Download Proximate perspectives on the evolution of female aggression: good

Downloaded from http://rstb.royalsocietypublishing.org/ on June 14, 2017
Proximate perspectives on the evolution
of female aggression: good for the
gander, good for the goose?
Kimberly A. Rosvall
rstb.royalsocietypublishing.org
Review
Cite this article: Rosvall KA. 2013 Proximate
perspectives on the evolution of female
aggression: good for the gander, good for the
goose? Phil Trans R Soc B 368: 20130083.
http://dx.doi.org/10.1098/rstb.2013.0083
One contribution of 14 to a Theme Issue
‘Female competition and aggression’.
Subject Areas:
behaviour, evolution, physiology,
neuroscience, genomics
Keywords:
female aggression, sex differences,
individual variation, androgen receptor,
aromatase, sexual conflict
Author for correspondence:
Kimberly A. Rosvall
e-mail: [email protected]
Electronic supplementary material is available
at http://dx.doi.org/10.1098/rstb.2013.0083 or
via http://rstb.royalsocietypublishing.org.
Department of Biology, Center for the Integrative Study of Animal Behavior, Indiana University,
Bloomington, IN 47405, USA
Female– female aggression often functions in competition over reproductive
or social benefits, but the proximate mechanisms of this apparently adaptive
behaviour are not well understood. The sex steroid hormone testosterone (T)
and its metabolites are well-established mediators of male–male aggression,
and several lines of evidence suggest that T-mediated mechanisms may
apply to females as well. However, a key question is whether mechanisms
of female aggression primarily reflect correlated evolutionary responses to
selection acting on males, or whether direct selection acting on females
has made modifications to these mechanisms that are adaptive in light of
female life history. Here, I examine the degree to which female aggression
is mediated at the level of T production, target tissue sensitivity to T, or downstream genomic responses in order to test the hypothesis that selection favours
mechanisms that facilitate female aggression while minimizing the costs of
systemically elevated T. I draw heavily from avian systems, including the
dark-eyed junco (Junco hyemalis), as well as other organisms in which these
mechanisms have been well studied from an evolutionary/ecological perspective in both sexes. Findings reveal that the sexes share many behavioural and
hormonal mechanisms, though several patterns also suggest sex-specific
adaptation. I argue that greater attention to multiple levels of analysis—
from hormone to receptor to gene network, including analyses of individual
variation that represents the raw material of evolutionary change—will be a
fruitful path for understanding mechanisms of behavioural regulation and
intersexual coevolution.
1. Introduction: why individual variation and
mechanisms matter
Evolutionary endocrinology employs both proximate and ultimate approaches to
understand how phenotypes evolve [1–5]. Proximate mechanisms generate many
of the constraints that underlie trade-offs, they create or constrain the potential for
phenotypic plasticity, and they harbour the ‘pool’ of inter-individual variation
that can be shaped by evolution [2–8]. In short, mechanisms determine what is
possible in evolution in a number of ways, and, as a consequence, a mechanistic
view is essential to a full understanding of the evolutionary process.
Several tools and perspectives from evolutionary biology and endocrinology
are central to this ‘evo-endo’ approach. For one, manipulating a hormone-mediated
trait with exogenous hormones or receptor antagonists is key to establishing the
functional significance of hormone–phenotype relationships. Further, a focus on
individual variation can shed light upon the proximate sources of natural phenotypic variation, because individual variation in quantitative traits is the raw
material of phenotypic evolution. Endocrinology has traditionally ignored individual variation as technical or environmental noise [9], though it has received greater
attention in recent years [6,7,10,11]. By illuminating covariation among endocrine
parameters, phenotypes and Darwinian fitness, research on among-individual
variation lends insights into how evolution can proceed [9]. For instance, if there
is no inter-individual variability in some component of mechanism, then evolution
cannot proceed via changes in that particular mechanism because the strength of
& 2013 The Author(s) Published by the Royal Society. All rights reserved.
Downloaded from http://rstb.royalsocietypublishing.org/ on June 14, 2017
(a)
H
The evo-endo approach just described can be profitably
applied to the evolution of female competition and aggression,
a topic that has received considerable recent attention [14–17].
Much of this research has emphasized ultimate perspectives,
and it has become clear that female–female aggression and
other exaggerated traits often function in competition over
reproductive and social benefits. Female–female aggression
has been shown to be beneficial in many contexts, including year-round territorial species, cooperative breeders and
sex-role reversed species where females compete for rank,
territories and access to mates. Even females in monogamous
and polygynous species compete for paternal care, limited
nesting sites or access to high-quality males [15,16]. Thus,
female–female aggression is widespread in the animal kingdom, and given that it is often individually repeatable
[18,19], this lays a strong foundation for aggression to be
heritable and evolutionarily significant. The arguments and
predictions below build upon this now well-supported view
that aggression is often selectively advantageous to females.
In addition to direct selection on females themselves,
female aggression and its underlying mechanisms ought to be
shaped by correlated responses to selection shaping aggression
in males, as well as coevolutionary dynamics between males
and females. It was long assumed that exaggerated female
traits and behaviours like female aggression were primarily
non-adaptive by-products of genetic correlations with males
[20]. After all, males and females share the vast majority of
their genome [21], and so, phenotypes and mechanisms that
are advantageous to males ought to be expressed in females
as well. If the sexes differ in the degree to which these phenotypes or mechanisms are advantageous, then this sexual
conflict will lead to coevolution between males and females
[22]. Coevolution may take many different forms, including
antagonistic directional change in one sex that is countered
by the other sex (e.g. increase versus decrease in trait X) [23],
as well as ‘palliative’ adaptations that break or minimize
sexual conflict by reducing the deleterious consequences for
one sex (e.g. one sex avoids the costs of an increase in trait X
via some adaptation in trait Y) [24].
In investigating how these evolutionary processes act on
mechanisms of aggression in males and females, the sex
steroid hormone testosterone (T) is a logical starting point.
The activational effects of T often masculinize behaviour,
either directly or after T has been converted to other
G
T
(b)
sensitivity and
metabolism
AROM
AR
target
tissue
E2
ER
altered
transcription
phenotypic response
Figure 1. Simplified overview of the mechanisms by which gonadal sex
steroids influence behaviour via genomic effects, emphasizing components
highlighted in this review. Phenotypic variation has been linked with variation in (a) hormone production, (b) hormonal responses in target tissues,
including local metabolism of T into other sex steroids and sensitivity to
these steroid hormones (e.g. binding of T to ARs or binding of E2 to ERs)
and (c) genomic responses, which are the changes in transcription brought
on by hormone – receptor binding. All three components also interact with
each other and with behaviour via complex feedback loops (denoted by
the double-lined arrows), as well as other endocrine mechanisms not
depicted here [28,29]. See §2b for elaboration.
hormones, such as oestradiol (E2). It is also well established
that T and its metabolites are proximate mediators of male
aggressive behaviour, and more and more studies are investigating relationships between T and aggression in females
[4,25]. Females naturally produce T in the gonads, brain and
adrenals, and T is often highest in circulation in association
with reproduction, in part because T is an intermediary on
the pathway to produce E2, which is required for ovulation
[26,27]. Furthermore, both sexes express androgen receptors
(ARs) in a number of neural and peripheral tissues, and both
sexes exhibit behavioural or physiological responses to T
[26,27]. Thus, T is naturally elevated in females at certain
times of the year, providing opportunities for selection
acting directly on females to shape T-behaviour relationships
in potentially adaptive ways.
(b) Mechanistic sources of variation
In general, T affects behaviour through a cascade of interconnected mechanisms (figure 1 represents a simplified version),
and this cascade can be parsed in different ways to study how
each contributes to behavioural variation [10,30]. Here, I
focus on three components of this cascade for which there
are abundant data from males and females in relation to
aggressive behaviour. (i) Behavioural variation may relate
to variation in hormone production (figure 1a). The major
source of T in circulation is the gonad, and gonadal T production is regulated by the hypothalamo-pituitary-gonadal
(HPG) axis, which is activated by external stimuli (day
length, conspecifics, etc.). Gonadotropin-releasing hormone
(GnRH) is released from the hypothalamus (H in figure 1a),
and acts on the pituitary (P) to release gonadotropins
Phil Trans R Soc B 368: 20130083
(a) Why female –female aggression, why testosterone?
P
production
(c) genomic
response
2. A mechanistic approach to the evolution of
female aggression
2
LH
GnRH
rstb.royalsocietypublishing.org
selection is proportional to variation among individuals
[12]. Importantly, analyses of covariation among endocrine
parameters and phenotype should be paired with experimental approaches that can demonstrate functional significance
(e.g. hormone manipulations, artificial selection, mutation analyses, etc. [1]), as well as comparative (phylogenetic) approaches
that highlight which particular mechanisms have most likely
been agents of phenotypic evolution in the past [13]. Thus, a
combination of proximate and ultimate perspectives can
reveal not only whether or why selection favours a particular
phenotype, but also how phenotypic change might occur in
the future, or how it has occurred in the past.
Downloaded from http://rstb.royalsocietypublishing.org/ on June 14, 2017
By investigating these mechanisms in females and how they
vary from T-mediated mechanisms of behaviour in males, we
can gain unique insights into the role of sex-specific adaptation
in intersexual coevolution and the mechanisms by which it
occurs. Accordingly, the principal goal of this article is to
assess the degree to which females use sex-specific mechanisms
that are adaptive in the light of female life history.
