Download Sexual segregation in bison: a test of multiple hypotheses

Sexual segregation in bison:
a test of multiple hypotheses
Michael S. Mooring1,2) , Dominic D. Reisig2) , Eric R. Osborne2) ,
Adam L. Kanallakan2) , Brent M. Hall2) , Eric W. Schaad2) ,
David S. Wiseman3) & Royce R. Huber4)
(2 Department of Biology, Point Loma Nazarene University, San Diego, CA, U.S.A.;
Bison Range, Moiese, MT, U.S.A.; 4 Fort Niobrara National Wildlife Refuge,
Valentine, NE, U.S.A.)
3 National
(Accepted: 14 June 2005)
Summary
Sexual segregation, in which males and females form separate groups for most of the year,
is common in sexually dimorphic ungulates. We tested multiple hypotheses to explain sexual segregation in bison (Bison bison) at National Bison Range, Montana and Fort Niobrara
National Wildlife Refuge, Nebraska during June-August of 2002-2003. Fieldwork involved
use of GPS to record space use by segregated groups, vegetation transects to measure forage
availability, fecal analyses to document diet composition and quality, and behavioural observations to characterize activity budgets. During sexual segregation, males in bull groups
used areas with greater per capita abundance of forage, higher proportion of weeds, and less
nutritious grasses (as indicated by lower % fecal nitrogen) compared with females in cow or
mixed groups. However, there was no difference between the sexes in activity budgets, predation risk factors, or distance to water. Single-sex bull groups were no more synchronized
in activity than mixed groups. These results support the ‘sexual dimorphism-body size hypothesis’, which proposes that males segregate from females because their larger body size
requires more abundant forage, while longer ruminal retention permits efficient use of lowerquality forage. The gastrocentric model, based on the digestive physiology and foraging requirements of dimorphic ungulates, supplies the most likely proximate mechanism for bison
sexual segregation. Our results would also partly support the ‘reproductive strategy-predation
risk hypothesis’ if females form large groups to reduce predation risk. The predictions of the
‘activity budget hypothesis’ were not supported for bison.
Keywords: bison, sexual segregation, sexual dimorphism-body size hypothesis, gastrocentric
model, reproductive strategy-predation risk hypothesis, activity budget hypothesis.
1)
Corresponding author’s address: Department of Biology, Point Loma Nazarene University, 3900 Lomaland Drive, San Diego, CA 92106, USA; e-mail address: mikemooring@
ptloma.edu
© Koninklijke Brill NV, Leiden, 2005
Behaviour 142, 897-927
Also available online -
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Introduction
Sexual segregation may be defined as the separation of males and females into different groups for most of the year (‘social segregation’; Conradt, 1998), whether those segregated groups occur in geographically distinct regions (‘habitat’ or ‘spatial segregation’) or use the same area nonsynchronously (‘temporal segregation’). Understanding the effects of large
vertebrates on ecosystem structure and function requires information on how
the sexes partition space, habitat, and forage (Bowyer, 2004). Large herbivores influence fundamental processes of ecosystems (e.g., nutrient cycling,
succession, biodiversity), and differential habitat and space use by male and
female groups can have important consequences for population dynamics if
accompanied by sex differences in survival and reproduction. Indeed, it has
been suggested that the niche requirements of sexes of polygynous ungulates
are sufficiently different to warrant being managed as if they were separate
species (Kie & Bowyer, 1999; Bowyer, 2004). Sexual body size dimorphism,
which is associated with polygynous mating systems in ungulates (Weckerly,
1998; Loison et al., 1999; Perez-Barberia et al., 2002), is likely to be a key
factor favoring sexual segregation (Mysterud, 2000; Ruckstuhl & Neuhaus,
2002).
