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All content in this area was uploaded by Juan Carlos Blanco on May 03, 2018
Content may be subject to copyright.
Dispersal patterns, social structure and mortality of wolves
living in agricultural habitats in Spain
J. C. Blanco & Y. Corte
´s
Wolf Project-Conservation Biology Consultants, C/Manuela Malasa ˜na, Madrid, Spain
Keywords
Canis lupus; grey wolf; dispersal; pack
formation; tenure; mortality.
Correspondence
Juan Carlos Blanco, Wolf Project-
Conservation Biology Consultants,
C/Manuela Malasa ˜na 24, 28004 Madrid,
Spain. Tel:+34 91 5930456;
Fax: +34 91 4460447
Email: jc.blanco@ya.com
Received 8 August 2006; accepted
4 January 2007
doi:10.1111/j.1469-7998.2007.00305.x
Abstract
Wolf Canis lupus dispersal, social structure and mortality have been extensively
studied in natural and semi-natural areas of North America and northern Europe
but have never been assessed in agricultural areas. From 1997 to 2004, 14 wolves
(11 in a wolf-saturated area and three in a low-density area) were radio-collared
with long-lasting transmitters in a Spanish agricultural area containing a high-
human-population density, a dense network of roads and a shortage of wild
ungulates. The wolves mainly feed on an overabundance of livestock carrion. Nine
wolves (one of them, three times) dispersed during the study period. The mean age
and distance of natal dispersal were 24.8 months and 32 km. The natal dispersal
period was much longer in wolves radio-collared in the saturated area (mean
414.6 months) than in the low-density area (o1 month). All three of the
dispersers living in the low-density area, and two of the six dispersers in the
saturated area settled and bred during the study. The average tenure of six breeders
was 4.5 years. The radio-collared wolves spent 72% of the monitoring time living
in packs and the rest living in pairs, as dispersers or as peripheral wolves, but the
percentage of loners was much higher in the saturated (33.5%) than in the low
density (1.6%) areas. The overall annual mortality was 18% (lower than in most
populations studied in less modified habitats), but lone wolves had a significantly
higher mortality than members of packs and pairs. Nine wolves died during the
study, none of them due to natural causes. In general, our results are very similar
to those obtained in less modified habitats, except for the dispersal distance, which
was much shorter than in other studies. We suggest that barriers and habitat
constraints may reduce dispersal distances in our study area.
Introduction
Dispersal is the principal means by which maturing grey
wolves Canis lupus leave their natal packs, reproduce and
potentially expand their population’s geographic range
(Fuller, Mech & Cochrane, 2003). In social animals such as
wolves, dispersal is an important mechanism for population
regulation (Lidicker, 1975), and is likely to influence survi-
val (Waser, 1996).
Grey wolf dispersal has been studied extensively in North
America (see review in Fuller et al., 2003). In Europe, the
only dispersal data available are from Scandinavia (Wabak-
ken et al., 2001) and Finland (Kojola et al., 2006). All these
studies have been carried out in wilderness or only partially
modified areas, where wolves prey on wild ungulates and
there are no obvious barriers to wolf dispersal. Nevertheless,
many wolf populations live in close contact with humans in
many parts of southern Europe, the Middle East and south-
ern Asia (Boitani, 2003), where habitat characteristics differ
considerably from those of the above-mentioned regions.
Moreover, considering the recent expansion of wolves in
much of their range (Mech, 1995), it is likely that in the
future they will increasingly occupy more modified regions.
In Spain, following centuries of severe persecution,
wolves reached their lowest point in the 1970s. Subse-
quently, they were partially protected and the largest re-
maining population started to increase and expanded
southward and eastward (Blanco, Reig & Cuesta, 1992).
During the 1980s, wolves appeared in flat, agricultural,
treeless and densely inhabited areas with a near-absence of
wild ungulates. In these areas, the habitat is patchy and
there is a dense network of highways, roads, railways, rivers
and transport infrastructures that sometimes act as semi-
permeable barriers for wolves (Blanco, Corte
´s & Virg ´
os,
2005). Wolves mainly feed on livestock carcasses they find in
dumps or scattered throughout the countryside, and during
most of the study period a surplus of food was available
(Corte
´s, 2001). These habitat conditions may influence
dispersal and population dynamics, parameters that have
not been documented previously in this kind of habitat.
This paper assesses the dispersal patterns of wolves living
in this agricultural area and other parameters related to
Journal of Zoology 273 (2007) 114–124 c2007 The Authors. Journal compilation c2007 The Zoological Society of London114
Journal of Zoology. Print ISSN 0952-8369
dispersal, such as social structure and mortality. Our aim is
to compare our results with those obtained in the studies
carried out during the last 40 years in wilderness and semi-
wilderness areas.
