Oxidative stress and life histories: unresolved issues and
current needs
John R. Speakman1,2, Jonathan D. Blount3, Anne M. Bronikowski4, Rochelle Buffenstein5,
Caroline Isaksson6, Tom B. L. Kirkwood7, Pat Monaghan8, Susan E. Ozanne9, Micha€el Beaulieu10,
Michael Briga11, Sarah K. Carr9, Louise L. Christensen1, Helena M. Cocheme12, Dominic L. Cram13,
Ben Dantzer14, Jim M. Harper15, Diana Jurk7, Annette King7, Jose C. Noguera8, Karine Salin8, Elin
Sild6, Mirre J. P. Simons16, Shona Smith8, Antoine Stier17, Michael Tobler6, Emma Vitikainen3,
Malcolm Peaker18 & Colin Selman8
1Institute of Biological and Environmental Sciences, University of Aberdeen, Tillydrone Avenue, Aberdeen AB24 2TZ, UK
2State Key Laboratory of Molecular Developmental Biology, Institute of Genetics and Developmental Biology, Chinese Academy of Sciences,
Beijing, China
3Centre for Ecology and Conservation, University of Exeter, Penryn Campus, Cornwall TR10 9FE, UK
4Department of Ecology, Evolution and Organismal Biology, Iowa State University, 251 Bessey Hall, Ames, Iowa 50011
5Physiology, Barshop Institute for Aging and Longevity Research, UTHSCSA, 15355 Lambda Drive, San Antonio, Texas 78245
6Department of Biology, Lund University, Solvegatan 37, Lund 223 62, Sweden
7The Newcastle University Institute for Ageing, Institute for Cell & Molecular Biosciences, Campus for Ageing and Vitality, Newcastle upon Tyne
NE4 5PL, UK
8Institute of Biodiversity, Animal Health and Comparative Medicine, University of Glasgow, Graham Kerr Building, Glasgow G12 8QQ, UK
9University of Cambridge Metabolic Research Laboratories and MRC Metabolic Diseases Unit, Level 4, Wellcome Trust-MRC Institute of Metabolic
Science, Addenbrooke’s Hospital, Cambridge CB2 0QQ, UK
10Zoological Institute and Museum, University of Greifswald, Johann-Sebastian Bach Str. 11/12, Greifswald 17489, Germany
11Behavioral Biology, University of Groningen, Nijenborgh 7, Groningen 9747 AG, The Netherlands
12MRC Clinical Sciences Centre, Imperial College London, Hammersmith Hospital Campus, Du Cane Road, London W12 0NN, UK
13Department of Zoology, University of Cambridge, Cambridge CB2 3EJ, UK
14Department of Psychology, University of Michigan, Ann Arbor, Michigan 48109
15Department of Biological Sciences, Sam Houston State University, 1900 Avenue I, LDB 100B, Huntsville, Texas 77341
16Department of Animal and Plant Sciences, University of Sheffield, Alfred Denny Building, Western Bank, Sheffield S10 2TN, UK
17Department Ecology, Physiology et Ethology, University of Strasbourg - IPHC (UMR7178), 23, rue Becquerel, Strasbourg 67087, France
18Rushmere, 13 Upper Crofts, Alloway KA7 4QX, UK
Keywords
Aging, disposable soma theory, free radicals,
life-history theory, oxidative stress.
Correspondence
John R. Speakman, Institute of Biological and
Environmental Sciences, University of
Aberdeen, Tillydrone Avenue, Aberdeen
AB24 2TZ, UK.
Tel: +1224 272879; Fax: +1224 272396;
E-mail j.speakman@abdn.ac.uk
and Colin Selman, Institute of Biodiversity,
Animal Health and Comparative Medicine,
College of Medical, Veterinary and Life
Sciences, Graham Kerr Building, University of
Glasgow, Glasgow G12 8QQ, UK.
Tel: +44 141 330 6077;
Fax: +44 141 330 5971;
E-mail: Colin.Selman@glasgow.ac.uk
Funding Information
This study was the result of a week-long
workshop sponsored by the Rank Prize Funds
attended by all the authors
Abstract
Life-history theory concerns the trade-offs that mold the patterns of investment
by animals between reproduction, growth, and survival. It is widely recognized
that physiology plays a role in the mediation of life-history trade-offs, but the
details remain obscure. As life-history theory concerns aspects of investment in
the soma that influence survival, understanding the physiological basis of life
histories is related, but not identical, to understanding the process of aging.
One idea from the field of aging that has gained considerable traction in the
area of life histories is that life-history trade-offs may be mediated by free radi-
cal production and oxidative stress. We outline here developments in this field
and summarize a number of important unresolved issues that may guide future
research efforts. The issues are as follows. First, different tissues and macro-
molecular targets of oxidative stress respond differently during reproduction.
The functional significance of these changes, however, remains uncertain. Con-
sequently there is a need for studies that link oxidative stress measurements to
functional outcomes, such as survival. Second, measurements of oxidative stress
are often highly invasive or terminal. Terminal studies of oxidative stress in
wild animals, where detailed life-history information is available, cannot gener-
ally be performed without compromising the aims of the studies that generated
the life-history data. There is a need therefore for novel non-invasive measure-
ª 2015 The Authors. Ecology and Evolution published by John Wiley & Sons Ltd.
This is an open access article under the terms of the Creative Commons Attribution License, which permits use,
distribution and reproduction in any medium, provided the original work is properly cited.
5745
Received: 9 September 2015; Accepted: 20
September 2015
Ecology and Evolution 2015; 5(24):
5745–5757
doi: 10.1002/ece3.1790
ments of multi-tissue oxidative stress. Third, laboratory studies provide unri-
valed opportunities for experimental manipulation but may fail to expose the
physiology underpinning life-history effects, because of the benign laboratory
environment. Fourth, the idea that oxidative stress might underlie life-history
trade-offs does not make specific enough predictions that are amenable to test-
ing. Moreover, there is a paucity of good alternative theoretical models on
which contrasting predictions might be based. Fifth, there is an enormous
diversity of life-history variation to test the idea that oxidative stress may be a
key mediator. So far we have only scratched the surface. Broadening the scope
may reveal new strategies linked to the processes of oxidative damage and
repair. Finally, understanding the trade-offs in life histories and understanding
the process of aging are related but not identical questions. Scientists inhabiting
these two spheres of activity seldom collide, yet they have much to learn from
each other.
Background
First formulated in the 1950s (Harman 1956), the free
radical damage theory of aging enjoyed a golden period
as the predominant mechanistic theory of aging (Beck-
man and Ames 1998). In the new millennium, however,
its luster has been somewhat tarnished, with a series of
studies providing data contrary to its main theoretical
expectations [e.g., (Yang et al. 2007; Doonan et al. 2008;
Jang et al. 2009; Perez et al. 2009c; Zhang et al. 2009)].
These findings have led to a number of review articles
pronouncing the theory dead, moribund, or at the very
least suffering a mid-life crisis (Buffenstein et al. 2008;
Gems and Doonan 2009; Perez et al. 2009a; Speakman
and Selman 2011; Stuart et al. 2014 but see Barja 2013a).
Many gerontologists now question the idea that oxidative
stress is the principal, generalized mechanism underlying
aging, although most agree it probably plays some role.
On the other hand, eco-physiologists have embraced this
concept as providing a physiological mechanism that
might play a key role in determining the outcome of life-
history trade-offs between activities, such as growth or
reproduction, and body maintenance and hence future
survival or reproduction (Costantini 2008; Dowling and
Simmons 2009; Metcalfe and Alonso-Alvarez 2010; Isaks-
son et al. 2011a,b). Over the last decade, there has been a
proliferation of studies investigating whether oxidative
stress plays a role in life-history evolution. Such investiga-
tions have been largely correlative, but sometimes experi-
mental, and undertaken across a diversity of organisms,
studied under natural, semi-natural, and laboratory con-
ditions (summarized and subjected to meta-analysis in
Blount et al. (2015)). The results, however, have often
been contradictory. The current absence of consensus and
direction has led to the need to crystallize the main issues
in the field, to avoid the area drifting into a shambles of
conflicting data, and to resolve contradictory claims about
what the data actually mean. This manuscript emerged
from a workshop attended by the authors and held in
April 2014, funded by the Rank Prize Organization. It
reflects a consensus on the main unresolved issues in this
field, as perceived by the participants of the workshop,
and the theoretical and empirical approaches required to
resolve them. Our hope is that this commentary will help
to move the field forward positively by stimulating
researchers to refine and expand the scope of existing
studies so that we do not simply continue to accumulate
the same types of data, resulting in repeated additions to
the confusion, as opposed to genuine illumination. In a
wider context, we hope that by focussing the attention of
the wide variety of researchers in this area it may addi-
tionally rejuvenate the interests that gerontologists have
in oxidative stress – who were perhaps too hasty to
declare the idea deceased (Kirkwood and Kowald 2012).
