BIRTH-SITE SELECTION BY ALASKAN MOOSE: MATERNAL
STRATEGIES FOR COPING WITH A RISKY ENVIRONMENT
R.
TERRY BOWYER, VICTOR V AN BALLENBERGHE, JOHN
G.
KIE, AND JULIE
A. K.
MAIER
Institute of Arctic Biology, University of Alaska Fairbanks, Fairbanks, AK 99775 (RTB, JAKM)
United States Forest Service, Pacific Northwest Research Station, 3301 C Street, Suite 200,
Anchorage, AK 99503 (WB)
United States Forest Service, Pacific Northwest Research Station, 1401 Gekeler Lane, La Grande,
OR 97850 (JGK)
We studied birth-site selection in Alaskan moose (Alces alces gigas) from 1990 to 1994 in
Denali National Park and Preserve in interior Alaska. Twenty percent of preparturient
females made extensive movements (2:::5 km) immediately before giving birth. Females
selected (use was greater than availability) sites for giving birth (n = 39) that were on
southerly exposures with low soil moisture and high variability in overstory cover. Moose
selected birth sites based on micro-site characteristics rather than on broad types of habitat,
which were used in proportion to their availability. Spatial distribution of birth sites did
not differ significantly from random locations. We hypothesize that such unpredictable
behavior by females is a strategy to avoid predators. Parturient females also selected sites
with high visibility that were located at high elevation, which ostensibly allowed them to
see and then hide from approaching predators. We rejected the hypothesis, however, that
moose in this population spaced themselves away from predators or avoided habitat types
favored by large carnivores. Likewise, we rejected the hypothesis that moose gave birth
close to human developments to avoid predators; random' sites were > 100 m closer to
human developments than were birth sites. Cover of forage, especially willows (Salix), was
more than twice as abundant at birth sites than random sites. Forage quality, as indexed
by nitrogen content and in vitro dry matter digestibility, was slightly but significantly higher
at birth sites. An inverse relationship between visibility and availability of forage indicated
that female moose made tradeoffs between risk of predation and food in selecting sites to
give birth. Thus, maternal females coped with a risky environment; they gave birth at sites
that helped them minimize risk of predation but exhibited risk-averse behavior with respect
to the forage necessary to support the high cost of lactation. We hypothesize that risk of
predation prevented moose from seeking birth sites with more forage and, hence, a greater
nutritional reward, which reduced the variance in forage availability at birth sites.
Key words: Alces alces, Alaskan moose, maternal strategies, birth-site selection, riskaverse foraging, risk of predation, tradeoffs, interior Alaska
Female mammals bear costs of both gestation and lactation (Millar, 1977; Pond,
1977). Moreover, among many polygynous
mammals, males contribute little more than
genes to their offspring; consequently, the
burden of rearing young in such species
rests entirely with females (Clutton-Brock,
1991). Ungulates, which are among the
most polygynous and sexually dimorphic
mammals (Ralls, 1977; Weckerly, 1998),
Journal of Mammalogy, 80(4):1070-1083, 1999
often follow this differential pattern of parental investment (Clutton-Brock, 1991).
Only recently, however, have effects of maternal behavior on performance of offspring
become of interest to evolutionary biologists (Bernardo, 1996), although that topic
has fascinated those studying the biology of
moose (Alces alces) for many years (Altmann, 1958, 1963; Peterson, 1955).
Female ungulates often encounter severe
1070
November 1999
SPECIAL FEATURE-UNGULATE LIFE-HISTORY STRATEGIES
environmental constraints on their ability to
conceive, gestate, provision, and rear offspring successfully (Bowyer, 1991; Rachlow and Bowyer, 1991, 1994). Most fetal
growth occurs in the last one-third of gestation (Schwartz and Hundertmark, 1993),
but females in northern environments must
incur costs of' sustaining such growth near
the end of winter when forage is relatively
unpalatable (Weixelman et al., 1998) and
body reserves are at yearly minima
(Schwartz, 1998). In addition, costs of lactation for large herbivores are enormous
compared with other aspects of maternal investment (White and Luick, 1984). Females
in poor physical condition may give birth
to young that are underweight and exhibit
low survivorship (Byers and Hogg, 1995;
Clutton-Brock et aI., 1987; Festa-Bianchet
and Jorgenson, 1998). Moose with low
body mass at birth may not exhibit compensatory growth and remain among the
smallest individuals in their cohort (Keech
et al., 1999). Indeed, females obtaining a
poor diet may reduce investment in neonates (Rachlow and Bowyer, 1994) or curtail it altogether (Langenau and Lerg,
1976). Undernourished mothers also may
fail to defend their young adequately from
predators (Smith, 1987). Losses of young
to predators can be substantial, and this
pressure has helped shape adaptations of
ungulates for coping with the environments
they inhabit (Bleich, 1999; Bowyer, 1987;
Bowyer et al., 1998a; Hirth, 1977; Kie,
1999; Van Ballenberghe and Ballard, 1994).
Indeed, the need to balance the requirement
of obtaining a nutritious diet against risk of
predation has been well documented for ungulates (Berger, 1991; Bleich et al., 1997;
Kohlmann et aI., 1996; Nicholson et al.,
1997).