Study area
We conducted the study in the provinces of Valladolid,
Zamora, Segovia and A
´vila, in north-central Spain (Fig. 1).
The study area comprises flat, almost treeless agricultural
land, with cereal and maize fields providing some cover for
wolves during certain seasons. Remnant forests only cover 7
and 26% of the area north and south of the River Duero
(which bisects the study area), respectively. These fragmen-
ted forest patches are generally privately owned, with
restricted public access. Wild boar Sus scrofa, the only wild
ungulate in the area, is common in the remnant forests
(c. 1 animal km
2
) but almost absent in agricultural areas
(Corte
´s, 2001). European rabbits Oryctolagus cuniculus are
locally abundant and Iberian hares Lepus granatensis reach
densities of 3.5 individuals km
2
in optimal areas (Calzada
& Mart ´
ınez, 1994). The human population (10–40 in-
habitants km
2
) engages in agriculture and, to a lesser
extent, in livestock farming.
Sheep flocks are usually protected by shepherds during
the daytime and locked in pens at night, and so damage to
livestock is low and people’s tolerance of wolves is higher
than in other parts of Spain (Blanco & Corte
´s, 2002).
The main food of wolves in our area is livestock carrion.
In 603 scats analysed, livestock carrion comprised 75.3% of
the biomass consumed by wolves, and wild boar just 5.7%
(Corte
´s, 2001). Until the end of 2000, most of the carcasses
of the livestock that died in the farms were accessible for
wolves. In 1999, we interviewed 129 shepherds, who de-
clared that they disposed of the dead livestock in open pits
(77.3%) or left them in the fields (8.5%) (Corte
´s, 2001). In
late 2000, bovine spongiform encephalopathy (BSE) was
detected in Spain; new laws that obliged farmers to inciner-
ate sheep and cattle carcasses were strictly enforced and
carrion pits were progressively closed, and so food avail-
ability for wolves started decreasing. In 2003, five of six
carrion pits located in the territories of radio-collared
wolves were closed. As a consequence, we consider two
periods in the study: until the end of 2000, when food was
apparently overabundant, and from 2001, when carrion
availability started to decrease.
At the beginning of the study, wolves were almost absent
south of the river Duero, with appreciable colonization of
this area from 1999 onwards. Throughout the whole study
period, wolf density was much higher to the north than
south of the river Duero, and in 2001 wolf density was
estimated at 1.63 and 0.77 packs/1000 km
2
, north and south
of the river Duero, respectively (Llaneza & Blanco, 2005).
North of the Duero, the population is apparently saturated.
North of the river Duero, low wolf hunting quotas are
allowed whereas to the south of the Duero they are fully
protected. Poaching is common on both sides of the river
(Corte
´s, 2001). North of the river Duero, the road density is
0.40 km km
2
when considering just paved roads, and
1.53 km km
2
when considering ‘permanent roads’ (Mla-
denoff et al., 1995; Merrill, 2000).
Methods
From March 1997 to May 2004, we radio-collared 14 wolves
in six different packs using the procedure described
Figure 1 Wolf Canis lupus range in Spain (grey)
(Blanco & Corte
´s, 2002) and our study areas (1) to
the north and (2) south of the Duero (dark grey).
Journal of Zoology 273 (2007) 114–124 c2007 The Authors. Journal compilation c2007 The Zoological Society of London 115
Wolf dispersal in SpainJ. C. Blanco and Y. Corte
´s
elsewhere (Corte
´s, 2001; Blanco et al., 2005). Eleven wolves
were radio-collared with VHF transmitters (Televilt, Lin-
desberg, Sweden) whose batteries have an average life of
6 years, two with VHF transmitters (Telonics, Mesa, AZ,
USA) with 2–3-year duration batteries and one with a GPS-
GSM transmitter (Vectronic, Berlin, Germany) that only
lasted 3 months. In addition, the radio-collared wolves were
sometimes observed or killed after their batteries expired,
and this information was integrated into our data. Age was
estimated by body size and appearance, tooth replacement
(Van Ballenberghe & Mech, 1975), tooth wear (Valverde &
Hidalgo, 1979; Landon et al., 1998; Gipson et al., 2000) and
date of capture. Wolves were divided into three age cate-
gories: pups (o12 months old), yearlings (12–24 months
old) and adults (424 months old). Wolves were located
from the ground or aircraft using standard triangulation or
homing techniques (Mech, 1983). We also monitored them
intensively during all-night sessions (Blanco et al., 2005).
Natal dispersal is the movement of an animal from its
natal home range to where it reproduces or would have
reproduced if it had survived or found a mate. Breeding
dispersal or secondary dispersal is the movement of an adult
between consecutive breeding sites or groups (Greenwood,
1980; Shields, 1987; Gese & Mech, 1991; Waser, 1996). We
assume that wolves o2 years old were captured in their
natal home range unless there was evidence to indicate
otherwise.
