American Journal of Primatology 73:1072–1081 (2011)
Female Marmosets’ Behavioral and Hormonal Responses to Unfamiliar Intruders
CORINNA N. ROSS1?AND JEFFREY A. FRENCH2
1Department of Cellular and Structural Biology, University of Texas Health Science Center, San Antonio, Texas
2Department of Psychology, University of Nebraska at Omaha, Omaha, Nebraska
The endocrine control mechanisms for female mammalian aggression have been largely unstudied.
Although it has been proposed that androgens may modulate female aggressive behavior in a similar
manner to males, very little conclusive evidence exists. Previous work in male marmosets found that
post-encounter increases in testosterone (T) were dependent on the intensity of aggression displayed
during the aggressive encounter. We exposed female marmosets (Callithrix kuhlii), a monogamous and
biparental primate, to aggressive interactions with unfamiliar intruders. Individual female marmosets
exhibited changes in T and estradiol (E2) that are associated with aggressiveness dependent on the
intensity of aggression displayed as well as their role during the encounter. Resident females exhibited
increased E2immediately following an encounter in which they displayed high rates of aggression. If
resident females received high rates of aggression from the intruder, the resident displayed increased T
24hr following the encounter. Interestingly, if the female was an intruder in the encounter, the
intensity of her aggression was associated with increased cortisol immediately following the trials,
whereas received aggression was associated with increased T and E2immediately following the trial.
Female primates do exhibit situation-dependent changes in gonadal steroids in association with
aggression that may serve to prime them for future aggressive interactions. Am. J. Primatol.
r 2011 Wiley-Liss, Inc.
Key words: challenge hypothesis; neuroendocrine; aggression; intruder challenge
In many species of birds, reptiles, and mammals,
both males and females engage in aggressive beha-
vior to protect territory, resources, mates, or off-
spring [see review Stockley & Bro-Jorgensen, 2011].
The examination of the neuroendocrine modulations
associated with aggression has focused on males and
testosterone (T), whereas females have been left
understudied. The impetus for this focus has been
the development and testing of the Challenge
Hypothesis proposed by Wingfield et al. . The
Challenge Hypothesis predicts variation in circulating
concentrations of T in association with aggressive
behaviors of male animals relative to their social and
parental status, in light of the higher costs associated
with maintaining T on immune function and
parental care behaviors [Hasselquist et al., 1999;
Peters, 2000; Wingfield et al., 1987]. Specifically,
males from polygynous species that are highly
engaged in territorial protection and mate acquisi-
tion, and spend very little time engaged in parental
care, are predicted to consistently show high con-
centrations of circulating androgens, including T
throughout the breeding season [Johnsen, 1998;
Wingfield et al., 1990; see review in Gleason et al.,
2009]. On the other hand, males in monogamous
social systems, which are more invested in paternal
care, are expected to only show increases in T when
engaged in aggressive behaviors, with T quickly
returning to baseline shortly after the interaction,
thus lessening the impact on paternal care. Evidence
in support of the Challenge Hypothesis has been
described for many species of lizards, birds, and
mammals, including nonhuman primates [Cavigella
& Pereira, 2000; Creel et al., 1993; Johnsen, 1998;
Ross et al., 2004; Wingfield et al., 1987, 1990].
In some species, females can be just as aggressive
as males during intraspecific encounters [Elekonich
& Wingfield, 2000; Mayer & Rosenblatt, 1987;
Stockley & Bro-Jorgensen, 2011; Vom Saal et al.,
1995]. As is the case in males, there are a number of
negative impacts associated with increased concen-
trations of androgens over time for females, including
decreased maternal care behavior, reduced fecundity,
Published online in Wiley Online Library (wileyonlinelibrary.com).
Received 7 January 2011; revised 31 May 2011
Contract grant sponsor: National Science Foundation; Contract
grant numbers: IBN: 97-23842; 00-91030.