Except during the rut, adult male and female bison (Bison bison) are
usually segregated into separate social groups (Post et al., 2001). During
the rut, bison males aggregate with females in order to compete for the
privilege of guarding (tending) estrus females. In our study populations, peak
rut (when most breeding occurs) falls during July and August. At National
Bison Range, peak rut is from late July through early August (Lott, 1981,
Figure 6), while at Fort Niobrara peak breeding takes place from mid-July to
late August (Wolff, 1998, Figures 1 and 11; Mooring et al., 2004, Figure 2;
Mooring, unpublished data). In this study, bison were studied during the prerut period of sexual segregation in June and early July, and during the rut
period of sexual aggregation in late July and early August. Large size and
fighting ability of male bison have been favoured by natural selection (Lott,
2002). Sexual body size dimorphism is therefore extreme in bison: adult
males (x = 900 kg) are about twice the mass of females (x = 450 kg);
indeed, bison males are the largest land mammals native to the Western
Hemisphere (Shaw & Meagher, 2000). For this reason, bison constitute an
ideal model for investigating the ecology and evolution of sexual segregation
in large, sexually dimorphic herbivores.
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Table 1 lists the hypotheses (and associated predictions) identified as the
most likely to provide useful explanations of sexual segregation in polygynous ungulates such as bison (Main et al., 1996; Bleich et al., 1997; Ruckstuhl & Neuhaus, 2000, 2002; Mooring et al., 2003). These explanations are
based on differences in reproductive strategy, predation risk, digestive physiology, and activity budgets between males and females. These hypotheses
are not necessarily mutually exclusive, and one or more factors could be responsible for sexual segregation. As pointed out by Main et al. (1996), if
sexual segregation confers advantages to reproductive success by improving
physical condition, then sexual segregation should be most pronounced during the time of year when physical condition is most influenced by habitat
choice and when energy requirements differ most between the sexes. In bison, this period corresponds to the spring and early summer, when males are
building up energy stores in preparation for rut and females are giving birth
to and nursing their offspring. The objective of this study was to investigate
the factors influencing bison sexual segregation by studying bison during
segregation in the early summer, prior to rut. To this end, we characterized
the space use, foraging behaviour, and activity budgets of bison in segregated
groups and related these factors to the associated vegetation cover and diet
quality of males and females as determined by % fecal nitrogen. Because
we were able to construct activity budgets for males and females, as well as
assess the synchrony of activity within bull and mixed groups, we could test
the predictions of the ‘activity budget hypothesis’ (Ruckstuhl & Neuhaus,
2000, 2002), not previously examined in bison.
Methods
Study sites
National Bison Range
Bison (Bison bison) were primarily studied at the National Bison Range
(NBR), Moiese, Montana, during June-August, 2002. The national wildlife
refuge consists of 9000 ha (86 km2 ) of primarily palouse prairie, at elevations from 820 to 1500 m. NBR is home to a diversity of large herbivores
in addition to bison, including white-tailed deer (Odocoileus virginianus),
mule deer (O. hemionus), elk (Cervus elaphus), pronghorn antelope (Antilocapra americana), and bighorn sheep (Ovis canadensis). Large carnivores at
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Table 1. Hypotheses to explain sexual segregation in ungulates. The predictions refer to the period of sexual segregation, when males and females form
separate groups.
1. Reproductive strategy-predation risk hypothesis (Main et al., 1996; Bleich et al., 1997;
Ruckstuhl & Neuhaus, 2000, 2002; Mooring et al., 2003): Males and females pursue different strategies to maximize reproductive success, with males maximizing body condition
and females maximizing offspring survival. Because males are less vulnerable to predation
than females, they can exploit areas of greater predation risk with more abundant and/or
higher quality forage, whereas females use areas of increased security that contain predictable
sources of food and water for offspring and to support lactation (e.g., bighorn sheep, Mooring
et al., 2003). Alternatively, bison females may use large group size to reduce predation risk,
which would enable females to choose both offspring security and higher-quality forage (but
not more abundant forage, because large groups will reduce per capita forage abundance).
Prediction 1: Males will choose sites with abundant and/or higher-quality food (even with
greater predatiorn risk).