To generate hypotheses regarding the nature of adaptive
mechanisms of aggression for females, I begin by considering
T-mediated mechanisms of aggression in males, and the
degree to which they are well suited to females. In males,
aggression is often associated with higher T in circulation
and greater sensitivity to T in the brain. Binding of T to AR
initiates a suite of transcriptional effects that are thought to
be advantageous in the face of protracted social competition,
e.g. by shifting resources towards greater activity and energy
mobilization [8,32]. On the other hand, elevated T can be
costly, e.g. in terms of immune function, stress physiology
or trade-offs with parental care [33]. Notably, these costs
have been repeatedly shown to be quite detrimental to females,
e.g. by delaying breeding, inhibiting maternal care or interfering with mate choice [34–37]. Combined with observations
from §2a that (i) female aggression is selectively advantageous
in a number of competitive contexts, (ii) females are naturally
capable of producing and responding to T and (iii) these mechanisms are probably influenced by coevolutionary processes
with males, a series of clear predictions emerge regarding
how selection might facilitate aggression in females without
some of these costs of T.
In short, if mechanisms of aggression in females are
adaptive (to females), then selection ought to modify the
connections between behaviour and the mechanisms from
figure 1 to minimize the costs of T [33]. There are several
ways that this might occur, including lower T levels in
sensitivity orr
metabolism
m
higher T
conflict
confl
f ict
(c) altered
genomic
responses
lower T
initial
conflict
palliative
adaptations
p
cooperation/conflict
along
g new dimensions
evolutionary
time
l ti
ti
Figure 2. Hypothetical steps in the evolution of mechanisms of aggression,
adapted from a general model of sexual conflict by Lessells [24]. Males and
females may have an initial conflict, depicted here as conflict over T levels
(a). Palliative adaptations that break the cycle of adaptation/counter-adaptation
may arise, and these adaptations are predicted to include female-driven changes
in target tissue sensitivity to, or metabolism of, hormones (b) as well as altered
genomic responses (c), among others (see §2c) . These new adaptations may be
adaptive for both males and females (cooperative coevolution) or they may lead
to sexual conflict along new dimensions.
circulation in females (figure 2a). A number of palliative adaptations that minimize the costs of high T on females also should
be favoured (figure 2b,c), including reduced sensitivity to T in
certain tissues (e.g. downregulation of AR expression in brain
areas controlling maternal care), increased sensitivity to T in
other tissues (e.g. in those controlling aggression), greater use
of neurosteroidogenesis or local conversion to synthesize T
within target tissues, or otherwise altered behavioural
response to T via several mechanisms, such as sexually
dimorphic transcriptional responses to T.
Notably, several studies of male aggression have demonstrated apparently adaptive seasonal changes in these very
mechanisms. For example, it is thought to be costly for
male birds to maintain high T outside of the breeding
season, and many male songbirds do have lower T in the
non-breeding season [33]. However, non-breeding season
aggression can still occur, and it is facilitated by seasonal
shifts in properties of neural target tissues, including altered
neurogenomic responses to social challenges, upregulation of
AR, and greater conversion of hormone precursors into T
within the brain [38–40]. The important insight from these
examples is that divergent behavioural mechanisms can
exist within the same genome, suggesting it is reasonable to
hypothesize that males and females also may differ in these
same mechanisms despite their near-identical genomes.
In the following sections, I test these predictions regarding adaptive mechanisms of female aggression by focusing
on T-mediated mechanisms, with an emphasis on birds and
other systems in which these mechanisms have been well
studied in an evolutionary/ecological context in both sexes.
I draw many examples from the dark-eyed junco (Junco hyemalis), the songbird that I have been studying for the past
few years, because T-mediated mechanisms and their phenotypic and evolutionary consequences have been particularly
well documented in both sexes in this system [5,27].
I first examine sex differences in T production (§3) and the
degree to which the relationship between T and behaviour is
similar or different in the two sexes, focusing on the behavioural effects of experimental elevation of T (§4) and
social modulation of T (§5). I then examine the degree to
which aggression is mediated by variation in target tissue
3
Phil Trans R Soc B 368: 20130083
(c) Hypotheses and predictions regarding adaptive
mechanisms for females
(b) altered
(a)
rstb.royalsocietypublishing.org
including luteinizing hormone (LH). LH binds LH-receptors
in the gonad (G) to stimulate steroid biosynthesis and
T release. (ii) Individuals also may differ in target tissue
sensitivity or metabolism of hormones (figure 1b), due to variation in the location or abundance of nuclear hormone
receptors (e.g. ARs, oestrogen receptors (ERs)) or enzymes
involved in local steroid synthesis or metabolism, such as
aromatase (AROM), which converts T to E2. Notably, many
of the behavioural effects of T are generated by E2
after local conversion via AROM [10]. (iii) Individuals also
may differ because different genes are ‘turned on’ by the
hormone –receptor complex, i.e. downstream genomic responses
(figure 1c), such that certain types of cellular and physiological effects are brought about by a hormone in one sex or one
individual, and not in another. When T or E2 bind nuclear
receptors, the hormone –receptor complex interacts with the
genome to cause massive changes in transcription and
translation, ultimately affecting phenotype [31]. Owing to
interactions among these endocrine components, they
should not be considered mutually exclusive (e.g. high T
can upregulate target tissue expression of AR, behaviour
can feed back upon any of these endocrine parameters,
etc.). This list and schematic are also not exhaustive (e.g.
see especially [28,29]), but they represent mechanisms that
have been most thoroughly studied in both sexes.
Downloaded from http://rstb.royalsocietypublishing.org/ on June 14, 2017
(a) Do the sexes coevolve in hormone production?
Male and female animals often differ in levels of T in circulation, providing one possible way for the sexes to differentially
modulate behaviour. Comparative analyses demonstrate that
female T levels covary with male T levels (birds: [27,41],
fishes: [42]), and these positive correlations between male
and female T levels are consistent with the hypothesis that
T levels in one sex reflect correlated evolutionary responses
between the sexes. If true, then another prediction is that
males and females will have arrived at these correlated T
levels due to shared underlying changes along the HPG
axis. For example, we might expect males and females to be
correlated in (i) abundance of GnRH-producing neurons in
the hypothalamus, (ii) secretion rates of LH from the pituitary
and/or (iii) abundance of gonadal LH-receptors or steroid
biosynthetic enzymes, to name a few possibilities.
When colleagues and I tested this prediction in the darkeyed junco, however, it was not fully supported. We found
that males from two phenotypically divergent subspecies of
junco differed in gonadal sensitivity to LH (LH-receptor
mRNA abundance) and hypothalamic sensitivity to androgenmediated feedback (AR mRNA abundance), but females from
the two subspecies did not differ in these or other HPG-related
parameters that we assessed [43,44]. Thus, males from these
two subspecies appear to have diverged in HPG axis regulation,
but females did not show correlated changes. On the other hand,
the mechanisms that differentiate ‘high T’ individuals from
‘low T’ individuals appear to be similar in the two sexes
[43,44]. For instance, among individual within each sex, we
found that endocrine parameters were not correlated across
multiple tiers of the HPG axis (e.g. a ‘high T’ individual was
not a ‘high LH’ individual or a ‘high LH-receptor’ individual). Further, an individual junco produces roughly the same
amount of T when the pituitary is stimulated with an exogenous
dose of GnRH as he or she does when the gonad is stimulated
with a standardized dose of LH. Thus, something downstream
of the pituitary (i.e. something about the gonad) holds the key
to individual differences in T production, and our analyses
suggest that it is the expression of steroid biosynthetic enzymes.
Because the gonad is the most sexually differentiated organ,
these findings also suggest that one or another sex could alter
T levels via changes that are localized to the gonad, rather
than also requiring concomitant changes in the pituitary and
hypothalamus, therefore permitting somewhat independent
changes in hormone secretion in the two sexes. Greater attention to individual variation in more species will elucidate
whether females tend to diverge in the same ways as males,
or whether sex-specific changes along the HPG axis
(b) Does female life history predict variation in T?
Male T levels tend to covary with mating system and parental
investment at an evolutionary scale, and T also varies with
season, being higher in association with mating or male–
male aggression and lower in association with paternal care
[32,45–47]. Similar to these patterns in males, several lines of evidence from birds lend support to the hypothesis that female T
levels are shaped by selection acting directly on females. First,
female T levels peak early in the breeding season, either prior
to laying when female competition is most intense, or during
egg laying when both T and E2 are high in association with
ovulation/egg production [27]. Second, female T levels are
higher in monogamous versus non-monogamous species [27],
and they are higher in colonial versus non-colonial species
[41], demonstrating elevated T in two mating systems where
female–female competition plays an important role (e.g. in
defense of monogamy or nesting sites, respectively). Perhaps
surprisingly, T levels are relatively low in females from polyandrous species [27]. However, if competition is especially intense
among these females, then selection may promote behavioural
mechanisms that do not require high T in circulation. This prediction is supported by findings from sex-role reversed species
in which females typically have lower T than males [48], but
females may instead have heightened sensitivity to T in areas
of the brain associated with aggression [49]. Thus, in addition
to any correlated responses to selection acting on males,
selection also appears to act directly on females to alter T
levels in association with female–female competition.