?Correspondence to: Corinna N. Ross, Department of Cellular
and Structural Biology, Barshop Institute for Longevity and
Aging Studies, University of Texas Health Science Center, 15355
Lambda Dr., STCBM Bldg, Ste 3.325, San Antonio, TX 78245.
rrrr 2011 Wiley-Liss, Inc.
and lowered immunity [Gerber et al., 2010; O’Neal
et al., 2008; Rutkowska et al., 2005; Zysling et al.,
2006]. In males, T is the main focus of study as the
dominant acting gonadal steroid, whereas in females
a number of hormones may play a role in behavioral
actions including T, estradiol (E2), progesterone (P4),
cortisol (CORT), and oxytocin (OT).
Female aggression, specifically associated with
late pregnancy status and postpartum protection of
pups (i.e. maternal aggression), has been associated
with abrupt increases in E2and decreases in P4for
rats [Leopoldo de Sousa et al., 2010; Mayer &
Rosenblatt, 1987]. The endocrine changes in rats are
in direct contrast to the evidence from mice in which
maternal aggression is associated with increased P4
and decreased E2 [Svare et al., 1986]. Maternal
increased circulating and brain concentrations of OT
in a number of mammals [see review, Campbell, 2007].
Finally, the HPA axis and circulating concentrations
of CORT and its correlates have been associated with
maternal aggressive interactions, as both a response to
stress [Castro & Matt, 1997; Mendoza & Mason, 1986;
Smith & French, 1997a] and a priming agent for the
actions of OT [Trainor et al., 2010].
The few studies that have been published
examining female aggression
responses not associated with a maternal role of the
female in lizards, birds, and mammals have had
contradictory results. Studies on birds have found
that steroid hormones, such as T and E2, are not
associated with aggressive interactions of females
[Canoine & Gwinner, 2005; Elekonich & Wingfield,
2000; Zysling et al., 2006]. Female–male aggression in
iguanas was associated with increased T during the
mating season and female–female aggression in the
iguana was associated with increased E2 and P4
[Rubenstein & Wikelski, 2005]. Increased E2was also
associated with female–female aggression in moun-
tain spiny lizards [Woodley et al., 2000]. Work in
Siberian hamsters found no association between
gonadal steroid hormones and female aggression
[Scotti et al., 2007], neither did a study of California
mice [Davis & Marler, 2003]. However, female bank
voles treated with exogenous T and E2 showed
elevated levelsof aggression
Increased T and aggression have been associated
with dominance rankings in wild female baboons
[Beehner et al., 2005], but no further studies have
examined nonhuman primate responses to aggressive
encounters and the neuroendocrinological correlates.
Callitrichines are small New World primates
that are cooperative breeders and tend to be socially
monogamous. Infant care and survival are highly
dependent on paternal care, with males being the
primary carrier of the infants from 2 weeks of age to
weaning [Digby, 1995a; Goldizen, 1987; Price, 1992].
T levels in male Callithrix kuhlii are lowest in males
during the period of maximal infant care, higher in
males without parental experience, and higher in
males that have lower rates of infant care [Nunes
et al., 2000, 2001, 2002]. Previous behavioral studies
have revealed varying responses to intruders asso-
ciated with both the gender and the age of the
(C. kuhlii) respond more aggressively to adult male
intruders than either juvenile male or female
intruders. Additionally, resident males have in-
creased T 2–6hr and 24hr following encounters with
adult male intruders during which the resident male
exhibited aggressive behavior [Ross et al., 2004]. The
magnitude of the hormonal response was positively
associated with the intensity of the aggression
during the male–male encounter.
Aggression of female callitrichids during intru-
der encounters varies by species. Female–female
encounters in golden lion tamarins (Leontopithecus
rosalia) were characterized by highly aggressive
interactions, including high rates of physical attacks
and chases [French & Inglett, 1989]. Cotton-top
tamarins (Saguinus oedipus), common marmosets
(Callithrix jacchus), saddleback tamarins (Saguinus
fuscicollis), red bellied tamarins (Saguinus labiatus),
and black tufted ear marmosets (C. kuhlii) all show
higher rates of aggression during female–female
encounters than female–male encounters; but this
aggression is often limited to low intensity noncon-
tact displays, including scent marking, genital dis-
plays, and tongue flicking. This variability in
response has been attributed to species differences
in the mechanisms of reproductive inhibition, with
those species that exhibit low intensity agonism
often being species in which subordinate females are
reproductively suppressed [French et al., 1995].
