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Movements and spatial use of habitat are
important for elucidating social behaviors and
intraspecific and interspecific relations among
animals (Gaulin and Fitzgerald 1988, Rachlow
et al. 1998). In species with polygynous mat-
ing systems, variation in sizes of home ranges
for males and females has been related to dif-
ferences in body size and access to resources
and mates (Cederlund and Sand 1994, Goodrich
and Buskirk 1998). Home ranges of females
must be large enough to include sufficient food
for successful reproduction, whereas home
ranges of males reflect a strategy to maximize
access to females during the breeding period
(Clutton-Brock 1989). In mate-defense polygy-
nous systems males often course over large
areas in search of females and gain access to
them by dominance displays sometimes associ-
ated with direct male-male competition (Berger
and Cunningham 1996). In contrast, the repro-
ductive success of females in these systems is
influenced more by life history constraints and
the energetics of producing surviving offspring
(Emlen and Oring 1977, Robbins 1993, Berger
and Cunningham 1994).
The North American porcupine (Erethizon
dorsatum) is a medium-sized mammal with a
range extending from northern Mexico into
Alaska and Canada (Woods 1973). The mating
system of this species has been characterized
as mate-defense polygyny in which the repro-
ductive success of males depends on the num-
ber of females they are able to monopolize
during the late summer–early fall mating sea-
son, rather than the amount or quality of re-
sources they control (Roze 1989, Sweitzer and
Berger 1997). Reproductive success among
female porcupines appears more related to life
history constraints (litter size in this species is
invariably one) and the availability of sufficient
resources to meet energetic needs during ges-
tation and lactation (Roze 1989, Sweitzer and
Western North American Naturalist 63(1), ©2003, pp. 1–10
BREEDING MOVEMENTS AND REPRODUCTIVE ACTIVITIES
OF PORCUPINES IN THE GREAT BASIN DESERT
Richard Alan Sweitzer1
ABSTRACT.—I assessed movements of North American porcupines (Erethizon dorsatum) in the Great Basin of north-
western Nevada in relation to reproductive activities during the late summer and fall periods of 1991 and 1992. Porcu-
pines exhibit a mate-defense polygynous mating system and I hypothesized that (1) competitively dominant males
would have larger home ranges than both subordinate males and adult females, and (2) variation in home range size
among adult male porcupines would be positively correlated with reproductive success. Results indicated that dominant
male porcupines ranged over larger areas (average 95% minimum convex polygon home range = 20.7 ha) than subordi-
nate males (average 95% MCP home range = 2.9 ha) and adult females (average 95% MCP home range = 8.2 ha).
Analyses of movements in relation to body size and energetic requirements revealed that home ranges of dominant male
porcupines were larger than predicted based on body size (approximately 10.2 ha). Breeding period home ranges of
dominant male porcupines encompassed portions of the home ranges of 3 to 10 adult females, and indices of reproduc-
tive success based on observations of mate-guarding behaviors suggested a strong positive relationship between home
range sizes of male porcupines and mating success. Together these data suggested that larger home ranges among domi-
nant males were related to increased mating opportunities and not increased metabolic requirements associated with
larger male body sizes. In the study area, however, female porcupines congregated around small, patchily distributed
riparian areas, and dominant males with relatively small home ranges encompassing riparian areas may have gained
mating access to multiple females. Finally, analyses of overlap among core home ranges (60% MCP) of adult male and
adult female porcupines suggested that both sexes maintained relatively exclusive core home range areas, with males
exhibiting significantly less range overlap with other males (x
–= 9.4%) than females with other females (x
–= 27.1%). It is
possible that the small, patchily distributed riparian areas in this desertlike area were such a limited resource that
females were unable to maintain exclusive use of their home range areas.
Key words: Erethizon dorsatum, Nevada, mating systems, reproductive behavior.
1Department of Biology, University of North Dakota, Grand Forks, ND 58202.
1
Holcombe 1993). Associated with mating be-
haviors, male porcupines do not defend territo-
ries, and they range widely during the breed-
ing period (mid-August to mid-November) in
search of pre-estrous females (Roze 1989).