(c) Individual variation in T
Contrasting males and females in patterns of individual variation can tell us whether there is existing variation in T that
could be shaped by selection. Techniques such as injections
with GnRH permit precise assessment of an individual’s physiological capacity to produce and secrete T [50]. Importantly, T
produced in response to this ‘GnRH challenge’ is repeatable in
individual males and females [43,44,50], building an essential
foundation upon which further questions about the functional
consequences or mechanistic sources of these stable individual
differences can be built. For example, one key question is
whether individual differences in T are more likely to relate
to aggression in males than in females. In dark-eyed juncos,
variation in T produced in response to a GnRH challenge is
positively correlated with individual differences in aggression
within each sex [51,52], but in the White’s skink (Egernia whitii),
circulating T is related to aggression in males but not in females
[53]. These are only two examples, and many more studies will
need to consider individual variation if we are to determine
whether the sexes differ in the degree to which levels of
circulating T predict behaviour.
4. Behavioural response to exogenous
testosterone
(a) Control versus testosterone-treated
Experimental elevation of T (EET) via implants or injections
[27] often leads to an increase in aggressive behaviour in
4
Phil Trans R Soc B 368: 20130083
3. Testosterone production in males and females
(possibly enabled via organizational effects) facilitate somewhat
independent mechanisms in males and females.
rstb.royalsocietypublishing.org
sensitivity to T (§6) and whether molecular mechanisms of
aggression and hormone response are similar or different in
the two sexes (§7). In each section, I relate findings on females
to comparable findings on males, and I assess evidence for
female-specific adaptation by relating findings to female life
history. Whenever possible, I emphasize individual-based
approaches that facilitate conclusions as to whether evolution
might change aggression via the same or different mechanisms
in the two sexes. In closing, I considering findings in the larger
context of molecular and behavioural evolution (§8), and I
outline critical next steps in this field of research (§9).
Downloaded from http://rstb.royalsocietypublishing.org/ on June 14, 2017
(b) Individual variation in behavioural response to T
At the inter-individual level, it is unclear how much variability
there is in aggressive response to EET, though some individuals certainly seem to be more affected than others in terms
of parental behaviour. For instance, EET leads to delayed
breeding in female juncos, and some females implanted
prior to egg laying fail to breed altogether [34]. Further,
T-females have fewer nests than controls, and among those
T-females who do have nests, they have fewer eggs and
hatchling than controls; however, among females who have
chicks, there is no EET effect on likelihood of survival to fledging [36]. One explanation for these patterns is that only those
T-females who were least sensitive to T were able to breed successfully, whereas females who were naturally most sensitive
were unable to nest to begin with. Interestingly, offspring of
T-females were more likely than those of control females to
return the following year as adults, consistent with natural
selection related to behavioural sensitivity to T. Future studies
should attempt to uncover the proximate mechanisms that
differentiate these most responsive and least responsive
females, and alternatives will need to be explored (i.e.
variation in T metabolism, implant placement, etc.).
5
Phil Trans R Soc B 368: 20130083
behavioural mechanisms have been shaped by direct selection on females. In other species, such as the tree swallow
(Tachycineta bicolor), females are behaviourally sensitive to
EET both in terms of (increased) aggression and (reduced)
incubation behaviour [35]. Female incubation is essential in
this species, and so this behavioural sensitivity to T seems
maladaptive in light of the essential parental care hypothesis.
However, if we instead apply the assumption typically applied
to males, i.e. that T-mediated aggression is advantageous
because it facilitates exclusive access to a mate and/or territory
[8,32,33], then behavioural sensitivity to T seems adaptive in
light of the life history of female tree swallows: heightened
female aggression increases the likelihood of obtaining a nesting cavity [19] and deters floater females that can kill or evict
resident females [64]. Under these circumstances, any female
that is able to increase aggression in response to either social
or seasonal changes in T ought to have a fitness advantage
over a behaviourally insensitive female.
An important caveat is that EET studies run the risk of
falsely concluding that elevated plasma T is the source of
female aggressiveness for three reasons. (i) Some studies use
male-like T doses, and other studies do not report T levels,
drawing into question whether all of these manipulations
should be seen as ecologically relevant. (ii) Some studies use
short-days as a means of hormonally castrating animals, but
enlarged steroid-synthesizing gonads are not the only difference between short- and long-day animals, and seasonal
neural plasticity may also affect how T is processed in the
brain [65]. (iii) EET studies can tell us that aggression is
mediated by T to some degree, but they cannot address
whether natural variation in T in circulation relates to female
aggression. For example, due to extensive interactions among
components of the endocrine system, EET might lead to
enhanced aggression by upregulating AR expression, increasing testosterone-AR binding, or affecting titers of other
hormones that directly mediate aggression [1]. Greater use of
sex-appropriate doses and other experimental manipulations,
such as receptor antagonists or AROM inhibitors, will be
essential to contextualizing these findings in the future.
rstb.royalsocietypublishing.org
males [4,25,33]. However, not all species follow this pattern,
and hypotheses proposed to reconcile these mixed results
have invoked species differences in the relative benefits of
aggression and paternal care (reviewed in [54]). Selection is
thought to favour behavioural insensitivity to T (or, lack of
behavioural response to T) in species in which (i) breeding
seasons are short and temporal partitioning of mating effort
and parental effort may be advantageous (‘single-brood’
or ‘short season’ hypotheses), or where (ii) male care is
especially important for offspring survival (‘essential parental
care’ hypothesis).
Because females often invest relatively more in parental
care than do males [55], this logic might also apply to females
that are faced with trade-offs between maternal care and
aggression. Provided that T is detrimental to maternal care,
then females ought to evolve behavioural insensitivity to T,
such that EET will not impact care or aggression. On the
other hand, if T is beneficial because it facilitates adaptive
physiological or behavioural responses to female–female
competition, then selection may favour behavioural sensitivity, with females responding to EET with an increase in
aggression and a reduction in care [35]. A third alternative
would invoke adaptive modularity of aggression and care,
allowing females to retain sensitivity to T in relation to
aggression but not provisioning [4,5], possibly by altering
receptor abundance in some brain areas and not others.
In most species studied to date, female breeding season
aggression increases in response to EET (see electronic supplementary material, table S1). In some species, certain
aspects of aggression are affected, whereas others are not
(e.g. displays versus overt attacks), suggesting that T influences
aggression, but other mechanisms fine-tune expression of one
or another behaviour (e.g. European robins (Erithacus rubecula)
[56], bank voles (Clethrionomys glareolus) [57]). In songbirds,
EET typically increases the incidence of spontaneous song or
other vocalizations (e.g. European starling (Sturnus vulgaris)
[58]), though these findings should not necessarily be viewed
as increased aggression without contextual information
about song use [56,59]. Interestingly, some exceptions to the
general EET-increase in female aggression suggest potential
links between function and mechanism. For example, female
rats (Rattus norvegicus) are not very aggressive except in the
context of maternal aggression (defense of young), and they
exhibit less female–female aggression in response to EET
[60]. Thus, one possibility is that T-independent mechanisms
may be more applicable when female–female aggression
functions in defense of young (instead of in defense of social
or reproductive benefits), a hypothesis that is ripe for more
rigorous experimental testing as additional species with more
diverse life histories are studied.
Thus, evolution has largely not produced sex-specific
behavioural insensitivity to T in spite of the fact that T
can be particularly costly to females and despite evidence
(from males) that behavioural sensitivity/insensitivity to T
is fairly evolutionarily labile. But, is this broad pattern adaptive? When these data are paired with data on the effects of
EET on parental behaviour, an apparently adaptive view
does begin to emerge. For example, provisioning and incubation are not affected by EET in female juncos [34,61], but
aggressive behaviour is increased [62]. Combined with evidence that male provisioning is reduced by EET in this
species [63], this example shows female-specific adaptive
modularity of behavioural sensitivity to T, suggesting that
Downloaded from http://rstb.royalsocietypublishing.org/ on June 14, 2017
5. Hormonal response to an aggressive challenge
Experimental manipulations using receptor antagonists, RNA
interference (RNAi), and knockouts widely support a causal
relationship between aggression and AR, ER, AROM and
other aspects of target tissue binding and processing of sex
steroids [25]. Comparative studies in males [83] and male–
female comparisons in sex-role reversed species [49] likewise
demonstrate that T and/or sensitivity to T can diverge alongside aggressive behaviour. Comparative analyses of T, AR
and aggression in both sexes are rare, though both T and hypothalamic AR abundance track species and sex differences in
aggression in Sceloporus lizards [84]. Collectively, these studies
suggest that target tissue sensitivity to T is likely to play a role
in the expression of aggression behaviour, though until
recently, there was limited direct evidence to extend these findings to explain individual differences (but see [85–88]), which
is a necessary step in understanding how aggression might
evolve. What evidence is there that individuals within a sex
naturally vary in hormone sensitivity in target tissues, and
does this variation predict functional variation in behaviour
that could be used by selection?
Phil Trans R Soc B 368: 20130083
6. T, sensitivity and natural variation
in aggression
6
rstb.royalsocietypublishing.org
Hormones may affect behaviour, and behaviour also may
alter hormone secretion. Among the best-studied effects of
behaviour on hormones is the transient rise in T associated
with social instability in many male vertebrates (i.e. the challenge hypothesis [32,45 –47]). T is thought to rise in response
to social challenges in order to facilitate physiological and behavioural changes that are adaptive for prolonged social
instability, such as altered stress reactivity, immune function,
metabolism or aggression [33].