We used an intruder paradigm to test the
C. kuhlii to encounters with unfamiliar conspecifics.
Resident females were exposed to encounters with
adult and juvenile intruders of both sexes. We
expected the intensity of agonistic displays to be
greatest in resident female–adult female intruder
encounters. If the intruder elicits an aggressive
response from the resident female, and female
marmoset aggression is governed by similar mechan-
isms as male marmoset aggression, then we pre-
dicted that exposure to a female intruder would elicit
high rates of aggressive and agonistic displays that
are associated with increases in T immediately
following the encounter. Additionally, as T is quickly
aromatized to E2 in females of many mammalian
species [Callard et al., 1978], we predicted that
circulating T and E2 would be increased 24hr
following the encounter. Because the reaction to
the encounter may differ for the resident and the
intruder female, we also monitored the intruder’s
behavioral and endocrine responses to these trials.
Finally, because agonistic encounters may serve as
potent stressors, we monitored changes in CORT
Am. J. Primatol.
Female Marmoset Aggression / 1073
following the encounter for both the resident and
Five pairs of Wied’s black tufted ear marmosets
(C. kuhlii), consisting of an adult breeding male and
an adult breeding female, served as resident subjects
for the study. The animals were housed at the
Research Facility. The animals were maintained in
pairs and small family groups in cages measuring at
least 1.2?0.9?2.4m. The cages contained natural
branches, a nest tube, a feeding platform, and
enrichment devices. Visual access between groups
was limited, but olfactory and auditory contact was
available with two to three other family groups. All
protocols were approved by the University of
Nebraska at Omaha Institutional Animal Care
Committee and adhered to the American Society of
Primatologists principles for the ethical treatment of
nonhuman primates. For further details of animal
husbandry and housing, see Schaffner et al. .
Twenty marmosets were used as intruder sub-
jects in our observations. Thirteen marmosets (six
males and seven females) were adult intruders (424
months) and were breeding adults in their own social
groups. Seven intruders (four males and three
females) served as juvenile intruders. Juveniles were
between the ages of 6 months and 2 years of age, and
resided in a social group that contained older adult
at Omaha Callitrichid
To protect the intruder from physical injury
during the encounters, they were placed in an
intruder cage (60?40?40cm) constructed of wire
mesh. The intruder cage eliminated the possibility of
physical contact between residents and intruders
while still providing good visual contact between
interacting marmosets. All residents were exposed
and habituated to the intruder cage before any
testing began, by placing the empty intruder cage
in each home cage for three 8hr time periods before
the first test. We did not test groups in which the
female was in the third trimester of pregnancy or
had nursing infants. Post hoc analysis of the female’s
reproductive status during testing based upon inter-
birth intervals found that all females were either
nonpregnant or in the first and second month of
pregnancy during the trials. The resident pair and
intruder were first observed during a 10-min pretrial
period. The intruders were removed from their home
cage by coaxing them into a small transport cage
attached to their home cage. From the transport
cage, the animals were transferred to the intruder
cage. After transfer, intruders were allowed to
acclimate to the cage for 5min in an empty room.
After this period, the intruder was placed on the floor
in the home cage of the resident pair to be tested.
Two observers recorded behaviors during the
intruder encounter; one observer focused on the
behavior of the intruder and the resident of the same
sex, and one recorded the behavior of the other
resident. Interobserver reliability was at least 90%
for all trials, as determined by comparing the scores
of the social interactions between resident partners.
Observations occurred at 20second intervals for
30min, and were recorded using Observer 3.0son
a laptop computer. Patterns of agonistic and aggres-
sive behavior toward the intruder, affiliative and
sexual behavior within the resident pair, and general
activity of both the resident and intruder were
recorded during each observation, as described in
Ross et al. ; see Table I. Following the intruder
trial, the intruder was released back into its home
cage and the behavior of both residents and intruder
was monitored for an additional 10min. All intruder
encounter trials started between 08:00 and 09:00hr.
Each resident pair was presented with two
different adult males, two adult females, and one
female and one male juvenile in a randomized order.