In environments with a positive relationship between quality and abundance of forage and risk of predation (i.e., where ungulates seek out areas with ample forage,
and predators concentrate their hunting in
such areas), a tradeoff between those environmental factors may occur (Bowyer et al.,
1071
1998b). Thus, female ungulates may be
forced to tradeoff adequate forage to support lactation against risk of predation for
them to rear young successfully (Bowyer et
al., 1998b; Kohlmann et al., 1996; Nicholson et al., 1997; Rachlow and Bowyer,
1998). Understanding the nature of that
tradeoff is necessary to comprehend how
maternal females cope with their environment while attempting to rear young.
Because nutritional requirements of female ungulates and risk of predation on
their neonates reach maxima during and
shortly after partUrition (Bowyer et al.,
1998a, 1998b; Rachlow and Bowyer,
1998), we chose that period to study maternal tradeoffs in Alaskan moose (A. a. gigas). We selected moose for our analysis
because they remain at or near the birth site
for several weeks following parturition
(Addison et al., 1990); therefore, measurements of forage biomass and quality are
simplified compared with situations where
neonates follow their mothers (Bowyer et
aI., 1998b). In addition, moose in Denali
National Park and Preserve, Alaska, where
we conducted our study, contend with a full
array of natural predators, including wolves
(Canis lupus) and grizzly bears (Ursus arctos-Bowyer et aI., 1998a; Miquelle et al.,
1992).
We tested if forage quality and abundance or risk of predation was more important in determining selection of birth
sites by moose. We determined if maternal
females avoided predators by spacing their
birth sites away from habitats used by predators, or located their birth sites near human
developments. We also tested for a tradeoff
between forage abundance and risk of predation by examining variation in forage
available to moose at birth and random sites
to infer if they followed a risk-prone or
risk-averse strategy (sensu Stephens and
Krebs, 1986). Finally, we discuss selection
of birth sites and the role of this behavior
in shaping patterns of maternal tradeoffs in
moose.
JOURNAL OF MAMMALOGY
1072
I' Ra.-._
• BrthSitas
DSIudy--ri-
FIG. I.-The eastern end of Denali National
Park and Preserve, Alaska, showing the three intensive areas where we sampled random (n = 61)
and birth (n = 39) sites of Alaskan moose from
1990 to 1994. Contour intervals are 100 m.
MATERIALS AND METHODS
Study area.-We studied birth-site selection
by moose in the eastern part of Denali National
Park and Preserve in interior Alaska during each
spring from 1990 to 1994. The study area extended from Highway 3 westward along the Denali Park Road to the Sanctuary River and included ca. 300 km2 of wilderness (Fig. 1).
Moose were distributed in a broad valley with
elevations ranging from 650 to 1,200 m. Rugged
foothills bounded that area to the south, and the
terrain of the Alaskan Range rose precipitously
to the north. Vegetation was dominated by
brushy tundra characterized by resin birch (Betula glandulosa) intermixed with stands of spruce
(Picea glauca and P. mariana) often with a willow (Salix) understory. Herbaceous tundra occurred at higher elevations, with low-lying areas
dominated by meandering creeks and dry
washes with stringers of willow. Trembling aspen (Populus tremuloides), poplar (P. balsamifera), and alder (Alnus) were more common in
the eastern part of our study area, although isolated stands occurred throughout the area (Fig.
2). More complete descriptions of topography
and vegetation of this area were provided elsewhere (Bowyer et aI., 1998a; Miquelle et al.,
1992; Molvar and Bowyer, 1994; Molvar et aI.,
1993). Our classification of habitat types was
modified from Viereck et aI. (1992); we recognized eight broad types of habitat that occurred
in the Park (Fig. 2).
Vol. 80, No.4
Summers in the Park were short and cool, and
winters were long, cold, and often severe. Average temperature ranged from -17°C in January to 12°C in July; yearly snowfall averaged
190 cm, with snow sometimes persisting for 9
months. Depth of snow was above average (41
cm) during the 5 years of our study, with winter
1991-1992 producing exceedingly deep snow
(>90 cm-Bowyer et aI., 1998a). Climatic conditions were highly variable among years (Bowyer et aI., 1998a; Rachlow and Bowyer, 1991,
1994, 1998).
About 150 moose were present during our
study. The population of moose was typical of
others in interior Alaska that were held below
carrying capacity by heavy predation (Gasaway
et al., 1992; Van Ballenberghe and Ballard,
1994). Low survivorship of young (ca. 0.20 by
20 days old) indicated that the population likely
was declining (Bowyer et al., 1998a). Moose appeared to be in excellent physical condition and
exhibited high rates of twinning (32-64% of
births-Bowyer et aI., 1998a).
The Park contained relatively high densities
of wolves and grizzly bears, the primary predators of moose (Albert and Bowyer, 1991; Mech
et aI., 1998; Miquelle et al., 1992). Grizzly bears
were responsible for most (53%) mortality of
young moose from 1990 to 1994 (Bowyer et al.,
1998a) and also killed many young caribou
(Rangifer tarandus-Adams et aI., 1995a,
1995b).