Upon locating a pre-estrous female, several
males may compete for guarding position, with
the dominant male guarding the female for
1–3 days until copulation (Roze 1989). Com-
petition for mate access among males often
escalates from auditory threats and visual dis-
plays of incisors to battles in which rivals impale
each other with quills and tear out patches of
the other’s hair and quills by biting (Dodge
1982, Sweitzer and Berger 1997). Thus, home
ranges of dominant male porcupines may over-
lap the ranges of multiple females as well as
the ranges of other males (Roze 1989). Among
female porcupines home ranges also overlap,
which is interesting because access to sufficient
forage resources is expected to be a key com-
ponent of female reproductive success. How-
ever, some evidence suggests that females main-
tain exclusive core areas within their home
ranges (Roze 1989).
As part of a larger 5-year study of the popu-
lation and behavioral ecology of porcupines, I
studied the home range movements and re-
productive behaviors of individual porcupines
in the Granite Range of northwestern Nevada
from April 1990 to November 1992. My objec-
tive was to test 3 hypotheses related to breed-
ing period movements among mammals with
mate-defense polygyny: (1) males should have
larger home ranges than females during the
breeding period, (2) home ranges among adult
males will be related more to mate access than
to metabolic demands, and (3) competitively
dominant males will have larger home ranges
than subordinate males, which will be positive-
ly correlated with indices of reproductive suc-
cess. I also examined patterns of overlap among
home ranges of males and females to assess
whether female porcupines are more territor-
ial than males associated with the importance
of resource access for females in polygynous
systems.
STUDY AREA
The research site was a 20-km2enclosed
basin (Granite Basin; 40°44′N, 119°20′W) be-
tween 1520 and 1640 m elevation (Fig. 1).
Granite Basin is characterized by sagebrush
steppe habitat interspersed with juniper trees
(Juniperus osteosperma) and multiple rock out-
croppings (Fig. 1). Within Granite Basin there
are 3 small spring areas, which provide impor-
tant riparian habitat for many vertebrates in-
cluding porcupines. The riparian habitat around
these springs, hereafter referred to as “groves,”
is a complex of buffalo-berry (Shepardia argen-
tea), willow (Salix sp.), and a variety of other
riparian shrubs and plants (Fig. 1). Porcupines
in Granite Basin focused their activities around
the 3 different groves, which provided forage,
water, protective cover, and den sites. Rock
outcroppings and juniper shrubland also were
used for den sites, and grassland/shrubland
habitats were frequented by porcupines dur-
ing spring and early summer when emergent
vegetation provided forage (Fig. 1). The 3 grove
areas were spatially separated such that most
individual porcupines included a single grove
area within their home ranges. Movements of
some females and several male porcupines en-
compassed more than 1 grove area.
MATERIALS AND METHODS
All porcupines in the Granite Basin were
captured and individually marked upon initial
observation; all resident adult males and
females were known. Individual porcupines
were located using dogs, a spotting telescope,
night-vision equipment, radiotelemetry, and
tracking when snowcover was present during
systematic surveys of foraging areas and den
sites. Additional details on surveys used to
assess population sizes are presented elsewhere
(Sweitzer 1996), but population size during the
April to November period was estimated at 75
in 1991 and 46 in 1992. Animals were captured
by ascending trees or approaching their posi-
tions in other habitats for immobilizations with
a jab stick loaded with ketamine hydrochlo-
ride (dosage 10 mg ⋅kg–1) and xylazine hydro-
chloride (4 mg ⋅kg–1). Each animal was fitted
with a numbered color ear tag (Allflex style
C12334-8 [Dallas, TX], modified and attached
with aluminum self-piercing domestic livestock
ear tags [National Band and Tag Company,
Newport, KY]), weighed, and measured for mid-
sternal chest circumference, middorsal and mid-
ventral body length from tip of nose to end of
tail, and length of the left footpad. Ages were
determined from body mass and body dimen-
sions or known from birth (Sweitzer and Berger
2 WESTERN NORTH AMERICAN NATURALIST [Volume 63
1992, 1993). Individuals were periodically re-
captured to monitor growth and hormone levels
in blood associated with reproduction (Sweitzer
and Holcombe 1993). Forty-one different por-
cupines were fitted with radio-collars (AVM
Instrument Company, Livermore, CA; Custom
Electronics, Urbana, IL) during the study period
(April 1991 to December 1993). Only adult-aged
animals (≥18 months) were selected for moni-
toring by radiotelemetry, due to a research
focus on different aspects of reproduction and
mating behaviors. However, locations of other
porcupines also were noted during the system-
atic surveys of the study area. Radio-collared
porcupines were located by homing to positions.
