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Burrows play an essential role for the European wild rabbit. They provide shelter, a safe breeding place and protection against climatic extremes. However, little is known about burrow structure and the environmental factors influencing it. In this work we analyse the structure of rabbit burrow tunnels in southwestern Spain and search for structural types. In addition, we study the influence of soil composition on burrow tunnel structure. For this purpose, a total number of 122 burrow tunnels were located and measured. A principal component analysis and a two-step cluster analysis were performed to identify burrow tunnel types. Three different types of natural burrow tunnels have been identified. Furthermore, the texture and composition of the soil were found to notably influence burrow tunnel structure. This study shows that rabbits tend to excavate bigger burrow tunnels in soils characterised by sandy soils of a mostly large particle composition, while the higher proportion of small particles in silty soils promotes shorter and narrower burrow tunnels.
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Ethology Ecology & Evolution
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Burrow types of the European wild rabbit in southwestern Spain
S. Serranoa; S.J. Hidalgo de Truciosb
a Dipartimento di Biologia Animale, Università degli Studi di Pavia, Pavia, Italy b Grupo de
Investigación en Recursos Cinegéticos y Biodiversidad, Facultad de Veterinaria, Universidad de
Extremadura, Cáceres, Spain
Online publication date: 13 January 2011
To cite this Article Serrano, S. and Hidalgo de Trucios, S.J.(2011) 'Burrow types of the European wild rabbit in
southwestern Spain', Ethology Ecology & Evolution, 23: 1, 81 — 90
To link to this Article: DOI: 10.1080/03949370.2010.534318
URL: http://dx.doi.org/10.1080/03949370.2010.534318
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Ethology Ecology & Evolution 23: 81–90, 2011
Burrow types of the European wild rabbit
in southwestern Spain
S. SERRANO 1,3 and S.J. HIDALGO DE TRUCIOS 2
1Dipartimento di Biologia Animale, Università degli Studi di Pavia, Piazza Botta 9,
27100, Pavia, Italy
2Grupo de Investigación en Recursos Cinegéticos y Biodiversidad, Facultad de Veterinaria,
Universidad de Extremadura, 10071 Cáceres, Spain
Received 13 April 2009, accepted 11 September 2010
Burrows play an essential role for the European wild rabbit. They provide shel-
ter, a safe breeding place and protection against climatic extremes. However, little is
known about burrow structure and the environmental factors influencing it. In this
work we analyse the structure of rabbit burrow tunnels in southwestern Spain and
search for structural types. In addition, we study the influence of soil composition on
burrow tunnel structure. For this purpose, a total number of 122 burrow tunnels were
located and measured. A principal component analysis and a two-step cluster analy-
sis were performed to identify burrow tunnel types. Three different types of natural
burrow tunnels have been identified. Furthermore, the texture and composition of the
soil were found to notably influence burrow tunnel structure. This study shows that
rabbits tend to excavate bigger burrow tunnels in soils characterised by sandy soils
of a mostly large particle composition, while the higher proportion of small particles
in silty soils promotes shorter and narrower burrow tunnels.
KEY WORDS:Oryctolagus cuniculus, burrow structure, soil composition, tunnel
types.
INTRODUCTION
The European wild rabbit (Oryctolagus cuniculus), an endemic species of the
Iberian Peninsula (MONNEROT et al. 1994), is a keystone species in Mediterranean
habitats (DELIBES-MATEOS et al. 2007) and an important ecosystem engineer (GÁLVEZ
BRAVO et al. 2009). Unfortunately, the species has suffered a drastic decline in its his-
torical range (MORENO et al. 2007; VIRGÓS et al. 2007) due to myxomatosis and rabbit
haemorrhagic disease (VILLAFUERTE et al. 1995) combined with the loss of suitable
habitat (VIRGÓS et al. 2003).
Without having important morphological adaptations for the excavation process,
digging is one of the main features of the rabbit (THOMPSON &KING 1994). Warrens
3Corresponding author: Sara Serrano, Dipartimento di Biologia Animale, Università degli Studi
di Pavia, Piazza Botta 9, 27100 Pavia, Italy (E-mail: sara.serrano@unipv.it).
