165
NOTES
Differences in key habitat use between dominant
and subordinate animals: intraterritorial
dominance payoffs in Eurasian badgers?
Eloy Revilla and Francisco Palomares
Abstract: Group-living territorial animals such as the Eurasian badger, Meles meles L., face the problem of intragroup
competition. Badgers are asymmetric in their access to reproduction (dominant individuals being the ones that reproduce), but little information exists about the extent of intragroup trophic competition. We studied badgers’ use of a key
trophic resource (the European rabbit, Oryctolagus cuniculus), as well as the use of the habitat where this resource is
located (key habitat, Mediterranean scrubland) by a low-density group-living population of badgers in Coto del Rey,
Doñana, southwestern Spain. During 1995–1996, there was a 2.2-fold reduction of rabbit density, which was reflected
in a significant diminution of rabbit use; despite this, rabbits continued to be the most used trophic resource. Notwithstanding the decrease in rabbit density, subordinate badgers reduced their use of the key habitat, while dominant badgers increased it. These results suggest that in Coto del Rey, badger groups exhibit a fully despotic system, with
dominance by some individuals not only in access to reproduction, but also in access to food through unequal use of
the key habitat that contains it.
Résumé : Les animaux terrestres qui vivent en groupes, tels le Blaireau d’Eurasie, Meles meles L., doivent faire face à
la compétition intra-groupe. Les blaireaux sont asymétriques quant à leur accès à la reproduction (les individus dominants sont les reproducteurs), mais il existe peu d’information sur l’importance de la compétition trophique intragroupe. Nous avons examiné l’utilisation d’une ressource trophique critique (le Lapin de garenne, Oryctolagus cuniculus) et l’utilisation de l’habitat où se trouve cette ressource (habitat optimal, maquis méditerrannéen), chez une population de petite densité de blaireaux vivant en groupes (Coto del Rey, Doñana, sud-ouest de l’Espagne). En 1995–1996, il
s’est produit une réduction de la densité des lapins par un facteur de 2,2. La réduction de la disponibilité des lapins a
causé une diminution significative de la prédation exercée sur eux, mais les lapins sont restés la ressource trophique de
prédilection. Indépendamment de la diminution de la densité des lapins, les blaireaux subordonnés se sont mis à moins
utiliser l’habitat optimal, alors que les dominants ont entrepris de les utiliser davantage. Ces résultats semblent indiquer
qu’à Coto del Rey, les groupes de blaireaux sont organisés en un système despotique où quelques individus dominent
l’accès à la reproduction et ont en plus l’avantage de l’accès à la nourriture à cause de l’utilisation inégale de l’habitat
optimal.
[Traduit par la Rédaction]
Introduction
Intraspecific territoriality is produced by asymmetric despotic competition for a given resource (Davies 1978; Gordon
E. Revilla1 and F. Palomares. Department of Applied
Biology, Estación Biológica de Doñana, Consejo Superior
de Investigaciones Científicas, Avenida Maria Luisa s/n,
41013 Sevilla, Spain.
1
Author to whom all correspondence should be sent at the
following address: Department of Ecological Modelling,
Umweltforschungszentrum (UFZ) (Centre for Environmental
Research), Leipzig–Halle, P.O. Box 2, D-04301 Leipzig,
Germany (e-mail: elsa@oesa.ufz.de).
1997). The main benefit to territory owners is a greater
probability of holding this resource and, eventually, greater
overall fitness (Stamps 1994). On the other hand, group living is favoured by such factors as high predation pressure and
patchy resource distribution (Alexander 1974; Wrangham and
Rubenstein 1986). Resource competition among group members has been reported in many social-living species (Barton
and Whiten 1993; Hall and Fedigan 1997; Packer and Caro
1997; Schmidt and Mech 1997). Therefore, the role of intragroup competition for the resource that promotes or reinforces territoriality might be an important factor in the ecology
of territorial species living in groups. Trophic resources have
been proposed as one of the most important determinants of
both territoriality and group living. In some cases, intragroup
competition is reduced through the exploitation of renewable
resources (Waser 1981). In others, it has been suggested that
patchy resource distribution allows the primary owner to
share the territory with other individuals at low cost (Macdonald 1983; Kruuk and Macdonald 1985; Carr and Macdonald 1986).