Upon visual contact, location information was
recorded and later mapped on a detailed draw-
ing of the Granite Basin study site. Subsequent-
ly, Universal Transverse Mercator (UTM) coor-
dinates were assigned to each position (accuracy
estimated at ±10 m), aided by an aerial photo-
graph of the Granite Basin area, which was
registered in a geographical information sys-
tem (TNT Mips, MicroImages, Inc., Lincoln,
NE). The aerial photograph was prepared by
scanning the image and assigning UTM coor-
dinates to multiple geographical landmarks
obtained from a 7.5-minute United States Geo-
graphical Survey map of the area.
In this study I generally had fewer than 25
locations for each individual during the sum-
mer to fall period (15 June to 31 November)
2003] BREEDING PERIOD MOVEMENTS OF PORCUPINES 3
Fig. 1. Aerial photograph of Granite Basin study area showing different habitats used by porcupines including 3 small
grove areas focused around permanent springs, several rock outcroppings, and juniper and sagegrush shrublands. Grove
areas were important habitats for a variety of activities (forage, refuge from predators, den sites, mating activities, etc.);
rock outcroppings were important for den sites; open grassland/shrublands were used for foraging during different peri-
ods of the year; and juniper shrubland habitats (interspersed with rock outcroppings) also provided den sites.
encompassing the breeding season. Boulanger
and White (1990) noted that minimum convex
polygon (MCP) methods can provide reliable
estimates of home range areas when number
of measurements is limited. Therefore, I esti-
mated 95% MCP home range areas for the
porcupines monitored during this study using
the computer program RANGES V (Kenward
and Hodder 1996). Individual animals included
in home range analyses were those for which
≥12 locations were available within the period
from 15 June to 31 November 1992 and 1993.
The “core” region of individual home ranges
(area of intensive use including den sites and
other resources necessary for survival) was
defined as the MCP home range area which
included 60% of an animal’s positions. I selected
the 60% MCP as the core home range based
on examination of multi-range utilization plots
produced by RANGES V (Kenward and Hodder
1996). As detailed by Kenward and Hodder
(1996), home range utilization plots for multi-
ple individuals may be used to identify the
core region of home ranges for animals as the
point at which variation in home range size
tends to a minimum, which is the percentage
of fixes that excludes most excursive activity.
Significant autocorrelation can occur if an ani-
mal moves either less or more between se-
quential locations than between nonsequential
locations, related to an individual’s past expe-
rience and knowledge of resources within
home ranges (Powell 1987). To avoid serial auto-
correlation, consecutive locations were sepa-
rated by at least 24 hours.
As part of behavioral monitoring of porcu-
pines during the 1991 and 1992 breeding sea-
sons, I noted mate-guarding episodes between
male and female porcupines and whether males
suffered foreign quill impalements or injuries
(detailed by Sweitzer and Berger 1997). Guard-
ing episodes were defined as close associations
between male and female porcupines (distances
of separation ≤5 m) for 1–3 days during the
mating season. Guarding episodes in this species
are often associated with vocalizations by
females that attract other adult males, some-
times leading to direct male-male competition
for guarding position (Roze 1989, Sweitzer and
Berger 1997). During contests for guarding
position, male combatants often suffer quill im-
palements and loss of patches of quills from
bites (quill patch injuries). Thus, I used data on
guarding associations, foreign quill impalements,
and quill patch injuries (noted when animals
were periodically captured during breeding
periods; Sweitzer and Berger 1997) to identify
adult male porcupines that were reproduc-
tively active (observed in mate-guarding asso-
ciations or quilled or injured) or not reproduc-
tively active (not observed in mate-guarding
associations nor quilled or injured). Assuming
that males that guarded females were relative-
ly dominant compared to those that did not
guard females, I compared the 95% MCP
home ranges of these 2 classes of males to test
the hypothesis that dominant males would have
larger home ranges than subordinate males
during the breeding period. I also assessed
whether adult male porcupines had larger
95% MCP home ranges than adult females.
