Animal Behaviour 80 (2010) 921e927
Contents lists available at ScienceDirect
Animal Behaviour
journal homepage: www.elsevier.com/locate/anbehav
Deciding to win: interactive effects of residency, resources and ‘boldness’
on contest outcome in white-footed mice
Matthew J. Fuxjager a, *, Jon L. Montgomery b,1, Elizabeth A. Becker b,1, Catherine A. Marler a, b,1
a
b
Department of Zoology, University of Wisconsin e Madison
Department of Psychology, University of Wisconsin e Madison
a r t i c l e i n f o
Article history:
Received 7 July 2010
Initial acceptance 4 August 2010
Final acceptance 17 August 2010
Available online 18 September 2010
MS. number: A10-00474R
Keywords:
aggression
boldeshy
context dependent
decision making
mice
Peromyscus leucopus
social behaviour
social status
territoriality
winning behaviour
Various environmental and social factors can bias who wins and who loses a fight, but less is known
about how these factors interact with each other to affect contest outcome. We examined this issue in the
white-footed mouse, Peromyscus leucopus, as males of this species illustrate great flexibility in aggressive
and territorial tactics in the field and in the laboratory. We found that the effects of both residency status
and resource abundance (food, water and cover from predators) increased winning ability; however,
there was no interaction between these two effects. As such, the impacts of residency and resource
abundance on winning might represent two distinct behavioural phenomena that manifest via different
mechanisms. We also found that mice that urine-marked an open arena at a high frequency were more
likely to win fights in resource-rich environments compared to resource-poor environments. Meanwhile,
mice that urine-marked the same type of environment at a substantially lower frequency did not show
this behavioural difference between contexts. Because urinary marking behaviour is allied with aspects of
risk-taking behaviour and social status, this result implies that males integrate information about their
relative ‘boldness’ with information from their environment to make context-appropriate tactical decisions about fighting. We speculate that our data illustrate real-time, decision-making processes that are
a necessary component of conditional strategies used to optimize fitness.
Ó 2010 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.
Aggression aids in the pursuit of resources vital for reproduction
and survival. Because aggressive behaviour is costly with respect to
energy use and the risk of injury or death (Wilkinson & Shank 1976;
Silverman & Dunbar 1980; Marler & Moore 1988, 1989; Briffa &
Sneddon 2007), selection is thought to favour mechanisms
through which animals make tactical decisions about when to fight
by gathering information about themselves, their opponents, and
their environment (Parker 1974). Numerous studies explore the
extrinsic and intrinsic factors that are integral to this decisionmaking process (Arnott & Elwood 2009); however, surprisingly less
is understood about the interactive effects among these factors and
their influence on decision making.
Two environmental factors that alter aggression and contest
outcome are residency status (i.e. the ‘resident advantage’) and the
presence of environmental resources. Animals that fight in their
* Correspondence: M. J. Fuxjager, Department of Zoology, 250 N Mills Street,
Madison, WI 53706, U.S.A.
E-mail address: [email protected] (M.J. Fuxjager).
1
J. L. Montogmery, E. A. Becker and C. A. Marler are at the Department of
Psychology, University of Wisconsin e Madison, 1202 W Johnson Street, Madison,
WI 53706, U.S.A.
own territory or to gain access to resources that enhance fitness are
more likely to win competitive disputes than they are without the
influence of these two factors (Davies 1978; Olsson & Shine 2000;
Sneddon et al. 2003; Kemp & Wiklund 2004; Aragon et al. 2006;
Arnott & Elwood 2007). Evidence suggests that, in some cases,
the effects of residency and resources on aggression and/or
winning are functionally interdependent, whereby individuals are
able to adjust their winning ability in response to resources only in
familiar environments (Buena & Walker 2008). In contrast, other
evidence suggests that the effects of residency and resources on
aggression and/or winning occur via different mechanisms,
implying that their impacts on behavioural output are functionally
distinct (see Takahashi et al. 2001; Arnott & Elwood 2007; Fayed
et al. 2008). Thus, there is a need for experimental research to
disentangle and measure the combined and separate effects of
residency and environmental resources on contest winning ability.