Like most polygynous ungulates, moose sexually segregate at the time of parturition (Bleich
et al., 1997; Bowyer 1984; Bowyer et aI, 1996;
1997; Kie and Bowyer, 1999; Miller and Litvaitis, 1992; Miquelle et al., 1992). Maternal females become solitary in early spring and seek
secluded areas for giving birth (Cederlund et al.,
1987; MacCraken et al., 1997; Molvar and Bowyer, 1994).
Sampling procedures.-We located birth sites
of moose by tracking adult females fitted with
radiotelemetry collars (Telonics, Mesa, AZ)
from early May to mid-June 1990-1994. Mean
date of birth for moose in the Park was 25 May,
and births were highly synchronized (95% of
births in 16 days-Bowyer et al., 1998a). During our 5-year study, 11-18 females wore telemetry collars each year. We attempted to locate
females twice each day by driving westward for
40 km and then returning east along the park
road. When two to three sequential telemetry
November 1999
SPECIAL FEATURE-UNGULATE LIFE-HISTORY STRATEGIES
1073
40'
o
10
20
~~~~iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii
c==l
Herbaceous tundra
Dwarf shrub
N
Kilometers
A
Low shrub . . Forest
. . Tall shrub
River bar
c=J
c=J
Alpine tundra
Snow & Ice
FrG. 2.-Habitat types in the eastern end of Denali National Park and Preserve, Alaska. Moose
did not give birth in the steep, precipitous terrain of alpine habitat.
fixes indicated a female was relatively stationary, we followed that signal to locate the female
and potentially her birth site. Some parturient
females also moved away from their previous
location immediately before they gave birth,
which also helped us to identify such individuals
and locate their birth sites. We recorded the proportion of moose that made such movements
and the linear distances they moved in 1993 and
1994. If a female had given birth, we took care
not to approach too closely .or to disturb the
mother or her young. All aspects of this research
were approved by the Institutional Animal Care
and Use Committee at the University of Alaska
Fairbanks.
We also located birth sites opportunistically
while driving along the park road by observing
lone females closely with binoculars or a spotting scope to determine if a neonate was present.
We also examined areas with trees or shrubs that
had their bark stripped recently by moose. Such
areas were obvious before leaf-out in early
spring. Bark stripping occurred around birth
sites because the female seldom ventured > 100
m from her young and rapidly depleted forage
around the site (Miquelle and Van Ballenberghe,
1989). We also searched for birth sites for ca.
3h using a fixed-winged aircraft. We mapped locations of all birth sites we discovered and often
placed flagging near the site (> 100 m) so we
could relocate it later. We never sampled a birth
site, however, until the female" and her young
had departed.
Birth sites of moose were concentrated in
three areas (X = 2,058 ha ± 1,424 SD) in the
eastern end of the Park (Fig. 1). Consequently,
we distributed our random samples in those
same three areas so that we included only habitat
variables that were available to calving moose.
More random sites were sampled than birth sites
because habitat characteristics were more variable at random locations and a larger sample
was required to describe those sites adequately.
Three areas were selected for intensive study be-
1074
JOURNAL OF MAMMALOGY
cause our previous observations, and those of
others (Miquelle et al., 1992), indicated that they
were used traditionally by moose for calving.
We sampled three birth sites, however, that were
located outside of our areas of intensive study.
We were unable to sample one birth site and one
random site because we could not cross the rising water in Riley Creek safely to reach those
areas. We sampled 39 birth sites and 70 random
sites, but because of some missing data, fewer
random sites were used in particular analyses.
We attempted to distribute samples of birth and
random sites throughout the spring so that a preponderance of samples did not occur in a particular week. Our desire to avoid birth sites until
moose were no longer using them, however,
sometimes caused us to modify that sampling
design.
At each birth or random site, we recorded the
location (2-5 m accuracy) using a global positioning system (GPS). We measured concealment cover at each site with a cover pole (Griffith and Youtie, 1988) that was 2 m in height
and divided into 20-cm segments. The pole was
observed from 10 m at each cardinal direction,
and percent cover was determined by noting the
proportion of segments on the pole that were
obscured (2':50% of each segment) by vegetation
or topographic features. We also recorded overstory cover at the center of a birth site or random
location using a spherical densiometer (Lemmon, 1957) that was read from each of the four
cardinal directions. We determined gravimetric
moisture of soil using a soil core that was 2 cm
in diameter and driven 10 cm into the substrate.
We also recorded wind speed at sites with a
windgauge by noting the maximum value obtained over a I-min interval. We noted if snow
was present at birth or random sites ($;2 m from
the center of the site).