We identify 6 Themes that are not mutually exclusive and
are not listed in any perceived order of importance. These
are (1) to understand the significance of different
responses to reproduction observed in different tissues,
(2) to refine the methodology for measuring oxidative
stress to allow studies in the field that do not require pre-
cious animals in longitudinal studies of life history to be
sacrificed, (3) to make laboratory studies more represen-
tative of conditions in the wild, (4) to develop good theo-
retical models enabling refinement of predictions of what
we expect experiments to show, and the development of
good alternative ideas, (5) to expand the range of species
and processes that are studied to gain new perspectives
and insights, and (6) to foster greater communication
between evolutionary/physiological ecologists and bioger-
ontologists. We stress that it is entirely possible for the
management of oxidative stress to play a role in life-his-
tory evolution and that the need to manage, minimize, or
repair oxidative damage might vary among different kinds
of tissues and organisms. That such management occurs
5746 ª 2015 The Authors. Ecology and Evolution published by John Wiley & Sons Ltd.
Oxidative Stress and Life Histories J. R. Speakman et al.
might well mean that oxidative stress, at least in simple
terms of accumulated damage to macromolecules, also
has a role in the aging process.
Theme 1: variable responses of different
tissues and different macromolecular
targets during reproduction
As data have accumulated, it has become clear that differ-
ent studies have utilized an extensive range of assays to
directly (and indirectly) quantify oxidative stress in differ-
ent tissues. Much has been written about the need to
include direct measures of oxidative damage and/or reac-
tive oxygen species (ROS), rather than only measure
antioxidants as a proxy measure, as it is the accumulated
damage that is presumed to be harmful (Monaghan et al.
2009; Isaksson et al. 2011a,b; Selman et al. 2012; Metcalfe
and Monaghan 2013). In response, a growing number of
studies have measured markers of oxidative damage in
relation to reproduction. Much of this research has
focussed on mammals, mostly rodents, and almost exclu-
sively on females. While some mammalian studies have
supported the idea that investment in offspring produc-
tion (during pregnancy and lactation) causes oxidative
damage (Sainz et al. 2000; Bergeron et al. 2011; Stier et al.
2012; Fletcher et al. 2013), others have found no effect or
that oxidative damage is actually reduced in lactating
compared to nonreproducing females (Garratt et al. 2011,
2013; Oldakowski et al. 2012; Schmidt et al. 2014). The
biological relevance of this diversity in response is unclear
and has often been explained by individual, species, tissue
or assay differences, and the difference between studies in
the laboratory and field (see Themes 2 and 3 below).
However, it has recently become apparent, by applying
multiple assays across multiple tissues, that it is possible,
in exactly the same individuals, to find some tissues and
macromolecules where reproduction elevates damage,
others where there is no impact, and yet others where
damage is reduced (Garratt et al. 2011, 2013; Oldakowski
et al. 2012; Yang et al. 2013). These complex patterns may
result from individuals selecting which tissues to protect
and which to leave vulnerable (Garratt et al. 2013; Yang
et al. 2013). An intriguing example of this complexity is
the lack of an effective antioxidant system in mammalian
pancreatic beta-cells, which renders them susceptible to
oxidative stress. Rashidi et al. (Rashidi et al. 2009) have
shown how this phenomenon can be explained in terms of
life-history trade-offs whereby reactive oxygen species
(ROS) in beta-cells, by their negative effect on insulin
synthesis/secretion, play a fitness-enhancing role for the
whole organism.
One key problem is that we do not know what the
long-term consequences (if any) of different forms of
oxidative damage are, for both organ function and ulti-
mately survival, and whether changes in oxidative damage
during reproduction are reversible/repairable. Further-
more, we do not know whether the consequences of
oxidative damage differ across the life course. An auto-
mobile analogy may help conceptualize this problem. In
the course of being driven, cars get dirty, and it is easy to
imagine that oxidative damage is akin to cellular ‘dirt’.
Where exactly the dirt lands on a car may have very
different consequences. For example, dirt on the body
work may be unsightly, and easy to quantify, but it has
little impact on the functionality of the car. Dirt on the
windscreen/windshield may be more serious, and impair
the performance of someone driving the car. But dirt in
the fuel line or carburetor could completely prevent the
car from operating. At present, we do not know which
parts of an animal, that become oxidatively damaged
during reproduction, or throughout life, are analogous to
the body work (which can be restored to pristine form
if washed and waxed), and which are the fuel lines and
carburetor. There is a strong need therefore for studies
that associate different measurements of damage with
functional outcomes such as organ function and fitness
outcomes such as survival and future reproductive
performance.
We do not wish to completely discourage people from
making studies where single tissues are sampled, and
single markers of damage and protection are employed;
in studies of natural populations of animals, this is some-
times the only possible sampling strategy (Selman et al.
2012) (see also Theme two). However, it is important to
recognize that the correlation of responses across different
tissues in the same individual can be poor, at least based
on the relatively small number of studies that have
explored such associations during reproduction (Yang
et al. 2013; Schmidt et al. 2014; Xu et al. 2014) or in
response to other stressors (Selman et al. 2002a,b, 2008;
Kaushik and Kaur 2003; Veskoukis et al. 2009; Meitern
et al. 2013). Hence, interpreting such studies in the
absence of a clear link to a functional outcome is fraught
with difficulty. In addition, we sorely need more studies
that expand the range of markers that are used and the
range of tissues that are measured, in response to experi-
mental manipulations of both reproductive status (allo-
cated to breed or not breed) and reproductive effort
(number of raised offspring or investment per offspring).
It is important when making such manipulations to
quantify the impact of the manipulation on the invest-
ment by the female, as simply adding or subtracting off-
spring does not necessarily cause an impact on female
investment (Speakman and Garratt 2014). This is because
individuals may continue to work at the same rate they
would have without the manipulation, generally with the
ª 2015 The Authors. Ecology and Evolution published by John Wiley & Sons Ltd. 5747
J. R. Speakman et al. Oxidative Stress and Life Histories
outcome of the manipulation being felt more acutely on
the offspring than the adult. At present, we have no idea
whether conservation exists across different organisms
when examining multi-tissue and multi-target patterns of
damage and protection. Presumably, organisms allocate
protection and repair to those things that are most criti-
cal, which may itself vary with life stage. Hence, patterns
of protection and repair may provide useful clues as to
the targets most likely to be functionally significant for
survival. At present, we do not know whether the same
targets will be universally important across all organisms,
or whether different taxa will have unique points of
vulnerability. Moreover, it is unclear whether the effects
will be the same across both sexes, in particular as males
and females may show pronounced differences in markers
of oxidative stress during reproduction (Alonso-Alvarez
et al. 2004; Wiersma et al. 2004; Isaksson et al. 2011a,b;
Stier et al. 2012; Isaksson 2013), and in their rates of
aging. Resolving these issues will be an important future
aim as our knowledge expands.
Theme two: the dilemma of field studies.
Longitudinal data on life histories make
animals too valuable to sample
destructively for oxidative stress assays
Linking together information on oxidative stress and life-
history strategies across the lifespan requires detailed
knowledge of individuals at different ages/stages of their
life histories. Longitudinal studies and experimental
manipulations of either status or effort provide the best
way to understand the nature of the underlying associa-
tions because cross-sectional surveys and correlational
studies may be prone to artifacts through differential sur-
vival and reproduction (Nussey et al. 2009; Bouwhuis
et al. 2012). This might, for example, lead to the illusion
that oxidative damage declines in relation to organismal
age, because those individuals with the highest levels of
damage die sooner. Additionally, this could suggest oxida-
tive damage is reduced by reproduction, because only
those individuals with pre-existing low damage initiate
attempts to reproduce [e.g., Beaulieu et al. 2014]. Field
studies that meticulously follow individuals throughout
their life course, quantifying reproduction and growth,
are growing in number. Repeatedly measuring oxidative
stress markers within individuals over time seems an
obvious way to test the role of oxidative stress on life his-
tories (Nussey et al. 2009; van de Crommenacker et al.
