Reports
Ecology, 97(8), 2016, pp. 1905–1912© 2016 by the Ecological Society of America

The importance of motivation, weapons, and foul odors in driving
encounter competition in carnivores
Maximilian L. Allen,1,2,3,6 Christopher C. Wilmers,1 L. Mark Elbroch,4 Julie M. Golla,5 andHeiko U. Wittmer2
1Center for Integrated Spatial Research, Environmental Studies Department, University of California, Santa Cruz, California 95064

USA

2School of Biological Sciences, Victoria University of Wellington, PO Box 600, Wellington, 6140 New Zealand

3Department of Forest and Wildlife Ecology, University of Wisconsin, 1630 Linden Drive, Madison, Wisconsin 53706 USA
4Panthera Puma Program, Panthera, 18th Floor, 8 W 40th St, New York, New York 10018 USA

5College of Natural Resources, Utah State University, College of Natural Resources, Utah State University, 5200 Old Main Hill,

Logan, Utah 84322 USA

Abstract. Encounter competition is interference competition in which animals directly contend for resources. Ecological theory predicts the trait that determines the resource holding potential (RHP), and hence the winner of encounter competition, is most often body size or mass. The
difficulties of observing encounter competition in complex organisms in natural ­environments,
however, has limited opportunities to test this theory across diverse species. We studied the outcome of encounter competition contests among mesocarnivores at deer carcasses in California todetermine the most important variables for winning these contests. We found some support for
current theory in that body mass is important in determining the ­winner of encounter competition,
but we found that other factors including hunger and species-­specific traits were also important. In
particular, our top models were “strength and hunger” and “size and hunger,” with models emphasizing the complexity of variables influencing o­ utcomes of encounter competition. In addition,
our wins above predicted (WAP) statistic suggests that an important aspect that determines thewinner of encounter competition is species-­specific advantages that increase their RHP, as bobcats(Lynx rufus) and spotted skunks (Spilogale gracilis) won more often than predicted based on mass.
In complex ­organisms, such as mesocarnivores, species-­specific adaptations, including strategic
behaviors, ­aggressiveness, and weapons, contribute to competitive advantages and may allow certain ­species to take control or defend resources better than others. Our results help explain how
interspecific competition shapes the occurrence patterns of species in ecological communities.
Key words: California; carrion; encounter competition; foraging arenas; interference competition;­interspecific interactions; mesocarnivore.

Introduction
Encounter competition occurs when animals directly
contend for resources, and one of the competitors suffers“harm” such as loss of feeding opportunities or energy,
injury, or death (Schoener 1983). Such contests overresources often involve fighting (e.g., Dickman 1991), andprevious work indicates that the winner of these contests
is most frequently determined by body size or mass (Connell
1983, Schoener 1983, Donadio and Buskirk 2006). The
ability of an individual to win a contest for a valuable
resource is referred to as its resource holding potential
Manuscript received 26 November 2015; revised 15 February
2016; accepted 29 April 2016. Corresponding Editor: E. T. Borer.
6 E-mail: maximilian.allen@wisc.edu(RHP, Briffa and Sneddon 2007). Aside from size, othertraits can determine RHP and therefore the outcome ofencounter competition between animals of asymmetric size
(Courtene-­Jones and Briffa 2014, Martin and Ghalambor2014). Certain species or individuals may be more aggressive
than others, while others have developed specialized
weapons or defenses (Courtene-­
Jones and Briffa 2014,
Martin and Ghalambor 2014). These should theoreticallyallow individuals to overcome a certain amount of size
differential, yet direct tests of interspecific contests in thewild are few. Therefore traits other than size should beconsidered to better c­ omprehend outcomes of encountercompetition contests in complex organisms including vertebrates (e.g., Briffa and Sneddon 2007, Martin andGhalambor 2014). Determining the various factors that1905

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MAXIMILIAN L. ALLEN ET AL.