While the challenge hypothesis has been well supported
in most vertebrate lineages, recent analyses in male songbirds
suggest that many species do not modulate T in response to
aggression [47,66]. Many of these exceptions occur in species
that show a discrete shift from investment in mating effort to
investment in parental effort, including species with only one
brood, a short breeding season, and/or essential paternal care
[47,66]. Therefore, the same life-history traits and hypotheses
that account for behavioural sensitivity to T also may account
for interspecific variation in T response to aggressive challenges (see §4). If these hypotheses are generally applicable
and therefore apply to females, and if female mechanisms
of behaviour are adaptive (for females), then these mechanisms should reflect the selective forces acting on female
aggression and female life-history trade-offs as well [35].
A review of this literature in females reveals mixed results
that provide key insights into the evolution of hormone–behaviour interrelationships (see electronic supplementary material,
table S2). In some species, breeding density and/or the frequency of aggressive interactions is positively correlated with
T levels in circulation or in egg yolks, suggesting that females
may socially modulate T [67,68]. Notably, though, yolk and
plasma T are not always correlated [69], and so apparent support for the challenge hypothesis assessed via indirect
measures such as eggs should be interpreted with caution.
When female T has been measured in circulation following an
aggressive challenge and compared to unchallenged controls,
only a few species show a significant elevation: cichlids (Neolamprologus pulcher) [70], dunnocks (Prunella modularis) [71],
marmosets (Callithrix kuhlii) [72] and buff-breasted wrens
(Thryothorus leucotis) [73]. Several other studies show no
change in T following aggressive encounters, and some species
that do not elevate T instead show socially induced changes in
progesterone (California mice (Peromyscus californicus) [74],
African black coucals (Centropus grillii) [75], Galapagos
marine iguanas (Amblyrhynchus cristatus) [76]) or corticosterone
(mountain spiny lizards (Sceloporus jarrovi) [77], European
stonechats (Saxicola torquata) [78]). Aggression-induced changes
in progesterone are especially intriguing because they again
suggest overlap with mechanisms of maternal aggression (see
[79]). Another notable finding is that some females exhibit
reduced plasma T levels after aggressive challenges (song sparrows (Melospiza melodia) [80], Eastern bluebirds (Sialia sialis)
[69], Galapagos marine iguanas (A. cristatus), [76]), suggesting
that T is actively removed from circulation during these encounters, probably via liver catabolism or conversion to other
hormones in target tissues.
Even more revealing is that there are four examples
in which males and females from the same species differ in
their hormonal response to aggressive challenges (see electronic supplementary material, table S2), and three of four
show that males elevate T and female do not. While this
pattern may well suffer from publication biases, it suggests
that evolutionary losses of social elevation of T might be
more common in females than males, a result that is consistent with a predicted palliative adaptation permitting females
to be aggressive independently of elevated T. The buffbreasted wren is the one exception where females elevate T
and males do not [73,81], and several details suggest that
female hormone–behaviour relationships are indeed adaptive
to these female wrens [73]. First, T secretion is highly contextspecific: females primarily elevate T levels after female or pair
(female þ male) intrusions during the pre-breeding season
when territorial challenges from floater females are most frequent. Second, the hormonal response seen in females in the
pre-breeding season dissipates in the nestling period, in accordance with the essential parental care hypothesis. This femalespecific social modulation of T is particularly striking in light
of the usual disconnect between T and aggression in other tropical (male) birds [4,82], further suggesting that this pattern of T
secretion relates to selection acting directly on females.
Tests of the challenge hypothesis in females have been
woefully biased towards species with fairly extensive
maternal care, limiting ability to ask whether the extent of
female care maps onto these mechanisms. Further study
of species with different life histories is needed, as there are
many open questions. Are socially induced T surges in
females more common in species with limited female care?
Are they more common when offspring are relatively resistant to parental neglect? Is social modulation adaptive in
females, and does it prepare them for success under persistent social instability, as it is thought to do in males [32,33]?
With strong evidence that female aggression is advantageous
in a number of contexts, such as competition for rank, territories or access to mates, the solution to this issue may lie
in finding the (possibly rare?) species in which female aggression has little to no benefit, and contrasting hormonal
mechanisms with closely related species that benefit from
female aggression.
Downloaded from http://rstb.royalsocietypublishing.org/ on June 14, 2017
Changes in the regulation of gene expression are thought to
be major drivers of phenotypic evolution [90]. Gene regulation (transcription) can be influenced by experience or by
organizational effects of hormones, so differential gene
expression is important in the context of sexual conflict because
it allows potentially adaptive trait modification over the backdrop of the same genome [91,92]. Indeed, sex differences in
gene expression are a common solution to sexual conflict
[21,93], one that is mediated by sex steroids to some degree
[94]. Genes with sexually dimorphic expression also tend to
evolve more rapidly that those without sex biases [95,96], and
this accelerated evolution supports an important stipulation
of sex-specific mechanisms of behaviour because it permits
molecular mechanisms to diverge (adaptively) between the
two sexes.
(a) Molecular mechanisms of aggression
Whole genome expression studies on males demonstrate a
strong connection between aggression and gene regulation at
short time-scales (i.e. immediate response to a rival) and at
evolutionary time-scales (i.e. between species comparisons)
[97,98]. Artificial selection further confirms that divergence
in aggression is accompanied by major changes in gene
expression, including genes whose products have known
roles in mediating aggression (serotonin receptor) or hormonal
response (ER-related transcription repressor) [99,100]. An elegant set of studies on Drosophila showed that only 8% of
genes differed consistently between replicate lines selected
for aggression [99], and only 10% of the aggression-associated
genes from this artificial selection paradigm were also differentially expressed among wild-derived lines that naturally varied
(b) Transcriptional response to androgens
Comparing male and female transcriptional response
to experimental androgen treatment can reveal whether
sex-specific responses to hormones underlie sex-specific
mechanisms of behaviour. Female and male mice that were
gonadectomized (GDX) and treated with dihydrotestosterone
(DHT, which is a non-aromatizable androgen with very high
affinity for AR), however, show comparable transcriptional
changes in liver and adipose tissues, compared to sex-specific
controls [94]. Not surprisingly, many of the genes affected
by DHT are related to lipid and steroid metabolism, and
there is a strong overlap in the list of DHT-affected genes
and sex-biased genes. Network coexpression analyses,
which are representations of genes whose expression is correlated and therefore likely to be co-regulated, show that liver
and muscle tissue have greater modularity in females than
in males, demonstrating some degree of sex-specific gene
regulation in response to hormones, possibly allowing
females to regulate certain sets of genes independently of
others. It may be exactly this sort of molecular mechanism
that allows dominant N. pulcher females to exhibit largely
male-like patterns of gene expression while maintaining
normal female reproductive behaviours (i.e. shifts in an
7
Phil Trans R Soc B 368: 20130083
7. Downstream genomic mechanisms
in aggressiveness (a result not significantly different from
the null expectation) [101]. While all of these genes may not
play a direct role in aggression, experimental manipulations
suggest that many do [101], and these studies collectively
suggest that, among males, there are multiple molecular
routes to greater aggressiveness.
Genome-wide expression analyses also show some similarities in the molecular architecture of male and female aggression.
In the same Drosophila experiment described above [99], there
was strong evidence of correlated responses between the
sexes, at both behavioural and molecular levels: female aggression also changed in response to artificial selection on males,
and 96% of the genes associated with aggression in males
were associated with aggression in females. Male and female
zebrafish (Danio rerio) likewise show similarities in the molecular signatures of aggression/dominance, layered atop sexual
dimorphism in gene expression [102]. Notably, genes associated with the aggressive/dominant phenotype in both sexes
include several related to HPG axis function and sensitivity to
hormones (GnRH, AROM, AR and some ERs). In the cichlid
fish N. pulcher, dominant females have high T and are also masculinized in neural gene expression, though they exhibit normal
female reproductive behaviours [92]. Thus, the neurogenomic
signature of the aggressive/dominant phenotype has strong
overlap in the two sexes, and cluster analyses reveal sex-specific
mechanisms as well. A similar conclusion was reached by
comparing neural gene expression from one cichlid fish
(Julidochromis transcriptus) that exhibits the typical pattern of
male dominance and territoriality to a close congener, the sexrole reversed J. marlieri [103]. The large, aggressive dominant
females from the sex-role reversed species were more similar
in gene expression to the dominant males from the other species
than they were to conspecific males or congeneric females.
Again, differentially expressed genes include those known to
affect aggression (isotocin, AROM), and the upregulation of
neural AROM in role-reversed females lends support to the
hypothesis that females may facilitate aggression by changes
in target tissue processing of T.
rstb.royalsocietypublishing.org
Using dark-eyed juncos, colleagues and I contrasted males
and females in the degree to which T levels and neural sensitivity to T predict individual differences in aggression [88].