2011). There is, however, a major dilemma because in
general such studies were not initiated as tests of the
oxidative stress theory, and they have alternative goals
and aims that may be conflicting. For example, the
currently available assays, apart from measurements in
blood and minor biopsies, require destructive testing.
Euthanizing animals that are part of long-term monitored
populations is, however, generally incompatible with the
primary goals of such longitudinal field studies. This
might not be a problem if it turns out that the markers
measured in blood are indeed the markers of greatest
functional significance (see Theme 1 above). This would
be indeed fortuitous. However, if these markers only
reflect “dirt on the chassis” then the opportunity to
exploit the thousands of person hours devoted to longitu-
dinal quantification of the life-history parameters of
known individuals will be lost. There is a major demand
therefore for the development of assays in accessible sam-
ples that will inform us about the multi-tissue, multi-tar-
get nature of protection and repair processes without the
need to kill (or severely impair) animals in the process.
For example, standard tissue biopsies may be sufficiently
traumatic to animals that even though the animal survives
the tissue collection, it may have a significantly elevated
risk of mortality when placed back into the field, such
that its contribution to the population demographic data
may be compromised. Alternatively, its behavior might be
altered leading to reduced likelihood of it being resam-
pled at a later age. Moreover, tracking, trapping, captur-
ing, and restraining wild animals may affect measures of
oxidative status. Nevertheless, some tissues (e.g., feathers,
hair, and small epidermal biopsies) may be readily sam-
pled repeatedly with minimal invasiveness, and consider-
able information can also be gleaned from urine and fecal
samples. We envisage that technological advances in the
future may come in several different forms. First, it is
already clear that compounds in the urine/feces may
provide an indication of oxidative damage (e.g., levels of
f2 isoprostanes, 8-OHdG). As metabolite profiling of
urine/feces becomes more sophisticated with the capacity
to monitor simultaneously thousands of compounds, the
prospect that multiple urinary markers that may provide
a “damage fingerprint” seems feasible. Second, animals
may be injected with compounds that are metabolized in
relation to aspects of “damage” or “protection/repair”
capacity and the products of such reactions in blood or
urine monitored. It may be possible to use radioactive
markers to localize tissue damage or protection in given
tissues using PET scanning or other imaging technology –
although admittedly applying such technology in the field
would prove challenging. Finally, as the costs of transcrip-
tomic profiling by RNAseq decline, it is now possible to
generate profiles of gene expression from nucleated blood
cells, mouth swabs and/or very small biopsy samples,
allowing simultaneous evaluation of the status of repair,
turnover, and protection systems, in multiple tissues.
Development of such methods would provide significant
benefits for both field- and laboratory-based studies. A
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Oxidative Stress and Life Histories J. R. Speakman et al.
major point, however, that we wish to emphasize is that
it is not a trivial matter to bring assays from the labora-
tory to the field, and needs to be done with great care.
Assays may be highly sensitive to environmental condi-
tions and individual factors that while easy to control
in the laboratory are much more problematical to control
in the field – including for example time as last meal (or
the composition of the meal itself), whether or not
the individual encountered a predator or a dominant
individual in the recent past, and the almost unavoidable
influence of pollutants in all but the most pristine envi-
ronments. Just because a particular assay works well
under controlled laboratory settings, we should not
assume that it will work equally well in the field.
Theme three: considerations for laboratory
studies
Laboratory studies provide unrivaled opportunities to
perform controlled experiments, with multiple detailed
measurements and terminal assays that are seldom possi-
ble in the field. Yet these studies come with their own
drawbacks, chief among which is the question whether
the situation in the laboratory adequately mimics the situ-
ation in the field with respect to the key elements that
generate the trade-off between reproduction and survival.
An important question is whether animals in the labora-
tory with ad libitum food supplies are in a situation
where it is necessary to trade-off somatic protection
against reproduction (e.g., Selman et al. 2012; Metcalfe
and Monaghan 2013). This could be a serious issue if ani-
mals in the field are routinely food limited. However, if
animals in the field during peak reproduction are operat-
ing at an intrinsically determined physiological capacity,
then this issue is less important because both laboratory
and field animals will be likely to be working at this same
limiting physiological capacity – that is the energy budget
within which different costly functions must be accom-
modated will be the same irrespective of food supply
(Speakman and Garratt 2014). There is considerable evi-
dence to suggest that some strains of laboratory mice and
other rodents are operating at a physiologically imposed
maximum when they are at peak lactation (reviewed in
Speakman and Krol 2011). However, the nature of this
ceiling is disputed. Similarly in the field, there is evidence
that some species of bird are operating at a physiological
ceiling (Daan et al. 1996; Tinbergen and Verhulst 2000;
Welcker et al. 2010; Elliott et al. 2014), although the
causes and definition of this ceiling are also unclear. How
commonly physiological factors limit expenditure, as
opposed to the level of food supply is uncertain, but
studies of the responses of animals to food supplementa-
tion indicate that the response is seldom to elevate energy
expenditure (Boutin 1990), instead opting to reduce the
time required to ingest a given level of resource, perhaps
indicating that physiological limitations may be the norm.
Nevertheless, there are other aspects of the laboratory
situation that could compromise experiments to establish
the oxidative costs of reproduction, and or the implications
of oxidative damage for survival. Among such issues are
the levels of antioxidants in laboratory rodent chow, when
compared with wild foods (assuming dietary antioxidants
do play a role in damage mitigation – which is currently
unclear). This could be a problem in different ways. Manu-
facturers of rodent chow generally add large amounts of
antioxidants, not as a nutritional additive but to improve
the shelf life of the product. Such levels may be unnaturally
high relative to foods available in the wild. As food intake
is greatly elevated during lactation, it is possible that repro-
ducing laboratory animals may inadvertently ingest enor-
mous levels of exogenous antioxidants in their diets, and
this may explain why a common observation in laboratory
studies is that oxidative damage at peak lactation appears
to be reduced. Should animals studied in a laboratory con-
text therefore be fed diets with extremely low levels of
antioxidants? The problem is that we do not presently
know the extent to which lactating mammals, or chick-
rearing birds in the wild, modulate their dietary intake to
enhance their antioxidant intake or intake fatty acids and
other nutritional components that are less susceptible to
attack by free radicals (Beaulieu and Schaefer 2013). There
is a need for information on the dietary intakes of antioxi-
dants of the diet in wild animals at critical times of peak
reproductive activity so that diets could be formulated to
allow laboratory studies to more closely mimic the situa-
tion in the wild [see also Isaksson et al. (2011a,b)].
Another issue relates to the ambient temperatures and
photoperiods at which laboratory animals are maintained.
In the wild, animals are exposed to fluctuating tempera-
tures and photoperiods which place them under variable
thermoregulatory demands (endotherms) or constraints
(ectotherms). This might compromise the ability to invest
in reproduction [e.g., Krol and Speakman (2003)]. Yet in
the laboratory, studies are commonly performed at room
temperature (c. 21–23°C) and a fixed photoperiod with
minimal fluctuations being part of the legal housing
requirements in many countries. The stability of this
temperature and its level and invariant photoperiod may
be completely inappropriate to mimic the experience of
animals in the wild. Levels of many hormones are driven
by photoperiod changes, and these often prepare the
organism for the change in reproductive status altering
not only the hypothalamic–pituitary gonad axis but also
metabolic rate, membrane composition appetite, and
stress responses. Moreover, melatonin, which directly
depends on photoperiod, and has powerful antioxidant
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J. R. Speakman et al. Oxidative Stress and Life Histories
properties, can affect the overall oxidative status of the
organism (Tan et al. 2010). For some animals which rou-
tinely occupy hypoxic environments in the wild, for
example, fossorial animals or those from high altitude,
well-ventilated facilities may actually expose them to
unnaturally high levels of oxygen and hence risk of oxida-
tive stress. A classic example in this regard are the high
levels of oxidative damage in naked mole rats (Hetero-
cephalus glaber) (Andziak and Buffenstein 2006; Andziak
et al. 2006), which is an exceptionally long-lived rodent.
Naked mole rats naturally live in large family groups in
sealed underground burrows where they experience pro-
found hypoxia and hypercapnia (Peterson et al. 2012). In
the laboratory, high levels of oxidative stress may be a
result of the relatively hyperoxic captive conditions. The
fact naked mole-rats in captivity tolerate this chronic high
level of oxidative damage and their extraordinary longev-
ity is based upon captive data (Lewis et al. 2013) may
suggest oxidative stress is not a key element of their
enhanced longevity. An under-recognized issue is also the
possibility that laboratory animals live continually in a
state of chronic stress, even in the best-managed facilities.