drive the outcome of encounter competition will help in
better understanding the evolution of behavior as well as
the forces structuring ecological communities.
Encounter competition occurs rarely and unpredictably, and is thus difficult to observe in natural environments; this is especially true for cryptic species, suchas carnivores. Previous research on the subject has relied
upon laboratory experiments and easily observed species
inhabiting open habitats to develop theory (Schoener
1983, Johnson et al. 1985, Cooper 1991, Dickman 1991).
This bias may have influenced our current perceptions on
encounter competition, which emphasizes “bigger isbetter” even in complex organisms with diverse adaptations (Martin and Ghalambor 2014). This bias may beespecially relevant in mesocarnivores, highly adaptedpredators that occasionally fall prey to bigger predatorsthemselves (Polis and Holt 1992, Palomares and Caro
1999, Donadio and Buskirk 2006). Their unique positions as predators and prey have led to distinctive adaptations and ecological niches. For example, the chemical
weapons of skunks, which are traditionally associatedwith defense (Hunter 2009), could also prove advantageous during encounter competition (e.g., Martin andGhalambor 2014).
Mesocarnivores commonly interact over importantsources of food, including carrion (Wilmers et al. 2003,
Selva et al. 2005, Allen et al. 2014). Competition may
increase when resources, such as carrion, are limited(Wiens 1977, Palomares and Caro 1999) or highly valued(Tanner and Adler 2009), or when niche overlap among
competitors is high (Case and Gilpin 1974, Schoener
1983). Carrion is also generally stationary, which allows
for repeated intra-­and inter-­specific encounter competition contests at a given carcass, and allows for prolonged
assessment. Because animals repeatedly compete forcarrion resources, it suggests that encounter competition
contests among mesocarnivores at carrion may not solely
depend on body size or mass. Instead, species might betesting each other or relying on other traits and factors thatmight influence the winner of these contests (e.g., Martin
and Ghalambor 2014). These factors may include order ofarrival at the carcass, which animal is defending thecarcass, and the motivation (i.e., hunger) and health of theindividuals involved. Therefore, determining the winner ofinterspecific encounter competition contests among mesocarnivores may prove complex in natural systems.
In order to test the ecological theory underlying
encounter competition, we compared the importance ofsize versus other traits in determining a species’ RHP ininterspecific contests. We created foraging arenas (Ahrens
et al. 2012), in the form of black-­tailed deer (Odocoileus
hemionus columbianus) carcasses, and recorded the outcomes of interspecific contests among mesocarnivores
with motion-­triggered video cameras. We quantified theanimal’s mass and seven other variables, including health,hunger, and whether the animal was defending thecarcass to determine if they contributed to winning contests. We then tested the variables both separately andEcology, Vol. 97, No. 8

together in multivariate models to test existing ecologicaltheory and determine if factors other than mass predicted
winners in interspecific encounter competition within amesocarnivore guild.
Materials and Methods
Study area
We conducted our study in an area of approximately
1024 km2 in and adjacent to the Mendocino National
Forest, California (Fig. 1). Characteristics of the study
area, including topography, climate, and habitat composition, have been described in detail in Allen et al. (2014).
The area supported two large carnivores, the puma
(Puma concolor) and black bear (Ursus americanus), anda diverse community of mesocarnivores including grayfoxes (Urocyon cinereoargenteus), coyotes (Canis latrans),
bobcats (Lynx rufus), western spotted skunks (Spilogale
gracilis), fishers (Martes pennanti), ringtails (Bassariscus
astutus), and raccoons (Procyon lotor).
Field methods
We collected road-­killed black-­tailed deer from highways
in Mendocino, Lake, Glenn, Sonoma, and Marin counties.
We only collected deer in good condition, with clear,unclouded eyes, no discoloration in the abdominal region,and no obvious external wounds. As requested by California
Fish and Wildlife to minimize the transmission of disease,
we removed the head and internal organs before placing thecarcasses in the field for our experiments. We placed eachcarcass in its entirety (n = 100) or cut in half (n = 71) stratified across different habitats. We anchored each carcass toa tree with a metal cable, and mounted a motion-­triggered
camera on a nearby tree. The cameras were set to record the
maximum amount of video: Cuddeback IR cameras(n = 112; Cuddeback IR, De Pere, Wisconsin, USA) were
set to take one photo and a 30-­s video with a 1-­min delay
between triggers, while Bushnell cameras (n = 59; Bushnell
ScoutCam, Overland Park, Kansas, USA) were set torecord a 60-­s video with a 1-­s delay between triggers. We
removed the cameras after three weeks or when there wereno longer edible remains.
We examined each video where an encounter occurred
between mesocarnivores and evaluated the encounter todetermine a winner and loser. Encounters were split into
two types of contests: direct encounters (n = 33) and“push-­offs” (n = 27). We defined direct encounters as interactions in which two animals directly interacted in thevideo. We defined push-­offs as contests in which one
animal left the carcass within 10 min of the arrival ofanother species, and when the arrival of the second animal
was likely the reason for the first species leaving. In
push-­off encounters we only used encounters that werecharacterized by apprehensive or agitated body languagein the animal leaving, or aggressive body language in theAugust 2016