North American porcupines are sexually
dimorphic in body size, with adult males in
the study population averaging 35% larger than
adult females during late summer (Sweitzer
and Berger 1997). Thus, potentially larger home
ranges among adult male porcupines may be a
function of increased metabolic requirements
related to body size. To test the hypothesis
that home ranges of male porcupines are larger
than home ranges of females due to mating
behaviors and not simply because of larger
male body sizes, I assessed whether observed
sizes of home ranges for males were equiva-
lent to those that would be predicted based on
energetic requirements (Gehrt and Fritzell
1997, Goodrich and Buskirk 1998). Adult female
porcupines raise offspring without the assis-
tance of males, and female reproductive suc-
cess is coupled with the availability of suffi-
cient resources to produce surviving offspring.
Therefore, I assumed that sizes of home ranges
of females were set by metabolic demands (the
area used by a female reflected that needed to
satisfy energetic requirements in the Granite
Basin area; Gehrt and Fritzell 1997), and I
used the mean size of observed home ranges
for females to calculate the predicted home
range of males based on body size from the
formula (Sandell 1989):
HRfemaleMean body mass0.75
male
HRmale = ___________________________
Mean body mass0.75
female
Data on body masses for males and females
used in this calculation were the mean body
masses for radio-collared adult females and
radio-collared adult males measured during
4 WESTERN NORTH AMERICAN NATURALIST [Volume 63
the late summer–early fall time periods of each
year.
To assess the extent to which home range
sizes among adult male porcupines may be
related to reproductive success (e.g., do males
with larger home ranges experience greater re-
productive success?), I calculated index scores
of male reproductive success based on (1) the
numbers of adult females included within the
100% MCP ranges of adult male porcupines
during the breeding seasons in 1992 and 1993
and (2) observations of mate-guarding associa-
tions (Table 2). It was possible that a few females
that were transient within a male’s range and
within Granite Basin in general were not de-
tected. However, because all resident or tran-
sient individuals that were observed were cap-
tured and marked and because survey efforts
were intensive and systematic during the breed-
ing period, the minimum number of females
scored as present within an individual male’s
home range likely approached the total (but
see Discussion for individual male R3). The
total number of adult females (radio-collared
and non-radio-collared females) that were
within ranges of individual males was deter-
mined by mapping the locations of all females
noted in the period from 1 September to 31
October (most active period of mating activi-
ties; Sweitzer and Holcombe 1993; unpublished
data on serum testosterone levels) within the
100% MCP home range outlines of each male
using the range overlap feature of RANGES V.
Observations of mate-guarding associations
then were used to assign point values repre-
senting the probability that an individual male
mated with each female observed within his
range. The reproductive success index score
for each male was calculated as the sum of point
values assigned for each female noted within
the range of the male. Assignments of repro-
ductive success point values were as follows:
1.0 if a male guarded a female for at least 2
days, 0.75 if a male guarded a female 1 day
and no other males guarded her, 0.5 if a male
guarded a female but other males also guarded
her, 0.25 if an individual female was observed
within a male’s home range but was not guarded
by the male or observed being guarded by
other males, 0.125 if an individual female ob-
served within a male’s home range was not
guarded by the male but 1 other male guarded
her, and 0 if an individual female observed
within a male’s home range was not guarded
by the male but at least 2 other males guarded
her (Table 2). A linear regression analysis was
used to assess the relationship between repro-
ductive success index scores and the 95% MCP
home range sizes of adult male porcupines.
Because female mammals require access to
sufficient resources to support energetic costs
of gestation and lactation, female porcupines
may maintain relatively more exclusive territo-
ries than male porcupines. To examine potential
differences in home range overlap related to
sex, I calculated overlap of 95% and 60% MCP
home ranges among radio-collared porcupines
using the range overlap analysis options in
RANGES V. Ranges of pairs of animals were
considered to overlap when at least 2% of an
animal’s range was within the range of another.