The annual dispersal rate was estimated in two different
ways: (1) by dividing the number of dispersers by the wolf-
years of monitoring (Gese & Mech, 1991; Mech et al., 1998);
(2) using the program MICROMORT (Heisey & Fuller,
1985), substituting the event death for the event dispersal, as
recommended by Arthur, Paragi & Krohn (1993) and
Ferreras et al. (2004). This method allows statistical com-
parisons to be made using the Zstatistic proposed by Heisey
& Fuller (1985).
Before leaving the area, some dispersers show pre-dis-
persal forays (Messier, 1985) or spent some time in the
periphery of their natal home range. This process is defined
as the pre-dispersal period. Similarly, some wolves, after
reaching their establishment home ranges, show incomplete
site fidelity for some weeks or months before finally settling.
This is the settlement period. We defined the total duration of
dispersal as the time between the start of dispersal and
establishment. The pre-dispersal period was not included in
the calculation of dispersal duration unless otherwise specified.
Dispersal distance is the distance between the arithmetic
centre of the natal home range, or the capture site, to
establishment home-range centre, the mortality site or the
last location while dispersing (Maehr et al., 2002). The
distances of effective dispersals were compared with
the average home range width of resident wolves (15 km),
which was calculated as the diameter of a 182 km
2
circle (the
average area of an adult wolf home range in our study area
using the 95% minimum convex polygon method, J. C.
Blanco & Y. Corte
´s, unpubl. data).
Pair formation was confirmed when the target wolf was
consistently seen with another wolf during or after the
dispersal period. Pack formation was deduced from the
presence of three or more wolves showing site fidelity,
assessed by observation of the wolves or through elicited
howling. Breeding was confirmed by visual observation of
the pups or from pups answering to simulated howling
(Harrington & Mech, 1982).
To study the social structure of the wolf population, we
recognized four categories of social status: (1) pack mem-
bers, (2) pair members, (3) dispersers (4) peripheral wolves,
defined by Mech (1970) as the individuals that rank so low
that they avoid the main pack members and stay near the
fringes of the pack social centre. Dispersers were recognized
as they do not show site fidelity. The three other categories
were established by assessing whether the radio-collared
wolves were consistently alone (peripheral wolves) or living
with other pack or pair members. With this objective we
tried to observe the wolves actively, used the simulated
howling method and checked tracks in the sand. During the
study, we carried out 528 ‘sit and wait’ observation sessions
(e. g. at rendezvous sites) and 253 simulated howling trials.
The percentage of wolves in each given category was
estimated by dividing the sum of those days spent by each
radio-collared wolf in the category by the number of radio-
days of all categories. The periods in which the social
category of wolves was uncertain were excluded from
calculations.
We estimated survival rates using the program MICRO-
MORT (Heisey & Fuller, 1985), and we compared survival
among social categories using the test proposed by these
authors. We assigned mortality dates as halfway between
the last known live location and the first indication that the
wolf had died, unless carcass evidence indicated otherwise.
Results
Dispersal rate, sex, age, distance and time of
dispersal
Between 1997 and 2004, we radio-collared 14 wolves (seven
males and seven females) that were followed for a total of
40.6 wolf-years (mean: 34.8 months, range: 3–71 months).
Four wolves were followed for 5–6 years and two other
wolves for 4–5 years. Nine wolves dispersed during the study
period (Table 1; Fig 2), and so the annual dispersal rate was
23.9% (using MICROMORT) or 27.4% (using the method
described by Gese & Mech, 1991). Although the annual
dispersal rate of males (32.0%) was higher than that of
females (16.3%), this difference was not significant
(Z=1.273, P=0.1014). Nine of the 11 dispersals (92%)
corresponded to natal dispersal and two others (8%) to
breeding dispersal. The average age of natal dispersal for
seven wolves of known age was 24.8 months (SD = 4.98),
24.5 months for three males and 25.0 for four females. Five
wolves dispersed when they were 2–3 years old and the
others as yearlings. The minimum average distance of natal
dispersal was 32 km (n= 8; range: 13–50 km). This is a
minimum figure because the signal of one male wolf was
lost during dispersal, suggesting a long-distance dispersal.
Journal of Zoology 273 (2007) 114–124 c2007 The Authors. Journal compilation c2007 The Zoological Society of London116
Wolf dispersal in Spain J. C. Blanco and Y. Corte
´s
Dispersal distances were similar for males (31.5 km, n=4)
and for females (32.5 km, n= 4). Of seven known cases, four
natal dispersals started between October and January and
the three others in July and August (Table 1).