Eurasian badgers, Meles meles L., are territorial, solitary
foragers (Kruuk 1989; Woodroffe and Macdonald 1993). At
high population density, they live in groups, sharing a communal territory but exhibiting little or no cooperation between individuals other than shared territory defence (Kruuk
1989; Woodroffe and Macdonald 1993). Under these circumstances, food competition within the group is presumed
to be low, making living in a common territory inexpensive.
Locally, group-living populations of badgers may rely on
one food type, mostly earthworms (Lumbricus spp.) or European rabbits (Oryctolagus cuniculus; for example, see Kruuk
and Parish 1981; Martín et al. 1995). These trophic resources
are concentrated in key habitats: deciduous woodlands and
pastures in the case of earthworms (Kruuk 1978; Hofer 1988;
Kruuk 1989) and Mediterranean scrubland in the case of
rabbits (Palomares et al.1996; Palomares and Delibes 1997).
Access to reproduction is asymmetric among group members,
determining social status (Kruuk and Parish 1987; Cresswell
et al. 1992; Woodroffe and Macdonald 1995). Female body
condition, which depends on resource availability, is, in combination with age, an important factor determining reproduction
probability (Cresswell et al. 1992; Woodroffe and Macdonald
1995). Under normal conditions, competition for trophic resources is thought to be negligible within groups (Woodroffe
and Macdonald 1993). However, the question of whether
intragroup competition occurs under trophic stress remains
unanswered, despite its importance in understanding the regulation of the social system of badgers. Two alternative answers to this question are possible. If the territory is equally
accessible to all individuals, there should be no asymmetry
in access to the key trophic resource and, hence, they would
be expected to suffer equally from food stress (scramble
competition). In contrast, if badgers are unequal competitors,
with a dominance hierarchy in access not only to reproduction but also to food when it is limiting, dominant animals
should preferentially use the key habitat that contains the
resource (contest competition).
We studied the use of the main trophic resource, and the
use of the key habitat holding that resource, by a groupliving population of badgers that uses rabbits as the staple
food. After a decline in the rabbit population, badgers were still
using this lagomorph as their main prey, but the use of the
key habitat changed between individuals of different reproductive status.
Methods
Study area
The study was carried out during 1995 and 1996 in an area
called Coto del Rey at the northern border of Doñana National
Park in southwestern Spain. The climate is Mediterranean subhumid
with Atlantic influence. Winters are wet and mild, while summers
are hot and dry; the normal annual rainfall is 500–600 mm/m2.
During the study there was a shift from a drought (252 mm/m2 of
rainfall during winter 1995) to a torrential rainfall period with frequent flooding events (1031 mm/m2 in winter 1996). Thus, we distinguish between a dry period (winter, spring, and summer 1995)
and a wet period (autumn 1995 and all of 1996). We considered
seasons to be as follows: winter, January to March; spring, April to
June; summer, July to September; and autumn, October to December. We defined the study area (42 km2) as a minimum convex
polygon enclosing all the radiolocations of marked badgers (see
below). This area is covered by 49.0% marshland, 14.7% pine
plantations, 12.1% Mediterranean scrubland, 11.6% pastureland,
8.2% Mediterranean savannah (“dehesa”), 2.6% ash stands, and
1.1% Eucalyptus spp. plantations (Revilla et al. 2000). Vegetation
types define the habitat types (for a further description of the study
area and vegetation types see Revilla et al. 2000).
Badger population
We trapped, measured, and weighed badgers, marked them with
a microchip, and equipped them with a radio collar (A. Wagener,
Telemetrieanlagen HF-NF Technik, Herwarthstraße 22, Köln, Germany). Marked animals belonged to three and four groups in 1995
and 1996, respectively. Territory size was, on average, 406 ha (defined as the 95% kernel isoline; Revilla 1998). Badger density was
0.83 and 0.75 individuals/km2 in 1995 and 1996, respectively (Revilla
et al. 1999). The average number of animals per social group was
3.2 (excluding yearlings, estimated using sightings at setts and
track censuses; for more details see Revilla et al. 1999). The number of animals present (for at least part of the time) in the population was 11 and 8, of which 9 and 7 were radio-marked (82–88%),
during 1995 and 1996, respectively. Animals were radiolocated via
triangulation (Revilla et al. 2000).