Statistical analyses of overlapping home ranges
were based on geometric mean overlaps (Minta
1992). For any 2 animals, aand b, the geomet-
ric mean overlap was calculated as the product
of the ratios of overlap size to the size of the
home ranges of the individuals (Minta 1992,
1993):
(Overlap area Overlap area )0.5
Mean overlap = ____________ ____________
Home range a Home range b
Home ranges of reproductively active males
were compared to home ranges of non-active
males, and both classes of males were com-
pared to females. All comparisons were made
using Mann-Whitney U tests. I pooled predicted
and observed home ranges for reproductively
active males for each year because of small
samples and used a paired ttest to compare
predicted and observed sizes of home ranges
of males. Means are presented ±1 sx
–. All sta-
tistical analyses were completed with SYSTAT
8.0 (SPSS Inc., Chicago, IL).
RESULTS
Overlays of the 95% MCP home ranges of
adult female porcupines and the composite
95% MCP ranges of adult males on an aerial
photograph of the study area (Figs. 2, 3) illus-
trate the general pattern of porcupine move-
ments in Granite Basin. All but 1 of 22 adult
female porcupines monitored during the study
occupied a portion of at least 1 grove area dur-
ing the breeding season (Figs. 2, 3). The com-
posite ranges of 4 and 6 adult male porcupines
in 1991 and 1992, respectively, encompassed
2003] BREEDING PERIOD MOVEMENTS OF PORCUPINES 5
parts, or the entire ranges, of all 22 adult
females. In general, the portions of female and
male home ranges away from grove areas in-
cluded den sites in rock outcroppings or
juniper trees. Although all 3 grove areas were
used by adult females in 1992, grove area 2
was not used by radio-collared adult females
in 1991.
Home ranges of adult male and adult female
porcupines in Granite Basin during the summer
and fall periods of 1991 and 1992 averaged 15.3
ha and 8.2 ha, respectively (Table 1; Mann-
Whitney U test = 135, df = 1, P = 0.31).
Although all radio-collared adult females were
reproductively active in one or both years, not
all adult male porcupines were reproductively
active (Table 2). Three of the 10 radio-collared
adult males were not detected guarding females
or observed with quill impalements or injuries
(Table 2). In relation to mating activities, repro-
ductively active adult male porcupines ranged
over larger areas than adult females (Table 1;
Mann-Whitney U test = 125, df = 1, P = 0.014),
whereas adult males not involved in mating
activities had smaller home ranges than adult
females (Mann-Whitney U test = 10, df = 1, P
= 0.054) and reproductively active adult males
(Mann-Whitney U test = 19.0, df =1, P=
0.053).
Large home ranges observed among repro-
6 WESTERN NORTH AMERICAN NATURALIST [Volume 63
Fig. 2. Plots of 95% minimum convex polygon home ranges for radio-collared adult female porcupines and a compos-
ite 95% MCP home range for radio-collared adult male porcupines in 1991. Home range outlines were overlaid on an
aerial photograph of the Granite Basin study area to illustrate patterns of habitat use in relation to different habitat fea-
tures (groves, rock outcroppings, juniper shrublands, and grassland/shrublands).
ductively active male porcupines were not
linked to body size. The average 95% MCP
home range size of reproductively active male
porcupines was 2 times greater than predicted
(10.2 ha) based on body size (t = 2.77, df = 6,
P = 0.032). In contrast, the average 95% MCP
home range size of 3 adult males that were not
active during the breeding season (x
–= 2.9 ±
2.0 ha) was over 3 times smaller than pre-
dicted (9.0 ha) based on body size (t = –3.14,
df = 2, P = 0.09). However, sizes of core home
ranges of adult male porcupines were similar
to predicted based on body size (Table 1; t =
0.37, df = 9, P = 0.72). The 60% MCP home
ranges of males were pooled in this analysis
because they were similar for both classes of
males.