The natal dispersal process
Pre-dispersal and natal pack abandonment
Two of the nine dispersers were radio-collared when they
were already dispersing, and so we do not know whether
they had a pre-dispersal process. In the other seven wolves,
we did not detect pre-dispersal forays sensu Messier (1985).
Nevertheless, two wolves used the peripheral areas of their
home range much more frequently in the weeks before their
dispersal. The two wolves that avoided the nucleus of the
pack before dispersing were F3 and F1. F3, after having
attended the breeder’s pups throughout summer 1998, left
the homesite area in October and occupied the periphery of
her natal pack territory for 37 days before dispersing. From
summer to the predispersal period, the average distance of
the diurnal rest sites between F3 and her pack’s breeding
female (F4, also radio-collared) increased from 2.2
(SD =2.1; n= 27) to 6.2 km (SD= 4.8; n=7) (U=2.619;
P= 0.002; Mann–Whitnney test). F1 spent 82 days mainly
using the periphery of her natal pack territory, between May
and July 1997 (breeding season), before dispersing. During
this period, only one of the 15 days in which she was located
was spent with the pups of the pack; in contrast, during the
same period, the radio-collared male M1(which was probably
her brother from the same litter) spent nine of the 20 days in
which he was located in his pack homesite (Yates corrected
w
2
=4.44; d.f.=1; P= 0.035).
Dispersal period
The dispersal period lasted from o9 days to 32 months. We
have recorded two different dispersal patterns. The wolves
living in the low-density area south of the river Duero
showed very rapid dispersal. Two of them suddenly left their
natal pack and when they were detected again, 25 and 9 days
after dispersing, they had already settled down in their new
areas. The third wolf (F1) also probably showed this
pattern, but after leaving her natal area she went missing
for several weeks and we cannot be sure. In contrast, in the
saturated area north of river Duero, the average dispersal
duration was 414.6 months (n= 5) (Table 1). Of the six
wolves that dispersed in this area, one disappeared during
dispersal and the other five were floating for months or years
before settling or dying. Two of them eventually bred, two
died before settling and one was still dispersing at the end of
the study.
The dispersers that were floating north of the river Duero
used different methods to find a vacant territory or be
adopted by a pack (Tables 1 and 2). F3 seemed to spiral
outward from the natal territory, until she finally established
in a corner of it. M2 used a nomadic pattern until he settled
Table 1 Study period, age, distance and duration of dispersal of the radio-collared wolves Canis lupus
Wolf
id.
a
Area
b
Radiocollaring
date Last data
Monitoring
period
Dispersed
during the
study
Month
of natal
dispersal
Natal
dispersal age
(months)
Natal
dispersal
distance (km) Predispersal
Dispersal
duration
M1 South Duero 01.03.1997 21.12.2002 5 years and 10 months Yes January 31 26 No o25 days
F1 South Duero 23.04.1997 23.01.2003 5 years and 9 months Yes July 26 42 Yes Unknown
F2 North Duero 10.10.1997 22.02.1998 4months and medio No
F3 North Duero 23.09.1997 23.07.2003 5years and 10 months Yes October 29 13 Yes 10.3 months
F4 North Duero 23.09.1997 23.06.2002 4years and 9 months No
M2 North Duero 08.03.1998 08.09.2002 4years and 6 months Yes 50 Unknown 412.7months
M3 North Duero 08.03.1998 23.10.1998 7.5months Yes 14 Unknown 47.5 months
F5 North Duero 06.05.1998 28.02.2001 2years and 9.5 months Yes August 27 45 No 32 months
M4 North Duero 15.05.1998 22.12.2003 5years and 7 months No
M5 North Duero 31.03.1999 20.08.1999 5months Yes May 24 Lost No 43 months
F6 North Duero 01.04.1999 19.01.2000 8.5months No
F7 South Duero 03.04.2002 10.01.2004 1 year and 9 months Yes November 18 30 No o9 days
M6 North Duero 31.05.2004 28.08.2004 3months No
M7 North Duero 31.05.2004 23.06.2005 13months Yes Oct–Jan 18.5 36 Unknown 47.5 months
a
M, males; F, females.
b
North Duero: high wolf density; South Duero: low wolf density.
Journal of Zoology 273 (2007) 114–124 c2007 The Authors. Journal compilation c2007 The Zoological Society of London 117
Wolf dispersal in SpainJ. C. Blanco and Y. Corte
´s
in a vacant territory. M3 showed a nomadic pattern but
mainly shifted between two packs. And F5 explored specific
areas and moved to contiguous ones every few months (Fig.
3). Floaters did not avoid other packs: F5 and M2 were each
detected in the core areas of three known wolf packs, F3 and
M3 in that of two packs and M7 in that of one pack.