ENCOUNTER COMPETITION IN MESOCARNIVORES

1907

animal arriving, in order to eliminate spurious data frompassive encounters during which a subdominant animal
may have fed while the dominant animal was absent. We
defined the winner of each encounter competition contest

as the species that controlled the carcass and was able tofeed at the end of the encounter; with each encounter being
recorded as a win, a loss, or a tie. In the event of a tie, where
both animals fed at the same time, the contest was

Table 1. The individual variables tested to predict the winner of encounter competition contests.
Name

Abbreviation

Unit

Description

Coefficient

P value

C scoreMass
Mass differential

MASS
MSDF

kg
kg

0.05470.1233

0.16
<0.01

0.57
0.67

Number of individuals

NUMB

number

−0.9400

0.29

0.52

Total time

TOTM

minutes−0.0001

0.83

0.47

Time before encounter

TBEN

minutes−0.0016

0.22

0.52

Visit duration

VSDR

minutes−0.2886

<0.01

0.78

Discoverer

DSCR

0 or 1−1.1056

<0.01

0.64

Controller

CONT

0 or 1−1.5581

<0.01

0.69

Health of animal

HLTH

0 or 1the mass of the given species
the difference in mass between
the given species and thespecies it is competing with
the number of individuals of thespecies present during the eventthe total time the species spent at
the carcass
the total time the species spent at
the carcass before theencounter occurred
the duration of the current visit
for the species at the carcass
before the event occurred
whether this species discovered
the carcass before the otherspecies
was the species in control of the
carcass when the encounteroccurred
was there any noticeable injury
or abnormal about the animal−1.2295

0.03

0.52

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Fig. 1. A map of the study area, in Mendocino National Forest in the North Coast Range of California, USA. The location ofeach of the 171 carcasses used to document encounter competition among mesocarnivores is noted.

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Ecology, Vol. 97, No. 8

Table 2. The base of the multi-variate models we tested to determine which hypotheses best explained the winner of interspecificencounter ­competition contests.
Name

Hypothesis

ReasonVariables

Size

The size of the animals will be
the most important influence.

MASS × MSDF

Strength

Animals with greater physicalstrength will be more likely towin.
Animals that are hungry will be
more aggressive and less likely
to cede the carcass to another.

Animals of larger size will have an
advantage over smaller animals, and thatwill determine who wins encounters.
The size, health, and number of animals
will be the most important influence of
who wins an encounter.
Animals that are hungry will be more
willing to engage others and risk injury,
and this will make them more likely to
win encounters.
The animal that is defending the carcass
will have an advantage, and this will
increase in cold weather with higher
metabolic need.