Although reproductively active adult male
porcupines ranged over larger areas than non-
reproductively active adult males (Fig. 2, 3),
they apparently did not encompass more adult
females within their larger ranges. Reproduc-
tively active adult males overlapped the ranges
of 7.4 ±1.1 adult females, whereas non-repro-
ductively active males overlapped the ranges
of 5.7 ±0.7 (Table 2; Mann-Whitney U test =
14.5, df = 1, P = 0.35). Nevertheless, males
with large home ranges were successful in mat-
ing activities because their larger home range
sizes were positively related to indices of
reproductive success based on mate-guarding
associations (adj R2= 0.77, df = 1,8, P = 0.01;
2003] BREEDING PERIOD MOVEMENTS OF PORCUPINES 7
Fig. 3. Plots of 95% minimum convex polygon home ranges for radio-collared adult female porcupines and a compos-
ite 95% MCP home range for radio-collared adult male porcupines in 1992. Home range outlines were overlaid on an
aerial photograph of the Granite Basin study area to illustrate patterns of habitat use in relation to different habitat fea-
tures (groves, rock outcroppings, juniper shrublands, and grassland/shrublands).
Table 2). The regression analysis identified a
single adult male porcupine as an outlier (W3
in 1992); this male had a small 95% MCP home
range but a relatively high score for reproduc-
tive success (Table 2). When this male was re-
moved from the analysis, the fit of the regres-
sion model describing the relation between in-
dices to reproductive success and home range
size was significantly improved (y = –0.274 +
0.125x; adj R2= 0.93, df = 1,7, P = 0.0001).
Data on home range overlaps during the
breeding season suggested that adult female
porcupines in Granite Basin maintained less
exclusive territories than adult males. As illus-
trated graphically in Figures 2 and 3, the 95%
MCP home ranges of adult female porcupines
overlapped most significantly (average female-
female 95% MCP range overlap = 20.4 ±3%)
in and around grove areas. In contrast, aver-
age 95% MCP home range overlap among
adult males was only 10.6 ±4.7%, which sug-
gested a trend for less overlap of 95% MCP
ranges among adult male than among adult
female porcupines (Mann-Whitney U test =
120, df = 1, P = 0.10). Similarly, the 60% MCP
core home range overlap was higher among
adult females (x
–= 27.1 ±3.6%;) than among
adult males (x
–= 9.4 ±2.8%; Mann-Whitney U
test = 22, df = 1, P = 0.008).
DISCUSSION
Previous research on this population indi-
cated that large, dominant male porcupines
that guarded females suffered fewer quill
impalements and injuries than subordinate
males, providing a mechanism for the evolu-
tion of sexual size dimorphism in this species
(Sweitzer and Berger 1997). This study extends
our understanding of mating behavior in por-
cupines by revealing that dominant adult male
porcupines ranged over larger areas than sub-
ordinate adult males and adult females, inde-
pendent of greater metabolic needs related to
larger body size (Table 1). This was important
because the breeding period movements of
dominant males overlapped the home ranges
of 3 to 10 adult females, potentially resulting
8 WESTERN NORTH AMERICAN NATURALIST [Volume 63
TABLE 1. Summary of home range data for porcupines in Granite Basin in summer and fall (June 15 to November 31)
1991 and 1992.
95% MCP range (ha) 60% MCP range (ha)
__________________________ __________________________
Group NObserved PredictedaObserved Predicted
Adult males 10 15.3 ±3.8 9.9 ±0.3 3.3 ±2.0 2.6 ±0.1
activeb7 20.7 ±3.7 10.2 ±0.2 4.6 ±2.8 2.7 ±0.1
inactive 3 2.9 ±2.0 9.0 ±0.4 0.3 ±0.1 2.4 ±0.1
Adult females 22 8.2 ±1.5 2.1 ±0.6
aPredicted home range based on body size.
bReproductively active adult males were observed guarding females during mating seasons and were noted with quill impalements or injuries. Reproductively
inactive adult males were not observed guarding females or noted to have quill impalements or injuries.
TABLE 2. Summary of reproductive activities of adult male porcupines in Granite Basin during the breeding seasons
in 1991 and 1992.