Using plasma and tissues collected from free-living males
and females just after a short simulated territorial intrusion,
we showed that neural gene expression for AR, ERa and
AROM predicted individual differences in aggression in both
sexes, but circulating T levels related to aggression only in
males. In addition, the sexes did not differ in variance in transcript abundance in any gene or brain area, but male T levels
had significantly higher variance than female T levels. Building
upon an evolutionary framework in which variance in quantitative traits is proportional to the potential strength of selection,
these data support the view that the potential for selection to
change aggressiveness via shifts in T levels may be higher in
males, whereas the potential for selection to change aggressiveness via shifts in AR, ERa or AROM expression may be equal in
the two sexes. Future studies should extend this line of research
to investigate (i) how sources of variation in female aggression
change as populations diverge and (ii) the degree to which
mechanisms of female aggression can change independently
of mechanisms of male aggression. Some of our recent work
on male juncos suggests that the endocrine mechanisms that
account for subspecies differences in aggression are not
simply extensions of intra-populations mechanisms [89], and
similar comparative approaches applied to females should
be quite informative in understanding how mechanisms of
behaviour evolve.
Downloaded from http://rstb.royalsocietypublishing.org/ on June 14, 2017
There are hurdles to overcome in assessing how individual
males and females vary in these molecular mechanisms.
Most notably, the establishment of repeatable individual
differences requires repeated sampling, but most studies
use tissues that cannot be resampled (brain, liver, etc.). The
use of commercial-grade arrays [104] and the discontinuation
of the practice of pooling individuals [99] mean that we are
well poised to learn from these individual differences,
should we choose to analyze them. Cluster analyses and heatmaps often do reveal high variability among individuals
within a population, sex or morph [92,102,104]. A handful
of studies also report that individual variation is large compared to inter-population variation in gene expression, and
at least some proportion of individual variation is genetic
[105]. ‘Understanding what genes do in their natural context
is a key issue’ [106, p. 10], and an increased focus on the individual at the molecular level will be critical to understanding
how complex behaviours such as aggression evolve.
8. Do shared mechanisms represent homology?
As we attempt to synthesize the above observations that
some hormonal and molecular mechanisms are the same in
the two sexes, while others are different, another key question
comes into focus: ‘What exactly do we mean by sameness?’
[107, p. 436]. When males and females repeatedly use the
same mechanisms across a wide range of species, can we
tell if the sexes have arrived at similar mechanisms because
they share an ancient homology, because they have coevolved using a derived mechanism, or because they have
independently co-opted common mechanisms?
While there seem to be almost infinite mechanisms by
which one or another trait might evolve, life repeatedly uses
the same mechanisms [13,107–110]. Many of the studies
reviewed here support this ‘toolkit’ or ‘hotspot’ view of
evolution—aggressive behaviour has again and again been
9. Conclusion
This review contrasts mechanisms of aggression in females
with those of males, focusing on variation in T production, as
well as target tissue hormonal and transcriptional responses.
Because T can be particularly costly to females, but aggression
has repeatedly been shown to be advantageous, I predicted
that selection would favour female mechanisms of aggression
that use variation at the level of target tissues (e.g. sensitivity,
metabolism, or genomic responses) more so than mechanisms
that require systemically elevated T. The literature supported
this prediction to some degree, though both sexes clearly mediate aggression at all of these levels. Some evidence suggested
that female mechanisms track male mechanisms, consistent
with correlated evolutionary responses between the sexes,
8
Phil Trans R Soc B 368: 20130083
(c) Individual variation in molecular mechanisms
associated with variation in the same mechanisms, including
T levels, abundance of AR, ER or AROM, and gene expression
related to production of or sensitivity to sex steroids. Surely, it
is most parsimonious to assume that sex similarities in mechanism are homologous. On the other hand, ‘mechanistic drift’
does occur in nature—even arthropod legs, which are well
established to be homologous structures, are not produced
via the same mechanisms in flies and beetles (for details, see
[107]). Thus, if there is ample genetic variation in a particular
gene in two lineages and the product of this gene is behaviourally relevant, then each lineage may independently arrive at
similar mechanisms of behaviour [109,110]
It seems reasonable to imagine that this logic could apply to
males and females for whom parts of their genome are differentially regulated and may evolve somewhat independently.
But, how does this fit with the coevolutionary dynamics
and predicted palliative adaptation discussed above (§2)? If
mechanisms are homologous in the two sexes, and represent
correlated evolutionary responses, then the sexes should not
simply share one determinant of phenotype (e.g. one gene,
one receptor, or one hormone), but they also ought to share
full networks of gene regulation [108] or multiple mechanisms
along the cascade linking the endocrine system with behaviour [3,5]. Further analyses of individual variation in gene
expression (and sex comparisons of that variation) in noninbred organisms will be particularly important in resolving
this issue, as will network analyses that assess interactions
among co-regulated genes [105]. Deviations from full overlap
at multiple levels of the endocrine system will be informative
in understanding why evolution might use one mechanism
instead of another. That is, if we accept that mechanistic drift is
possible, then adaptive changes in mechanisms (‘mechanistic
selection’) ought to occur to suit one or another sex, yielding
neuroendocrine and genomic mechanisms of female behaviour
that are adaptive for females, and vice versa. Notably, any
palliative adaptation on the part of females might also lead to
new dimensions of conflict and/or coevolution with males
(figure 2, far right panel). If derived mechanisms of aggression
break the cycle of conflict by reducing deleterious effects, then
these adaptations should be advantageous for both parties and
they should therefore take hold in the population [24]. Consistent
with this view, many of mechanisms of aggression predicted
to be beneficial for females have also been reported in males in
contexts when aggression is advantageous but elevated T is
not [38–40], though more research is needed to determine the
sequence of adaptations leading to this outcome.
rstb.royalsocietypublishing.org
‘aggression module’ of genes that occur independently of
shifts in a ‘reproduction module’ [92]).
Notably, the GDX male and female mice described above
were treated with the same (male-like) androgen dose. While
this approach can tell us how male-like hormone levels
affect tissues with male and female genomic backgrounds, it
cannot tell us whether androgen levels that are at the high
end of the sex-appropriate physiological range also engender
similar effects in the two sexes. Peterson and colleagues [104]
reported such a study in the dark-eyed junco and come
to a somewhat different conclusion, looking at neural tissue.
T-treatment affected expression of many genes that have
been functionally linked with behaviour, including AROM
and monoamine oxidase A, among others, but males and
females had very little overlap in the specific genes affected
by T-treatment (,1%). While future studies are needed to
explore alternative explanations for the difference between
this study and the mouse study above (e.g. dosage effects,
mammals versus birds, GDX versus intact, etc.), these results
provide some of the first direct evidence that males and females
can fine-tune behavioural mechanisms via sex-specific effects
of T on particular genes. A key next question is whether
these divergent transcriptional responses are adaptively
suited to each sex.
Downloaded from http://rstb.royalsocietypublishing.org/ on June 14, 2017
Acknowledgements. Many thanks to E. D. Ketterson, G. E. Demas,
M. P. Peterson, K. E. Cain, C. M. Bergeon Burns, A. Campbell and
two anonymous reviewers for critical feedback, and to the guest
editors A. Campbell and P. Stockley for inviting this contribution.
Funding statement. K.A.R. supported by NIH R21HD073583.
References
1.
2.
3.
4.
5.
6.
Zera AJ, Harshman LG, Williams TD. 2007
Evolutionary endocrinology: the developing
synthesis between endocrinology and evolutionary
genetics. Annu. Rev. Ecol. Evol. Syst. 38, 793–817.
(doi:10.1146/annurev.ecolsys.38.091206.095615)
Adkins-Regan E. 2012 Hormonal organization and
activation: evolutionary implications and questions.
Gen. Comp. Endocrinol. 176, 279–285.
(doi:10.1016/j.ygcen.2011.12.040)
Adkins-Regan E. 2008 Do hormonal control
systems produce evolutionary inertia? Phil.
Trans. R. Soc. B 363, 1599 –1609. (doi:10.1098/
rstb.2007.0005)
Hau M. 2007 Regulation of male traits by
testosterone: implications for the evolution of
vertebrate life histories. BioEssays 29, 133–144.
(doi:10.1002/bies.20524)
Ketterson ED, Atwell JW, McGlothlin JW. 2009
Phenotypic integration and independence:
hormones, performance, and response to
environmental change. Integr. Comp. Biol. 49,
365–379. (doi:10.1093/icb/icp057)
Williams TD. 2008 Individual variation in endocrine
systems: moving beyond the ‘tyranny of the Golden
Mean’. Phil. Trans. R. Soc. B 363, 1687 –1698.
(doi:10.1098/rstb.2007.0003)
7.
8.
9.
10.
11.
12.
13.
Kempenaers B, Peters A, Foerster K. 2008 Sources of
individual variation in plasma testosterone levels.
Phil. Trans. R. Soc. B 363, 1711–1723.
(doi:10.1098/rstb.2007.0001)
Ketterson ED, Nolan V. 1999 Adaptation, exaptation,
and constraint: a hormonal perspective. Am. Nat.
154, S4 –S25.
Bennett AF. 1987 Interindividual variability: an
underutilized resource. In New directions in
ecological physiology (eds ME Feder, AF Bennett,
WW Burggren, RB Huey), pp. 147 –169. Cambridge,
UK: Cambridge University Press.
Ball GF, Balthazart J. 2008 Individual variation and the
endocrine regulation of behaviour and physiology in
birds: a cellular/molecular perspective. Phil. Trans. R.