Psychosocial factors may also contribute to decisions animals
make about when and how to behave aggressively. For instance,
individuals that are more likely to take risks and explore novel
environments (i.e. ‘bold’ personality types) are usually more aggressive and socially dominant, as opposed to individuals less inclined to
take risks and explore novel environments (i.e. ‘shy’ personality
0003-3472/$38.00 Ó 2010 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.anbehav.2010.08.018
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M.J. Fuxjager et al. / Animal Behaviour 80 (2010) 921e927
types) (Brain & Nowell 1969; Huntingford 1976; Verbeek et al. 1996;
Sundstrom et al. 2004; Reaney & Backwell 2007). These two behavioural syndromes (bold and shy personalities) can be adaptive (Sih
et al. 2004a, b), and there is a growing interest in how they might
interact with the environmental context (Coleman & Wilson 1998) to
drive animal decision making (Frost et al. 2007; Jones & Godin 2010).
In this study, we used the promiscuous white-footed mouse,
Peromyscus leucopus (Xia & Millar 1991), to investigate how residency
status, resource abundance and boldness affect the ability to win
aggressive contests, by either acting alone or in combination with
each other. Males of this species are capable of using different
reproductive tactics, including territoriality and wandering in search
of mates (Wolff 1986; Morris 1989; Wolff & Cicirello 1990; Halama &
Dueser 1994). This idea is supported by field and laboratory studies
that show that white-footed mice vary greatly in the degree to which
males defend home ranges and act aggressively towards conspecifics; in other words, some males vigorously fight off intruders,
while other males act peacefully towards intruders (Wolff et al.1983;
Oyegbile & Marler 2006). An individual’s decision to adopt either of
these tactics over the other may involve assessment of environmental factors, such as population density or resource competition
(Wolff 1986; Morris 1989; Halama & Dueser 1994). It is also possible
that these tactics are related to individual differences in personality.
For example, territorial mice may have bolder personalities
compared to nonterritorial males, because territorial holders must
continually risk their own survival by patrolling territory boundaries
and scent marking (Koivula & Viitala 1999; Gosling et al. 2000; Probst
et al. 2002; Hurst & Beynon 2004). Given the risks associated with the
exploration and marking of environments, urine-marking behaviour
itself is likely to be a reliable indicator of bold versus shy personality
types (Wilson et al. 1994; Greenberg 1995; Sih et al. 2004a, b) and,
thus, may affect decision-making processes that shape aggression.
Based on the behavioural repertoire of white-footed mice
described above, we predicted that residency and resource abundance would positively affect winning ability in male mice. We
reasoned that if these two factors increased winning ability by
interacting with each other, their effects on behavioural output may
be functionally coupled (e.g. Buena & Walker 2008). Using a similar
logic, we also assessed whether boldness or shyness (as measured
by scent-marking behaviour) impacted winning ability by either
acting alone or in conjunction with environmental cues (residency
and/or resource level). In the case of the latter, we reasoned that
this would imply that personality types are able to modulate
aggression in a context-dependent manner.
‘opponent,’ then paired and housed with a single female in a standard cage. Pairs were each provided nesting material and food and
water ad libitum.
On day 9, urinary scent-marking behaviour of each focal male
was assessed to examine aspects of boldness or shyness before the
test encounter. This consisted of removing each focal male from its
standard cage and placing it in a glass aquarium (60 30 30 cm)
that was lined with an unsoiled sheet of filter paper (Fisher, Qualitative P8, flow rate: fast). Each male was given 30 min to freely
explore the open aquarium and urinate/scent-mark before it was
placed back into its standard cage. The filter paper was immediately
examined under ultraviolet light (20 W) to illuminate urine marks
(Desjardins et al. 1973). Each mark was circled with pencil, and the
total marks deposited by each mouse were recorded. The distribution of marking frequency was roughly bimodal, such that
roughly half of the focal mice marked frequently (N ¼ 26;
median ¼ 128 marks, range 40e537 marks; Fig. 1a) and the other
half deposited few marks (N ¼ 30; median ¼ 3 marks, range 0e19
marks; Fig. 1b). As a result, we used this naturally occurring
dichotomy in marking behaviour to classify mice as either highmarking or low-marking.
On day 10, high- and low-marking mice were assigned at
random to treatment groups. In each group, focal mice experienced
an aggressive social encounter (on day 11) against an opponent that
was matched for sexual opportunity (time in which mating could
occur with a female before the test encounter, see above) and body
size (within an average of 0.7% body mass (0.2 g, SE ¼ 0.19 g)).