ISSN 0394-9370 print/ISSN 1828-7131 online
© 2011 Dipartimento di Biologia Evoluzionistica dell’Università, Firenze, Italia
DOI: 10.1080/03949370.2010.534318
http://www.informaworld.com
Downloaded By: [Hidalgo de Trucios, S J] At: 16:06 14 January 2011
82 S. Serrano and S.J. Hidalgo de Trucios
have an important role as shelter (PARER 1977; KOLB 1994), providing a safe breeding
place (MYKYTOWYCZ &GAMBALE 1965) and protection against climatic extremes. The
presence of the warren is particularly important in open habitats with no shrub protec-
tion (PALOMARES &DELIBES 1997; LOMBARDI et al. 2003). Additionally, rabbit warrens
may play an important ecological role in some areas where the species has been intro-
duced. The European rabbit replaces the role of some native burrowers in some areas
of Australia, where its burrows provide shelter for many species (READ et al. 2008).
Although one single rabbit can dig a long burrow network of 2 m in one night,
as described by MYERS (1958), warrens are usually the result of the excavating effort
of a family group over several years. The continuous digging by a group of rabbits
can build a structure of high complexity, consisting of a ramified network with several
chambers. A warren is a dynamic structure that depends on several factors, such as the
number of rabbits living in it, the social relationship between them and the variation
of rabbit density during the years (KOLB 1985). Recent studies point to the need to also
take into account spatial factors, such as proximity to nearby warrens (BARRIO et al.
2009). Soil type also influences warren structure. Sandy soils are the most favourable
for rabbit excavation, and strongly influence warren structure and spatial distribution
(MYERS &PARKER 1975a; PARER &LIBKE 1985). On the other hand, according to
other authors, warrens in sandy soils are usually smaller than warrens in heavy soils
(KOLB 1985). The low water-holding capacity of sandy soils prevents the killing of young
rabbits by flooding or collapsing of the warren due to heavy rains (MYERS &PARKER
1965). Other strategies to stabilise the warren structure in sandy soils are the excavation
of the warren in an elevated position (MYERS &PARKER 1975b) or under supporting
structures, such as of tree or shrub roots and rocks (PALOMARES 2003; GEA-IZQUIERDO
et al. 2005).
Recent work has been carried out to explain rabbit warren habitat selection at
different space scales in the Iberian Peninsula (PALOMARES 2003; GEA-IZQUIERDO
et al. 2005; DELLAFIORE et al. 2008). However, little empirical research on the rabbit
warren structure and the influence of soil composition on it has been conducted. Most
of the few studies concerning burrow structure refer to subterranean rodents such as
the black-tailed prairie dog (Cynomys spp.) (WILCOMB 1954; SHEETS et al.1971) or the
tuza (Thomomys bottae)(V
LECK 1981; SEABLOOM et al.2000), and most of the scarce
studies about rabbit warren structure have been limited to analysing stops (LLOYD
&MCCOWAN 1968) or very few warrens (OLIVER &BLACKSHAW 1979; PARER et al.
1987). On the other hand, most studies relating to warrens and soil influence upon
them have been conducted in countries where the species has been introduced (PARER
&LIBKE 1985; ROGERS et al. 1994). Thus, KOLB (1985) proposed three variables that
summarised rabbit burrow structure in warrens of Scotland: number of entrances,
an inverse relationship between number of burrow entrances and depth, and an
inverse relationship between length of burrow entrances and number of junctions.
More recently, some authors have provided new records of variables of the warren
structure, such as the number of burrow entrances, burrow entrance size and clustered
distribution of the warrens, in order to improve the building of artificial warrens
(GEA-IZQUIERDO et al. 2005; PAULA &PALOMARES 2007) or to provide insights into
the warren ripping effect in vegetation communities (ELDRIDGE et al. 2006).
However, little is known about the structure of rabbit burrows in the Iberian
Peninsula and far fewer studies analyse burrow structure in relation to soil compo-
sition. The purpose of this study was to analyse the structure of rabbit burrow tunnels
in southwestern Spain and to search for structural types. We also study the influence of
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Burrow types of the European rabbit 83
soil composition on burrow tunnel structure. The relationship of burrow structure and
species behaviour is also discussed.