Even such apparently subtle changes such as rotation of
staff caring for the animals [e.g., Sorge et al. 2014] may
influence the stress experienced by captive animals.
Finally, if the intention is to establish the linkage
between aspects of damage and survival, it is important
to recognize that oxidative stress may only be a survival
issue under certain environmental conditions. For exam-
ple, if animals sustain damage to their skeletal muscle,
this might only compromise individuals if they have to
perform physical exercise to obtain their food or avoid
predation – as is generally the case in the wild, but sel-
dom so in captivity. Another example is if oxidative stress
acts to reduce the functionality of the immune system.
However, for animals raised in specific pathogen-free
(SPF) facilities such an effect would be unlikely to trans-
late into a survival impact.
Theme four: the lack of specificity in the
theoretical predictions or of good
alternative hypotheses
The idea that the link between reproduction and future
survival may be mediated by oxidative stress predicts that
oxidative damage should increase as a consequence of
reproduction and that this should limit future survival or
future reproductive performance. These predictions are
not the same as the predictions surrounding the role of
oxidative stress as a mediator of aging. Hence, tests of the
free radical theory of aging are not necessarily equivalent
to tests of the mediating role of oxidative damage in life
histories. Moreover, a problem with this prediction is that
it is completely nonspecific regarding the locations and
targets of such damage. The organism is viewed as a
single integrated and homogeneous unit that is predicted
to be damaged or not damaged. In reality, organisms
comprise a complex set of organs that perform very dif-
ferent functions, have different metabolic rates, different
levels of oxidants and antioxidants, repair mechanisms
and ROS, and these organs are potentially affected very
differently by the process of reproduction. For example,
for mammals and birds, a major consequence of repro-
duction is elevated food intake. This food needs to be
digested by the alimentary tract and the digested sub-
strates then processed by the liver. We might imagine
major impacts of reproduction on the metabolic activities
of these tissues. Indeed in mice, at peak lactation the liver
doubles in size and the small intestine grows longer by
around 50% (Johnson et al. 2001). In contrast, the brain
needs to function continuously whether the organism
reproduces or not. It seems unlikely that major changes
occur in brain metabolic rate during reproduction, and
its size is unaffected. These changes in size and potentially
also the metabolic rate of the component body tissues
during reproduction may be coupled with alterations in
where the animals decide to selectively allocate their pro-
tection and repair processes. However, we do not have
any clear predictions regarding what the consequences of
these differences might be. Hence, it is difficult to evalu-
ate the data that are being generated. Does the theory
predict that damage should be uniformly elevated across
all tissues? If so we probably have enough data to reject it
already. However, perhaps the model does not predict
this. For example, the proliferation of newly constructed,
and presumably undamaged, liver tissue during reproduc-
tion may mean that the average concentration of mea-
sured damage is reduced – and this would be consistent
with the empirical observations in several previous studies
(Garratt et al. 2011, 2013; Oldakowski et al. 2012; Yang
et al. 2013; Xu et al. 2014). In fact, many organs in
lactation show hyperplasia and this may explain reduced
damage reported more widely in other tissues such as the
kidney (Oldakowski et al. 2012; da Silva et al. 2013).
Hence, these reduced levels of damage may not be incon-
sistent with the life-history predictions, despite on the
face of it being in the completely opposite direction to
the simplistic prediction that “damage will be increased”.
Furthermore, even within the cell, there may be nonuni-
formity in the distribution of sites at which oxidative
damage occurs, is detected by conventional assays, and
actually matters. There is a strong need therefore for a
more refined conceptualization of the idea, and deriva-
tion of testable mathematical models which make predic-
tions at the tissue/organ level, at a particular stage of
development and with respect to different macromolecu-
5750 ª 2015 The Authors. Ecology and Evolution published by John Wiley & Sons Ltd.
Oxidative Stress and Life Histories J. R. Speakman et al.
lar targets of damage. We envisage these models may gen-
erate different predictions depending on the actual
mechanics of reproduction. For example, pregnancy and
lactation might be expected to involve very different
physiological processes and consequences, than oviparity
and chick feeding in birds. Viviparous and oviparous
reptiles may similarly differ in the patterns of oxidative
stress that are predicted by the theory.
A second element of the current theoretical shortcom-
ings is that not only are the predictions of the oxidative
stress model for life-history effects poorly defined, but
there are few alternative hypotheses to be evaluated. Even
the changes that are anticipated under the null hypothesis
remain uncertain, as for example the changes in tissue
proliferation during reproduction highlighted above
might be expected to alter apparent measured damage,
even if there was no actual change in the underlying pro-
cesses. Hence, the required modeling needs not only to
refine the predictions of the oxidative stress model but
also the expectations under the null and alternative
hypotheses. One promising potential alternative is the
“oxidative shielding theory” (Blount et al. 2015). This
idea suggests that there may be major consequences of
oxidative damage early in life. Hence, a priority of the
parents may be to ensure that damage to their soma(s) is
not transferred to their offspring. Even though oxidative
protection and repair may be costly, it may be more ben-
eficial for parents to upregulate these during reproduction
to reduce their own oxidative damage, thereby minimiz-
ing damage to their offspring, because the fitness benefits
in offspring quality, offset the sacrifice in terms of off-
spring numbers. Clearly, transfer of somatic damage from
parents to offspring may be more likely during some
aspects of reproduction (e.g., in pregnancy where the
fetus and mother are in close physiological contract and
the mother passes chemicals from her body to the fetus)
compared with others like chick rearing in birds where
there is no direct physiological contact of parents and off-
spring. This may suggest shift in focus toward egg pro-
duction and laying (Vezina and Williams 2002; Williams
et al. 2009) might be warranted (see also Velando et al.
2008).
Theme five: a wider diversity of animals
and experimental conditions on which to
test the ideas may provide useful insights
Tests of the oxidative stress theory for the trade-off in
life-history evolution have been performed on a wide
variety of vertebrate species including small mammals
(Bergeron et al. 2011; Garratt et al. 2011; Fletcher et al.
2013; Cram et al. 2015), including bats (Wilhelm et al.
2007; Schneeberger et al. 2014), large mammals (Castillo
et al. 2005; Nussey et al. 2009; Rizzo et al. 2013), reptiles
(Robert et al. 2007; Isaksson et al. 2011a,b), and birds
(Wiersma et al. 2004; Costantini et al. 2006, 2010; Bize
et al. 2008; Marko et al. 2011; Isaksson 2013). Yet, despite
this large range, there are a wide variety of unstudied
reproductive strategies within the endotherms, including
semelparity, and brood parasites where contrasting behav-
iors of the participants may make useful tests of the
model. Moreover, within ectotherms, the growing number
of long-term mark/recapture studies (e.g., Robert and
Bronikowski 2010; Schwanz et al. 2011) should allow a
more complete understanding of oxidative stress and
damage with respect to physiological mode and tempera-
ture sensitivity of the animals. Moreover, future analyses
may enable partition of the contrasting results between
mammals and birds into causes due to phylogeny and
physiology (reviewed in Schwanz et al. 2011).
Expanding the diversity of models used may provide
unexpected insights into the processes involved that are
not immediately obvious from the limited set of animals
studied thus far. Insights from the long-lived social breed-
ing naked mole rat, for example, have been particularly
important regarding the complexity of the process of
damage and protection, as the extant levels of oxidative
damage are surprisingly high for such a long-lived organ-
ism (Andziak et al. 2006): if the oxidative stress theory of
aging is correct. Moreover, contrasting the role of oxida-
tive stress in life histories, levels of damage are similar
among breeding females and nonbreeding subordinates,
despite a 3–5 fold change in metabolic rate during preg-
nancy and lactation (Urison and Buffenstein 1995). High
levels of oxidative damage even in young animals have
also been observed in long-lived bats and birds (Hamilton
et al. 2001; Hermes-Lima and Zenteno-Savin 2002; Buf-
fenstein et al. 2008) and suggests an understudied strategy
for coping with oxidative stress may just be to tolerate the
damage by mitigating its functional impact (Lewis et al.