Three weeks separated all trials with an animal,
whether the animal served as a resident or an
intruder. Additionally, at least 2 days separated all
trials in the colony in order to prevent a general
disturbance to the colony. Control trials were
randomly scheduled among experimental trials while
leaving 3 weeks between each use of an animal.
A control resident trial was conducted by placing an
empty intruder cage in the resident cage and
observations were performed in the same manner
as described above. After the 30min trial period, the
intruder cage was removed from the room and a
10-min post-observation was conducted. A control
intruder trial was conducted by placing the intruder
animal in the intruder cage and then placing the
intruder cage in a novel empty cage the size of a
standard family housing. This trial was used to
control for nonencounter-related handling effects.
Observations were made as described for the experi-
mental trial. The animal was then released into its
home cage and observed. These trials served as
control for both behavioral responses to novel
situations, as well as a control for the daily circadian
rhythms of hormones to be assayed.
Before testing (06:00–08:00hr), urine was col-
lected noninvasively from all resident and intruder
subjects, using the procedures previously outlined in
French et al. . Urine was collected every 2hr
following the trial from each of the animals until
17:00hr and then again the following morning. All
samples were centrifuged at 7,000rpm for 2min to
remove debris and the supernatant was transferred
to a clean minivial. The samples were then stored at
?201C until the assays were performed.
Am. J. Primatol.
1074 / Ross and French
CORT concentrations were measured in all
urine samples using an enzyme immunoassay devel-
oped and validated for use in C. kuhlii, as previously
described in Smith and French [1997a,b]. Recovery
of all standards (range 1.95–1,000pg) added to
quality control pools was 10172%. The intra-assay
coefficients of variation for medium and low concen-
tration pools were 4.46 and 3.47%, respectively
(n520). The interassay coefficients of variation for
the medium and low concentration pools were 14.29
and 17.75%, respectively (n520).
T and E2 concentrations were measured in
samples using an enzyme immunoassay adapted for
C. kuhlii, as outlined in Nunes et al.  and Fite
and French , respectively. Briefly, samples
were hydrolyzed with 20ml b-glucuronidase, and then
extracted with 5ml diethyl ether. The ether was
evaporated and samples reconstituted in phosphate-
buffered saline. Recovery of the T standards was
96.973% and E2was 93.272%. Intra-assay coefficients
of variation for the high and low control pools were 5.16
and 3.85%, respectively (n519), for T and 2.37 and
3.06%, respectively (n518), for E2. The interassay
coefficients of variation for the high and low control
pools were 9.68 and 8.34%, respectively (n519), for T
and 9.51 and 17.68%, respectively (n518), for E2.
Since residents experienced two separate intru-
der encounters with adult intruders, the replicate
trials with adult intruders were collapsed into a
composite score for each intruder sex. All behavioral
data were standardized to a frequency per 10min to
allow direct comparisons between pre-/trial-/post-
values. Several aggressive displays and behavioral
patterns were summed to produce a composite
measure of aggression. Previous research with this
species has found that the behaviors, including
attack, chase, erh-erh vocalizations, cage mark,
genital display, and piloerection, correlate strongly
with each other and have previously been collapsed
into a single category, the high-level aggression
category [Ross et al., 2004; Schaffner & French,
1997]. This composite score will be simply referred to
as aggressive displays throughout [Ross et al., 2004].
A four-factor analysis of variance was used to
determine differences among trial conditions, and
Bonferroni adjustments were used for post hoc tests
TABLE I. Definitions of Behaviors Collected During the Intruder Trials for Both the Residents and Intruders
State of resident
Focal animal travels to a new position
Focal animal is not traveling
Focal animal is consuming food or water
Focal animal is scratching/cleaning body
Focal animal is in apparent play behavior alone or with others.
Usually includes chase behaviors
Grooming or being groomed
Resident pair in physical contact
Focal animals is within 10cm of mate
Focal animal engaged in mating behavior
Proximity to cage
NearI Focal animal is within 10cm of intruder cage
Gutteral vocalization accompanied by attack
Contact call, long in duration and high in pitch
Genital rub on surfaces
Exposing genital area by lifting tail
Focal animal bites intruder cage and chases
Focal animal’s hair is extended from body
Intruder follows movement makes no response
Intruder is nonresponsive
Intruder actively attacks and chases resident
Method of data collection was either instantaneous (I) or all occurrences (A) (as defined in French et al., 1995; Ross et al., 2004; Schaffner & French, 1997).