The settlement process
Of the nine wolves that started the dispersal process, two
died during their dispersal, one disappeared and one was
still dispersing at the end of the study. Four of the five
wolves that established successfully, settled just after
Figure 2 Dispersal of radio-collared wolves Canis lupus (1997–2005). The locations where wolves were fitted with transmitters and where
dispersers reproduced or died are shown. The river Duero and the Highway 6 are also shown.
Table 2 Pairing and breeding of dispersers, tenure of territorial breeders and fate of all the radio-collared wolves Canis lupus
Wolf id. Pairing Breeding Tenure after settling Fate of the wolf
M1 Unknown 15 months Battery expired
F1 Paired after dispersal Bred 4th breeding season after settlin g 62 months Killed by car
F2 Illegally poisoned
F3 Paired after dispersal Bred 2nd breeding season after settlin g 52months Killed by car
F4
a
457 months Illegally shot
M2 Paired before or during dispersal Bred 1st breeding season after settling 442 months Battery expired
M3 Did not pair Died before breeding Killed by car
F5 Did not pair Died before breeding Illegally shot
M4
a
490 months Legally shot
M5 Unknown Unknown Lost during dispersal
F6 Illegally shot
F7 Paired after dispersal Bred 1st breeding season after settling 14 months Killed by dogs
M6 Collar failure
M7 Paired before or during dispersal Did not breed 1st breeding season after dispersal Ongoing monitoring
a
Radio-collared when they were territorial breeders.
Journal of Zoology 273 (2007) 114–124 c2007 The Authors. Journal compilation c2007 The Zoological Society of London118
Wolf dispersal in Spain J. C. Blanco and Y. Corte
´s
arriving and were not detected in their natal home ranges
anymore. For the fifth wolf (F3), the settlement was a
progressive process that lasted 7.5 months. During this time,
in eight of the 27 locations (29.6%) she had returned to visit
her natal territory.
Pairing, pack formation, breeding and tenure
after settling
We have data on pair formation on five of the nine
dispersers: two males (M2 and M7) paired before or at the
beginning of their dispersal, and three females (F1, F3 and
F7) paired after or at the end of the dispersal. Of the five
successful dispersers, three of them formed their own pack:
M2 and F7 reproduced in the first breeding season after
settling and F3 in the second breeding season. Two other
dispersers joined existing packs: F1 formed a pair in an
existing pack, bred 4 years after settling and formed a new
pack (the former one split into two). M1 joined another
pack and then dispersed twice in subsequent years.
The different dispersal success had distinct consequences
for the population trends on the different sides of the river.
The three wolves radio-collared south of the river Duero
dispersed and eventually formed their own packs, but only
two of the six wolves (33.3%) that dispersed north of the
Duero formed a new pack. The characteristics of the newly
formed territories were also very different on both sides of
the river. South of the river Duero, the three new packs
formed at the edge of the distribution area, in areas not
occupied by other wolves and of much better quality
habitats than the two packs formed in the north. Agricultur-
al and other very modified habitats formed 21, 68 and 19%
of the three new packs’ territories to the south of the Duero,
and 78 and 99% of those north of the Duero. North of the
river, one of the new packs was formed by budding. F3 set
up a territory occupying the worst-quality portion of her
mother’s (F4) territory, which was also bisected by a four-
lane fenced highway (Blanco et al., 2005). The second pack
north of the Duero was formed by M2, who carved out a
territory in a practically treeless area after floating nomadi-
cally for more than a year.
Seven radio-collared adult females were monitored for an
average of 3.2 years (range: 0.5–5.9 years) until they died,
and their complete reproductive histories are known. Three
adult females never reproduced. The other four females bred
once, twice, four and Zsix times, respectively, during their
lifespan.
Four of the five wolves that settled after dispersing
retained their territory until they died, or until the radio-
collar battery expired (Table 2). In addition, two wolves that
Figure 3 Dispersal of F5 (May 1998–February 2001). She successively occupied five home ranges and died before settling. The broken lines
indicate back and forth movements.
Journal of Zoology 273 (2007) 114–124 c2007 The Authors. Journal compilation c2007 The Zoological Society of London 119
Wolf dispersal in SpainJ. C. Blanco and Y. Corte
´s
were radio-collared when they were already territorial bree-
ders (M4 and F4) retained their territory until they were
shot. The average recorded tenure of these six breeders was
4.5 years (range: 1.1–7.5 years).
Breeding dispersal
Only one case of breeding dispersal was detected. M1
dispersed from his natal pack in January 1998 (when he
was about 31 months old) and joined another pack 26 km
away. In October 1999, he moved to a contiguous territory
17 km away and stayed there for 2 more years. In 2001, he
moved again to another contiguous territory 18 km away
from the last one and remained there at least until 2002,
when his battery expired. In the two breeding dispersals,
he moved a similar distance to the diameter of an average
territory (15 km). These breeding dispersals were gradual
and the new territories overlapped the previous ones
(Fig. 4).