We sampled woody vegetation (browse) available to moose at each birth and random site by
aligning a 50-m transect, which was centered on
the GPS location, in a random direction. Percent
cover of forage within the reach of moose (2.5
min height-Weixelman et aI., 1998) was sampled with the line-intercept method (Canfield,
1941). We recorded cumulative crown cover of
forage as an index of available forage to moose
but considered small gaps «4 cm) in crowns of
small trees and shrubs as continuous cover. We
also recorded the relative amount of foraging by
moose on each species of browse by ranking
Vol. 80. No.4
each contiguous area of cover along the transect
as high (>50%), moderate (26-50%), or low
($;25%). Those ranks corresponded to the percentage of leaders of current annual growth that
was browsed by moose. Such rankings of use
have been used as an index to browsing intensity
for many years (Aldous, 1944). We created a
mean index to browsing for each transect by
weighting the rank for browsing intensity by the
percent cover of a particular contiguous grouping of forage. We did not sample herbaceous
vegetation because it was uncommon in early
spring, and moose in the Park eat mostly browse
at that time of year (Van Ballenberghe et aI.,
1989).
We also clipped samples of current annual
growth for forage species if they were available
at birth or random sites. We collected a minimum of 15 leaders of current annual growth
from willows at each site, which were composited into a single sample for analysis. Those
samples were placed in plastic bags and stored
frozen until they could be analyzed for forage
quality. Before analysis, samples were dried to
a constant weight at 50°C, and ground with' a
Wiley mill so fragments would pass through a
1-mm mesh screen. Forage quality was indexed
by determining in vitro dry matter digestibility
(IVDMD-Van Soest, 1982) with rumen liquor
from a caribou fed a diet that included willows.
Percent nitrogen (N) also was determined using
standard techniques (Van Soest, 1982) at the Institute of Arctic Biology of the University of
Alaska Fairbanks.
We used the Geographic Information System
(GIS) ARCIINFO (Environmental Systems Research Institute, Redlands, CA) to derive several
variables that were indicative of a broader scale
than data collected at birth and random sites,
including distance of random and birth sites to
streams, forest, and human developments. Human developments included campgrounds, the
park road, and other Park facilities. We used a
LANDSAT-TM scene that was classified to determine eight broad habitat types. We used the
GRIDS module of ARCIINFO with a cell size
of 80 m to determine slope, aspect, and steepness around sites. Aspect was transformed to the
sine and cosine of the direction (in degrees) of
the slope face. We also obtained elevation from
a digital elevation model (United States Geological Survey, scale = 1:250,000). We determined
terrain ruggedness by multiplying the angular
November 1999
SPECIAL FEATURE-UNGULATE LIFE-HISTORY STRATEGIES
deviation of aspect by the SD of slope steepness
(Nicholson et al., 1997). We also calculated the
"viewshed," which was the area a standing
moose (2 m in height) might view to a maximum distance of 300 m based on topography
around a birth or random site. That value was
corrected for cover of vegetation by multiplying
the proportional cover in the top 40 cm of the
2-m tall cover pole by the viewshed. Our index
to visibility increased with the unobstructed
view from the site.
Statistical analysis.-We used stepwise (n to
enter and remain = 0.15) logistic regression to
identify variables important in discriminating
birth (coded 1) from random sites (coded 0-Agresti, 1990). Because moose modify surrounding vegetation and substrate at most birth
sites, we were confident that no random sites
were used by moose for parturition. We eliminated one of any pair of variables with a r 2:
0.5 to control for multicollinearity. We assured
that our logistic model was apt by examining a
Homser-Lemeshow test for goodness-of-fit. Because year (P > 0.15) failed to enter our logistic
regression, we pooled years to compare birth
with random sites. For descriptive purposes
only, we present selection of habitat features as
use minus availability. We also performed twotailed t-tests for unequal variances (Zar, 1984)
on some of the individual variables. We used
multivariate analysis of variance (MANOVA) to
test for differences in forage use between birth
and random sites for the four most common species of browse eaten by moose. Quality of forage (IVDMD, N) was tested with multivariate
analysis of covariance (MANCOVA) with site
(birth or random) as the main effect and Julian
date and year as covariates. That approach was
necessary because plant phenology and, hence,
quality of forage in the Park varied among years
(Bowyer et aI., 1998a). Female moose remained
at some birth sites longer than at others thereby
determining dates on which sites could be sampled. We use the SAS statistical package for
those analyses (SAS Institute Inc., 1988). We
examined differences in use of habitat types for
birth sites compared with their relative availability with a G-test (Zar, 1984).
We used multi-response permutation procedures (MRPP) to test for differences between locations of birth sites of moose and random locations within and across years (Slauson et aI.,
1991; Zimmerman et al., 1985). We subset our
1075
random sites for this analysis so they equaled
the number of birth sites in our three areas of
intensive sampling.
RESULTS
Movements of preparturient females
were sampled only in 1993-1994; 20% of
20 females made unusual movements immediately before giving birth (7.3 km ± 2.3
SD). We also examined spatial distribution
of birth and random sites within years
(1990-1994) and strata (Fig. 1; three outlying birth sites withheld from analysis).
Nearest-neighbor distance for birth sites
was 1.1 ± 0.6 km, whereas the distance between random sites was 0.9 ± 0.5 km.
MRPP analysis indicated no significant difference (P > 0.6) between the spatial arrangement of birth and random sites for
within-year data pooled, or for the 3 individual years for which we had sufficient
data to allow analyses: 1992 (P > 0.9),
1993 (P > 0.4), and 1994 (P > 0.3).