Am. J. Primatol.
Female Marmoset Aggression / 1075
to evaluate all between-subjects main effects. Specifi-
cally, for resident behaviors the design was a 2
(resident sex)?2 (intruder sex)?2 (intruder age)?3
(time of the observation: pretrial, during, post-trial).
Control trials were not included in these analyses,
because no aggressive behaviors were displayed during
any of the trials in the presence of the empty cage. All
results are reported as means7standard error.
The hormonal data for both residents and
intruders were analyzed using the percent change
relative to the baseline morning samples, for the
samples collected 2–6hr after the trial (samples
averaged together) and samples 24hr after the trial,
accounting for both shorter and longer term changes
in steroid production and excretion. These times were
chosen based on previous evidence that excretion of
steroids may commence as early as 2–6hr following a
change in the plasma concentrations [Smith &
French, 1997a,b]. Changes from baseline values in
CORT, T, and E2concentrations of female residents
were analyzed using an analysis of variance, specifi-
cally 5 (exposure conditions: adult male intruder,
juvenile male intruder, adult female intruder, juve-
nile female intruder, control)?2 (time after the trial:
2–6hr, 24hr). The change in CORT, T, and E2for
intruder females was tested with a 2 (age of the
intruder: adult, juvenile)?2 (condition: intruder
trial, control)?2 (time after the trial: 2–6hr, 24hr).
The controls were included in the ANOVA in order to
determine whether changes in hormone concentra-
tions throughout the day after an intruder trial were
distinct from changes that are associated with normal
circadian rhythms [Smith & French, 1997b].
To compare the relationships between aggression
and hormone changes, as well as examining the
relationship between changes in CORT, T, and E2,
partial correlations were conducted controlling the ID
of both the resident and intruder. For these analyses,
the samples were not collapsed or averaged between
trials. Each resident was analyzed using every trial in
which they participated; i.e. two trials with adult male
intruders and two trials with adult female intruders.
Hormone samples were averaged for the 2–6hr time
period and analyzed as changes from the baseline
values. Analyses first examined the relationship be-
tween female resident aggression and CORT, T, or E2
for all intruders; further analysis restricted the
comparison to exposure to intruder females only.
Similar tests were used to compare changes in CORT,
T, and E2over time, as well as relationships between
aggression, CORT, T, and E2for intruders. All analyses
were done using SPSS 13.0 and an a level of 0.05 was
set for all analyses.
Resident marmosets responded to intruder trials
in a sex specific manner with higher rates of aggressive
displays in the presence of a same-sex intruder
(F(1,8)510.50, P50.012), whereas the status of the
intruder as juvenile or adult did not result in
significantly different levels of aggressive displays
(Fig. 1). Female residents showed increased rates of
scent marking during trials with female intruders
(female intruder: 0.3370.07 scent marks/10min, male
intruder: 0.1470.02 scent marks/10min; F(1,4)59.29,
P50.038). We found that the number of instantaneous
samples in which the females were in contact with their
mate was significantly increased during trials in which
they were exposed to a male intruder (male intruder:
0.3870.08 contact/10min, female intruder: 0.267
0.09 contact/10min; F(1,4)526.66, P50.007). No
other behaviors scored for the female resident differed
owing to the sex of the intruder.
Physical proximity to the females’ social partner
revealed that females spent considerably more time
near, but not in contact with, their male partners
during the intruder phase of the trial, relative to the
post-trial observation (trial: 0.8370.09, post-trial:
0.4770.08, P50.01). A significant interaction was
found between trial type and time for the behavioral
score of move (F(8,32)52.87, P50.016) with mar-
mosets in all the intruder trial phases of the
experiment showing more movement than before or
after the intruder trial (trial: 0.8070.85, pre:
0.4070.21, post: 0.3670.24). Females also displayed
higher rates of genital displays during the intruder
(F(2,8)55.17, P50.036, trial: 0.0570.02 genital
displays/10min, post-trial: 0.0170.01 genital dis-
plays/10min, P50.05). No other behaviors scored
during the intruder trials, including allogrooming,
time away from the mate, sex, self-grooming, feeding,
playing, twitter calls, long calling, or time on the
intruder cage, differed significantly owing to the
phase of the trial or the trial condition.