Prediction 2: (a) Females will choose sites of reduced predation risk (even with lower
forage abundance and/or quality), or (b) females will form large groups, which allow female
groups to select areas of high-quality (but not more abundant) forage.
Prediction 3: Females with young will occur closer to water sources than mature males.
2. Sexual dimorphism-body size hypothesis, a.k.a. Forage selection hypothesis (Main et
al., 1996; Bleich et al., 1997; Ruckstuhl & Neuhaus, 2000, 2002; Mooring et al., 2003):
Metabolic and digestive differences between the sexes enable larger-bodied males to exploit
greater per capita abundance of lower-quality forage than smaller-bodied females, who must
be more selective for less common, high-quality forage. Two mechanisms may be involved,
either (a) indirect, or scramble, competition (Clutton-Brock et al., 1987; Conradt et al., 1999,
2001), or (b) gastrocentric processes (Barboza & Bowyer, 2000, 2001; Bowyer, 2004). The
indirect competition mechanism involves competitive exclusion of males from areas of highquality forage by females (males are forced to accept inferior forage), whereas the gastrocentric mechanism is based upon inherent nutritional advantages of different diet selection by
males and females.
Prediction 1: Males will select more abundant and lower-quality (higher-fiber) forage.
Prediction 2: Females will selectively feed on less abundant, higher-quality forage.
Prediction 3: If indirect competition is operating, male groups will be found in areas of
high-quality forage primarily when female groups are absent. Alternatively, if a gastrocentric
mechanism is in effect, male groups will choose more abundant, lower-quality forage without
regard to female groups.
NBR are mountain lion (Felis concolor), bobcat (Lynx rufus), coyote (Canis
latrans), and black bear (Ursus americanus). The vegetation is composed
of about 70% grasses, 20% forbs, and 10% woody vegetation by standing crop biomass (Belovsky & Slade, 1986). The dominant native palouse
prairie grasses are bluebunch wheatgrass (Agropyron spicatum), Idaho fes-
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Table 1. (Continued).
3. Activity budget hypothesis (Conradt, 1998; Ruckstuhl, 1998; Ruckstuhl & Neuhaus, 2000;
Mooring et al., 2003): Larger males cannot forage with smaller females due to differences in
activity budgets that result from body size differences in digestive physiology.
Prediction 1: Males will spend more time lying down or ruminating to digest higher-fiber
diet.
Prediction 2: Females will spend more time foraging and moving to obtain a high-quality
diet.
Prediction 3: Females will be more selective (e.g., taking more steps while foraging).
Prediction 4: Subadult males will forage more like females than mature males.
Prediction 5: Activity synchrony will be greater in same-sex groups than in mixed groups.
cue (Festuca idahoensis), and rough fescue (F. scabrella), with junegrass
(Koeleria macrantha) and needle-and-thread (Stipa comata) as subdominant palouse components. Other native grasses include needlegrass (Stipa
sp.), slender and western wheat grass (Agropyron trachycaulum, A. smithii),
Sandberg bluegrass (Poa secunda), basin wildrye (Elymus cinereus), foxtail
and little barley (Hordeum jubatum, H. pusillum), and buffalograss (Buchloe
dactyloides). Non-native (invasive weed) grasses present at NBR include
cheatgrass (Bromus tectorum), Kentucky and bulbous bluegrass (Poa pratensis, P. bulbosa), crested and intermediate wheatgrass (Agropyron cristatum,
A. intermedium), and wild oat (Avena fatua). Dominant forbs include lupine
(Lupinus sp.), yarrow (Achillia millefolium), Dalmatian toadflax (Linaria
dalmatica), salsify (Tragopogon dubius), and arrowleaf balsamroot (Balsamorrhiza sagittata). In addition to the predominant grassland communities, forest and shrub communities are found at higher elevations or along
drainages. Woody plants include fringed sagebrush (Artemisia frigida), western snowberry (Symphoricarpos occidentalis), prairie rose (Rosa woodsii),
Douglas fir (Pseudotsuga menziesii), and Ponderosa pine (Pinus ponderosa).