Soc. B 363, 1699–1710. (doi:10.1098/rstb.2007.0010)
McGlothlin JW, Ketterson ED. 2008 Hormonemediated suites as adaptations and evolutionary
constraints. Phil. Trans. R. Soc. B 363, 1611–1620.
(doi:10.1098/rstb.2007.0002)
Lande R. 1979 Quantitative genetic-analysis of
multivariate evolution, applied to brain-body size
allometry. Evolution 33, 402–416. (doi:10.2307/
2407630)
O’Connell LA, Hofmann HA. 2012 Evolution of
a vertebrate social decision-making network.
14.
15.
16.
17.
18.
19.
Science 336, 1154– 1157. (doi:10.1126/science.
1218889)
Clutton-Brock T. 2009 Sexual selection in females.
Anim. Behav. 77, 3– 11. (doi:10.1016/j.anbehav.
2008.08.026)
Stockley P, Bro-Jørgensen J. 2011 Female
competition and its evolutionary consequences in
mammals. Biol. Rev. 86, 341 –366. (doi:10.1111/j.
1469-185X.2010.00149.x)
Rosvall KA. 2011 Intrasexual competition
in females: evidence for sexual selection?
Behav. Ecol. 22, 1131 –1140. (doi:10.1093/
beheco/arr106)
Tobias JA, Montgomerie R, Lyon BE. 2012 The
evolution of female ornaments and weaponry:
social selection, sexual selection and ecological
competition. Phil. Trans. R. Soc. B 367, 2274–2293.
(doi:10.1098/rstb.2011.0280)
While GM, Isaksson C, McEvoy J, Sinn DL, Komdeur J,
Wapstra E, Groothuis TGG. 2010 Repeatable intraindividual variation in plasma testosterone
concentration and its sex-specific link to aggression in
a social lizard. Horm. Behav. 58, 208–213. (doi:10.
1016/j.yhbeh.2010.03.016)
Rosvall KA. 2008 Sexual selection on aggressiveness
in females: evidence from an experimental test with
9
Phil Trans R Soc B 368: 20130083
without changes in circulating T, i.e. via neurosteroidogenesis, or modified transcriptional responses to social stimuli?
Do the sexes differ in the degree to which rapid (nongenomic) effects [28] or binding globulins [29] modulate
behaviour? Also needed are investigations of the mechanisms
that engender these individual, sex, and species differences,
and organizational effects of sex steroids are an obvious
candidate [2].
Of particular note is the shortage of analyses that contrast
the pool of individual variation in males and females, an
approach that can reveal whether the endocrine system harbours the same raw material for phenotypic evolution in
the two sexes. An individual-based approach is also essential
to relating mechanistic variation to natural selection in the
wild. In the junco, for example, individual differences in
HPG axis reactivity [51] and gene expression related to
neural sensitivity to T [88] predict individual differences
in aggressiveness, and more aggressive females are more
likely to successfully fledge offspring [51], suggesting a selection gradient on these endocrine parameters. I hope that this
review will stimulate further use of these individual-based
approaches to shed light on the mechanisms by which
behaviour evolves. After all, ‘real individuals are unique
combinations of traits, some above and some below average.
It is time to recognize the uniqueness of the individual and to
turn it to our advantage as biologists.’ [9, p. 161].
rstb.royalsocietypublishing.org
e.g. intersexual correlations in T levels, overlap in molecular
mechanisms of aggression. But other patterns were more
consistent with selection acting on females themselves, e.g.
apparently adaptive context specificity of T levels or social
regulation of T, behavioural modularity in response to EET,
and some divergence in transcriptional responses to sexspecific elevations in T. I reviewed several studies on the
organism that I study, the dark-eyed junco, combining results
across multiple levels of analyses. These studies collectively
showed that males and females share many of the same sources
of hormonal and behavioural variation, but male and female
juncos diverge in aspects of HPG axis regulation, in the
degree to which circulating T relates to aggression, and in
their response to T, both behaviourally and transcriptionally.
These findings provide a novel dataset in support of the hypothesis that the sexes can implement somewhat independent
mechanisms of behaviour, despite sharing the same genome.
A few limitations of this review highlight critical next
steps for research. Most importantly, phylogenetic representation and diversity of female life history in study species is
remarkably inadequate. Second, many questions have not
yet been sufficiently asked or answered. Future studies
should ensure that endocrine parameters and behaviour are
assessed in an evolutionarily relevant context by carefully
selecting the study system, dosage of hormone treatments
and methods for assessing female– female aggression.
Third, this review has addressed many connections between
behaviour and the endocrine system, but it has not addressed
them all. For example, do socially mediated T elevations in
females enable physiological or behavioural priming for success in future social instability (as hypothesized in males), or
have females found mechanisms to accomplish this endpoint
Downloaded from http://rstb.royalsocietypublishing.org/ on June 14, 2017
21.
22.
24.
25.
26.
27.
28.
29.
30.
31.
32.
33.
34.
35.
37.
38.
39.
40.
41.
42.
43.
44.
45.
46.
47.
48.
49.
50.
51.
52.
53.
54.
55.
56.
57.
58.
59.
60.
61.
62.
(Centropus grillii). Dev. Neurobiol. 67, 1560– 1573.
(doi:10.1002/dneu.20528)
Jawor JM, McGlothlin JW, Casto JM, Greives TJ,
Snajdr EA, Bentley GE, Ketterson ED. 2006 Seasonal
and individual variation in response to GnRH
challenge in male dark-eyed juncos (Junco
hyemalis). Gen. Comp. Endocrinol. 149, 182–189.
(doi:10.1016/j.ygcen.2006.05.013)
Cain KE, Ketterson ED. 2012 Competitive females are
successful females; phenotype, mechanism, and
selection in a common songbird. Behav. Ecol.
Sociobiol. 66, 241–252. (doi:10.1007/s00265011-1272-5)
McGlothlin JW, Jawor JM, Ketterson ED. 2007
Natural variation in a testosterone-mediated
trade-off between mating effort and parental
effort. Am. Nat. 170, 864– 875. (doi:10.1086/
522838)
While GM, Uller T, Wapstra E. 2011 Variation in
social organization influences the opportunity for
sexual selection in a social lizard. Mol. Ecol. 20,
844–852. (doi:10.1111/j.1365-294X.2010.04976.x)
Lynn SE. 2008 Behavioral insensitivity to
testosterone: why and how does testosterone alter
paternal and aggressive behavior in some avian
species but not others? Gen. Comp. Endocrinol. 157,
233–240.
Trivers RL. 1972 Parental investment and sexual
selection. In Sexual selection and the descent of
man, 1871 –1971 (ed. B Cambell), pp. 136–179.
London, UK: Heinemann.
Kriner E, Schwabl H. 1991 Control of winter song
and territorial aggression of female robins (Erithacus
rubecula) by testosterone. Ethology 87, 37– 44.
(doi:10.1111/j.1439-0310.1991.tb01186.x)
Kapusta J. 1998 Gonadal hormones and intrasexual
aggressive behavior in female bank voles
(Clethrionomys glareolus). Aggress. Behav. 24,
63– 70. (doi:10.1002/(sici)1098-2337(1998)
24:1,63::aid-ab6.3.0.co;2-t)
Sandell MI. 2007 Exogenous testosterone increases
female aggression in the European starling (Sturnus
vulgaris). Behav. Ecol. Sociobiol. 62, 255 –262.
(doi:10.1007/s00265-007-0460-9)
Searcy WA. 1988 Do female red-winged blackbirds
limit their own breeding densities? Ecology 69,
85– 95. (doi:10.2307/1943163)
Debold JF, Miczek KA. 1981 Sexual dimorphism in
the hormonal-control of aggressive behavior of
rats. Pharmacol. Biochem. Behav. 14, 89 – 93.
(doi:10.1016/0091-3057(81)90108-8)
O’Neal DM, Reichard DG, Pavilis K, Ketterson ED.
2008 Experimentally-elevated testosterone, female
parental care, and reproductive success in a
songbird, the dark-eyed junco (Junco hyemalis).
Horm. Behav. 54, 571 –578. (doi:10.1016/j.yhbeh.
2008.05.017)
Zysling DA, Greives TJ, Breuner CW, Casto JM,
Demas GE, Ketterson ED. 2006 Behavioral and
physiological responses to experimentally elevated
testosterone in female dark-eyed juncos (Junco
hyemalis carolinensis). Horm. Behav. 50, 200 –207.
(doi:10.1016/j.yhbeh.2006.03.004)
10
Phil Trans R Soc B 368: 20130083
23.
36.
maternal care co-occur. PLoS ONE 8, e54120.
(doi:10.1371/journal.pone.0054120)
Gerlach NM, Ketterson ED. 2013 Experimental
elevation of testosterone lowers fitness in female
dark-eyed juncos. Horm. Behav. 63, 782–790.
(doi:10.1016/j.yhbeh.2013.03.005)
McGlothlin JW, Neudorf DLH, Casto JM, Nolan V,
Ketterson ED. 2004 Elevated testosterone reduces
choosiness in female dark-eyed juncos (Junco
hyemalis): evidence for a hormonal constraint on
sexual selection? Proc. R. Soc. B 271, 1377–1384.
(doi:10.1098/rspb.2004.2741)
Mukai M, Replogle K, Drnevich J, Wang G, Wacker
D, Band M, Clayton DF, Wingfield JC. 2009 Seasonal
differences of gene expression profiles in song
sparrow (Melospiza melodia) hypothalamus in
relation to territorial aggression. PLoS ONE 4, e8182.