HungerReports

DefenseAnimals defending the carcass
will be more likely to win.

considered a win for each species in our statistical models,as they both received energetic gain. We provide thepairings of species in all contests in Appendix S1: Table S1.
For each encounter we then quantified eight variables(Table 1) to determine the best predictors of winners inencounter competition contests between mesocarnivores.
We determined “mass” for each species from the mean
body mass reported by Jameson and Peeters (2004). We
then calculated “mass differential” based on the difference in body mass between the two species in a given
encounter, with the large species given a positive number
and the smaller species a negative number. We determined “number of individuals” by how many individualsof each species were present in the encounter. We determined “total time” as the sum amount of time that eachspecies spent at the given carcass to the closest minute.
We calculated “time before encounter” as the combined
time for all visits the first species spent at the carcass
before the encounter occurred, and “visit duration” as
the amount of time during the specific visit the first species
was present at the carcass before the encounter occurred.
We defined the “defender” as the species that was currently feeding and hence defending the carcass when theencounter occurred. We noted “health” as either positiveor negative, with negative used when an individual animal
had a noticeable injury. We calculated “temperature” asthe mean daily temperature in the study area for the dayof the encounter using data available from National
Oceanic and Atmospheric Administration (Mendocino
Pass, California weather station).
Statistical analyses
We used program R version 3.0.0 (R Core Team 2013)
for all statistical analyses, and following R guidelines, wecited any associated packages used in analyses. We
employed an a priori approach to first test the influenceof select individual covariates of biological importance.
Second, we proceeded with meaningful combinations ofthese covariates to test more complex ecological

MSDF × NUMB × HLTH
VSDR × TBENDFND × TEMP

questions (Table 2). We employed generalized linear
models with a binomial link in the rms package (Harrell2013). We used win/loss as our binomial dependent variable, and first used each of the variables in turn as our
independent variable, before progressing to multivariate
models.
We determined the best predictors among individual
variables using their P values and C index scores (Hosmer
and Lemeshow 2000), and considered C index scores of
0.8 to be strong, and 0.6 to be relatively strong. For eachof our multivariate models, we calculated Akaike’s
Information Criterion (hereafter AIC) scores (Akaike1974), and used AIC weights (AICw) to compare models
to each other. We considered the top model and any subsequent model differing by < 4 ΔAIC units to have produced substantial empirical support for explaining
variation in the dependent variable, and any model
within 4–7 ΔAIC units of the top model to have produced
considerably less support, but to still hold biologicalinformation potentially relevant for ecological interpretation (Burnham and Anderson 2002). For each variablein our biologically relevant models, we then provided
their coefficient, odds ratio, and we calculated their
Cohen’s d score as follows:√
3d = LogOddsRatio ×pi
as suggested by Sanchez-­Meca et al. (2003).
In order to understand species-­
specific trends inencounter competition contests, we calculated a post hoc
statistic we termed “Wins Above Predicted” (WAP) for
each mesocarnivore species recorded. For each species,
we calculated a predicted score and an actual score,
assigning a point value (2 = win, 1 = tie, 0 = loss) for theoutcome of each encounter. Our predicted values werebased on encounter competition theory that predicts thatthe species with greater body mass will win encountercompetition contests (Connell 1983, Schoener 1983). The
species final WAP score was then the difference between
these predicted and actual scores.

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ENCOUNTER COMPETITION IN MESOCARNIVORES1909

Results
We recorded a total of 27, 314 videos at black-­tailed
deer carcasses, and documented 60 interspecific encountercompetition contests among mesocarnivores. Species
involved in contests included gray fox (n = 31), coyote(n = 28), bobcat (n = 27), western spotted skunk (n = 16),
fisher (n = 13), ringtail (n = 3), and raccoon (n = 2).

Table 3.

The best significant predictor of winning an
encounter competition contest among individual variables was “visit duration” (P < 0.01, C = 0.78), the only
variable with a strong correlation (Table 1). This suggests that animals that had been feeding for relatively
short durations were more likely to win, as they wanted
to continue to feed. Other variables with significant
and moderately strong correlations included:

The results from our comparisons of competition models.