2013) (see Theme 1). Tolerance of damage may be attrib-
uted to subcellular localization of the oxidatively damaged
macromolecules, a phenomenon seldom measured in
studies of oxidative damage and life history. Naked mole-
rats, in particular, have an enhanced network of mecha-
nisms to maintain protein stability when challenged with
oxidative stressors, as indicated by findings that their liver
proteins are significantly more resistant to urea-induced
protein damage (Perez et al. 2009b). Moreover, it appears
that certain proteins bear the brunt of the oxidative dam-
age (e.g., triosephosphate isomerase and peroxiredoxin 1),
yet their functionality does not appear to be compromised
(De Waal et al. 2013). These proteins may serve as oxida-
tive sinks, which lead to better protection of the cytosolic
environment from the formation of harmful oligomers or
larger aggregates (Rodriguez et al. 2014).
ª 2015 The Authors. Ecology and Evolution published by John Wiley & Sons Ltd. 5751
J. R. Speakman et al. Oxidative Stress and Life Histories
Although different species have different life-history
strategies and these life-history differences may engender
different responses to oxidative stress, comparisons across
species are accompanied by a suite of still-debated diffi-
culties, such as the problems of how to deal with body
size effects and phylogenetic independence (Speakman
2005; Chamberlain et al. 2012; Barja 2013b). Studying the
co-variation of life-history strategies and oxidative dam-
age within single species avoids the allometry and scaling
issues that arise when making comparisons across species.
For example, contrasts between “fast” and “slow” living
garter snakes have been particularly informative with
respect to the evolution of physiological divergence
including patterns of oxidative and other stressors
(Schwartz and Bronikowski 2011, 2013a,b). Their con-
trasting reproductive strategies and stress responses could
expand our understanding of the universality of oxidative
stress as mediator of life histories, particularly interpreted
within a coherent theoretical framework (Theme 4) that
generates contrasting predictions of what we might expect
to happen. Overall, we see distinct advantages to broaden-
ing the scope of existing studies to include a wider diver-
sity of organisms exploiting a more expansive diversity of
life-history strategies.
Theme six: evolutionary/physiological
ecologists and gerontologists need to speak
to each other more deeply and more often
The idea that oxidative stress might underpin the phe-
nomenon of aging was the dominant mechanistic theory
of aging for almost 50 years. Although the idea that oxida-
tive stress may play a role in life-history trade-offs is not
the same, it is clear that the two are related, as reproduc-
tive lifespan, reproductive senescence, and mortality risks,
hence lifespan, are all components of life histories. How-
ever, these theories do make different predictions, and
testing them requires different approaches. The popularity
of the “free radical oxidative damage” theory of aging
among gerontologists waned, almost in parallel with the
increase in popularity of the role of oxidative stress in
influencing the outcome of life-history trade-offs among
eco-physiologists. It is apparent from the literature cited in
many ecophysiology texts, which frequently include the
original paper on free radicals by Harman (1956) and even
the rate of living theory by Pearl (Pearl 1928), that many
of the more recent contributions to the literature by bio-
gerontologists, which have fueled the reduction in the
popularity of the idea in that field, are either not being
read, or are being selectively ignored. Examples include
the impressive body of work by Arlan Richardson and col-
leagues showing that knocking out or over expressing
many of the major protective enzymes against oxidative
stress, such as superoxide dismutase, catalase and glu-
tathione peroxidase, leads to the expected changes in
oxidative damage, but with no impact on lifespan (Perez
et al. 2008, 2009c; Jang et al. 2009; Zhang et al. 2009) (but
recall discussion above under Theme 3 regarding the prob-
lems of laboratory studies). However, when these geneti-
cally manipulated models are subjected to high-fat diets or
other unfavorable environmental conditions, increased
oxidative stress accelerates pathology and shortens lifespan,
and reduced oxidative stress and overexpression of antiox-
idants reduces age-associated pathology and increases lifes-
pan (Salmon et al. 2010). These examples clearly suggest
that the role that oxidative stress plays in aging depends
on environmental conditions. Under optimized laboratory,
husbandry conditions animals can tolerate oxidative dam-
age. However, under environmental conditions including
suboptimal nutrition, oxidative damage, in keeping with
the oxidative stress theory of aging, may overwhelm the
cytoprotective defences with concomitant effects on age-
related diseases and lifespan (Salmon et al. 2010).
Moreover, many studies by both evolutionary/physio-
logical ecologists and biogerontologists still reiterate the
idea that increases in energy expenditure lead to increases
in free radical production, despite the evidence in favor
of that being at best equivocal. Papers predicting, on the
basis of our improved knowledge of mitochondrial func-
tioning, that the opposite is likely to pertain in many cir-
cumstances were published 15 years ago (Brand 2000)
and empirical studies showing that high metabolism is
linked to greater longevity or is unrelated to lifespan, in
both correlational and experimental settings, have been
published also within the last decade (Speakman 2004;
Selman et al. 2008; Keipert et al. 2011). It seems that the
precise mitochondrial pathway responsible for the
increased energy expenditure (i.e., mitochondrial proton
leak vs. ATP synthesis) is likely to determine the sign of
the relationship between energy expenditure and oxidative
stress levels (Stier et al. 2014). This selective blindness to
contrary evidence occurs despite the fact that several
reviews of the field (e.g., Monaghan et al. 2009; Selman
et al. 2012; Speakman and Garratt 2014) have pointed
out the problem with the assumption of a direct positive
link in considerable detail.
Recent findings in gerontology that have emphasized
the roles of particular signaling pathways in the process
of aging and senescence (such as the insulin/IGF-1 signal-
ing, sirtuin, and mTOR pathways), which play highly
conserved roles in growth metabolism and reproduction
(Kenyon 2011; Gems and Partridge 2013), and the poten-
tial role of multiplex resistance to stressors based on
responses of cultured primary fibroblasts (Salmon et al.
2005; Harper et al. 2007, 2011) have much to offer eco-
physiologists as they strive to understand the physiologi-
5752 ª 2015 The Authors. Ecology and Evolution published by John Wiley & Sons Ltd.
Oxidative Stress and Life Histories J. R. Speakman et al.
cal basis of life-history trade-offs. In addition, many
biobiogerontology laboratories have access to equipment
for quantifying various aspects of stress that are beyond
the reach of a dedicated ecophysiology laboratory, and
hence, collaborations between the two fields may be extre-
mely useful. But the flow of information and technology
need not be only in one direction. Furthermore, it is now
becoming clearer that ROS themselves participate in sig-
naling pathways. In addition to the long-recognized role
of ROS in host defences against pathogens, ROS have also
been shown to be an important feature in the regulation
of the entry of mammalian cells to the state of replicative
senescence. The senescent state of the cell is not merely
the result of intrinsic failure of the mitotic machinery,
but appears to be a highly regulated outcome that results
from activation of a pathway that integrates input from
mitochondrial dysfunction, telomere erosion, and DNA
damage (Passos et al. 2010).
Biogerontologists may have been too hasty to dismiss
the role of oxidative stress as an important functional
component of aging, based on studies of model organisms
in protected laboratory environments (Theme 3). Studies
of wild animals living in the field with all its wonderful
complexity and challenges may help redress this balance
and re-kindle interest in this idea among the community
from whence it emerged. After all more than 99.999% of
all organisms on earth live and age in the field, and not
in a protected laboratory, and the processes that cause
them to do so may indeed include oxidative stress in one
form or another. So studies taking advantage of the huge
diversity in aging rate and lifespan in free-living animals
may be particularly relevant and important for gerontolo-
gists to be aware of. Finally, free radicals not only play a
role in damage, and potentially aging, but are also essen-
tial positive components of living systems involved in sig-
naling pathways and in immune function (bactericidal
killing). Understanding these functions more clearly may
be important in interpreting the confusion of current
data. Therefore, meeting people working in these areas to
discuss research aims and findings as well may be equally
important. However, this cross-fertilization of disciplines
will not happen unless there is a council by which such
interactions might be facilitated. There is an urgent need
for a forum where ecophysiologists, biogerontologists,
and other scientists working on oxidative stress can meet
and exchange ideas and data with respect to aging, physi-
ology, ecology, and the evolution of life histories.
Summary
The role of oxidative stress as a factor influencing aging
and longevity, and in driving the evolution of optimal
investment patterns between reproduction and survival
(life-history theory) are distinct areas of research with dif-
fering underlying theoretical bases. Yet they are intercon-
nected at many different levels. We have formulated a
series of questions and propose the collection of some
different types of data that will enhance our ability to
understand the role of oxidative stress in life histories,
that will also impact our understanding of oxidative stress
and aging. The six themes highlighted here provide a
framework for moving forwards along this alternative
path. Significant synergies would be enabled, new insights
gained, and much confusion eliminated if there was more
dialogue between evolutionary/physiological ecologists
and biogerontologists.