(doi:10.1371/journal.pone.0008182)
Soma KK, Scotti MAL, Newman AEM, Charlier TD,
Demas GE. 2008 Novel mechanisms for
neuroendocrine regulation of aggression. Front.
Neuroendocrinol. 29, 476– 489. (doi:10.1016/j.yfrne.
2007.12.003)
Canoine V, Fusani L, Schlinger B, Hau M. 2007 Low
sex steroids, high steroid receptors: increasing the
sensitivity of the nonreproductive brain. Dev.
Neurobiol. 67, 57– 67. (doi:10.1002/dneu.20296)
Møller AP, Garamszegi LZ, Gil D, Hurtrez-Bousses S,
Eens M. 2005 Correlated evolution of male and
female testosterone profiles in birds and its
consequences. Behav. Ecol. Sociobiol. 58, 534– 544.
(doi:10.1007/s00265-005-0962-2)
Mank JE. 2007 The evolution of sexually selected
traits and antagonistic androgen expression in
actinopterygiian fishes. Am. Nat. 169, 142– 149.
(doi:10.1086/510103)
Bergeon Burns CM, Rosvall KA, Hahn TP, Demas GE,
Ketterson ED. In press. Examining sources of
variation in HPG axis function among individuals
and populations of the dark-eyed junco.
Horm. Behav.
Rosvall KA, Bergeon Burns CM, Hahn TP,
Ketterson ED. In press. Sources of variation in HPG
axis reactivity and individually consistent elevation
of sex steroids in a female songbird. Gen. Comp.
Endocrinol.
Oliveira RF, Hirschenhauser K, Carneiro LA, Canario
AVM. 2002 Social modulation of androgen levels in
male teleost fish. Comp. Biochem. Physiol. B 132,
203 –215. (doi:10.1016/s1096-4959(01)00523-1)
Hirschenhauser K, Oliveira RF. 2006 Social modulation
of androgens in male vertebrates: meta-analyses of
the challenge hypothesis. Anim. Behav. 71, 265–
277. (doi:10.1016/j.anbehav.2005.04.014)
Goymann W. 2009 Social modulation of androgens
in male birds. Gen. Comp. Endocrinol. 163,
149 –157. (doi:10.1016/j.ygcen.2008.11.027)
Eens M, Pinxten R. 2000 Sex-role reversal in
vertebrates: behavioural and endocrinological
accounts. Behav. Process. 51, 135 –147.
(doi:10.1016/S0376-6357(00)00124-8)
Voigt C, Goymann W. 2007 Sex-role reversal is
reflected in the brain of African black coucals
rstb.royalsocietypublishing.org
20.
tree swallows. Anim. Behav. 75, 1603 –1610.
(doi:10.1016/j.anbehav.2007.09.038)
Wallace AR. 1891 Natural selection and tropical
nature. London, UK: Macmillan and Co.
Ellegren H, Parsch J. 2007 The evolution of
sex-biased genes and sex-biased gene expression.
Nat. Rev. Genet. 8, 689–698. (doi:10.1038/
nrg2167)
Lande R. 1980 Sexual dimorphism, sexual selection,
and adaptation in polygenic characters. Evolution
34, 292–305. (doi:10.2307/2407393)
Rice WR. 2000 Dangerous liaisons. Proc. Natl Acad.
Sci. USA 97, 12 953–12 955. (doi:10.1073/pnas.97.
24.12953)
Lessells CM. 2006 The evolutionary outcome of
sexual conflict. Phil. Trans. R. Soc. B 361, 301–317.
(doi:10.1098/rstb.2005.1795).
Adkins-Regan E. 2005 Hormones and animal social
behavior. Princeton, NJ: Princeton University Press.
Staub NL, DeBeer M. 1997 The role of androgens in
female vertebrates. Gen. Comp. Endocrinol. 108,
1– 24. (doi:10.1006/gcen.1997.6962)
Ketterson ED, Nolan V, Sandell M. 2005 Testosterone
in females: mediator of adaptive traits, constraint
on sexual dimorphism, or both? Am. Nat. 166,
S85–S98.
Cornil CA. 2009 Rapid regulation of brain oestrogen
synthesis: the behavioural roles of oestrogens and
their fates. J. Neuroendocrinol. 21, 217–226.
(doi:10.1111/j.1365-2826.2009.01822.x)
Malisch JL, Breuner CW. 2010 Steroid-binding
proteins and free steroids in birds. Mol. Cell.
Endocrinol. 316, 42 –52. (doi:10.1016/j.mce.2009.
09.019)
Wingfield JC. 2012 Regulatory mechanisms that
underlie phenology, behavior, and coping with
environmental perturbations: an alternative look at
biodiversity. Auk 129, 1–7. (doi:10.1525/auk.2012.
129.1.1)
McEwan IJ. 2004 Molecular mechanisms of
androgen receptor-mediated gene regulation:
structure-function analysis of the AF-1 domain.
Endocr. Relat. Cancer 11, 281–293. (doi:10.1677/
erc.0.0110281)
Wingfield JC, Hegner RE, Dufty AM, Ball GF. 1990
The challenge hypothesis—theoretical implications
for patterns of testosterone secretion, mating
systems, and breeding strategies. Am. Nat. 136,
829–846. (doi:10.1086/285134)
Wingfield JC, Lynn SE, Soma KK. 2001 Avoiding the
‘costs’ of testosterone: ecological bases of
hormone –behavior interactions. Brain Behav. Evol.
57, 239–251. (doi:10.1159/000047243)
Clotfelter ED, O’Neal DM, Gaudioso JM, Casto JM,
Parker-Renga IM, Snajdr EA, Duffy DL, Nolan V,
Ketterson ED. 2004 Consequences of elevating
plasma testosterone in females of a socially
monogamous songbird: evidence of constraints on
male evolution? Horm. Behav. 46, 171 –178.
(doi:10.1016/j.yhbeh.2004.03.003)
Rosvall KA. 2013 Life history trade-offs and
behavioral sensitivity to testosterone: an
experimental test when female aggression and
Downloaded from http://rstb.royalsocietypublishing.org/ on June 14, 2017
89. Bergeon Burns CM, Rosvall KA, Ketterson ED. 2013
Neural steroid sensitivity and aggression: comparing
individuals of two songbird subspecies. J. Evol. Biol.
26, 820 –831. (doi:10.1111/jeb.12094)
90. Carroll SB. 2005 Evolution at two levels: on genes
and form. PLoS Biol. 3, e245. (doi:10.1371/journal.
pbio.0030245)
91. Bell AM, Robinson GE. 2011 Behavior and the
dynamic genome. Science 332, 1161–1162.
(doi:10.1126/science.1203295)
92. Aubin-Horth N, Desjardins JK, Martei YM,
Balshine S, Hofmann HA. 2007 Masculinized
dominant females in a cooperatively breeding
species. Mol. Ecol. 16, 1349– 1358. (doi:10.1111/
j.1365-294X.2007.03249.x)
93. Mank JE, Wedell N, Hosken DJ. 2013 Polyandry
and sex-specific gene expression. Phil. Trans.
R. Soc. B 368. 20120047. (doi:10.1098/rstb.
2012.0047)
94. van Nas A et al. 2009 Elucidating the role of
gonadal hormones in sexually dimorphic gene
coexpression networks. Endocrinology 150,
1235– 1249. (doi:10.1210/en.2008-0563)
95. Mank JE, Hultin-Rosenberg L, Axelsson E, Ellegren H.
2007 Rapid evolution of female-biased, but not
male-biased, genes expressed in the avian brain.
Mol. Biol. Evol. 24, 2698–2706. (doi:10.1093/
molbev/msm208)
96. Naurin S, Hansson B, Hasselquist D, Kim YH,
Bensch S. 2011 The sex-biased brain: sexual
dimorphism in gene expression in two species
of songbirds. BMC Genom. 12, 37. (doi:10.1186/
1471-2164-12-37)
97. Sanogo YO, Band M, Blatti C, Sinha S, Bell AM.
2012 Transcriptional regulation of brain gene
expression in response to a territorial intrusion.
Proc. R. Soc. B 279, 4929– 4938. (doi:10.1098/rspb.
2012.2087)
98. Alaux C et al. 2009 Honey bee aggression supports
a link between gene regulation and behavioral
evolution. Proc. Natl Acad. Sci. USA 106, 15 400–
15 405. (doi:10.1073/pnas.0907043106)
99. Edwards AC, Rollmann SM, Morgan TJ, Mackay TFC.
2006 Quantitative genomics of aggressive
behavior in Drosophila melanogaster. PloS Genet.
2, 1386 –1395. (doi:10.1371/journal.pgen.
0020154)
100. Kukekova AV et al. 2011 Sequence comparison
of prefrontal cortical brain transcriptome from a
tame and an aggressive silver fox (Vulpes vulpes).
BMC Genom. 12, 482. (doi:10.1186/1471-216412-482)
101. Edwards AC, Ayroles JF, Stone EA, Carbone MA,
Lyman RF, Mackay TFC. 2009 A transcriptional
network associated with natural variation in
Drosophila aggressive behavior. Genom. Biol. 10,
R76. (doi:10.1186/gb-2009-10-7-r76)
102. Filby AL, Paull GC, Hickmore TFA, Tyler CR. 2010
Unravelling the neurophysiological basis of
aggression in a fish model. BMC Genom. 11, 498.