NameStrength and hunger
Size and hunger
Hunger and control
HungerInvestment and control
Strength and control
ControlSize and control
Strength
Strength and investment
Size and investment
Size
Investment

Model

C index

AICc

ΔAIC

AICw

MSDF + NUMB + HLTH + VSDR + TBENVSDR + TBEN + MASS + MSDF
VSDR + TBEN + CONT + TEMPVSDR + TBENDSCR + TOTM + TEMP + CONTCONT + TEMP + MSDF + HLTH + NUMBCONT + TEMPCONT + TEMP + SIZE + MSDF
MSDF + NUMB + HLTHDSCR + TOTM + TEMP + MSDF + NUMB + HLTHDSCR + TOTM + TEMP + MASS + MSDF
MASS × MSDFDSCR + TOTM + TEMP

0.80
0.79
0.77
0.77
0.74
0.77
0.70
0.73
0.71
0.72
0.73
0.70
0.65

142.6
145.4
148.5
149.0
150.8
151.6
154.4
155.2
156.7
156.9
157.4
158.1
164.3

0
2.8
5.9
6.4
8.2
9
11.8
12.6
14.1
14.3
14.8
15.5
21.7

0.73
0.18
0.04
0.03
0.01
0.01
0.00
0.00
0.00
0.00
0.00
0.00
0.00

Notes: For each model we report the variables in the model, the C index score, and the Akaike information criterion weight(AICw). We determined the best model through comparison of AIC weights.

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Fig. 2. An illustrative figure of the dynamics of encounter competition between specific species. The difference between actualand predicted wins are noted on the lines between species. The two interspecific interactions most driving our results were bobcats
winning more often over coyotes than predicted, and spotted skunks winning more often over gray foxes than predicted.

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MAXIMILIAN L. ALLEN ET AL.

Table 4. The contributions of individual variables for our fourbiologically relevant models.
Variables

Coefficient

Odds ratio

D

2.98870.1236−2.24612.98870.1236−0.001619.8598
1.13160.10581.25130.76650.9984

–
0.07
−1.24
0.12
−0.15
0.00

1.1342−0.2548−0.0011−0.0544
0.1232

3.10870.7751
0.99890.94711.1311

–
−0.14
0.00
−0.03
0.07

1.1711−0.2117−0.0009−0.9353−0.00623.22550.80920.9991
0.39250.9938

–
−0.12
0.00
−0.52
0.00

0.7580−0.2868−0.0008

2.13400.75070.9992

–
−0.16
0.00

Strength and hunger
Intercept
MSDF
NUMB
HLTH
VLNT
TBEN

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Our wins above predicted (WAP) statistic showed strong
trends for four species (Table 5). Two species consistently
won more often than predicted: bobcats (WAP = +25) andspotted skunks (WAP = +19). Two species consistently lost
more often than predicted: coyotes (WAP = −27) and grayfoxes (WAP = −19). The WAP scores for these species were
influenced by two interspecific pairings; bobcats won
against coyotes during 12 encounter competition contests,
and spotted skunks won against gray foxes during sevenencounter competition contests (Fig. 2).

Size and hunger
Intercept
VLNT
TBEN
MASS
MSDF

Discussion

Reports

Hunger and control
Intercept
VLNT
TBEN
CONT
TEMP
HungerIntercept
VLNT
TBEN

“defender” (P < 0.01, C = 0.69), “mass differential”(P < 0.01, C = 0.67), and “discoverer” (P < 0.01,
C = 0.64; Fig. 2).
Our multivariate model comparisons identified two
top models (ΔAIC = 3.2), and an additional two models
that provided biologically relevant information(ΔAIC < 7; Table 3). Both “strength and hunger” and“size and hunger” produced considerable support forpredicting the outcome of encounters. The complexity ofvariables in the top-­ranked models (Table 4) emphasizedthat much more than size influenced the outcomes ofencounter competition contests.