MA MJ FA FJ
Aggressive Displays / 10 min
Fig. 1. Aggression directed toward intruders as a function of the
resident sex, intruder sex, and intruder age. MA, male adult; MJ,
male juvenile; FA, female adult; FJ, female juvenile.
Am. J. Primatol.
1076 / Ross and French
Female residents had no significant hormonal
changes from baseline at either 2–6hr post or 24hr
post for CORT, T, or E2as a function of the gender of
the intruder as tested by ANOVA. Variation in
aggression displayed during a trial was not signifi-
cantly associated with variation in baseline hormone
concentrations of CORT, T, or E2for the resident
females. When we further examined just the trials
involving female intruders, female residents dis-
played a significant positive relationship between
sexual bouts during a trial and the number of
P50.037, n515, controlled ID n55). Additionally,
female residents were found to have a significant
positive relationship between the change from the
baseline of E22–6hr post and the number of sexual
bouts (sex r50.815, P50.000) and the number of
aggressive displays during the trial (aggressive
displays r50.533, P50.05; when the outlier is
removed, the positive relationship remains but is
not significant r50.45, n.s.) (Fig. 2). The number of
sexual bouts during the trial were also significantly
positively associated with changes from the baseline
in T 2–6hr after the trial (r50.57, P50.033). There
was also a significant positive relationship between
the changes from the baseline in CORT 24 and E224
for females exposed to female intruders (r50.625,
P50.04). Although there was no relationship be-
tween the number of aggressive displays performed
by the resident and those done by the intruder, there
was a significant positive relationship between the
number of aggressive displays done by the female
intruder during the trial and the female residents’
change from the baseline for T 24 following the trial
(r50.79, P50.004; when the two outliers are
removed, the relationship remains but is marginally
significant r50.56, P50.1) (Fig. 3).
Female intruders responded quite differently to
the encounter than did the residents. Female
intruders showed a significant positive relationship
between the number of aggressive displays done by
the female resident during the trial and the change
from the baseline in T 2–6 as well as E22–6 (T 2–6
r50.79, P50.033; E22–6 r50.78, P50.049; n57)
(Fig. 4). Female intruders also showed a significant
positive relationship between the number of aggres-
sive displays that they exhibited during the trial and
their change from the baseline for CORT 2–6
Female marmosets exhibited sex-specific beha-
vioral andhormonal responses
intruders. Specifically, female residents displayed
more aggressive behaviors toward adult female
intruders than to any other type of social intruder.
Resident Female Aggressive Displays / 10 min
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
Resident Female Estradiol 2-6 hours
Post Intruder Encounter (% Change from Baseline)
Fig. 2. Percent change from baseline for resident females’ E2
2–6hr following an encounter with a female intruder and the
aggressive displays exhibited by the resident female during the
intruder encounter (partial correlation; r50.533, P50.05),
individuals are represented by each symbol.
Intruder Female Aggressive Displays/10 min
0 5 10 15 20 25 30
Resident Female T 24 Hrs Post Intruder Encounter
(% Change from Baseline)
Fig. 3. Percent change from baseline for resident females’ T
24hr following an encounter with a female intruder and the
number of aggressive displays received by the resident female
during the intruder encounter (partial correlation; r50.79;
P50.004), individuals are represented by each symbol.
Resident Aggressive Displays / 10 min
0.0 0.2 0.4 0.6 0.8 1.0
Hormonal Change 2-6 Hrs Post Intruder Encounter
(% Change from Baseline)
Fig. 4. Percent change from baseline for intruder females’ T 2–6
and E2 2–6hr following an intruder challenge and the number of
aggressive displays received by the intruder during the encoun-
ter (partial correlations; T2–6 r50.794, P50.033; E2 2–6,
Am. J. Primatol.