MATERIALS AND METHODS
Areas of study
The study was carried out in five different locations within the region of Extremadura
(Spain) between 2003 and 2004. Extremadura is located in western Spain at the south plateau
of the Iberian Peninsula between 37.95N and 40.48N of latitude and 4.65W and 7.55Wof
longitude. This region presents a Mediterranean type Csa according to the Köppen climatic clas-
sification. The monthly rainfall ranges between 73.8 L/m2in November and 5.5 L/m2in August
(NUÑEZ &SOSA 1999). The altitude of the sampling sites ranges from 168 to 524 m. The 10 ha
sampling sites were selected within those identified in previous works as sites with a high rab-
bit density and the presence of measurable burrows (SERRANO PÉREZ 2009). This sampling size
could be considered the home range of wild rabbits in southern Spain (LOMBARDI et al. 2003). The
current vegetation of sites 1, 2, 4 and 5 consists of the typical Mediterranean mosaic of open holm
oak woodlands, shrub lands and xerophytic grasslands called “dehesa”, while the remaining site,
3, is characterised by rocky grassland.
Warren surveys
Within each sampling site all warrens and burrows were surveyed. Warrens and burrows
were located and identified from November 2003 to March 2004. All measures were performed
without digging or modifying the original warren and burrow entrance structure. To solve the
problem of assigning entrances to their corresponding warren, a Stihl SH 85 blowing air machine
was used. For each warren, entrance location and the maximum and minimum distance between
entrances of the same warren were registered. In addition, the burrow height and width were mea-
sured at the entrance of the burrow and at a 30 and 60 cm distance using a two-piece graduated
bar. The length of each burrow tunnel, defined as the distance between the entrance and its first
angle or nest chamber, was measured, and its slope was recorded by using a clinometer. The bur-
row depth was calculated using the length and slope of each burrow tunnel. The temperature and
humidity at 0, 30 and 60 cm from the entrance were measured with a thermometer-hygrometer
fixed to a graduated bar, and the variation of these parameters with distance was calculated.
All measures were performed without digging or modifying the original warren and burrow
entrance structure. Stops and refuges were not studied because of their different function and
more restricted use by the species.
Soil measurements
In order to characterise the soil composition, a representative soil sample of each area of
study was collected. These soil samples were taken one metre from the edge of the nearest war-
ren to avoid the rabbit excavation effect (PARER &LIBKE 1985) and their volume was calculated.
First, the soil samples were dried at 64 C for as long as required to reach a constant weight. They
were sieved and classified into six categories depending on particle size: >25 mm, 3–25 mm, 1.5–3
mm, 1–1.5 mm, 0.5–1 mm, and <0.5 mm. The finest particles (<3 mm) were analysed at the
Laboratorio Agroalimentario y de Residuos (Junta de Extremadura) to determine clay, sand and
silt percentages. Textural determination was ascertained by means of the Bouyoucos hydrometer
method. Bulk density, defined as the ratio between oven-dried soil mass and its volume, was cal-
culated. Afterwards, this real bulk density was compared to the optimal level. The optimal bulk
density for root growth depending on texture as proposed by the United States Department of
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84 S. Serrano and S.J. Hidalgo de Trucios
Agriculture (USDA 1999) was used as the optimal bulk density for rabbit digging, since no other
reference values are available. A sample of soil was considered to have a high compaction level
when its real bulk density was higher than the optimal level and a low compaction level was
determined when lower than the optimal bulk density.
Data analysis
A cluster analysis was used to identify burrow types. In order to obtain uncorrelated vari-
ables that account for non-redundant information and therefore, to improve the understanding
of the cluster results, a principal component analysis (PCA) with Varimax rotation was first per-
formed. The variables included in the PCA were: height and width of the burrow tunnel at 0 and
30 cm, burrow tunnel length, slope, humidity increase, distance between the nearest entrances
and distance between the furthest entrances of the same warren. Height and width of the bur-
row tunnel at 60 cm were not included in the PCA because of the reduced number of tunnels
reaching this length and the strong correlation of the two variables with those at 30 cm. The five
first principal components explaining 75% of the total variance were retained and underwent a
two-step clustering method as proposed by MILLIGAN (1980). This clustering method consists of
a hierarchical agglomerative analysis followed by a k-means analysis. The hierarchical agglom-
erative analysis was performed using the algorithm of the unweighted pair-group method with
arithmetic average (UPGMA) cluster analysis and the Pearson correlation coefficient as the dis-
tance measure. This first analysis determines the final number of clusters and the location of
the centroids. Subsequently, a k-means analysis starting with the previously obtained centroids
was performed. This combined two-step method has been revealed as more effective than the
hierarchical or the k-means methods applied individually (DAVIS &KALKSTEIN 1990). A one-way
ANOVA was performed to check for differences in the structural variables between clusters. Each
significant cluster corresponds to a different burrow tunnel type.