This is among the first extensive studies of encountercompetition among mesocarnivores in North America, andextends our knowledge of encounter competition theory tointerspecific contests and natural ecological systems withmultiple carnivorous mammals. Our results show that bodymass is not the only trait influencing the outcome of interspecific encounter competition, and that there are multiple
traits that are important in determining the winner. For
dominant species, the cost of encounter competition is low,while the value of gaining exclusive access to a resource
tends to be high (Dickman 1991). Carrion is such a valuable
source of nutrition that it may increase the motivation ofsubordinate species to directly compete for it (e.g., Connell1983) despite the potential costs involved (Briffa andSneddon 2007). Mesocarnivores are also highly adapted
species with species-­
specific adaptations that include
strategy, aggressiveness, and weapons that may influence
the outcome of encounter competition contests (Martin andGhalambor 2014). Indeed, our WAP statistic suggests thatsome species, including bobcats and spotted skunks, hadadvantages over other species despite their smaller mass.
The best models explaining which species won competitive encounters included “hunger” (the feeding timeoverall and during a specific feeding bout) and “strength”(mass differential, the number of individuals in a group,and significant injuries), while “hunger” was also part of
the next best model. Body mass and the number of individuals clearly matter in determining the winner ofTable 5. The “wins above predicted” (WAP) for each of the seven mesocarnivores we recorded during encounter competition
contests.
Predicted

ActualSpecies

Win

Loss

Tie

Points

Win

Loss

Tie

Points

WAP

BobcatSpotted skunk
FisherRingtail
RaccoonGray fox
Coyote

9
0
0
0
2
21
28

18
16
13
3
0
10
0

0
0
0
0
0
0
0

18
0
0
0
4
42
56

21
7
2
0
1
8
14

5
4
11
3
1
17
13

1
5
0
0
0
5
1

43
19
4
0
2
23
29

+25
+19
+4
0
−2
−21
−27

Note: First are the predicted points based on the mass of the species competing, followed by their actual points, and their total WAP.

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consequences (e.g., Briffa and Sneddon 2007, van derMeer et al. 2011), especially if they are losing resourcesthat they acquired themselves. Our experiments werebased on carrion, where the “loser” experienced a limited
loss beyond the nutrition that could be gained from theresource itself and the time spent attempting to acquire it.
An animal that actually kills prey, however, would suffer
significant costs if they lost encounter competition contests that led to a loss of the resource (Krofel et al. 2012,
Allen et al. 2015, Elbroch et al. 2015). In contrast, species
can also respond positively to interspecific competition(e.g., Connell 1983). For example, even though grayfoxes lost encounter competition contests more often
than predicted, interspecific interactions may have ledthem to discover carrion resources they otherwise would
not have found, leading to a net gain from the resource.
Therefore, despite frequently losing encounter competition contests, interspecific interactions may lead to anincrease in the acquisition of carrion.
Our study was based on a relatively small sample
despite considerable efforts invested in the field. We
recorded mesocarnivore encounter competition contests
during only 0.0022% of videos recorded, although eventhis might be considered a high rate of return on ourinvestment when compared with other research on thesubject (e.g., Dickman [1991] did not record a single
contest between insectivorous mammals in enclosuresover 116 h of observation). Thus the use of motion-­triggered video cameras at important resources, including
carcasses, create foraging arenas (Ahrens et al. 2012) thatmay provide exceptional opportunities for the study ofencounter competition in vertebrates in natural environments. Our small sample size could also be interpreted tomean that interspecific encounter competition contests
were too costly to engage in frequently, as is the case for
intraspecific contests as well (Briffa and Sneddon 2007).
Certain species appeared to avoid encounters with specificspecies that were clearly dominant. For example, ringtails
appeared to avoid encounters with any other carnivore
whenever possible, leading to only two recorded contests.
We also never recorded encounter competition between
coyotes and gray foxes, which could be explained by thefact that coyotes are significant predators of gray foxes(Fedriani et al. 2000). That specific species pairings neveroccurred or were very rare, does support existing theory,and suggest “bigger is better” in some cases, as withinfamilies of mesocarnivores with morphologically similar
weapons. We recorded more encounters between speciesof relatively similar size and overlapping diet (Donadio
and Buskirk 2006), which may make the outcome lessdecided, and encourage interspecific competition for
species to exploit their species-­specific advantages.
Understanding spatial and temporal occupancy patterns of species is an important focus of many ecologicalstudies (e.g., Lesmeister et al. 2015, Wang et al. 2015),
and here we have informed that process through the creation of foraging arenas (Ahrens et al. 2012) to investigate the mechanisms underlying interspecific encounter