Birth sites of moose were typically small
areas (1-3 m across) in which the ground
had been pawed thereby exposing fresh
soil. Hair from the molting female often
was scattered across the site, and feces of
the female and her offspring were present.
Heavy use of forage around birth sites was
obvious with willows being most consumed
(Table 1). Females remained near birth sites
(:=:; 100 m) if undisturbed in all but one instance (n = 39). On that occasion, a female
moved> 125 m to a nearby hillside to eat
snow during the unusually warm spring of
1993 eX = 10.4°C for May-June). Females
that did not lose young to predators (n =
5) remained at the birth site for 3-4 weeks,
but most young moose (78%) were killed
by predators and did not survive >20 days
of age (Bowyer et al., 1998a). Female
moose never used the same birth site twice.
Female moose did not select broad habitat types for giving birth; proportional occurrence of birth sites and random ones
were distributed equally among those habitats (Fig. 3). Additionally, maternal females did not position themselves closer to
JOURNAL OF MAMMALOGY
1076
TABLE 1.-Relative use (proportional cover x
index offorage use) of browse by Alaskan moose
at birth and random sites in Denali National
Park and Preserve, Alaska, in spring 19901994. Multivariate analysis of variance (MANOVA) revealed an overall difference in use between birth and random sites (F = 15.63, d.f. =
4, 104, P < 0.0001); P-values are from planned
contrasts following MANOVA.
Browse
species
Willow
Poplar
Alder
Aspen
Birth sites
(n = 39)
Random sites
(n = 70)a
Vol. 80, No.4
G=1.37,d.f. =5,P= .
en
w
t:
en
•
BI~:~ ~~~ES
~ R~~D,,?~ )SITES
u..
o
w
<!l
<C
fZ
W
U
a:
w
0..
FOREST
TUNDRA
X
SD
X
SD
P-value
1.72
0.23
0.10
0.13
0.76
0.59
0.38
0.34
0.86
0.14
0.06
0.04
0.53
0.35
0.23
0.20
<0.0001
0.32
0.44
0.10
, Sample size differs slightly for random sites from that use
in logistic regression (Fig. 4) because of missing variables in
the regression analysis.
human developments such as campgrounds,
the Visitor Center, or the park road to avoid
predators; random locations were on average > 100 m closer to human developments
than were birth sites (t = 9.53, dj = 80, P
< 0.001; Table 2).
Selection of birth sites may have been
based on habitat characteristics' that occurred at a much smaller scale than broad
habitat types (Fig. 2). Thus, we examined a
suite of micro-site characteristics for birth
and random sites that were related to the
geographic, topographic, and climatic conditions in the Park, forage availability, and
risk of predation (Table 2). From that list,
stepwise logistic regression identified three
variables that discriminated birth from random sites: forage, aspect, and visibility
(Fig. 4). Female moose selected sites to
give birth with more forage (especially willow), better visibility, and southeasterly exposures (Table 2, Fig. 4). Selection of
southerly aspects likely related to climatic
conditions in spring. Some snow was on the
ground in all years when females were giving birth, but differences in snow cover between birth and random sites were small
(Table 2). Soil moisture, however, was 50%
lower at birth than random sites (t = 16.8,
HABITAT TYPES
FIa. 3.-A comparison of broad habitat types
available to (random sites) and used (birth sites)
by Alaskan moose in Denali National Park and
Preserve, Alaska, from 1990 to 1994.
d.! = 93, P < 0.001; Table 2). Thus, aspect
probably influenced whether neonates were
likely to get wet. There also was greater CV
in overstory cover at birth sites than at random sites (t = 8.37, d.! = 42, P < 0.001;
Table 2).
Visibility, which also entered the logistic-regression model (Fig. 4), probably was
related to the ability of parturient females
to observe predators before those carnivores approached birth sites closely. On average, birth sites were 96 m higher in elevation than were random ones (t = 20.8,
dj = 80, P < 0.001; Table 2), which likely
contributed to a better view. Concealment
cover, however, was similar at birth and
random sites (Table 2). Finally, birth sites
had more than twice the available forage
than did random sites (Table 2). Willow was
primarily responsible for that relationship
(birth sites, 19.4% ± 21.1 SD; random sites,
8.1 % ± 9.8 SD); other species of browse
were < 1% on all sites. The CV of willow
cover on birth sites (109%) was less than
on random sites (121 %). There also was a
weak but inverse correlation between visibility and percent cover of willows (r =
-0.15, P = 0.10) for random sites.
Female moose selected sites with a higher quality of forage (e.g., willows) to give
birth. Forage at birth sites was slightly but
November 1999
SPECIAL FEATURE-UNGULATE LIFE-HISTORY STRATEGIES
1077
TABLE 2.-Summary statistics for habitat characteristics of random and birth sites of Alaskan
moose in Denali National Park and Preserve, Alaska, during spring 1990-1994. Suite of variables
was analyzed with stepwise logistic regression.
