight
biomechanics and physiology. Moreover, patterns of intra- and
interspecific variation in bird wing length across altitudinal
gradients suggest that wing length is increased to offset the
higher induced power costs at low air densities (see Dudley
and Chai, 1996). The recent use of load-lifting methods to
assay maximum performance in hummingbirds (Chai et al.
1997), when combined with hypodense manipulations of
hovering flight, promises to be of great utility in analyses of
inter- and intraspecific adaptation across altitudinal gradients.

For insects, future experimental manipulations might
include total density reduction under both normoxic and
hyperoxic conditions to determine whether increased oxygen
availability augments maximum metabolic rate. Allometric
and phylogenetic comparisons among volant insects and
vertebrates are also warranted. Sphingid moths match


1048 R. DUDLEY

hummingbirds for body mass and hovering ability; a study of
these two taxa under conditions of maximal oxygen
consumption would enable direct comparisons of design limits
to tracheal and pulmonary respiration. To date, no hyperoxic
manipulations have been implemented on flying insects near
maximal performance, although the metabolic rate of hovering
honeybees is independent of oxygen partial pressure under
normobaria (Joos et al. 1997). The use of hyperdense and
hyperbaric mixtures (Fig. 2) may also reveal biomechanical
compensation and energetic responses not well-documented in
flying animals. For example, hyperbaric but normoxic gas
mixtures reduce the metabolic costs of hovering flight in
honeybees (Withers, 1981), consistent with reduced
aerodynamic costs of flight at higher air densities.

Moreover, the ontogenetic responsiveness of the tracheal
system of insect larvae to both hypo- and hyperoxia (Loudon,
1988, 1989; Greenberg and Ar, 1996) suggests that an
experimental evaluation of the flight performance of adult
pterygotes grown under conditions of variable oxygen
availability would be informative for paleobiological
reconstructions. Ontogenetic and chronic adult exposure of bats
and birds to hyperoxia might also reveal compensatory responses
in growth and flight physiology. For example, the growth of
avian embryos is enhanced under hyperoxia in some, but not all,
species (Temple and Metcalfe, 1970; Metcalfe et al. 1981;
Williams and Swift, 1988), and adult birds chronically exposed
to hypobaria exhibit acclimation of blood variables (McGrath,
1971; Pionetti and Bouverot, 1977). The ninefold set of discrete
combinatorial interactions between oxygen availability and air
density (Fig. 2) thus provides ample experimental opportunity to
evaluate the phenotypic plasticity of flight performance in
relation to variable atmospheric composition.

Whereas selection for enhanced aerobic performance is
generally viewed as a positive evolutionary outcome by
locomotor physiologists, the indirect consequences of aerobic
metabolism may pose specific biochemical challenges. For
example, metabolically generated oxidants, and particularly
superoxide radicals, are an important proximate cause of
accumulated biochemical damage and ultimately of senescence
(Shigenaga et al. 1994; Sohal and Weindruch, 1996). The
primary biochemical response to such damage is the presence
of enzymes, including catalases, peroxidases and superoxide
dismutases, that specifically mitigate the deleterious presence
of superoxide radicals (Fridovich, 1978; Packer, 1995;
Beckman and Ames, 1997). That this response is under genetic
control and potentially subject to the powerful effects of
natural selection has been elegantly demonstrated using
transgenic methods – overexpression of catalase and
superoxide dismutase in Drosophila melanogaster reduces
oxidative damage and significantly extends lifespan (Orr and
Sohal, 1994). If the proximate controls of longevity depend
substantially on such mitigation of oxidative damage, then the
evolutionary regulation of lifespan probably reflects variable
selective regimes acting on the expression of these enzymes.
Indeed, selection for enhanced lifespan in Drosophila also
increases superoxide dismutase activity (Tyler et al. 1993).

Given the enhanced longevities of flying animals (e.g.
Pomeroy, 1990), coupled with the greatly increased rate of
activity metabolism associated with flight, a combined
evolutionary and biochemical analysis of senescence in volant
forms will be directly relevant to the evolution of longevity.

The use of gas mixtures of variable composition permits
detailed experimental protocols to be applied to this important
issue. For example, the imposition of normobaric hyperoxia is
known to increase oxidative damage and to reduce lifespan in
Drosophila melanogaster (Sohal et al. 1993; Baret et al. 1994).
Longevity in hypo- and hyperdense flight media can thus be
compared under hyperoxic conditions to evaluate directly the
deleterious oxidative effects of total metabolic expenditure,
permitting energetic expenditures during flight to be decoupled
experimentally from ambient oxygen exposure. Selection
schemes for increased flight performance (e.g. Weber, 1996)
can also be effected at variable oxygen levels and at variable
air densities, permitting metabolic expenditure to be decoupled
from hyperoxic effects. The use of Drosophila permits a
parallel analysis of concomitant selection for enhanced
superoxide dismutase expression and/or effectiveness. Such
selection experiments would potentially link mechanistic
interpretations of senescence with evolutionary theories of
aging (e.g. Partridge and Barton, 1993).

Conclusions

The adequacy of geophysical information concerning
atmospheric composition is intrinsic to paleorespiratory analysis.
A substantial reduction of atmospheric carbon dioxide levels
through the Paleozoic coupled with a late Paleozoic oxygen pulse
seem likely, given the congruence of global climate models and
specific isotopic results, yet further analyses are warranted,
particularly for terrestrial as distinct from marine facies.
Correlational analyses of trends in body size evolution should
extend beyond documentation of the occurrence of gigantism to
include a detailed analysis of phyletic size change in relation to
postulated environmental parameters. The present-day
physiological simulations of ancestral respiratory patterns
supplement paleontological reconstructions and, together with
experiments involving ontogenetic exposure to hyperoxia and
specific selection regimes, may provide insight into the plasticity
of respiratory design as well as the nature of evolutionary
responses to oxidative damage. Experimental manipulations of
air density and oxygen concentration can be used to evaluate the
biomechanics of maximal flight performance and can specifically
place physiological performance within the ecological
framework of adaptation across altitudinal gradients. A diversity
of physiological analyses relevant to both past and present-day
adaptation are enabled by the simple recognition that atmospheric
composition has varied substantially through geological time.

I thank Doug Altshuler, Peng Chai, Carl Gans, Dmitry
Grodnitsky, Jon Harrison, Austen Riggs and Steve Roberts for
comments on the manuscript, and the NSF (IBN-9601089) for
financial support.


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