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encounter competition contests (e.g., Connell 1983,
Schoener 1983), and may be an important evolutionary
reason for sexual dimorphism where larger mass decides
the RHP of intraspecific contests (Briffa and Sneddon2007). The value of the resource being competed for also
appears to matter. Food resources, especially those thatare highly valued and only available for short duration
like carrion (Wiens 1977, Tanner and Adler 2009), may
encourage fierce interspecific encounter competition, as
compared with resources like dens or refugia that may
not translate across species. Our results suggested thatthe animal that was hungrier was also more motivated to
expend energy to gain a carrion resource.
Our WAP statistic suggested that important aspects
that determine the winner of interspecific encounter competition contests are species-­specific advantages. Bobcats
and spotted skunks won more encounters than were predicted based on size alone, while coyotes and gray foxes
lost more encounters than expected. Their higher RHP
appears to be linked to both behavioral traits and better
weapons (i.e., hooked claws for grappling in addition toteeth for bobcats, chemical weapons for skunks), or
simply an increased ability to defend carcasses than otherspecies. For example, we observed bobcats attacking
coyotes with their front claws, especially in cases where
the bobcat was defending the carcass. Spotted skunk’s
chemical defenses and asymptomatic coloring (Hunter2009) can also be used in offensive behaviors to increase
their RHP and ability to win encounter competition contests (e.g., Courtene-­Jones and Briffa 2014, Martin andGhalambor 2014); skunks need not actually use theirchemical weapons, but only need to be abundant enoughthat potential predators have learned the consequences ofinteracting with one (Hunter 2009). We observed skunks
approaching gray foxes at carcasses, and foxes retreating
5–7 m from the carcass while the skunk fed. A spottedskunk was documented usurping a kill from a puma
nearly 100 times its size (Allen et al. 2013), and so it is not
surprising that they were able to win encounters withmesocarnivores such as gray foxes only eight times larger.
The two species, coyotes and gray foxes, which lost more
often than expected, are both canids, and this suggeststhat canids may have similar behavioral strategies.
Alternatively, canids may simply be more risk averse
because they have developed fewer weapons than othermesocarnivores, as they can only attack with their teeth,
as opposed to the claws of bobcats or chemical attacks ofskunks. Some species have developed traits to increase
their RHP in interspecific encounter competition contests, including larger muscles and claws (Martin andGhalambor 2014). In intraspecific contests as well, individuals of some species have developed strategies toincrease their RHP despite their smaller size (Neems et al.
1990, Zamudio and Sinervo 2000). These adaptations arelikely to shape the behavioral ecology of the species,
including the mesocarnivores in our study.
Species or individuals that regularly lose interspecificencounter competition contests may experience fitness

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competition. This is the first experimental study of­interspecific encounter competition among mesocarnivores, extending our understanding of interspecificencounter competition based on studies involving invertebrates (e.g., Courtene-­
Jones and Briffa 2014). We
found that, similar to other taxonomic groups, bodymass is important, but this can be overshadowed bymotivation and other species-­
specific traits such as
weapons. Future studies should try to better understandthe role of species-­
specific adaptations in encountercompetition contests including possible evolutionary
trade-­
offs (e.g., Martin and Ghalambor 2014), andunderstanding the strategies of subordinate species thatallow them to exploit resources in the presence of dominant species. Our results suggest that access to importantresources provides life-­
history benefits that outweighrisks of injury or death, and these results inform ourunderstanding of the abundance and distribution patterns of species in ecological communities.

Reports

Acknowledgments
The California Department of Fish and Wildlife and Victoria
University of Wellington generously provided funding for the
study. We thank D. Casady, B. Evans, and the numerous fieldtechnicians that helped with the project. V. Yovovich and two
anonymous reviewers provided comments on previous drafts
that greatly improved the manuscript.
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Supporting Information
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doi/10.1002/ecy.1462/suppinfo