Birth sites (n = 39)
Habitat variables
Geographical
Distance to streams (m)
Distance to forest (m)
Distance to human
developments (m)
X
SD
Random sites (n = 61)
X
SD
362
159
277
196
383
109
255
147
559
396
453
357
0.250
-0.450
3.9
6.24
829
0.534
0.686
3.3
9.20
174
0.163
-0.147
4.1
5.91
733
0.625
0.759
3.5
8.48
170
145
8.0
7.7
12.3
51.0
135
15
6.4
27.0
17.0
61.1
172
147
8.3
12.9
13.6
37.3
185
14
9.2
33.7
23.3
58.4
275
20.2
20.7
8.8
9.7
49.5
53.9
10.1
19.6
33.2
11.3
43.4
61.5
7.2
25.4
45.7
9.5
Topographical
E-W aspect (radians)
N-S aspect (radians)
Slope (%)
Terrain ruggedness (index)
Elevation (m)
Climatic
Julian date
Windspeed (km/h)
Presence of snow (%)
Overstory cover (%)
CV overstory cover (%)
Soil moisture (%)
Forage
Cover of browse (%)
Risk of predation
Concealment cover (%)
CV concealment cover (%)
Visibility (index)
significantly higher in Nand IVDMD than
at random sites (Table 3).
DISCUSSION
Although one variable (visibility) that related to predation entered our logistic-regression model (Table 2, Fig. 4), the hypothesis concerning the role of human developments in protecting young moose
from predation was rejected. Visitors to the
Park observed grizzly bears pursuing female moose and their offspring, (and sometimes killing them) especially in the eastern
end of the Park where a large campground
(Riley Creek), train station, the Visitor Center, and Park Headquarters concentrated human activities (Albert and Bowyer, 1991).
Parturient moose were thought to select
such areas for giving birth because high
levels of human activity around those developments deterred bears. Random locations, however, were significantly closer to
human developments than were the birth
sites of moose (Table 2). In addition, moose
selected some areas in the Park for giving
birth that had few human developments except the park road. Finally, low survivorship of young moose (Bowyer et al., 1998a)
and bears killing them adjacent to human
developments (Albert and Bowyer, 1991)
indicated that location of birth sites near
such developments had little affect on reducing bear-moose encounters. Langley and
Pletscher (1994) also observed no relationship between birth sites of moose and their
distance to human habitation in northwestern Montana and southeastern British Columbia.
35
30
Z
0
f()
W
....J
OVERALL LOGISTIC
MODEL, P = 0.0001
P= 0.002
25
20
15
Vol. 80, No.4
JOURNAL OF MAMMALOGY
1078
72.7% CONCORDANT
PREDICTIONS FOR
39 BIRTH SITES AND
61 RANDOM SITES
w 10
C/)
P=0.036
TABLE 3.-Forage quality of willow (Salix)
available to Alaskan moose at birth and random
sites in Denali National Park and Preserve,
Alaska, 1992-1994. Least-square means corrected for Julian date and year are presented.
The overall Multivariate analysis of covariance
(MANCOVA) comparing differences between
sites was significant (F = 2.679, d.f. = 68, 104,
P = 0.0001).
P=0.033
(INDEX)
(SIN)
(% COVER)
HABITAT VARIABLES
FIG. 4.-Selection (use minus available) of
habitat variables associated with birth sites of
Alaskan moose in Denali National Park and Preserve, Alaska, from 1990 to 1994. Stepwise logistic regression indicated moose selected (use
> availability) birth sites with more forage and
greater visibility but avoided (use < availability)
north-facing slopes (Le., selected south-facing
ones).
The most likely explanation for numerous visitors observing grizzly bears preying
on young moose was that rates of people
visiting the Park have increased dramatically in recent years, and most campgrounds and other facilities were constructed in prime habitat or along routes of travel
for bears (Albert and Bowyer, 1991). Moreover, the eastern end of the Park, which is
the most intensively developed, is a traditional calving area for moose (Miquelle et
aI., 1992; Fig. 1). Thus, humans, female
moose and their neonates, and grizzly bears
co-occurred in the same area, and observations of bears preying on moose increased.
Risk of predation was related to birth-site
selection in moose, as other authors have
proposed (Addison et al., 1990; Bailey and
Bangs, 1980; Langley and P1etscher, 1994;
Leptich and Gilbert, 1986). Our index of
visibility (Table 2, Fig. 4) likely related to
the ability of a female moose to locate predators before the predator became aware of
her presence. Female moose and their neonates attempt to hide at the birth site to
Measures of
quality
Birth sites
(n = 16)
X
Nitrogen (%)
2.5
In vitro dry matter
digestibility (%) 39.9
Random sites
(n = 20)
SD
X
SD
P-value
0.5
2.4
0.6
0.0001
8.5
34.9
6.0
0.0012
elude predators rather than to flee immediately (R.T. Bowyer, in litt.). Offspring may
join their mother if she flees, or if too
young and small, remain motionless at the
birth site when predators approach (Bowyer
et aI., 1998a). Female moose sometimes
stand their ground and attempt to defend
their young from predators but may be
killed themselves in doing so (Bowyer et
al., 1998a). Although concealment cover at
birth sites was similar to that at random
sites (Table 2), there likely was sufficient
vegetative cover to conceal an adult female
from view when she was lying down. Consequently, female moose that observed
predators before they were observed would
have the opportunity to hide. This pattern
of maternal care and defense does not fit
traditional concepts related to the hider-follower dichotomy proposed for ungulates by
Lent (1974) and Walther (1984). Indeed,
several authors have questioned the usefulness of this concept as an organizing principle for understanding mother-young relationships among ungulates (Bowyer et al.,
1998b; Green and Rothstein, 1993).