Social structure and mortality
From 1997 to 2004, we made 267 observations of the radio-
collared wolves and other members of their packs, per-
formed 253 sessions of simulated howling close to them and
wolves replied 88 times (34.8%). At the end of the study, the
radio-collared wolves were found 72% of the observations
living in packs and the rest living in pairs, as dispersers or as
peripheral wolves. (Table 3).
The overall annual mortality rate was 18%, but it varied
depending on social category. Members of packs and pairs
showed the lowest mortality rates and dispersers and
peripheral wolves had the highest (Table 4). Lone wolves
(i.e. dispersers plus peripheral wolves) had a significantly
higher mortality (44% annual) than members of packs and
pairs (12%) (Z=1.6803, P=0.046). Nine radio-collared
wolves died during the study (Table 2), none of them due to
natural causes. Illegal killing (44.4%) and traffic kills
(33.3%) were the main causes of mortality. One wolf was
apparently killed by dogs on a cattle-rearing estate.
Discussion
Wolf dispersal occurs normally once in the life of the animal
and is difficult to detect using normal VHF radiocollars,
which only have a 1–2-year lifespan (Mech, 1987). However,
we used radiocollars with a battery life of 6 years, which
allowed us to follow some of the wolves throughout the
greater part of their lifespan. In fact, six wolves were
followed for 4–6 years, and nine of the 14 dispersed while
their radiocollar was active.
Figure 4 Dispersal of M1. In 1998, he dispersed from his natal home range (natal dispersal), and in subsequent years he dispersed again twice
(breeding dispersal).
Journal of Zoology 273 (2007) 114–124 c2007 The Authors. Journal compilation c2007 The Zoological Society of London120
Wolf dispersal in Spain J. C. Blanco and Y. Corte
´s
In addition, our study differs in several ways from those
undertaken previously on wolves. The area is highly modified,
had (at least during the majority of the study period) a food
surplus deriving from livestock carrion, it is a highly
fragmented landscape, with few woodland patches where
wolves can breed safely, and also has a considerable number
of roads and other man-made barriers (Blanco et al., 2005).
The road density north of the river Duero is 1.53 km km
2
,
that is, the highest ever recorded for a wolf population
(Merrill, 2000). Several studies carried out in North America
in the 1980s suggested that wolf populations could not
survive in areas with road densities 40.58 km km
2
(Thiel,
1985; Jensen, Fuller & Robinson, 1986; Mech et al., 1988).
Thus, the conditions of our study area contrast markedly
with the studies carried out in North America and in north-
ern Europe where the forested habitat is continuous, the
wolves feed on natural prey and there are no obvious
barriers to their dispersal.
Dispersal patterns and success
Despite these differences, our results were very similar to
those studies undertaken in less modified habitats, except
for the dispersal distance. The dispersal rate in this study
(23.9–27.4%) is very similar to the average dispersal rate for
eight studies reviewed by Fuller et al. (2003, p. 179) in North
America (25.4%), and closely approaches the 28% annual
rate found by Mech et al. (1998) in the wilderness location of
Denali National Park (Alaska), which has characteristics
almost completely different from those in our study area.
Similar to most other studies, we found no sex-biased
dispersal rate (Mech & Boitani, 2003). Regarding dispersal
age, five of the seven known-age wolves in our study
dispersed when between 2 and 3 years old, and the other
two as yearlings. The last two cases dispersed after the
closure of carrion pits when the food surplus had disap-
peared. The last two cases may be evidence of dispersal due
to resource competition, and the rest dispersal due to mate
competition (Gese, Ruff & Crabtree, 1996). In most other
studies, wolves dispersed as yearlings (Mech & Boitani,
2003), but the dispersal age was delayed when food was
abundant (Ballard, Whitman & Gardner, 1987; Mech et al.,
1998; Boyd & Pletscher, 1999), as occurred in our study
(Table 5).
In contrast, the mean dispersal distance in our study
(32.0 km) was 2.4–7 times smaller than that of wolves in less
modified habitats of North America and Europe (Table 5).
A probable cause of these shorter dispersal distances is the
fragmentation of the habitat and the presence of highways,
rivers, transport arteries and other barriers typical of habi-
tats with high human densities (Blanco et al., 2005).