Kruskal–Wallis non-parametric tests were performed to check for differences in the soil
particle composition and texture between burrow tunnel types. Differences in soil composition
between high and low compacted soils were tested by means of the Mann–Whitney test. In addi-
tion, the chi-square test was used to analyse the relationship between the type of the burrow tunnel
and the soil compaction level.
Finally, the Mann–Whitney Unon-parametric test was also performed to check for dif-
ferences in tunnel depth between different compacted soils and also between burrow types.
Significant differences were considered with a confidence level α=0.05. The analyses were carried
out with the SPSS 14.0 statistical package.
RESULTS
Burrow tunnel structure
As a result of the PCA, five principal components were extracted. The new vari-
ables were renamed according to the variable with the highest weight, as: PC1, entrance
(ENT); PC2, tunnel size at 30 cm (T30); PC3, humidity increase (HI); PC4, tunnel length
(TL); and PC5, dispersion of the entrances (DIS). The two-step clustering analysis classi-
fied the data into three groups of 31, 32 and 59 cases, respectively. There were significant
differences in the value of the five variables between clusters (P<0.05), except for the
tunnel size at 30 cm (T30) between clusters 2 and 3, and for the tunnel length (TL)
between clusters 1 and 3. These differences between variables of each cluster allowed
us to characterise three basic types of burrow tunnel. Fig. 1 illustrates the dispersion
of the variables depending on cluster type. Tunnel type A corresponds to a big burrow
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Burrow types of the European rabbit 85
Fig. 1. — Variable dispersion depending on the cluster: cluster 1 (circles), cluster 2 (triangles) and cluster
3 (crosses). ENT (entrance dimension), T30 (tunnel dimension at 30 cm), HI (humidity increase), TL
(tunnel length) and DIS (dispersion of the entrances).
tunnel (large height and width at the entrance and at 30 cm). Tunnel type B corresponds
to small burrow tunnels (height, width and length lower than the mean values). Tunnel
type C corresponds to the mean values of the structural variables. In this case, only
the distance between the nearest burrow entrances was lower than the mean value. All
types of tunnel become narrower as they approach the interior and change direction at
a maximum distance of 60 cm (see Fig. 2(a)–(c)).
Influence of soil composition on burrow structure
All three types of burrow tunnel were dug in soils with similar particle size compo-
sition, characterised by a high percentage of small particles (<0.5 mm) (48.3%, 55.8%
and 53.2% in tunnel types A, B and C, respectively). However, the soils preferred for dig-
ging burrow tunnels of type B and C presented a higher quantity of the smallest particles
and a lower quantity of the biggest ones than soils with burrow tunnels of type A (M–W
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86 S. Serrano and S.J. Hidalgo de Trucios
Fig. 2. — Sketch of the three types of burrow tunnels: (a) type A tunnel, (b) type B tunnel, (c) type C
tunnel. Tunnel dimensions correspond to the mean value of the structural variables with the highest
weight in the PCs.
between type B and A: Z=3.43, P=0.001; M–W between type C and A: Z=2.58, P=
0.01). No differences were found in medium size particles between soils with different
burrow tunnel types (M–W between type B and A: Z=0.46, P=0.644; M–W between C
and A: Z=1.85, P=0.064), except for tunnel types B and C (M–W: Z=2.67, P=0.008).
Results showed differences in the composition of sand (K–W, χ2=16.94, P<
0.001), clay (K–W, χ2=6.93, P=0.031) and silt (K–W, χ2=26.17, P<0.001) of the
soils where each type of burrow tunnel was dug. Sampled soils were classified into two
textural groups, loamy sand and silty loam, depending on the composition of sand,
clay and silt. Thus, it was found that tunnels of type A and B were dug in loamy sand
soils characterised by a high percentage of sand and silt. Sand constituted the main
particle making up this type of soil (56.5% on soils of tunnels of type A; 51.6% on soils
of tunnels of type B). Silt was the second particle type, with percentage values of 38.9%
in soils corresponding to tunnels of type A and 45.5% in soils of tunnels of type B. Type
C burrow tunnels were dug preferentially in silty loam soils, with a mean value of 54.0%
of silt and 42.0% of sand.