Acknowledgments
This study was the result of a week-long workshop spon-
sored by the Rank prize funds attended by all the authors.
We are grateful to the Rank prize funds for supporting
this meeting.
Conflict of Interest
None of the authors has any conflict of interests to
declare.
References
Alonso-Alvarez, C., S. Bernars, G. Devevey, J. Prost, B. Faivre,
and G. Sorci. 2004. Increased susceptibility to oxidative
stress as proximate cost of reproduction. Ecol. Lett.
7:363–368.
Andziak, B., and R. Buffenstein. 2006. Disparate patterns of
age-related changes in lipid peroxidation in long-lived
naked mole-rats and shorter-lived mice. Aging Cell
5:525–532.
Andziak, B., T. P. O’Connor, W. Qi, E. M. DeWaal, A. Pierce,
A. R. Chaudhuri, et al. 2006. High oxidative damage levels
in the longest-living rodent, the naked mole-rat. Aging Cell
5:463–471.
Barga, G. 2013a. Updating the mitochondrial free-radical
theory of aging: an integrated view, key aspects and
confounding concepts. Antioxd. Redox Signal. 19:1420–1445.
Barja, G. 2013b. Correlations with longevity and body size: to
correct or not correct? J. Gerontol. A Biol. Sci. Med. Sci.
69:1096–1098.
Beaulieu, M., and H. M. Schaefer. 2013. Rethinking the role of
dietary antioxidants through the lens of self-medication.
Anim. Behav. 86:17–24.
Beaulieu, M., S. Mboumba, E. Willaume, P. M. Kappeler, and
M. J. E. Charpentier. 2014. The oxidative cost of unstable
social dominance. J. Exp. Biol. 217:2629–2632.
Beckman, K. B., and B. N. Ames. 1998. The free radical theory
of aging matures. Physiol. Rev. 78:547–581.
ª 2015 The Authors. Ecology and Evolution published by John Wiley & Sons Ltd. 5753
J. R. Speakman et al. Oxidative Stress and Life Histories
Bergeron, P., V. Careau, M. M. Humphries, D. Reale, J. R.
Speakman, and D. Garant. 2011. The energetic and
oxidative costs of reproduction in a free-ranging rodent.
Funct. Ecol. 25:1063–1071.
Bize, P., G. Devevey, P. Monaghan, B. Doligez, and P. Christe.
2008. Fecundity and survival in relation to resistance to
oxidative stress in a free-living bird. Ecology 89:2584–2593.
Blount, J. D., E. I. Vitikainen, I. Stott, and M. A. Cant. 2015.
Oxidative shielding and the cost of reproduction. Biol. Rev.
Camb. Philos. Soc. 28; 9:e103286.
Boutin, S. 1990. Food supplementation experiments with
terrestrial vertebrates - patterns, problems, and the future.
Can. J. Zool. 68:203–220.
Bouwhuis, S., R. Choquet, B. C. Sheldon, and S. Verhulst.
2012. The forms and fitness cost of senescence: age-specific
recapture, survival, reproduction, and reproductive value in
a wild bird population. Am. Nat. 179:E15–E27.
Brand, M. D. 2000. Uncoupling to survive? The role of
mitochondrial inefficiency in ageing. Exp. Gerontol. 35:811–
820.
Buffenstein, R., Y. H. Edrey, T. Yang, and J. Mele. 2008. The
oxidative stress theory of aging: embattled or invincible?
Insights from non-traditional model organisms. Age (Dordr)
30:99–109.
Castillo, C., J. Hernandez, A. Bravo, M. Lopez-Alonso, V.
Pereira, and J. L. Benedito. 2005. Oxidative status during
late pregnancy and early lactation in dairy cows. Vet. J.
169:286–292.
Chamberlain, S. A., S. M. Hovick, C. J. Dibble, N. L.
Rasmussen, B. G. Van Allen, B. S. Maitner, et al. 2012. Does
phylogeny matter? Assessing the impact of phylogenetic
information in ecological meta-analysis. Ecol. Lett.
15:627–636.
Costantini, D. 2008. Oxidative stress in ecology and evolution:
lessons from avian studies. Ecol. Lett. 11:1238–1251.
Costantini, D., S. Casagrande, S.De Filippis, G. Brambilla, A.
Fanfani, J. Tagliavini, et al. 2006. Correlates of oxidative
stress in wild kestrel nestlings (Falco tinnunculus). J. Comp.
Physiol. B. 176:329–337.
Costantini, D., L. Carello, and A. Fanfani. 2010. Relationships
among oxidative status, breeding conditions and life-history
traits in free-living Great Tits Parus major and Common
Starlings Sturnus vulgaris. The Ibis 152:793–802.
Cram, D. L., J. D. Blount, and A. J. Young. 2015. Oxidative
status and social dominance in a wild cooperative breeder.
Funct. Ecol. 29:229–238.
van de Crommenacker, J., J. Komdeur, and D. S. Richardson.
2011. Assessing the cost of helping: the roles of body
condition and oxidative balance in the Seychelles warbler
(Acrocephalus sechellensis). PLoS ONE 6:e26423.
Daan, S., C. Deerenberg, and C. Dijkstra. 1996. Increased daily
work precipitates natural death in the kestrel. J. Anim. Ecol.
65:539–544.
De Waal, E. M., H. Liang, A. Pierce, R. T. Hamilton, R.
Buffenstein, and A. R. Chaudhuri. 2013. Elevated protein
carbonylation and oxidative stress do not affect protein
structure and function in the long-living naked-mole rat: a
proteomic approach. Biochem. Biophys. Res. Commun.
434:815–819.
Doonan, R., J. J. McElwee, F. Matthijssens, G. A. Walker, K.
Houthoofd, P. Back, et al. 2008. Against the oxidative
damage theory of aging: superoxide dismutases protect
against oxidative stress but have little or no effect on life
span in Caenorhabditis elegans. Genes Dev. 22:3236–3241.
Dowling, D. K., and L. W. Simmons. 2009. Reactive oxygen
species as universal constraints in life-history evolution.
Proc. Biol. Sci. 276:1737–1745.
Elliott, K. H., M. Le Vaillant, A. Kato, A. J. Gaston, Y. Ropert-
Coudert, J. F. Hare, et al. 2014. Age-related variation in
energy expenditure in a long-lived bird within the envelope
of an energy ceiling. J. Anim. Ecol. 83:136–146.
Fletcher, Q. E., C. Selman, S. Boutin, A. G. McAdam, S. B.
Woods, A. Y. Seo, et al. 2013. Oxidative damage increases
with reproductive energy expenditure and is reduced by
food-supplementation. Evolution 67:1527–1536.
Garratt, M., A. Vasilaki, P. Stockley, F. McArdle, M. Jackson,
and J. L. Hurst. 2011. Is oxidative stress a physiological cost
of reproduction? An experimental test in house mice. Proc.
Biol. Sci. 278:1098–1106.
Garratt, M., N. Pichaud, E. D. King, and R. C. Brooks. 2013.
Physiological adaptations to reproduction. I. Experimentally
increasing litter size enhances aspects of antioxidant defence
but does not cause oxidative damage in mice. J. Exp. Biol.
216:2879–2888.
Gems, D., and R. Doonan. 2009. Antioxidant defense and
aging in C. elegans: is the oxidative damage theory of aging
wrong? Cell Cycle 8:1681–1687.
Gems, D., and L. Partridge. 2013. Genetics of longevity in
model organisms: debates and paradigm shifts. Annu. Rev.
Physiol. 75:621–644.
Hamilton, M. L., H. Van Remmen, J. A. Drake, H. Yang, Z.
M. Guo, K. Kewitt, et al. 2001. Does oxidative damage to
DNA increase with age? Proc. Natl. Acad. Sci. USA
98:10469–10474.
Harman, D. 1956. Aging: a theory based on free radical and
radiation chemistry. J. Gerontol. 11:298–300.
Harper, J. M., A. B. Salmon, S. F. Leiser, A. T. Galecki, and R.
A. Miller. 2007. Skin-derived fibroblasts from long-lived
species are resistant to some, but not all, lethal stresses and
to the mitochondrial inhibitor rotenone. Aging Cell 6:1–13.
Harper, J. M., M. Wang, A. T. Galecki, J. Ro, J. B. Williams,
and R. A. Miller. 2011. Fibroblasts from long-lived bird
species are resistant to multiple forms of stress. J. Exp. Biol.