Opponents were assigned to focal mice blindly with respect to
opponent personality; thus, any effect of opponent boldness or
shyness on focal mouse contest behaviour was distributed
randomly across treatment groups. Furthermore, there was no
(a) High-marking mouse
METHODS
(b) Low-marking mouse
Animals
White-footed mice from our laboratory colony at the University
of Wisconsin (UW) e Madison were maintained according to the
NIH Guide for the Care and Use of Laboratory Animals. The colony was
kept on a 14:10 h light:dark cycle. Animals were housed in samesex groups of three to four per cage (28 18 12 cm) and provided
food and water ad libitum. The UW-Madison IACUC approved the
research detailed here.
One week before the study, 112 male and 112 female mice were
moved to a separate room for behavioural testing (same light cycle
as above). Behavioural manipulations occurred at least 1 h after the
dark cycle’s onset and under dim red light.
Procedure
The study took place over 11 days. On day 1, each male mouse
was weighed, randomly assigned as a ‘focal mouse’ or an
Figure 1. Sample marking patterns of (a) high-marking mice and (b) low-marking
mice. This assay was used to elucidate personality, as individuals that mark more in
a novel environment demonstrate a greater propensity for risk taking compared to
individuals that mark less (Koivula & Viitala 1999; Gosling et al. 2000; Roberts et al.
2001; Probst et al. 2002). Accordingly, we assumed high-marking mice to be individuals with relatively bolder personalities and low-marking mice to be individuals
with relatively shy personalities (Wilson et al. 1994; Greenberg 1995).
M.J. Fuxjager et al. / Animal Behaviour 80 (2010) 921e927
Table 1
Treatment groups to which focal mice were randomly assigned
Group
Test cage familiarity
Resource level
1; N¼7 high marking (bold)
N¼7 low-marking (shy)
2; N¼8 high marking (bold)
N¼7 low-marking (shy)
3; N¼6 high marking (bold)
N¼8 low-marking (shy)
4; N¼5 high marking (bold)
N¼8 low-marking (shy)
Familiar/Resident
Abundant
Familiar/Resident
Few
Unfamiliar/Nonresident
Abundant
Unfamiliar/Nonresident
Few
relationship between body size and marking behaviour (linear
regression: F1,50 ¼ 0.051, P ¼ 0.823), suggesting that focal mice and
opponents in a particular size class were not biased to have one
personality type over another. Treatment groups differed in terms
of the test encounter’s context (Table 1), occurring in an environment that either (1) was familiar to focal males and contained
abundant environmental resources, (2) was familiar to focal males
and contained few environmental resources, (3) was unfamiliar to
focal males and contained abundant resources, or (4) was unfamiliar to focal males and contained few resources. In all groups, test
encounters occurred in transparent test cages (30 50 30 cm,
W L H) that were separated by a divider into large
(30 30 30 cm) and small (30 20 30 cm) chambers. This
divider could be adjusted to either prevent or permit sight and
passage between chambers.
For groups 1 and 2 (see above), familiarity with the test environment was established by placing focal mice in the large chamber
of the test cage for 24 h prior to the test encounter (i.e. from day 10
to day 11; see Gleason & Marler 2010 for justification of 24 h
period). During this time, focal males were restricted to the large
chamber, as the divider prevented sight or passage into the adjacent, small chamber. Also, focal males were not accompanied in the
test cage by their female mates. To balance any effect that this
single day of social isolation might have had on aggression, focal
mice that were assigned to fight in unfamiliar test cages (groups 3
and 4) were placed without their female mates in a standard cage
for the same 24 h period (i.e. days 10e11); otherwise, these individuals were transferred to the test cage immediately before the
test encounter (see below).
In all groups, focal mice had access to food and water for the
entire 24 h isolation period (i.e. days 10e11). In group 1, focal mice
had access to extra chow and water simply because they were
exposed to the ‘abundant resources’ treatment (see below). In
groups 2, 3 and 4, focal mice had access to water and standard
amounts of food for 1 day; these amounts were substantially less
than those provided to focal mice with abundant resources (see
below). In group 2, these standard amounts of food and water were
removed immediately before the onset of the test encounter (see
below). In groups 3 and 4, these standard amounts of food and
water were effectively removed as well, because focal mice were
taken from the isolation cage and placed into the test arena that
held either abundant resources (group 3) or no resources (group 4).
No opponents were isolated for the 24 h prior to the test encounter;
thus, any effect of differences in temporary social isolation between
focal mice and opponents was distributed across all treatment
groups.