(doi:10.1186/1471-2164-11-498)
103. Schumer M, Krishnakant K, Renn SCP. 2011
Comparative gene expression profiles for highly
11
Phil Trans R Soc B 368: 20130083
76. Rubenstein DR, Wikelski M. 2005 Steroid hormones
and aggression in female Galapagos marine
iguanas. Horm. Behav. 48, 329– 341. (doi:10.1016/
j.yhbeh.2005.04.006)
77. Woodley SK, Matt KS, Moore MC. 2000
Neuroendocrine responses in free-living female and
male lizards after aggressive interactions. Physiol.
Behav. 71, 373 –381. (doi:10.1016/S0031-9384(00)
00345-0)
78. Canoine V, Gwinner E. 2005 The hormonal response
of female European stonechats to a territorial
intrusion: the role of the male partner. Horm. Behav.
47, 563–568. (doi:10.1016/j.yhbeh.2004.12.007)
79. Bosch OJ. 2013 Maternal aggression in rodents:
brain oxytocin and vasopressin mediate pup
defence. Phil. Trans. R. Soc. B 368, 20130085.
(doi:10.1098/rstb.2013.0085)
80. Elekonich MM, Wingfield JC. 2000 Seasonality and
hormonal control of territorial aggression in female
song sparrows (Passeriformes: Emberizidae:
Melospiza melodia). Ethology 106, 493–510.
(doi:10.1046/j.1439-0310.2000.00555.x)
81. Gill SA, Costa LM, Hau M. 2008 Males of a
single-brooded tropical bird species do not show
increases in testosterone during social challenges.
Horm. Behav. 54, 115–124. (doi:10.1016/j.yhbeh.
2008.02.003)
82. Goymann W, Moore IT, Scheuerlein A,
Hirschenhauser K, Grafen A, Wingfield JC. 2004
Testosterone in tropical birds: effects of
environmental and social factors. Am. Nat. 164,
327 –334. (doi:10.1086/422856)
83. Dijkstra PD, Verzijden MN, Groothuis TGG,
Hofmann HA. 2012 Divergent hormonal responses
to social competition in closely related species of
haplochromine cichlid fish. Horm. Behav. 61,
518 –526. (doi:10.1016/j.yhbeh.2012.01.011)
84. Hews DK, Hara E, Anderson MC. 2012 Sex and
species differences in plasma testosterone and in
counts of androgen receptor-positive cells in key
brain regions of Sceloporus lizard species that
differ in aggression. Gen. Comp. Endocrinol. 176,
493 –499. (doi:10.1016/j.ygcen.2011.12.028)
85. Schlinger BA, Callard GV. 1989 Aromatase-activity in
quail brain—correlation with aggressiveness.
Endocrinology 124, 437–443. (doi:10.1210/endo124-1-437)
86. Silverin B, Baillien M, Balthazart J. 2004 Territorial
aggression, circulating levels of testosterone, and
brain aromatase activity in free-living pied
flycatchers. Horm. Behav. 45, 225 –234.
(doi:10.1016/j.yhbeh.2003.10.002)
87. Trainor BC, Greiwe KM, Nelson RJ. 2006 Individual
differences in estrogen receptor alpha in select brain
nuclei are associated with individual differences in
aggression. Horm. Behav. 50, 338–345.
(doi:10.1016/j.yhbeh.2006.04.002)
88. Rosvall KA, Bergeon Burns CM, Barske J,
Goodson JL, Schlinger BA, Sengelaub DR,
Ketterson ED. 2012 Neural sensitivity to sex steroids
predicts individual differences in aggression:
implications for behavioural evolution. Proc. R. Soc.
B 279, 3547– 3555. (doi:10.1098/rspb.2012.0442)
rstb.royalsocietypublishing.org
63. Ketterson ED, Nolan V, Wolf L, Ziegenfus C. 1992
Testosterone and avian life histories—effects of
experimentally elevated testosterone on behavior
and correlates of fitness in the dark-eyed junco
(Junco hyemalis). Am. Nat. 140, 980–999.
(doi:10.1086/285451)
64. Leffelaar D, Robertson RJ. 1985 Nest usurpation and
female competition for breeding opportunities by
tree swallows. Wilson Bull. 97, 221–224.
65. Demas GE, Cooper MA, Albers HE, Soma KK. 2007
Novel mechanisms underlying neuroendocrine
regulation of aggression: a synthesis of rodent, avian,
and primate studies. In Handbook of neurochemistry
and molecular neurobiology (ed. JD Blaustein),
pp. 337–372. New York, NY: Springer.
66. Goymann W, Landys MM, Wingfield JC. 2007
Distinguishing seasonal androgen responses from
male– male androgen responsiveness—revisiting
the challenge hypothesis. Horm. Behav. 51,
463–476. (doi:10.1016/j.yhbeh.2007.01.007)
67. Chapman JC, Christian JJ, Pawlikowski MA,
Michael SD. 1998 Analysis of steroid hormone levels
in female mice at high population density. Physiol.
Behav. 64, 529–533. (doi:10.1016/S0031-9384
(98)00115-2)
68. Whittingham LA, Schwabl H. 2002 Maternal
testosterone in tree swallow eggs varies with
female aggression. Anim. Behav. 63, 63– 67.
(doi:10.1006/anbe.2001.1889)
69. Navara KJ, Siefferman LM, Hill GE, Mendonca MT.
2006 Yolk androgens vary inversely to maternal
androgens in eastern bluebirds: an experimental
study. Funct. Ecol. 20, 449–456. (doi:10.1111/j.
1365-2435.2006.01114.x)
70. Desjardins JK, Hazelden MR, Van der Kraak GJ,
Balshine S. 2006 Male and female cooperatively
breeding fish provide support for the ‘Challenge
Hypothesis’. Behav. Ecol. 17, 149– 154.
(doi:10.1093/beheco/arj018)
71. Langmore NE, Cockrem JF, Candy EJ. 2002
Competition for male reproductive investment
elevates testosterone levels in female dunnocks,
Prunella modularis. Proc. R. Soc. B 269,
2473–2478. (doi:10.1098/rspb.2002.2167)
72. Ross CN, French JA. 2011 Female marmosets’
behavioral and hormonal responses to unfamiliar
intruders. Am. J. Primatol. 73, 1072 –1081.
(doi:10.1002/ajp.20975)
73. Gill SA, Alfson ED, Hau M. 2007 Context
matters: female aggression and testosterone in
a year-round territorial neotropical songbird
(Thryothorus leucotis). Proc. R. Soc. B 274, 2187–
2194. (doi:10.1098/rspb.2007.0457)
74. Davis ES, Marler CA. 2003 The progesterone
challenge: steroid hormone changes following a
simulated territorial intrusion in female Peromyscus
californicus. Horm. Behav. 44, 185 –198.
(doi:10.1016/s0018-506x(03)00128-4)
75. Goymann W, Wittenzellner A, Schwabl I, Makomba
M. 2008 Progesterone modulates aggression in sexrole reversed female African black coucals.
Proc. R. Soc. B 275, 1053 –1060. (doi:10.1098/rspb.
2007.1707)
Downloaded from http://rstb.royalsocietypublishing.org/ on June 14, 2017
ecology and evolution of personality? Phil.
Trans. R. Soc. B 365, 4001 –4012. (doi:10.1098/rstb.
2010.0185)
109. Whitehead A. 2012 Comparative genomics in
ecological physiology: toward a more nuanced
understanding of acclimation and adaptation.
J. Exp. Biol. 215, 884– 891. (doi:10.1242/
jeb.058735)
110. Martin A, Orgogozo V. 2013 The loci of repeated
evolution: a catalog of genetic hotspots of
phenotypic variation. Evolution 67, 1235–1250.
(doi:10.1111/evo.12081)
12
Phil Trans R Soc B 368: 20130083
material for evolution. Mol. Ecol. 15, 1197–1211.
(doi:10.1111/j.1365-294X.2006.02868.x)
106. St-Cyr S, Aubin-Horth N. 2009 Integrative and
genomics approaches to uncover the mechanistic
bases of fish behavior and its diversity. Comp.
Biochem. Physiol. Part A Mol. Integr. Physiol. 152,
9 –21. (doi:10.1016/j.cbpa.2008.09.003)
107. Moczek AP. 2008 On the origins of novelty in
development and evolution. BioEssays 30,
432 –447. (doi:10.1002/bies.20754)
108. Bell AM, Aubin-Horth N. 2010 What can whole
genome expression data tell us about the
rstb.royalsocietypublishing.org
similar aggressive phenotypes in male and female
cichlid fishes (Julidochromis). J. Exp. Biol. 214,
3269–3278. (doi:10.1242/jeb.055467)
104. Peterson MP, Rosvall KA, Choi J, Ziegenfus C,
Tang H, Colbourne JK, Ketterson ED. 2013
Testosterone affects neural gene expression
differently in male and female juncos: a role for
hormones in mediating sexual dimorphism and
conflict. PLoS ONE 8, e61784. (doi:10.1371/journal.
pone.0061784)
105. Whitehead A, Crawford DL. 2006 Variation
within and among species in gene expression: raw