214:1902–1910.
Hermes-Lima, M., and T. Zenteno-Savin. 2002. Animal
response to drastic changes in oxygen availability and
5754 ª 2015 The Authors. Ecology and Evolution published by John Wiley & Sons Ltd.
Oxidative Stress and Life Histories J. R. Speakman et al.
physiological oxidative stress. Comp. Biochem. Physiol. C
Toxicol. Pharmacol. 133:537–556.
Isaksson, C. 2013. Opposing effects on glutathione and reactive
oxygen metabolites of sex, habitat, and spring date, but no
effect of increased breeding density in great tits (Parus
major). Ecol. Evol. 3:2730–2738.
Isaksson, C., B. Sheldon, and T. Uller. 2011a. The challenges of
integrating oxidative stress into life-history biology.
Bioscience 61:194–202.
Isaksson, C., G. M. While, M. Olsson, J. Komdeur, and E.
Wapstra. 2011b. Oxidative stress physiology in relation to
life history traits of a free-living vertebrate: the spotted snow
skink, Niveoscincus ocellatus. Integr. Zool. 6:140–149.
Jang, Y. C., V. I. Perez, W. Song, M. S. Lustgarten, A. B.
Salmon, J. Mele, et al. 2009. Overexpression of Mn
superoxide dismutase does not increase life span in mice.
J. Gerontol. A Biol. Sci. Med. Sci. 64:1114–1125.
Johnson, M. S., S. C. Thomson, and J. R. Speakman. 2001.
Limits to sustained energy intake II. Inter-relationships
between resting metabolic rate, life-history traits and
morphology in Mus musculus. J. Exp. Biol. 204:1937–1946.
Kaushik, S., and J. Kaur. 2003. Chronic cold exposure affects
the antioxidant defense system in various rat tissues. Clin.
Chim. Acta 333:69–77.
Keipert, S., A. Voigt, and S. Klaus. 2011. Dietary effects on
body composition, glucose metabolism, and longevity are
modulated by skeletal muscle mitochondrial uncoupling in
mice. Aging Cell 10:122–136.
Kenyon, C. 2011. The first long-lived mutants: discovery of the
insulin/IGF-1 pathway for ageing. Philos. Trans. R. Soc.
Lond. B Biol. Sci. 366:9–16.
Kirkwood, T. B., and A. Kowald. 2012. The free-radical theory
of ageing–older, wiser and still alive: modelling positional
effects of the primary targets of ROS reveals new support.
BioEssays 34:692–700.
Krol, E., and J. R. Speakman. 2003. Limits to sustained energy
intake. VII. Milk energy output in laboratory mice at
thermoneutrality. J. Exp. Biol. 206:4267–4281.
Lewis, K. N., B. Andziak, T. Yang, and R. Buffenstein. 2013.
The naked mole-rat response to oxidative stress: just deal
with it. Antioxid. Redox Signal. 19:1388–1399.
Marko, G., D. Costantini, G. Michl, and J. Torok. 2011.
Oxidative damage and plasma antioxidant capacity in
relation to body size, age, male sexual traits and female
reproductive performance in the collared Xycatcher
(Ficedula albicollis). J. Comp. Physiol. B 181:73–81.
Meitern, R., E. Sild, K. Kilk, R. Porosk, and P. Horak. 2013.
On the methodological limitations of detecting oxidative
stress: effects of paraquat on measures of oxidative status in
greenfinches. J. Exp. Biol. 216:2713–2721.
Metcalfe, N. B., and C. Alonso-Alvarez. 2010. Oxidative stress
as a life-history constraint: the role of reactive oxygen
species in shaping phenotypes from conception to death.
Funct. Ecol. 24:984–996.
Metcalfe, N. B., and P. Monaghan. 2013. Does reproduction
cause oxidative stress? An open question. Trends Ecol. Evol.
28:347–350.
Monaghan, P., N. B. Metcalfe, and R. Torres. 2009. Oxidative
stress as a mediator of life history trade-offs: mechanisms,
measurements and interpretation. Ecol. Lett. 12:75–92.
Nussey, D. H., J. M. Pemberton, J. G. Pilkington, and J. D.
Blount. 2009. Life history correlates of oxidative damage
in a free-living mammal population. Funct. Ecol.
23:809–817.
Oldakowski, L., Z. Piotrowska, K. M. Chrzascik, E. T.
Sadowska, P. Koteja, and J. R. Taylor. 2012. Is reproduction
costly? No increase of oxidative damage in breeding bank
voles. J. Exp. Biol. 215:1799–1805.
Passos, J. F., G. Nelson, C. Wang, T. Richter, C. Simillion, C.
J. Proctor, et al. 2010. Feedback between p21 and reactive
oxygen production is necessary for cell senescence. Mol.
Syst. Biol. 6:347.
Pearl, R. L. 1928. The rate of living. Alfred Knopf, New York,
USA.
Perez, V. I., C. M. Lew, L. A. Cortez, C. R. Webb, M.
Rodriguez, Y. Liu, et al. 2008. Thioredoxin 2
haploinsufficiency in mice results in impaired mitochondrial
function and increased oxidative stress. Free Radic. Biol.
Med. 44:882–892.
Perez, V. I., A. Bokov, H. Van Remmen, J. Mele, Q. Ran, Y.
Ikeno, et al. 2009a. Is the oxidative stress theory of aging
dead? Biochim. Biophys. Acta 1790:1005–1014.
Perez, V. I., R. Buffenstein, V. Masamsetti, S. Leonard, A. B.
Salmon, J. Mele, et al. 2009b. Protein stability and resistance
to oxidative stress are determinants of longevity in the
longest-living rodent, the naked mole-rat. Proc. Natl. Acad.
Sci. USA 106:3059–3064.
Perez, V. I., H. Van Remmen, A. Bokov, C. J. Epstein, J. Vijg,
and A. Richardson. 2009c. The overexpression of major
antioxidant enzymes does not extend the lifespan of mice.
Aging Cell 8:73–75.
Peterson, B. L., J. Larson, R. Buffenstein, T. J. Park, and C. P.
Fall. 2012. Blunted neuronal calcium response to hypoxia in
naked mole-rat hippocampus. PLoS ONE 7:e31568.
Rashidi, A., T. B. Kirkwood, and D. P. Shanley. 2009.
Metabolic evolution suggests an explanation for the
weakness of antioxidant defences in beta-cells. Mech. Ageing
Dev. 130:216–221.
Rizzo, A., E. Ceci, M. Pantaleo, M. Mutinati, M. Spedicato, G.
Minoia, et al. 2013. Evaluation of blood and milk oxidative
status during early postpartum of dairy cows. Animal
7:118–123.
Robert, K. A., and A. M. Bronikowski. 2010. Evolution of
senescence in nature: physiological evolution in populations
of garter snake with divergent life histories. Am. Nat.
175:147–159.
Robert, K. A., A. Brunet-Rossinni, and A. M. Bronikowski.
2007. Testing the ‘free radical theory of aging’ hypothesis:
ª 2015 The Authors. Ecology and Evolution published by John Wiley & Sons Ltd. 5755
J. R. Speakman et al. Oxidative Stress and Life Histories
physiological differences in long-lived and short-lived
colubrid snakes. Aging Cell 6:395–404.
Rodriguez, K. A., P. A. Osmulski, A. Pierce, S. T. Weintraub,
M. Gaczynska, and R. Buffenstein. 2014. A cytosolic protein
factor from the naked mole-rat activates proteasomes of
other species and protects these from inhibition. Biochim.
Biophys. Acta 1842:2060–2072.
Sainz, R. M., R. J. Reiter, J. C. Mayo, J. Cabrera, D. X. Tan,
W. Qi, et al. 2000. Changes in lipid peroxidation during
pregnancy and after delivery in rats: effect of pinealectomy.
J. Reprod. Fertil. 119:143–149.
Salmon, A. B., S. Murakami, A. Bartke, J. Kopchick, K.
Yasumura, and R. A. Miller. 2005. Fibroblast cell lines from
young adult mice of long-lived mutant strains are resistant
to multiple forms of stress. Am. J. Physiol. Endocrinol.
Metab. 289:E23–E29.
Salmon, A. B., A. Richardson, and V. I. Perez. 2010. Update
on the oxidative stress theory of aging: does oxidative stress
play a role in aging or healthy aging? Free Radic. Biol. Med.
48:642–655.