We defined the social status of individuals in terms of their reproductive asymmetry: dominants were those that showed reproductive activity. In the case of females, pregnancy and lactation
during any capture was sufficient for them to be considered dominant, since there was no case of double reproduction within any
territory (Revilla et al. 1999). The oldest males, which also had the
largest descended testes, were considered dominants. Subordinates
were animals born on the territory in previous years, but that showed
no reproductive activity (despite being sexually mature; Revilla et
al. 1999). In total, we radio-tracked 12 different individuals, of
which 6 were considered dominants and 6 subordinates.
Resource variation
Rabbits are the main prey species for a large number of predators in Mediterranean ecosystems (Delibes and Hiraldo 1981), and
badgers are no exception (Martín et al. 1995; Fedriani et al. 1998;
Revilla 1998). The rabbit population of Coto del Rey has been the
subject of long-term monitoring (Palomares et al. 1996, 2000;
Palomares and Delibes 1997). Rabbit abundance was estimated by
line-transect sampling (Buckland et al. 1993) carried out in randomly selected areas within the habitat types considered. Transects
ranged in length from 1100 to 2040 m per habitat and were slowly
(ca. 1.5–2.5 km/h) surveyed at dusk. We grouped observation distances of rabbits into 10-m intervals and truncated the distance at
50 m to improve the robustness of the estimator of density. The
truncation distance and grouping options were chosen after three
pilot samplings. Transects were always the same, did not follow
any track or road in the study area, followed a straight line, and
were always walked by the same observer. Rabbits were counted
seasonally during the period of study. We estimated rabbit density
(and 95% confidence intervals) with the program TRANSECT (Burnham
et al. 1980). (For more information about differences between habitat types, characteristics of the rabbit population, and methodology
see Palomares et al. 2000.)
Resource use
Diet
To detect changes in the importance of rabbits as a trophic resource during the two periods, we studied diet through analysis of
faeces collected at setts. We followed the general methodology
used in other studies of badger feeding habits (see Kruuk and
© 2001 NRC Canada
Notes
167
Fig. 1. Use of Mediterranean scrubland habitat (key habitat) during the foraging period by dominant and subordinate Eurasian badgers
(Meles meles), and rabbit abundance (number/ha) in Mediterranean scrubland, in Coto del Rey, Doñana, southwestern Spain. Data are
shown as the mean and standard error (95% confidence intervals for rabbit abundance) for winter (WI), spring (SP), summer (SU), and
autumn (AU) of 1995 (95) and 1996 (96).
Parish 1981; Martín et al. 1995), except for estimating ingested
biomass, for which we used correction factors calculated in a feeding
trial (Revilla 1998). In total we analysed 624 and 382 scats from
1995 and 1996. Differences in the pattern of use (presence–absence
of rabbit in every scat) between seasons and years were analysed
with multiway analysis of frequencies (logit models, procedure
CATMOD; SAS Institute Inc. 1990b). The significance level for posthoc comparisons was set with the Bonferroni correction.
Habitat
Mediterranean scrubland is the only habitat type significantly
preferred by badgers in Coto del Rey. Badgers located their territories so as to include as much of this habitat as possible, while
intraterritorial use was also biased towards Mediterranean scrubland during both foraging and resting (Revilla et al. 2000). This
pattern of habitat selection and preferences is strongly correlated
with rabbit abundance (Revilla et al. 2000). Within every territory
the areas of available Mediterranean scrubland were similar, about
142 ± 28.63 ha (mean ± 1 SD, N = 5). Percent use of Mediterranean scrubland was calculated using radiolocations (N = 889), separated by at least 4 h for the same individual to ensure statistical
independence (Revilla 1998), obtained during the period of activity
of the animals. We analysed the use of this key habitat with a general linear model to detect any variation between periods (dry and
wet), seasons, sexes, and reproductive status of the animals. The
angular-transformed percentage of radiolocations in Mediterranean
scrubland was the dependent variable. Independent variables were
season, period, and class of reproductive dominance. We also included individuals as a nested independent variable to account for
any variability between the animals. The initial model corresponded
to a factorial complete design with type III sum of squares (procedure GLM in SAS; SAS Institute Inc. 1990a). From the initial
complete model, we eliminated the nonsignificant factors (those
with the lowest sum of squares) one by one, beginning with the
most complex interactions, until we obtained a final model with
only significant predictors. We did not standardise the percentage
of locations into a selection ratio because Mediterranean scrubland
was present in all territories considered and it was the preferred
habitat type for every individual (Revilla et al. 2000), so percentages were comparable among territories.