For conditions in which mice fought in the presence of abundant
resources (groups 1 and 3), the large chamber of the test cage
contained a 20-ounce (0.6-litre) bottle of water, extra chow
(mean SE ¼ 234.3 5.0 g), nesting material, a covered nestbox
(14 7.5 6.5 cm), a wooden block (15 4 1.5 cm) and a plastic
tube (4 cm diameter, 20 cm long). Muroid rodents highly value the
923
acquisition of these resources for reproductive purposes and
survival (Nelson et al. 1995; Gray et al. 2000, 2002). For conditions
in which focal mice fought in the presence of few resources (groups
2 and 4), the large chamber of the test cage contained none of the
resources described above.
Test encounters were staged using a residenteintruder paradigm. For groups in which focal mice were familiar with the test
cages, opponents were placed in the small chamber of the test cage
and given 2 min to acclimate with the divider positioned so that
neither mouse could look or move into the adjacent chamber. After
this acclimation period, the divider was altered to allow the mice to
pass between chambers. Mice were then given 15 min to interact.
For groups in which focal males were unfamiliar with test cages,
males were placed into the larger chamber of the test cage and
given 15 min to explore the surroundings. Preliminary observations
indicated that 15 min was sufficient for focal mice to contact all the
resources in the large chamber, which confirmed that the focal
mouse was aware of all environmental resources. During this time,
the divider was positioned to prevent focal mice from seeing or
exploring the smaller chamber. After this 15 min exploration,
opponents were placed in the small chamber of the test cage and
given 2 min to acclimate. Then, the divider was lifted and the mice
were given 15 min to interact. It is important to note that in all
conditions, intruders never had access to the large chamber or its
resources (either abundant or few) prior to the test encounter itself.
All social encounters were videotaped.
Behavioural Analysis and Statistics
A single observer blind to treatment group watched the videotaped test encounters and recorded for each individual (1) attack
latency (time at which mouse first attacked its opponent); (2) total
attacks (sum of bites, chases, wrestling bouts, lunges directed at
opponent); and (3) total losing/submissive behaviour (sum of
retreats, jumps away and freezes in response to opponent). The
observer determined which mouse won the encounter. A winner
was defined as the mouse that directed at least three consecutive
attacks towards an opponent that each elicited losing/submissive
behaviour (Oyegbile & Marler 2006; Fuxjager et al. 2009; Fuxjager
& Marler 2010). Based on these data, a score on the winner index
was calculated for each focal mouse. The winner index is
a continuous dependent measure of overall winning ability during
a social aggressive contest. To compute a winner index score, total
losing/submissive behaviour is subtracted from total attacks, and
this figure is then divided by the sum of total attacks and total
losing/submissive behaviour. In effect, each score indicates an
animal’s relative display of winning-typical behaviour to losingtypical behaviour (see Eisenberg 1962 for operational definitions),
such that scores closer to 1 suggest an increased likelihood of
victory and scores closer to 1 suggest an increased likelihood of
loss (Oyegbile 2006; Fuxjager et al. 2010).
Binary logistic regression was used to confirm that winner index
scores predict an individual’s actual ability to win the test
encounter. Thus, ‘win’ or ‘no win’ was used as the dichotomous,
categorical dependent variable, and winner index scores were used
as the continuous independent variable. Three-way ANOVAs were
used to test whether residency, resource abundance or urinary
marking behaviour affects not only winner index scores, but also
total losing/submissive behaviour, total attacks and attack latency.
These latter analyses were important because they provide insight
into how mice adjust their behavioural repertoires (i.e. either
increase attack behaviour, decrease losing behaviour, or both) to
modulate winning ability. Significant interactions were explored
statistically using simple main effects tests. Attack latency, total
attack and total losing/submissive behaviour data were natural log
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M.J. Fuxjager et al. / Animal Behaviour 80 (2010) 921e927
transformed (ln (X þ 1)), since QeQ plots revealed that this transformation yielded more normally distributed data (Zar 1999).
RESULTS
Winner Ability
An individual’s score on the winner index predicted whether it
emerged as the winner of an aggressive social encounter (c21 ¼ 39.1,
P < 0.001). Individuals that obtained scores closer to 1 were more
likely to win a fight, whereas individuals that obtained scores closer
to 1 were more likely to either fail to win or altogether lose a fight.
This result suggests that scores on the winner index are indicative
of winning ability and can therefore be used as a continuous proxy
measure for winning ability in further analyses (see below).