Moose (Stephens and Peterson, 1984)
and other cervids (Bergerud, 1985; Bergerud and Page, 1987) are thought to "space
away" from predators at the time of parturition. Birth sites were located at higher
elevations than random sites (Table 2), as
November 1999
SPECIAL FEATURE-UNGULATE LIFE-HISTORY STRATEGIES
others have reported for moose (Wilton and
Garner, 1991). Moreover, predators tend to
be less abundant in the Park at high elevations (Adams et aI., 1995a; Mech et al.,
1998), and bears and wolves occur infrequently in the alpine zone (Rachlow and
Bowyer, 1991). Other cervids presumably
seek high-elevation sites to space away
froIP predators (Adams et aI., 1995a; Bergerud et al., 1984). Evidence we collected,
however, did not support that hypothesis.
Shrub tundra occurred at higher elevation
than other habitats used for parturition (Fig.
2), yet moose did not select that high-elevation habitat for giving birth (Fig. 3).
Likewise, moose did not use high-elevation
slopes of rugged and steep alpine habitat
(Fig. 2) for calving. Dry washes and river
bars were used extensively by grizzly bears
as routes of travel (Albert and Bowyer,
1991), but moose did not avoid those habitats for birth sites (Fig. 3). Thus, use of
higher-elevation sites (Table 2) likely related to the better view such sites provided,
and moose selected such sites independently of the broad habitat types in which they
occurred (Figs. 3 and 4). Perhaps other female ungulates that spaced themselves
away from predators to give birth also
could have been selecting sites that provided a superior view of approaching predators. Moose also locate birth sites on islands
presumably to avoid predators (Addison et
aI., 1990), but lakes and ponds were too
rare in the eastern part of the Park (Fig. 2)
to test that hypothesis.
Although moose did not space away
from predators to elude them, maternal females may have attempted to negate hunting tactics of bears and wolves by behaving
unpredictably at the level of the landscape.
Twenty percent of preparturient females
made extensive movements before giving
birth. Females also did not select broad
habitat types for parturition but used such
habitats in proportion to their availability.
The spatial arrangement of birth sites we
located did not differ from random locations. Those behaviors would prevent pred-
1079
ators from keying on previous locations of
some females to aid in locating their neonates. Likewise, predators could not focus
their hunting activities profitably in particular habitats or localized areas. We hypothesize that such unpredictable behavior by
maternal females is a strategy to thwart
some hunting tactics by predators, especially grizzly bears, which are the primary
cause of death for young moose.
Females remaining at or near the birth
site may represent an anti-predator strategy.
Female moose are nearby to defend their
young from predators, although such defense is not always successful (Bowyer et
aI., 1998a). We hypothesize that remaining
near the birth site would reduce scent trails
deposited by the female that would lead to
the birth site, thereby making neonates
more difficult to locate. Moose possess interdigital glands (Chapman, 1985), and
grizzly bears are thought to locate prey by
following their scent (Craighead and Mitchell, 1982).
Another variable that may relate to risk
of predation is the variability (CV) in overstory cover at a birth site (Table 2), which
also was proposed for birth-site selection in
black-tailed deer (Odocoileus hemionusBowyer et al., 1998b). Such variability in
crown cover from a tree or tall shrub would
create broad patches of sun and shade at the
birth site, and that contrast might help camouflage hiding neonates. Similarly, Eastland
et al. (1989) proposed that the high contrast
produced by a patchy cover of snow might
help conceal young caribou from view. Another possibility is that the lower portion of
the tree or shrub that produced variability
in overstory cover also would help break
up the silhouette of a female moose standing against a hill top or skyline, thereby
making her more difficult for a predator to
locate visually. A standing female moose
that was readily visible might provide a cue
to predators as to the location of her neonate. After grizzly bears located the general
area of a birth site, young moose seldom
survived (Bowyer et aI., 1998a).
1080
JOURNAL OF MAMMALOGY
Variability in overstory cover (Table 2)
also may have been associated with the
thermal environment of the birth site, as
Bowyer et al. (1998b) hypothesized for
sites used by neonatal black-tailed deer.
Southeast-facing slopes entered our logistic-regression model (Fig. 4) and may have
been correlated with other climatic variables. For instance, slopes with a more
southerly exposure undoubtedly were
warmer than other aspects, and birth sites
had significantly lower soil moisture than
random sites (Table 2). Perhaps variability
in overstory cover allowed neonates to thermoregulate more effectively by providing
patches of sun and shade at the birth site.
We cannot discriminate between that hypothesis and one related to predation from
our data.