Results
During the study period, maximum rabbit densities occurred in Mediterranean scrubland habitat (41.5 rabbits/ha
in June 1995) and minimum densities in pine plantations
(<0.1 rabbits/ha in October 1996). The difference in rabbit
density between Mediterranean scrubland and other habitat
types varied between 1.5- and 23-fold. After the beginning
of the rains in autumn 1995, there was a 2.2-fold reduction
in rabbit density in Mediterranean scrubland (Fig. 1). This
diminution was even greater in other habitat types such as
ash stands or pasturelands, where rabbit density was between 5.4 and 11.1 times lower than during the previous
year.
Resource use: diet
Rabbits were the most important prey during both dry and
wet periods, accounting for 65.4 and 44.7% of ingested biomass, respectively. As expected, there was less use of rabbits
in 1996 than in 1995 (67.8 and 62.5% occurrence and 58.9
and 49.2% biomass ingested during 1995 and 1996, respectively; Fig. 2). Multiway analysis of frequencies showed a
significant effect of year and season (χ2 = 7.75, df = 1, P =
0.0054, and χ2 = 118.57, df = 3, P < 0.0001), but not of their
interaction (χ2 = 5.18, df = 3, P = 0.1590). Differences between years were due to the lower percent occurrence during
1996 (Fig. 2). All seasons differed from one another (posthoc comparisons, χ2 > 13.27, df = 1, P = 0.0003 in all cases)
except for winter and spring (χ2 = 0.81, df = 1, P = 0.3689),
when badgers most frequently consumed rabbits (Fig. 2).
Resource holding: key-habitat use
Mediterranean scrubland was the most used habitat yearround. Its use was lowest during summer, with 62% of active
radiolocations, and greatest in winter, with 87%. In the final
model of the use of Mediterranean scrubland (r2 = 0.67, F =
3.45, df = 16, 27, P = 0.0023), dominance status, individuals,
© 2001 NRC Canada
Fig. 2. Use of rabbits as a trophic resource by badgers in Coto del Rey, represented as percent occurrence and ingested biomass, for
winter (WI), spring (SP), summer (SU), and autumn (AU) of 1995 (95) and 1996 (96). The numbers above the bars show the number
of scats analysed per season.
Table 1. Results of the final general linear model of the use of
Mediterranean scrubland by Eurasian badgers (Meles meles) in
Coto del Rey, Doñana, Spain.
Variable
df
Type III sum
of squares
F
P
Dominance status
Period × dominance status
Individual
1
2
13
0.1491
1.0013
1.1501
4.43
14.86
2.63
0.0449
0.0001
0.0166
and the interaction between dominance status and period
were significant (Table 1). The strong effect of the interaction between status and period was due to a change in the
use of Mediterranean scrubland between the dry and wet
periods, with subordinates decreasing and dominants increasing their use of this habitat in a complementary way
(post-hoc t test on the least-squares means, t > 3.02, P <
0.005; Fig. 1).
Discussion
Badgers are generalists, but locally may depend on one
trophic resource, such as rabbits (Martín et al. 1995; Fedriani
et al. 1998). Mediterranean scrubland is the key habitat for
rabbits (Palomares et al. 1996; Palomares and Delibes 1997),
hence it contains the key trophic resource for badgers in the
Coto del Rey area (Revilla et al. 2000). During the study
period, there was a 2.2-fold reduction of rabbit density in
Mediterranean scrubland habitat. This diminution was even
greater in other types of habitat such as dehesa or ash stands
(Palomares et al. 2000). Therefore, Mediterranean scrubland
probably played an increasing role as the most important
habitat for badgers during the wet period. After the decline
in the rabbit population, badgers reduced their use of this
food; however, rabbits were still the main trophic resource
in winter 1995 and spring 1996.