As predicted, mice altered their ability to win in response to
different extrinsic factors associated with the environment of their
social dispute. Both residency status (ANOVA: F1,48 ¼ 6.54, P ¼ 0.014;
Fig. 2a) and resource abundance (ANOVA: F1,48 ¼ 9.23, P ¼ 0.004;
Fig. 2b) had significant and positive main effects on winner index
scores, whereas urinary marking behaviour (index of personality)
prior to the test encounter did not (ANOVA: F1,48 ¼ 1.85, P ¼ 0.18).
However, there was a significant interaction between marking
1
(a)
behaviour and resource abundance (ANOVA: F1,48 ¼ 13.41, P ¼ 0.001;
Fig. 2c). High-marking mice that fought in the presence of abundant
resources scored significantly higher on the winner index compared
to either high-marking mice that fought in the presence of low
resources (simple main effect: F1,48 ¼ 23.47, P < 0.001) or lowmarking mice that fought in the presence of abundant resources
(simple main effect: F1,48 ¼ 15.22, P < 0.001). Although this threeway ANOVA model showed no other significant interactions (resiP ¼ 0.33;
residency*resource:
dency*marking:
F1,48 ¼ 0.96,
F1,48 ¼ 0.69, P ¼ 0.41; residency*resource*marking: F1,48 ¼ 2.22,
P ¼ 0.14), visual assessment of the interaction between residency
status and resource abundance suggests that these two factors
influence winning behaviour in an additive manner (Fig. 2d).
Antagonistic Behaviour
Contest environment and marking behaviour (personality) had
diverse effects on various metrics of social aggression. For example,
both residency status (ANOVA: F1,48 ¼ 0.5.21, P ¼ 0.027; Fig. 3a) and
resource abundance (ANOVA: F1,48 ¼ 6.24, P ¼ 0.016; Fig. 3b) had
a significant, negative main effect on total losing/submissive
behaviour. Although marking behaviour had no main effect on
losing/submissive behaviour (ANOVA: F1,48 < 0.001, P ¼ 0.99), it
(b)
1
Winner index score
*
*
0.5
0.5
0
0
−0.5
−0.5
−1
−1
Home cage
Winner index score
1
Unfamiliar
cage
Abundant
resources
(c)
*
0.5
0
0
−0.5
−0.5
High-marking (bold) mice
Low-marking (shy) mice
Abundant
resources
Few
resources
(d)
1
0.5
−1
Few
resources
Abundant resources
−1
Few resources
Home cage
Unfamiliar
cage
Figure 2. Effects of extrinsic and psychosocial factors on focal mouse winning behaviour. (a) Effect of residency on winner index scores. (b) Effect of resource abundance on winner
index scores. (c) Interaction between resource abundance and marking behaviour on winner index scores. (d) Interaction between residency and resource abundance on winner
index scores. Figure shows untransformed data, but transformed data were used for analysis. Data represent means SE. *P < 0.015, three-way ANOVA or simple main effects test.
M.J. Fuxjager et al. / Animal Behaviour 80 (2010) 921e927
30
20
10
Home cage
Unfamiliar
cage
*
40
30
20
10
0
Abundant
resources
Few
resources
Total attacks
Total attacks
30
20
10
0
Abundant
resources
Few
resources
(e)
40
40
30
20
30
20
10
10
0
40
(c)
High-marking (bold) mice
Low-marking (shy) mice
50
(d)
50
50
(b)
Total losing/submissive
behaviour
*
40
0
50
(a)
Total losing/submissive
behaviour
Total losing/submissive
behaviour
50
925
Home cage
Unfamiliar
cage
0
Abundant
resources
Few
resources
Figure 3. Effects of extrinsic and psychosocial factors on focal mouse antagonistic behaviour. (a) Effect of residency on losing/submissive behaviour. (b) Effect of resource abundance
on losing/submissive behaviour. (c) Interaction between resource abundance and scent-marking behaviour on losing/submissive behaviour. (d) Effect of residency on total attacks.