Bison segregate into distinct social groups throughout most of the year.
The oldest males (7 years) are solitary, male groups are composed of males
>2 years old, and mixed groups consist of females and their offspring along
with a few males (Berger & Cunningham, 1994; Shaw & Meagher, 2000;
Post et al., 2001). Nursery herds of females and calves (without older males)
form during the calving season (April-June). For this study, solitary and male
groups are termed ‘bull groups’, while mixed and nursery groups are termed
‘cow groups’. Bison at NBR are rotated among 8 large pastures on a monthly
basis to avoid overgrazing. During the study the bison were primarily in the
upper west pasture. The bison population during summer 2002 consisted of
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420 yearlings, subadults, and adults, plus young of year calves. All bison
were branded on the right or left hindquarters with the last digit of the
date of birth (the side alternating by decade), allowing us to know the age
of all animals. Because bison were not individually marked, we sampled
individuals from as many groups as possible in order to avoid re-sampling the
same animals, and we limited the number of focal observations conducted to
the number of individuals of each age/sex class to avoid pseudoreplication.
Bison were located by driving 4-wheel drive vehicles along refuge roads
and tracks during all daylight hours. Because the herds frequently moved
out of sight of roads, and management policy prevented us from leaving the
road system, group activity observations were conducted at another site (see
below).
Group locations and habitat evaluation
We recorded group locations from 4 June to 5 July, during the prerut period
of sexual segregation. Individuals that were on average <10 body lengths
(approx. 30 m) from one another and moved together in a coordinated fashion were considered members of the same group. At NBR, whenever we
sighted a group, the following information was recorded: group size, composition (when possible), and the Universal Transverse Mercator (UTM) coordinates. To record the location of each group, we used an ‘Earthmate’ Global
Positioning System (GPS) receiver linked to a laptop computer loaded
with the XMap 3.5 map engine and using 3-D TopoQuad (digital USGS
7.5-minute quadrangle maps) and Sat 10 (10-m colorized satellite imagery)
for Montana West (all products from DeLorme; Yarmouth, ME). The laptop
and GPS were powered by an APC adaptor cable running off the 12-volt
battery of the vehicle. Both topographic map and satellite imagery could be
viewed simultaneously, the location of each group was recorded on Draw
layers using symbols and labels, and linear distances and areas could be
measured with Draw tools for subsequent data analysis. When in the field,
the GPS unit gave a continuous reading of the vehicle location; using a Leica
LRF 1200 Rangemaster rangefinder and Suunto compass, we used the distance/bearing tool in XMap to pinpoint the map location, UTM coordinates,
and elevation of each group. Locations where groups were observed were
assessed for openness using a subjective score (open, intermediate, closed).
Group locations were later analyzed on XMap for terrain ruggedness, distance to the nearest water source, and distance to the nearest woodland cover.
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As a measure of ruggedness, we counted the number of contour lines intersecting the 4-ha grids containing a given group (as displayed in XMap at a
zoom level of 15-0), as per Bleich et al. (1997). We used the measure tool to
compute the distance in meters from the group location to the nearest watercourse (on TopoQuad) or tree cover (on Sat 10). We used satellite imagery
to identify vegetation because it used the most recent data (2001).
Behavioural observations
Behavioral observations at NBR were conducted during pre-rut sexual segregation (4 June to 5 July, 2002: activity budgets: 6-28 June; forage selectivity and efficiency: 4 June-5 July) and the rut period of sexual aggregation (22 July to 10 August). Observations were made from the vehicle with
10× binoculars and 15-60× telescopes during 20-min focal animal samples
(Altmann, 1974). During 20-min samples, activity budgets were recorded
by instantaneous sampling at 1-min intervals (Altmann, 1974). Focal animal
observations focused on 144 females (adult females 3 years old), 99 males
(adult males 4 years old), and 53 subadult males (males 2-3 years old)
in the herd. Ages of focal animals were known from brands (as mentioned
above). Every effort was made to avoid repeat sampling of the same individuals in a group based on brands and individual markings that could be recognized during an observation session. Activity scans at 1-min intervals were
used to compute the mean percentage of time that focal animals spent engaged in feeding, standing, moving, lying down, ruminating, and vigilance.