There was a clear shift in habitat use between the two status classes of badgers. Dominants increased their use of
Mediterranean scrubland at the beginning of the rains and
kept using it throughout the period of low rabbit density.
Subordinates followed the opposite pattern (Fig. 1), reducing
their use of the areas that still contained the main resource.
The most reasonable interpretation of these results is that
during the period of low rabbit density, dominants were excluding subordinates from rabbit-rich areas. If this interpretation is correct, badgers would be asymmetric not only in
access to reproduction (Revilla et al. 1999) but also in access
to the key trophic resource when the carrying capacity of the
territory is reached.
Suppression of reproduction is one of the mechanisms by
which group size in Eurasian badgers is adjusted to the local
availability of resources (Cresswell et al. 1992; da Silva
et al. 1994; Woodroffe and Macdonald 1995). For females,
good body condition is a necessary precondition for reproduction (Woodroffe and Macdonald 1995). Our findings suggest that dominant females may use habitat exclusion as a
means of suppressing reproduction of subordinates via body
condition. In accordance with this interpretation, subordinate
badgers in Doñana had poorer body condition than dominants
(Revilla 1998).
The badger population in Coto del Rey is characterised by
low density and the importance of rabbits in the diet (Revilla
et al. 1999). However, in areas of Central Europe and the
British Isles, badgers feed on earthworms, and reach much
higher densities. Earthworms are spatially and temporally
less predictable than rabbits. Predictability is probably important in the competition for food, as dominants should be able
to evict subordinates from resource-rich patches. If resourcerich habitat patches were less predictable, this exclusion would
be more difficult, or even impossible. Doñana badgers feed
© 2001 NRC Canada
mainly on rabbit kittens inside warrens, where they are accessible through digging (Martín et al. 1995). Areas of high
rabbit density within Mediterranean scrubland habitat follow
a gradient of increasing density towards the ecotones with
pastureland and marshland (Palomares et al. 1996; Palomares
and Delibes 1997). This spatial distribution makes the availability of rabbits very predictable between years, as clusters
of warrens do not change substantially in the short term. The
breeding season of rabbits also follows a very predictable
temporal pattern (Rogers et al. 1994). In contrast, earthworm
availability depends on day-to-day factors such as temperature and humidity, local factors such as grass height, and
landscape factors such as habitat type (Kruuk 1989). Therefore, we might expect that in areas where badgers have an
earthworm-based diet, it would be difficult for dominants to
control the use of patches with low predictability. If this
holds true, in these high-density populations, food stress will
produce a pattern closer to scramble competition than to
contest competition, which should be reflected in poorer
body condition for all individuals (including dominants) during periods of prey scarcity. This has been shown to some
extent by Rogers et al. (1997) in Gloucestershire, England,
and by Kruuk and Parish (1985) in northern Scotland. However, if badgers shift to more predictable resources (such as
cereal crops) during periods of scarcity, there may be a
change from scramble to contest competition, where dominant individuals would be at an advantage, as is shown by
data on reproduction during a poor year and a good year in
England (Woodroffe and Macdonald 1995).
Models of badger social organisation suggest that group
living has evolved as a response to the low or absent competition between group members, owing to the patchy distribution
of resources and (or) high rates of prey renewal (Woodroffe
and Macdonald 1993; da Silva et al. 1994). If carrying capacity is reached or reduced, our results suggest that different levels of competition might be expected, depending on
the predictability of the patches of key resource, following a
gradient between scramble and contest competition, in which
dominants would be at an advantage.
Acknowledgements
This research was founded by the Dirección General de
Investigación Científica y Técnica (projects PB94-0480 and
PB97-1163) and sponsored by Rover España. E.R. was supported by a predoctoral grant from the Spanish Ministry of
Education and Culture. J. Ayala, J. Calzada, and numerous
students helped with field and laboratory work. The comments of J. Calzada, M. Delibes, and two anonymous reviewers greatly improved the manuscript.
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