The final variable that entered our model
was availability of forage (Fig. 4); selection
for that variable was driven by abundance
of willows, which were an important component in the diet of moose in the Park
(Molvar and Bowyer, 1994; Molvar et al.,
1993; Van Ballenberghe et aI., 1989). Our
conclusion is supported by the heavy use of
willows at birth sites compared with random sites (Table 1). Moose also selected
sites to give birth with slightly but significantly higher-quality forage than at random
sites (Table 3). Maternal females experience
tremendous nutritional demands associated
with lactation (White and Luick, 1984).
Even slight differences in quality of forage
can be crucial in successfully provisioning
young cervids (White, 1983); this especially holds for moose, which remain with the
neonate at the birth site for several weeks.
That some females stripped bark to feed
rather than consuming current annual
growth of browse species indicated they experienced nutritional stress. Bark stripping
is associated with undernutrition in moose
during winter (Miquelle and Van Ballenberghe, 1989). Thus, forage quality and
quantity played a major role in determining
selection of birth sites by Alaskan moose.
Moreover, moose selected larger, more pal-
Vol. 80, No.4
atable stems when foraging than stems they
left behind (Bowyer and Bowyer, 1997; Vivas et al., 1991). Consequently, stems we
sampled after moose already had foraged at
birth sites may have underestimated forage
quality of the stems eaten by moose.
Three characteristics of the environment
were generally responsible for birth-site selection in moose: risk of predation, microclimate, and forage abundance and quality.
Moose apparently dealt with the needs for
rearing young under climatic conditions
that were hospitable by selecting slopes
with a southerly exposure, which were
available across an array of broad habitat
types (Fig. 3). Meeting nutritional needs of
females while avoiding predators, however,
was more complex. Visibility, which ostensibly varied inversely with risk of predation, also was related inversely to abundance of forage-risk of predation varied
directly with abundance of food. Sites were
not available that allowed females to maximize forage while minimizing risk of predation. Thus, parturient females made a
tradeoff between those variables in selecting sites where they gave birth.
Risk of predation, as indexed by visibility, was an important component of habitat
selection by parturient females (Fig. 4);
young moose may experience high rates of
predation (Ballard et al., 1981; Bowyer et
al., 1998a; Franzmann et aI., 1980; Gasaway et aI., 1992). We believe, however, that
too little attention has been given to nutritional needs of maternal females in understanding where they give birth and how this
selection relates to survivorship of their
young.
More research is needed to understand
how changes in population density or climatic variability affect habitat selection by
females. For example, female DalI's sheep
(Ovis dalli) selected areas with steep terrain
in a year with good growing conditions for
forage, but selected areas with more food
in a year when growth of forage was limited by cool weather (Rachlow and Bowyer,
1998). Models of habitat selection also will
November 1999
SPECIAL FEATURE-UNGULATE LIFE-HISTORY STRATEGIES
not include variables that are in sufficient
supply in the environment, but such variables may be essential for rearing young
successfully. Studies seldom make that distinction, and interannual variation in important components of habitat may be necessary to identify those factors or understand their value.
Moose tried to cope with a risky environment in both meanings of that term.
First, they attempted to minimize risk of
predation by selecting birth sites that allowed them to detect approaching predators
at a sufficient distance to elude those large
carnivores. Second, females were risk
averse (Stephens and Krebs, 1986) with respect to selecting sites that allowed them to
meet high nutritional costs of lactation.
Failure to satisfy both of those demands has
huge implications for their reproductive
success. Female moose attempted to find a
balance between those variables and, in doing so, made important maternal tradeoffs.
Indeed, the lower CV for willows at birth
sites is likely the result of moose avoiding
sites with too little forage to meet requirements for lactation. The positive relationship between abundance of willow and risk
of predation (inverse of visibility) also indicates that there were areas with ample
food that were too dangerous to be used for
birthing. Thus, we hypothesize that predation risk and demands of lactation help to
cause risk-averse foraging by female moose
at birth sites.
Much emphasis has been placed on
mother-infant relationships (Stringham,
1974) and the role of maternal care in determining survivorship of young (Byers and
Hogg, 1995; Clutton-Brock et al., 1987;
Festa-Bianchet and Jorgenson, 1998). We
suggest that habitat selection related to birth
sites may help regulate type and amount of
care given (Rachlow and Bowyer, 1994,
1998), and that birth-site selection is especially important in survival of young,
particularly in environments with effective
predators. Finally, a growing body of evidence indicates the hider-follower dichoto-
1081
my is too simplistic to explain complex patterns of maternal behavior exhibited by ungulates; environmental conditions may play
a larger role in influencing maternal behavior than previously recognized.
ACKNOWLEDGMENTS
We thank the personnel of Denali National
Park and Preserve for their assistance during our
field research, especially their help in arranging
for the necessary permits. We also thank E.
Rexstad for a helpful discussion concerning this
manuscript. We are grateful to V. Baxter for assisting with the field work. B. M. Pierce and G.
L. Kirkland, Jr. provided helpful comments on
this manuscript. This research was funded in
part by the Institute of Arctic Biology at the University of Alaska Fairbanks, and the United
States Forest Service.
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