(d) Effect of resource abundance on total attacks. Figure shows untransformed data, but transformed data were used for analysis. Data represent means SE. *P < 0.015, yP < 0.06,
three-way ANOVA or simple main effects test.
significantly interacted with the effect of resource abundance to
impact losing/submissive behaviour (ANOVA: F1,48 ¼ 4.15,
P ¼ 0.047; Fig. 3c). High-marking mice displayed significantly less
losing/submissive behaviour when fighting in the presence of
abundant resources compared to fighting in the presence of few
resources (simple main effect: F1,48 ¼ 7.96, P ¼ 0.007). Additionally,
high-marking mice fighting among abundant resources displayed
marginally less losing/submissive behaviour than low-marking
mice fighting among abundant resources (simple main effect:
F1,48 ¼ 3.23, P ¼ 0.079). There were no other significant interactions
in this model with respect to losing/submissive behaviour (residency*marking: F1,48 ¼ 0.002, P ¼ 0.96; residency*resource:
F1,48 ¼ 0.41, P ¼ 0.53; residency*resource*marking: F1,48 ¼ 1.31,
P ¼ 0.26). In addition, the total number of attacks that focal mice
directed towards their opponents was only marginally affected by
residency status (ANOVA: F1,48 ¼ 3.75, P ¼ 0.059; Fig. 3d) and
resource abundance (ANOVA: F1,48 ¼ 3.93, P ¼ 0.053; Fig. 3c). This
model contained no other significant main effects or interaction
terms (marking: F1,48 ¼ 2.74, P ¼ 0.10; residency*marking:
F1,48 ¼ 1.10, P ¼ 0.30; residency*resource: F1,48 ¼ 0.058, P ¼ 0.81;
resource*marking: F1,48 ¼ 2.64, P ¼ 0.11; residency*resource*
marking: F1,48 < 0.001, P ¼ 0.99). Lastly, attack latency was unaffected by any of the examined variables or interactions among these
variables (residency: F1,48 ¼ 0.11, P ¼ 74; resource: F1,48 ¼ 1.55,
P ¼ 0.22; marking: F1,48 ¼ 1.56, P ¼ 0.22; residency*marking:
F1,48 ¼ 0.017, P ¼ 0.90; residency*resource: F1,48 ¼ 0.59, P ¼ 0.45;
resource*marking: F1,48 ¼ 2.15, P ¼ 0.15; residency*resource*
marking: F1,48 ¼ 0.42, P ¼ 0.52).
DISCUSSION
In male white-footed mice, the ability to win aggressive social
disputes was positively affected by both residency status and
resource abundance, but not by an interaction between these two
factors. Winning ability was also positively influenced by personality trait (determined by urinary scent-marking behaviour in an
open-field), working in concert with the effect of resource abundance. Together, these data demonstrate that specific environmental factors are capable of independently and additively
regulating the expression of winning behaviour during fights and
that some of these effects are modulated by psychological factors
related to differences in bold and shy personalities.
Effects of the Environment on Winning Behaviour
Our findings suggest that the effects of residency and environmental resources on winning behaviour are functionally discrete
phenomena. Thus, mice are able to adjust their ability to win fights
in response to either of these factors, regardless of the other factor’s
immediate saliency. In other words, males can probably increase
winning ability in both (1) home environments, even if they
contain few resources, and (2) resource-rich environments, even if
they are unfamiliar. This idea that residency and environmental
resources independently regulate winning is also consistent with
the lack of an interaction between the two factors in terms of
winning. Indeed, the combined behavioural effect of residency and
resources was additive; mice fighting in home cages with abundant
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M.J. Fuxjager et al. / Animal Behaviour 80 (2010) 921e927
resources achieved average winner index scores that roughly
equalled the sum of the separate effects of residency and resources
on winner index scores (see Fig. 2d). Interestingly, these findings
are in contrast to those presented by Buena & Walker (2008),
whereby only male crickets (Acheta domesticus) fighting in
a familiar location are able to increase aggressive behaviour in
response to resources in the surrounding environment.
One explanation for our results is that the effects of residency and
resource levels on winning manifest via different proximate mechanisms; thus, selection probably favoured each phenomenon
without functionally coupling them over the course of time. Precedent for this hypothesis is anchored in prior research that shows that
territory ownership can increase a resident’s odds of victory by
providing him or her with a mechanical advantage during a fight
(Takahashi et al. 2001; Fayed et al. 2008), whereas resources can
increase these odds by enhancing an individual’s motivation to
engage in combat (Arnott & Elwood 2007). These dual mechanisms
are also likely to be germane to muroid rodents. For example, studies
show that the structural complexity of a territory can determine the
behavioural tactics residents are able to use to evict intruders (Gray
et al. 2000; Jensen et al. 2005), and other work shows that individuals prefer to live in and are more motivated to defend habitats that
contain structures that provide shelter and protection from predators (Gray et al. 2000, 2002; Jensen et al. 2003).