Female Marmoset Aggression / 1077
Females displayed higher rates of agonistic displays
reported for this species [French et al., 1995;
Schaffner & French, 1997]. Female marmosets also
specifically displayed high rates of aggression toward
adult female intruders. These aggressive behaviors
were positively associated with changes in hormonal
concentrations dependent on whether the female
was a resident giving or receiving aggression.
Although the sample size for this study was small
and it may be difficult to extrapolate our results to
female aggression in general, we find it very
interesting that rather than hormonal shifts being
associated simply with the female’s presence in an
female’s role in the aggressive encounter. For female
residents, high rates of displayed aggression was
associated with increases in E2immediately following
the encounter, whereas high rates of aggression
received was associated with increased T 24hr
following the encounter. Interestingly, if the female
was an intruder, high rates of aggression displayed
was associated with increased CORT immediately
following the trial, whereas increased aggression
received was associated with increased T and E2.
Unlike other studies that have found little
association between steroid hormone concentrations
and aggression in females, we found situationally
dependent relationships. Previous work on female
vertebrate aggression has focused on seasonal bree-
ders with strict nest building, mate acquisition, egg
laying, egg care, and offspring care seasons. These
females often have high levels of T or E2during the
mating and nest building season as a baseline, and
more closely resemble the aggressive interactions
and hormonal responses of males of polygynous
species [Canoine & Gwinner, 2005; Elekonich &
Wingfield, 2000; Rubenstein & Wikelski, 2005]. Very
little work has been done with females from mono-
gamous species for which breeding, mate guarding,
and infant care may occur simultaneously. Recent
studies of California mice have demonstrated that
female residents exhibit an immediate increase in
plasma corticosterone and OT following a 10min
exposure to an unfamiliar same-sex intruder [Trainor
et al., 2010]. The changes in corticosterone and OT
were not correlated with the aggression of the
encounter and were associated with the length of
the photoperiod that the females were maintained
in. Artificial manipulation of hormones through
implants in California mice has found decreases in
P4and the P4/T ratio owing to the presence of an
intruder that was not associated with the amount of
aggression displayed by the resident [Davis &
Marler, 2003]. This suggested that decreasing the
P4/T ratio serves to prime the female for the next
intruder interaction; however, animals were sacri-
ficed 30min following the trial for blood collection,
has been previously
which did not allow enough time to verify priming
effects. These rodent studies have controlled the
reproductive cycle of the female either by using
nulliparous [Davis & Marler, 2003] or naı ¨ve diestrus
mice [Trainor et al., 2010]. Work in hamsters
suggests that lactating females display higher rates
of aggression than cycling females [Siegal et al.,
1983], and many rodent studies have suggested that
nipple stimulation is directly linked to increased
female aggression [Gandelman & Simon, 1980; Svare
& Mann, 1983].
Unfortunately, as a result of colony constraints,
we were unable to control the reproductive status of
the marmoset females. However, we avoided testing
groups with females in the third trimester of
pregnancy or with nursing infants, as these time
periods would be most likely associated with sig-
nificant pregnancy-induced hormonal shifts and
likely maternal aggression. The post hoc analysis of
the interbirth intervals found that all females were
either nonpregnant or in the first and second month
Although it is possible that early pregnancy may
affect female aggression and hormonal responses to
an intruder, we do not suspect that this is the case.
Marmosets are socially monogamous and are often
referred to as pair-bonded primates [Gerber et al.,
2002], and the pairs tested in this experiment were
long-term pair mates, not new pairs. Pair-bonded
mammals typically display high levels of aggression
toward unfamiliar intruders, including sexually
receptive females, rather than soliciting them as
future mates. This maintenance of the pair bond
occurs whether the female of the bond is currently
pregnant or nonpregnant, as the male represents a
resource not only as a mate but also as a care
provider for future infants [Fernandez-Duque et al.,
1997; Young et al., 2011]. Furthermore, marmoset
pregnancies are unusual in that they display a brief
pause for the first month of pregnancy in which the
embryo does not begin growth and development
[Merker et al., 1988]. Therefore, we do not believe
that the early pregnancy status is likely to play a
large role in the aggressive responses of female
marmoset residents to intruders, but future research
should attempt to control this variable.