Table 3 Percentage of radio-days that the radio-collared wolves Canis
lupus spent in packs, pairs, as dispersers or as peripheral wolves in
different periods of the study
Pack Peripheral Disperser Pair
March 99 67.2 8.8 25.0 0.0
March 01 53.0 9.2 23.5 14.3
March 03 71.2 5.3 13.7 9.8
March 05 72.2 4.9 12.7 10.2
Table 4 Annual mortality rates of radio-collared wolves Canis lupus
(Heisey & Fuller, 1985)
Social category Radio-days Deaths Mortality rate 95% CI
Overall 14763 8 0.18 0.059–0.285
Pack 10051 4 0.14 0.003–0.250
Pair 1694 0 0 0.000–0.000
Disperser 1952 2 0.31 0.000–0.591
Peripheral 1066 2 0.50 0.000–0.805
CI, confidence interval.
Table 5 Dispersal data from various studies
Study area n
Mean age (months) Mean distance (km)
ReferencesMales Females
Both
sexes Males Females Both sexes
Spain 9 24.5 25.0 24.8 31.5 32.7 32.0 This study
North-west Minnesota 8 20–390 Fritts & Mech (1981)
North-east Minnesota 75 18.6 88 65 77 Gese & Mech (1991)
Wisconsin 16 65 144 114 Wydeven, Schults &
Thiel (1995)
South-central Alaska 38 30 33 84 114 Ballard et al. (1987)
Alaska 21 154 123 Ballard et al. (1997)
Rocky Mountains 17 33 23 27 152 264 Boyd et al. (1995)
Rocky Mountains 31 35.7 113 78 96.3 Boyd & Plestcher (1999)
Denali National Park 56 30 28 133 Mech et al. (1998)
Scandinavia 15 323 123 Wabakken et al. (2001)
Finland 50 13.5 109 99 98.5 Kojola et al. (2006)
Israel 7 50–150 Hefner & Geffen (1999)
Yellowstone National Park 30 25 Mech & Boitani (2003)
Journal of Zoology 273 (2007) 114–124 c2007 The Authors. Journal compilation c2007 The Zoological Society of London 121
Wolf dispersal in SpainJ. C. Blanco and Y. Corte
´s
Landscape constraints seem to be the reason why carnivore
populations studied in very modified habitats have shorter
dispersal distances than those living in undisturbed areas
(Maehr et al., 2002; Ferreras et al., 2004).
In general, the dispersal process in our study area fits into
the pattern described for other wolf studies. In areas of low
wolf density with many vacant territories, the dispersal
period is short, but in saturated wolf habitats the dispersal
period is longer and the dispersal success is lower (Fritts &
Mech, 1981; Messier, 1985; Gese & Mech, 1991). In our
study, the three wolves that dispersed in the low-density area
south of the river Duero found a vacant territory or were
adopted by a pack a few days or weeks after leaving their
natal territory. In contrast, in the saturated area north of the
river Duero, the average dispersal duration of five wolves
was 14.0 months, and only two of them eventually secured a
territory and reproduced. The duration of dispersal in this
saturated population is much greater than the mean dis-
persal duration recorded by Gese & Mech (1991) in Minne-
sota (4.1, 2.0 and 2.9 months for pups, yearlings and adults,
respectively) and is even longer than the maximum dispersal
duration (12 months) of the 75 dispersers followed by these
authors.
Some authors found that dispersers frequent areas along
the interstices among territories (Rothman & Mech, 1979;
Fritts & Mech, 1981; Meier et al., 1995) but other authors
have not observed this pattern (Messier, 1985; Boyd et al.,
1995). In our study, five of the floaters dispersing north of
the river Duero were detected once or several times visiting
other packs’ homesites. On one occasion, we even located
the floater M2 in summer together with the pups of another
pack with radio-collared wolves. In other areas, trespassers
are frequently killed by territory owners (Mech, 1977; Mech
et al., 1998). It is possible that the food surplus in our study
area results in the wolves not needing to vigorously defend
their territories against trespassers, as suggested by Boyd
et al. (1995).
As a consequence of the different dispersal success on
both sides of the river (3/3 south and 2/6 north of the river),
the population grew rapidly in the south during the study
period, increasing from three packs detected in 1997 to 20
packs in 2001 (Blanco et al., 2005; Llaneza & Blanco, 2005)
but north of the Duero the population did not obviously
increase during the same period.
Solitary wolves and mortality
Another consequence of the different dispersal patterns on
both sides of the river is the difference in the percentage of
lone wolves. The three wolves radio-collared south of the
river Duero were found as loners (dispersers plus peripheral
wolves) 1.6% of the observations (13.3 wolf-years’ monitor-
ing) and the rest of the time as territorial wolves living in
packs or in pairs. In contrast, the 11 wolves radio-collared in
the saturated population north of the Duero were found as
loners 33.5% of the observations (27.3 wolf-years’ monitor-
ing). The reasons for the high percentage of loners north of
the river Duero might be the semi-permeable river Duero
barrier (Blanco et al., 2005; Fig. 1), the food surplus, which
delays dispersal, and the poor vegetation cover, which may
limit breeding possibilities and formation of new packs.