Schmidt, C. M., J. D. Blount, and N. C. Bennett. 2014.
Reproduction is associated with a tissue-dependent
reduction of oxidative stress in eusocial female damara
land mole-rats (Fukomys damarensis). PLoS ONE 28; 9:
e103286.
Schneeberger, K., G. A. Czirjak, and C. C. Voigt. 2014.
Frugivory is associated with low measures of plasma
oxidative stress and high antioxidant concentration in free-
ranging bats. Naturwissenschaften 101:285–290.
Schwanz, L., D. A. Warner, S. McGaugh, R. Di Terlizzi, and A.
Bronikowski. 2011. State-dependent physiological
maintenance in a long-lived ectotherm, the painted turtle
(Chrysemys picta). J. Exp. Biol. 214:88–97.
Schwartz, T. S., and A. M. Bronikowski. 2011. Molecular stress
pathways and the evolution of life histories in reptiles. Pp.
193–209 in T. Flatt and A. Heyland, eds. Mechanisms of
Life History Evolution, Oxford University Press, Oxford,
UK.
Schwartz, T. S., and A. M. Bronikowski. 2013a. Dissecting
molecular stress networks: identifying nodes of divergence
between life-history phenotypes. Mol. Ecol. 22:739–756.
Schwartz, T. S., and A. M. Bronikowski. 2013b. Plasticity and
evolution of stress response networks in divergent life-
history phenotypes. Integr. Comp. Biol. 53:E192–E192.
Selman, C., T. Grune, A. Stolzing, M. Jakstadt, J. S. McLaren,
and J. R. Speakman. 2002a. The consequences of acute cold
exposure on protein oxidation and proteasome activity in
short-tailed field voles, Microtus agrestis. Free Radic. Biol.
Med. 33:259–265.
Selman, C., J. S. McLaren, A. R. Collins, G. G. Duthie, and J.
R. Speakman. 2002b. Antioxidant enzyme activities, lipid
peroxidation, and DNA oxidative damage: the effects of
short-term voluntary wheel running. Arch. Biochem.
Biophys. 401:255–261.
Selman, C., J. S. McLaren, A. R. Collins, G. G. Duthie, and J.
R. Speakman. 2008. The impact of experimentally elevated
energy expenditure on oxidative stress and lifespan in the
short-tailed field vole Microtus agrestis. Proc. Biol. Sci.
275:1907–1916.
Selman, C., J. D. Blount, D. H. Nussey, and J. R. Speakman.
2012. Oxidative damage, ageing, and life-history evolution:
where now? Trends Ecol. Evol. 27:570–577.
da Silva, A. C. A., T. B. Salomon, C. S. Behling, J. Putti, F. S.
Hackenhaar, P. V. G. Alabarse, et al. 2013. Oxidative stress
in the kidney of reproductive female rats during aging.
Biogerontology 14:411–422.
Sorge, R. E., L. J. Martin, K. A. Isbester, S. G. Sotocinal, S.
Rosen, A. H. Tuttle, et al. 2014. Olfactory exposure to
males, including men, causes stress and related analgesia in
rodents. Nat. Methods 11:629–632.
Speakman, J. R. 2004. Oxidative phosphorylation,
mitochondrial proton cycling, free-radical production and
aging. Pp 35–68 in M.P. Mattson, eds. Energy metabolism
and lifespan determination. Elsevier series Advances in Cell
Aging and Gerontology. Elsevier, Amsterdam.
Speakman, J. R. 2005. Body size, energy metabolism and
lifespan. J. Exp. Biol. 208:1717–1730.
Speakman, J. R., and M. Garratt. 2014. Oxidative stress as a
cost of reproduction: beyond the simplistic trade-off model.
BioEssays 36:93–106.
Speakman, J. R., and E. Krol. 2011. Limits to sustained energy
intake. XIII. Recent progress and future perspectives. J. Exp.
Biol. 214:230–241.
Speakman, J. R., and C. Selman. 2011. The free-radical damage
theory: accumulating evidence against a simple link of
oxidative stress to ageing and lifespan. BioEssays 33:255–
259.
Stier, A., S. Reichert, S. Massemin, P. Bize, and F. Criscuolo.
2012. Constraint and cost of oxidative stress on
reproduction: correlative evidence in laboratory mice and
review of the literature. Front. Zool. 9.
Stier, A., P. Bize, C. Habold, F. Bouillaud, S. Massemin, and F.
Criscuolo. 2014. Mitochondrial uncoupling prevents cold-
induced oxidative stress: a case study using UCP1 knockout
mice. J. Exp. Biol. 217:624–630.
Stuart, J. A., L. A. Maddalena, M. Merilovich, and E. L. Robb.
2014. A midlife crisis for the mitochondrial free radical
theory of aging. Longev. Healthspan 3:4.
Tan, D. X., R. Hardeland, L. C. Manchester, S. D. Paredes, A.
Korkmaz, R. M. Sainz, et al. 2010. The changing biological
roles of melatonin during evolution: from an antioxidant to
signals of darkness, sexual selection and fitness. Biol. Rev.
Camb. Philos. Soc. 85:607–623.
Tinbergen, J. M., and S. Verhulst. 2000. A fixed energetic
ceiling to parental effort in the great tit? J. Anim. Ecol.
69:323–334.
Urison, N. T., and R. B. Buffenstein. 1995. Metabolic and
body-temperature changes during pregnancy and lactation
5756 ª 2015 The Authors. Ecology and Evolution published by John Wiley & Sons Ltd.
Oxidative Stress and Life Histories J. R. Speakman et al.
in the naked mole-rat (Heterocephalus glaber). Physiol. Zool.
68:402–420.
Velando, A., R. Torres, and C. Alonso-Alvarez. 2008. Avoiding
bad genes: oxidatively damaged DNA in the germ line and
mate choice. BioEssays 30:1212–1219.
Veskoukis, A. S., M. G. Nikolaidis, A. Kyparos, and D.
Kouretas. 2009. Blood reflects tissue oxidative stress
depending on biomarker and tissue studied. Free Radic.
Biol. Med. 47:1371–1374.
Vezina, F., and T. D. Williams. 2002. What drives the
metabolic costs of egg production in birds? Integr. Comp.
Biol. 42:1327–1328.
Welcker, J., B. Moe, C. Bech, M. Fyhn, J. Schultner, J. R.
Speakman, et al. 2010. Evidence for an intrinsic energetic
ceiling in free-ranging kittiwakes Rissa tridactyla. J. Anim.
Ecol. 79:205–213.
Wiersma, P., C. Selman, J. R. Speakman, and S. Verhulst.
2004. Birds sacrifice oxidative protection for reproduction.
Proc. Biol. Sci. 271(Suppl 5):S360–S363.
Wilhelm, D., S. L. Althoff, A. L. Dafre, and A. Boveris. 2007.
Antioxidant defenses, longevity and ecophysiology of South
American bats. Comp. Biochem. Physiol. C Toxicol.
Pharmacol. 146:214–220.
Williams, T. D., F. Vezina, and J. R. Speakman. 2009. Individually
variable energy management during egg production is repeatable
across breeding attempts. J. Exp. Biol. 212:1101–1105.
Xu, Y. C., D. B. Yang, J. R. Speakman, and D. H. Wang. 2014.
Oxidative stress in response to natural and experimentally
elevated reproductive effort is tissue dependent. Funct. Ecol.
28:402–410.
Yang, W., J. Li, and S. Hekimi. 2007. A measurable increase in
oxidative damage due to reduction in superoxide
detoxification fails to shorten the life span of long-lived
mitochondrial mutants of Caenorhabditis elegans. Genetics
177:2063–2074.
Yang, D. B., Y. C. Xu, D. H. Wang, and J. R. Speakman. 2013.
Effects of reproduction on immuno-suppression and
oxidative damage, and hence support or otherwise for their
roles as mechanisms underpinning life history trade-offs, are
tissue and assay dependent. J. Exp. Biol. 216:4242–4250.
Zhang, Y. Q., Y. Ikeno, W. B. Qi, A. Chaudhuri, Y. Li, A.
Bokov, et al. 2009. Mice deficient in both Mn superoxide
dismutase and glutathione peroxidase-1 have increased
oxidative damage and a greater incidence of pathology but
no reduction in longevity. J. Gerontol. A Biol. Sci. Med. Sci.
64:1212–1220.
ª 2015 The Authors. Ecology and Evolution published by John Wiley & Sons Ltd. 5757
J. R. Speakman et al. Oxidative Stress and Life Histories