Depending on compaction, soils were classified as high compacted or low com-
pacted. High compacted soils were characterised by a higher quantity of sand (M–W;
Z=5.78, P<0.001) while low compacted soils were composed of more silt (M–W; Z
=8.30, P<0.001). No differences were found in the clay proportion between different
types of soil. Seventy-five per cent of the burrow tunnels of type A were excavated in
high compacted soils, while 83% of type C tunnels were located in low compacted soils.
Burrow tunnels of type B were found in both types of soils, although their occurrence
was relatively higher in low compacted ones (χ2=33.47, df =2, P=0.05).
It was found that the burrow tunnel depth was not related to soil compaction level
(M–W: Z=1635.00, P=0.605), but to burrow type (K–W, χ2=22.32, P<0.001). The
average depth of burrow tunnels of type A and C were similar: 18.4 ±9.8 and 16.7 ±
7.5 cm (mean ±SD) respectively, but significantly lower than the depth of burrows of
type B, 29.4 ±13.3 cm (mean ±SD) (M–W between type B and A: Z=3.19, P=0.001;
M–W between type B and C: Z=4.71, P<0.001).
DISCUSSION
As far as we are aware, the 122 burrows of the present study represent the largest
number of burrows analysed for a study of their structure in Mediterranean habitats.
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Burrow types of the European rabbit 87
Burrow tunnel structure
In this study, three well-defined types of burrow tunnels have been identified.
All tunnels showed some characteristics which may be related to rabbit anti-predator
behaviour. Thus, entrances of type B and C tunnels were small and all tunnels rapidly
became narrower approaching the interior, resulting in a final tunnel of adult rab-
bit size. In fact, predation is known to be one major factor affecting wild rabbit
behaviour (PONGRACZ &ALTBAECKER 2000) and specifically the utility of the warrens
(MYKYTOWYCZ &GAMBALE 1965; VILLAFUERTE 1994). Therefore, it might be expected
that some aspects of the burrow structure in natural warrens could be influenced
by the anti-predator behaviour, as PAULA &PALOMARES (2007) found for artificial
warrens.
In the case of type A tunnels there is a larger distance between entrances and,
therefore, longer tunnels. This favours escape in cases where a predator enters the war-
ren. More spread entrances could also favour escape in larger areas in the case of an
external attack. In the case of the shorter tunnels shown by types B and C, the narrower
size of the entrances excludes carnivore predators, without the effort of excavating
longer tunnels.
Influence of soil composition on burrow structure
All the burrows located in the present study were dug in sandy soils. This fact
is not surprising if we bear in mind that many previous studies have identified tex-
ture as the major factor influencing the penetration resistance of the soil (MYERS &
PARKER 1965; LOCKLEY 1976; CHAPUIS 1980). Consequently, sandy soils are considered
to be ideal for excavation, because of their low resistance (MYERS &PARKER 1975b;
ROGERS &MYERS 1979; PARER &LIBKE 1985). The results of this study showed that
not only the texture but also soil density influence rabbit burrow structure. Although
the sandy soils analysed were all compacted soils, the high density showed no negative
influence on the excavation effort. On the contrary, the high compaction level proba-
bly contributes to burrow structure stability, allowing the digging of bigger entrances
and tunnels and an internal network of longer tunnels with higher distances between
entrances. These results are consistent with the earlier findings of KOLB (1985) and
PARER et al. (1987). Thus, sandy soils would enable the species to dig larger and safer
burrows for breeding (KOLB 1985).
On the other hand, the high infiltration rate and the low water-holding capacity
of sandy soils allow rain to penetrate deeply, reducing the risk of burrow col-
lapse (PARER &LIBKE 1985). All three types of burrow tunnels were excavated
in soils with a high percentage of small particles. Nevertheless, soils of type A
tunnels showed a higher quantity of sand and big particles than type B and C
tunnels. As suggested by KOLB (1985), it seems that the structure of the soil deter-
mines burrow form by influencing the excavation and the amount of soil that
can be moved. Thus, the big particles of soils in tunnels of type A would facil-
itate the removal of a greater quantity of soil in the building of bigger burrow
tunnel. In humid conditions, the higher quantity of silt in tunnel types B and
C reduced the effective porosity of the soil, making the excavation process dif-
ficult. This effect, combined with the greater presence of smaller sized particles,
would result in a shorter and narrower burrow tunnel network with closer warren
entrances.