These characteristics are relatively rare in the other areas
where wolves have been studied and as a result the percen-
tage of loners found in our population north of the river
Duero (33.3%) is higher than that in other studies in less
modified habitats. Fuller et al. (2003), in an extensive review,
concluded that the average percentage of non-resident
individuals in North American studies was 12% (range:
7–20%).
The high percentage of loners has several management
consequences. On the one hand, they are a buffer for a
population, making it less vulnerable to exploitation as they
are adult individuals that can quickly replace breeders when
these die (Fuller et al., 2003). In addition, floaters are
undetectable without extensive radiotracking, and so they
form a ‘shadow population’ (Rohner, 1997), which is almost
impossible to calculate in the population surveys such as
those that are carried out in Spain. These surveys are based
on the detection of pups in summer but radio-tracking is
rarely used and snow is almost lacking in winter (Blanco &
Corte
´s, 2002; Llaneza & Blanco, 2005).
Surprisingly, the annual mortality rate (18%) in our area
was lower, and the tenure of the breeders (4.5 years) higher
than in the wilderness of Denali National Park (mortality,
27%; tenure, 4 years: Mech et al., 1998). In Denali, most
wolves died when killed by other wolves, while in our area
all mortalities were caused by humans, except for one wolf,
which was apparently killed by dogs. None of our wolves
died from natural causes. In our area, the mortality due to
humans perhaps compensates for the natural mortality, the
food surplus may reduce intraspecific competition and
the habitat conditions may be more predictable than in the
protected, natural habitats of Denali National Park or Isle
Royale (Michigan) where food availability varies according
to changes in the severity of the winter (Mech et al., 1998;
Post et al., 1999).
In our study, loners had a higher mortality rate than the
territorial wolves integrated in packs and pairs, as occurs in
other areas. In Minnesota, Mech (1977) also reported that
the annual mortality of territorial adults (18%) was much
lower than that of loners (34%), and other studies have
shown that dispersers suffer a higher mortality than resident
individuals (Peterson, Woolington & Bailey, 1984; Messier,
1985; Fuller, 1989; Pletscher et al., 1997). Both peripheral
wolves and dispersers are low-ranking individuals and
are likely forced to occupy the worst areas. In our study
area, the human-caused mortality is the proximate cause of
the wolf population regulation, but intraspecific competi-
tion appears to be the ultimate factor responsible for this
regulation.
The overall annual mortality in our area was very low
compared with other studies. Other wolf populations with
15–20% annual mortality rates showed 16–49% annual
rates of increase (see the review in Fuller et al., 2003). The
same occurred in our study area. During 1988–2001, in the
whole region of Castile and Leon, wolf density obviously
Journal of Zoology 273 (2007) 114–124 c2007 The Authors. Journal compilation c2007 The Zoological Society of London122
Wolf dispersal in Spain J. C. Blanco and Y. Corte
´s
increased in some areas and the range expanded by 35%
(from 57 000 to 77 300 km
2
) (Llaneza & Blanco, 2005).
Although we lack accurate data on wolf numbers, these
figures show an obvious increase in the wolf population.
This means that wolves can live and increase without the
support of immigrants in an agricultural area with a very
high road density, almost depleted of wild ungulates, as long
they have other food sources and the human tolerance is
high. In our area, the livestock management system meant
that losses to depredation are minimal and there are conse-
quently relatively low levels of wolf persecution.
Interestingly, the conditions that were prevalent in the
majority of our study area are now changing. After the
appearance of BSE towards the end of 2000, the carrion pits
have started to be closed and the food availability for the
wolves is apparently declining. If this decrease continues,
the population dynamic parameters could change and in the
near future could result in fewer loners, smaller pack sizes
and a general reduction in the wolf population.
Acknowledgements
We thank J.M. de Alba, M. Bezos, A. de la Fuente, J.M.
Garc ´
ıa, G. Garrote, A. de la Puente, J. Rodr´
ıguez and M.A.
Rodr ´
ıguez for field assistance. P. Ferreras, R. Villafuerte
and John Linnell provided guidance regarding statistical
analysis and corrected previous drafts of the paper. John
Muddeman and John Linnell improved the English. Two
anonymous reviewers commented on the paper. C. Morillo,
B. Asensio, B. Heredia, J.J. Areces and M. Aymerich
(Ministry of Environment), J. Mu ˜
noz, J.A. Arranz and
I. Molina (Junta de Castilla y Le ´
on) promoted this research
at various stages. This project was funded by the Spanish
Ministry of the Environment with additional aid from the
Regional Government of Castilla y Le ´
on.
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