Adult females within Number females Reproductive success 95% MCP home
ID Year 100% MCP range observed guarding index scores range (ha)
W18 1991 5 0 0 0.8
013 1991 5 0 0 0.9
B1 1991 10 3 3.5 24.9
W3 1991 8 3 3.125 28.7
O67 1992 10 1 2.0 21.1
W5 1992 7 2 2.25 21.4
W3a1992 4 3 2.75 0.9
B2 1992 10 3 3.375 30.3
Y9 1992 7 0 0 6.9
R3 1992 3 3 2.25 17.4
aIdentified as a statistical outlier; see text.
in greater reproductive success among domi-
nant males compared to subordinate males
(Table 2). Roze (1989) noted a similar associa-
tion between relatively large male porcupines
and reproductive success. Although my data
on indices of reproductive success support a
positive relationship between home range size
and reproductive success in porcupines, mating
access to receptive females by males is gained
primarily by dominance during male-male com-
petition (Sweitzer and Berger 1997) and not
solely by inclusion of females within a home
range. For example, 3 subordinate adult males
in this study moved over areas that overlapped
with 5 to 7 different females, but behavioral
data suggested they were unsuccessful at gain-
ing access to any of those females during the
mating season (Table 2).
Dominant males with large home ranges in
Granite Basin appeared to experience relative-
ly high reproductive success, but results from
2 of these males merit further discussion. One
of these males (R3) had a range which over-
lapped with at least 3 adult females during the
mating season. However, a significant portion
of this animal’s range extended into a region of
Granite Basin that was not a focal area of re-
search (northeastern area of Granite Basin; Fig
1). Although I occasionally surveyed the north-
eastern area of Granite Basin for porcupines
during winter when snow conditions facili-
tated locating animals, I had little information
on porcupines in this area. It is likely that the
range of male R3 overlapped the ranges of sev-
eral females unknown to the study, and his
reproductive success index score was probably
underestimated. The other male (W3) was
tracked in both 1991 and 1992 (Table 2). In
1992 the range of male W3 was much smaller
than in 1991, but his estimated reproductive
success index score was relatively high in both
years (Table 2). In 1991 the home range of male
W3 encompassed portions of 2 of 3 grove areas
in Granite Basin (Fig. 2), whereas in 1992 his
range was focused around 1 grove area (Fig. 3).
The relatively small grove area encompassed
by the range of male W3 in 1992 was used by
at least 4 different females, and he apparently
had sole access to all of these females because
I detected no other adult males in the area in
1992. Thus, by focusing his activities around a
single small grove area in 1992, male W3 was
able to gain mating access to multiple females
and avoid the energetic cost of moving over a
larger area in search of mates.
Data on home range overlaps suggested that
female porcupines in Granite Basin were less
territorial than males, even though females
have higher resource requirements for repro-
duction associated with gestation and lactation.
Overlap of both 95% MCP and 60% MCP home
ranges with other individuals of the same sex
was less for adult male than adult female por-
cupines. In a study of porcupines in the decid-
uous forests of northeastern North America,
Roze (1989) found that the non-winter core
home ranges of females overlapped very little
with other females, which he interpreted as
due to agonistic behavior and competition
among unrelated females for resources required
for successful reproduction (natal dispersal is
female-biased in this species; Roze 1989,
Sweitzer and Berger 1998). Roze (1989) fur-
ther noted that the non-winter ranges of males
overlapped more than those of females, even
though some males appeared to avoid each
other. In our Granite Basin study site several
small grove areas provide critically important
habitat for porcupines (Figs. 2, 3). Both male
and female porcupines congregated around
these grove areas, where they foraged, denned,
engaged in mating activities, and sought refuge
from predators (Sweitzer 1996). It is possible
that the patchily distributed grove habitats
around springs in this desert environment are
such a limited resource that females were
unable to maintain exclusive use of even small
core home ranges. Males, on the other hand,
may have been able to maintain relatively
exclusive home range areas associated with
their large body sizes and dominance.
ACKNOWLEDGMENTS
This field portion of this study was sup-
ported by grants from Sigma Xi, the American
Society of Mammalogists, the Theodore Roose-
velt Fund of the American Museum of Natural
History, Molly Knudtsen, and the Department
of Environmental and Resource Sciences of the
University of Nevada, Reno, through J. Berger.
Thanks to those organizations and individuals
for their support. C. Sorenson was instrumental
in helping to compile data for analyses. V. Velez,
A. Hodgson, and R. Loggins provided techni-
cal assistance. Thanks to Uldis Roze for com-
ments that improved the manuscript.
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Received 17 July 2001
Accepted 24 April 2002
10 WESTERN NORTH AMERICAN NATURALIST [Volume 63