Female marmosets displayed a unique hormonal
response in which the changes in steroid hormone
concentrations were associated not simply with the
level of aggression during the encounter, but also
with the role the female played in the encounter. The
marmosets’ responses most closely resemble those
reported for lizards, in which E2directly correlated
with aggression, whereas T seemed indirectly asso-
ciated [Rubenstein & Wikelski, 2005]. The authors
concluded that in lizards rapid aromatization of T to
E2 occurred during a fighting encounter. In male
Peromyscus, winners of an encounter have signifi-
cantly different behavioral and hormonal responses
Am. J. Primatol.
1078 / Ross and French
than do the losers of the encounters [Fuxjager et al.,
2010; Gleason et al., 2009]. Winners of an encounter
are significantly more likely to exhibit increased T,
increased activity of androgen receptor in several
brain regions, and to display aggressive behaviors in
future encounters. Female Syrian hamsters have
been found to be less likely to show repeated
submissive behaviors following the loss of an
encounter if they are implanted with E2 and T
capsules [Faruzzi et al., 2005]. Resident female
marmosets showed dramatic increases in urinary
E2immediately following an aggressive encounter,
suggesting rapid aromatization and usage of T to
facilitate aggressive actions during the encounter.
Although urinary assays of hormones might not
account for small transient changes in plasma
concentrations, we were able to detect the longer
term changes in the steroids. Furthermore, resident
females that were not aggressive during an encoun-
ter and yet encountered high rates of aggression
from the female intruder showed increased concen-
trations of T 24hr following a trial, which may reflect
hormonal priming for future aggressive interactions.
In the wild, high rates of intergroup encounters have
been recorded for several species of callitrichids [Digby,
1995b]. Thus, hormonal priming may be particularly
necessary for females to protect their mating status
and family cohesion following an encounter with a
particularly aggressive female intruder.
In male C. kuhlii, it was the intensity of the
aggression displayed by the individual that was
associated with increases in T both 2–6 and 24hr
following an encounter [Ross et al., 2004]. Although
the females displayed alterations in steroid concen-
trations associated with aggression, the changes
were not identical to those expressed by the males.
The neuroendocrine changes associated with aggres-
sion for females seem to be more sensitive to the
intricacies of the interaction than were changes in
the males. In many ways, this is to be expected in
light of both the social system of the marmoset and
the reproductive pressures upon the females. High
concentrations of T in males have been linked
to negative outcomes on immune function and
decreased levels of parental care; thus, there are
predicted trade-offs to increasing T in favor of
aggression [Hasselquist et al., 1999; Peters, 2000;
Wingfield et al., 1990]. For females, these trade-offs
may be even more delicate [O’Neal et al., 2008].
Female marmosets are not seasonal breeders, and
changes in T and E2may have deleterious impacts on
early and mid-pregnancies. Additionally, as female
marmosets may spend at least part of their lifetime
reproductively suppressed by a dominant female
[Smith & French, 1997b], females may be particu-
larly sensitive to receiving aggression and their roles
in these social encounters. Interestingly, the role of
the HPA axis may be particularly important for
intruder females, with immediate increases in CORT
being found only in these females. No changes in
CORT were found in the male marmosets at all,
regardless of status of the individual, aggression
received, or aggression displayed [Ross et al., 2004].
So, although females display an androgen-associated
response to aggression, it is a context-specific path-
way that is much different from male marmosets.
In conclusion, we found that, although females
show marked changes in the hormonal concentra-
tions following an intruder encounter, the modula-
tions were specific to not only the aggression of the
encounter but also the role the female played in
the encounter. The female marmoset offers a unique
model to examine the impact of social status and the
role of aggression in maintenance of social groups
and the underlying neuroendocrine mechanisms
associated with these behaviors.
We thank the following people for help in data
collection and animal management: Kim Patera,
Denise Hightower, Danny Revers, Dan Jorgensen,
Chad Hansen, Scott Nunes, and Jeffrey Fite. Support
for this projectcame
French from the National Science Foundation
(IBN: 97-23842 and 00-91030).
from grantsto Jeffrey
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