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88 S. Serrano and S.J. Hidalgo de Trucios
Warren depth
Regarding warren depth, a noticeably low mean value of 20 cm was found. This is
significantly lower than values reported for Australia (PARER &LIBKE 1985). It is widely
known that warren depth is useful for isolating rabbits from high external temperature,
providing a colder and humid ambience that avoids evaporative loss. Since the depth
protects rabbits from climatic extremes, differences in depth are probably related to
the differences in climatic conditions between the two areas. In fact, while the monthly
mean maximum air temperature in Australia ranged from 36 to 39 C(PARER &LIBKE
1985), in our areas of study it is under 34 C(INSTITUTO NACIONAL DE METEOROLOGÍA
2001). Differences in habitats and rabbit behaviour could also influence warren depth.
This study reports reliable experimental measurements of several structural vari-
ables of burrows and identifies well-defined patterns in burrow tunnels. The possible
relationship of these patterns to rabbit anti-predator behaviour is also discussed.
Such information contributes to a better knowledge of natural warrens and burrow
structures.
ACKNOWLEDGEMENTS
We thank D. Eldridge and two anonymous reviewers for their useful comments on a earlier
drafts of the paper. We are indebted to A. Meriggi and A. Serrano for their constructive comments
and suggestions on a previous version of the manuscript. We also thank A. Serrano for improving
the English version. The authors would also like to thank F. Salas, J.A. Velarde, M. López and
G. Arenas for field assistance. SSP was supported by a pre-doctoral grant from the Spanish
Ministry of Education and Science.
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... Kaninerna lever på stora ytor med över tio meter mellan varje håla (Schepers et al., 2009). Hålorna är en mycket viktig resurs för kaninen då dessa inte enbart erbjuder skydd mot otjänligt väder och predatorer utan även fungerar som en trygg plats för födsel av ungar (Serrano & de Trucios, 2011). I hålorna spenderar djuren största delen av dagen och de kommer upp för att födosöka först i gryning eller skymning och studier har visat att de dominanta hanarna kommer upp ett par timmar innan resterande gruppmedlemmar (Schepers et al. 2009). ...
... Eftersom djuren inte får komma ut regelbundet skulle en situation där de tas ut i koppel anses som stressande då de plötsligen utsätts för en stor mängd exogena stimuli som de inte har möjlighet att fly ifrån på grund av kopplet och de troligen inte har sitt gömställe tillgängligt. Det senare kan anses viktigt då hålorna, som tidigare nämnts, har visat sig utgöra en av de mest centrala resurserna för denna art (Serrano & de Trucios, 2011). Dock kan utevistelsen i sig ses som en form av aktivering vilket gör det tänkbart att stressen på lång sikt inte är negativ utan kanske har en påverkan på den låga andel onormala beteenden som visas i resultaten av den här studien. ...
... Detta kan i sin tur leda till såväl sjukdomstillstånd som uppkomst av onormala beteenden (Broom, 1991). Studier har visat att kaniner gräver olika långt beroende på underlag och i sandiga substrat som håller samman bättre har djuren visat sig bygga djupare hålor (Serrano & de Trucios, 2011). Att majoriteten djurägare har angivit att de använder sågspån som strömaterial tyder på att kaninen kanske inte ges möjlighet att bygga några djupa hålor av materialet då det inte håller samman på samma sätt som sand. ...
... We expect that rabbit occurrences would be favored in nonintervention sites near agricultural or natural pastureland areas, in areas with higher shelter availability and in areas with natural water availability (Lombardi et al., 2007;Sarmento et al., 2012;Trout, Langton, Smith, & Haines-Young, 2000). In addition, we expect that rabbit presence would be incremented by soils with favorable conditions for digging (Serrano & Hidalgo de Trucios, 2011;Williams et al., 2007), smooth slopes (Calvete et al., 2004;Delibes-Mateos et al., 2010;Farfán et al., 2008) and warmer and drier terrain orientations (Delibes-Mateos et al., 2010;Godinho et al., 2013). ...
... We identified the soil types from the official Portuguese soils map (1:25,000; IHERA, 1999) and categorized them as favorable or unfavorable for digging (soils). Favorable soils are deep, soft, well drained and rich in carbonates and sand content (Parer & Libke, 1985;Serrano & Hidalgo de Trucios, 2011;Williams et al., 2007). We classified the humic and nonhumic litholic soils of syenite as favorable soils. ...
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