Effects of Personality and the Environment on Winning Behaviour
Our results demonstrate that high-marking males are more
likely to win fights in resource-abundant environments compared
to resource-poor environments, whereas low-marking mice do not
show this behavioural discrepancy between contexts. In mice, scent
marking is costly in terms of energetic demand (Gosling et al. 2000)
and risk of predation (Koivula & Viitala 1999; Roberts et al. 2001;
Probst et al. 2002); thus, individuals that mark with high
frequency in a novel open environment are more likely to take risks
and, thus, have a personality that is relatively bold compared to
individuals that are less willing to scent-mark in the same environment (Wilson et al. 1994; Greenberg 1995). As such, our results
suggest that personality traits are capable of modulating winning
behaviour in a context-dependent manner. This conclusion is
further supported by the fact that we controlled for other factors
known to affect personality traits, such as prior context experience
(Frost et al. 2007), by randomly assigning focal individuals to the
various treatment groups.
More broadly, we speculate that our findings reflect a conditional strategy that white-footed mice use to gate behavioural
tactics related to social aggression (Gross 1996; West-Eberhard
2003). Certain populations of white-footed mice are similar to
many other mammals (Wolff 2008) in that they choose to acquire
mates by either acting territorial or wandering in search of females
(Wolff 1986; Morris 1989; Halama & Dueser 1994), and research in
a number of other taxa suggests that personality traits may affect
an individual’s reproductive tactics (Budaev et al. 1999; Gosling
2001; Martin & Fitzgerald 2005; Reaney & Backwell 2007). Given
this, our results potentially illustrate how personality traits drive
tactical decision-making processes with regard to reproduction. In
other words, high-marking mice that are likely to have bolder
personalities may determine that acting territorial and socially
dominant is an optimal reproductive strategy and, thus, are willing
to incur costs associated with such behaviour in habitats that make
suitable territories (i.e. resource-abundant cages). In contrast, lowmarking mice that have shyer personalities may determine that
acting territorial is a suboptimal reproductive strategy because they
are less willing to incur costs associated with aggression, regardless
of the environmental context. This interpretation is consistent with
our finding that winning ability is not influenced by an interaction
between marking behaviour (i.e. personality) and residency status,
particularly if the ‘resident advantage’ is mediated mostly by
a mechanical advantage provided to residents.
Behavioural Mechanisms of Shifts in Winning Ability
How do mice adjust their behavioural strategy in order to
increase the odds of winning a fight? In theory, mice may alter
losing behaviour or attack behaviour independently of each other
to modify overall winning ability, or by altering both types of
behaviour in concert. Our results suggest that the latter scenario is
most accurate. The same main and interactive effects that increased
winning ability had the opposite influence (i.e. decreased) on
losing/submissive behaviour, and there was a tendency for residency and resources to increase the total attacks focal mice directed
at opponents. Interestingly, we found no relationship between
residency, resources or marking behaviour (boldness) on attack
latency, suggesting that this metric of aggression is less important
for winning contests given the experimental conditions of this
study (but see also Jackson 1991; Hsu et al. 2009; Oliveira et al.
2009). With respect to antagonistic behaviour, our results are
consistent with other studies in wild and laboratory populations of
white-footed mice, in which attack and losing behaviour during
contests emerge as better predictors of contest outcome than
measures of attack latency (Wolff et al. 1983; Oyegbile & Marler
2006).
Conclusion
In summary, our results suggest that the effects of residency and
environmental resources do not interact to affect winning behaviour in white-footed mice. Our results also suggest that the effect of
bold and shy personality types modulates winning ability in
a context-dependent manner, implying that this species uses
a conditional strategy to appropriately gate displays of social
aggression. Finally, mice are probably able to modify their winning
behaviour, in part, by altering displays of losing/submissive and/or
submissive behaviour and attack behaviour during battle.
Acknowledgments
We thank Kyla Davidoff, Jan Davidoff, Benjamin Pawlisch and
Josh Pultorak for helpful comments on this manuscript, and Kirsten
Okpoguma for technical assistance with the animals. National
Science Foundation (NSF) Graduate Research Fellowship (to M.J.F.)
and NSF grant ISO-0620042 (to C.A.M.) funded this research.
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