In addition, we recorded the ‘nearest neighbor distance’ (distance in body
lengths from focal to nearest bison) during the 1-min scans. Assuming a random distribution, we calculated density using the formula d = (1/2(NND))2 ,
in which d = density in the same units as the nearest neighbor distance, and
NND = average nearest neighbor distance in bison body lengths. To convert bison body lengths to meters, we multiplied NND by 3 (body length
around 3 m; Nowak, 1999), and to convert bison per m2 to bison per ha,
we multiplied by 10,000 (1 ha = 10,000 m2 ). Wrist watches with repeating
alarm function were used for timing instantaneous scans. Data were written
directly into notebooks in the field and entered onto laptop computers back
at camp.
Separate observations at NBR characterized foraging selectivity and foraging efficiency. For selectivity, the time spent foraging by the focal animal during 10-min focal observations was recorded to the nearest second;
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whenever the focal animal stopped foraging, the stopwatch was stopped until foraging resumed. Cumulative foraging bouts of <5 min were discarded.
Following Risenhoover & Bailey (1985) and Ruckstuhl (1998), the number
of steps made by both forelegs during foraging was counted with a hand tally,
resulting in a measure of the number of steps taken per minute of foraging.
More steps taken during foraging was interpreted as greater foraging selectivity. Foraging efficiency was computed as the percent of active time spent
foraging during a 10-min focal sample (Berget et al., 1983; Stockwell et al.,
1991). Altogether, we collected 110 h of activity data and 46 h of foraging
selectivity and efficiency data during the prerut period of sexual segregation
at NBR.
Vegetation transects
To characterize forage availability at NBR, we conducted sixteen 40-m vegetation transects in representative areas of the reserve using the Daubenmire frame technique (Daubenmire, 1959). For the safety of the investigators, transects were sampled between 9-27 July, after the herd had been rotated to another grazing unit. The general location of transects was based
on the group data from the previous month (5-28 June), with those areas
that received the highest use by bull and cow groups selected for transects.
Briefly, for Daubenmire sampling a 40-m measuring tape was laid out with
the starting point and direction of transit determined by a random process.
A 0.33 m2 frame was then placed on the ground at 1-m intervals. For each of
the 40 frame measurements, the percent cover for every plant species was
estimated according to the following cover classes: 0-1%, 1-5%, 5-25%,
25-50%, 50-75%, 75-95%, and 95-100%. Plants were identified with the aid
of standard references (USDA Forest Service, 1937; Hitchcock, 1950; Hermann, 1966; Stubbendieck et al., 1997; Kershaw et al., 1998). Subsequently,
information on each plant species identified was gleaned from these references, including life span, native or introduced, invasive or noxious weed,
forage value, and palatability.
‘Palatability’ refers to the relative attractiveness of plants to a feeding animal, and is determined by a variety of factors, including fiber content, flavor,
nutrient and chemical content, and morphological features such as roughness. ‘Preference’, the actual selection of plants, is influenced by palatability.
In this study, palatability refers to the percentage of accessible forage that is
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typically grazed, based upon the following association between forage value
and percent grazed: worthless = 0, practically worthless = 5, poor = 5-15,
fair = 20-35, fairly good = 40-50, good = 55-70, very good = 75-85,
excellent 90 (USDA Forest Service, 1937). For each species, forage values
were converted into these percentages, taking the midpoint of each range,
and the resultant value termed the palatability index (PI). The PI was used
as an estimate of relative quality of forage species present at each transect.
Because structural carbohydrates and protein content of plants varies widely
with plant age and rainfall, palatability did not give information on the actual diet quality of forage at a particular time. Likewise, because preference
is influenced by a variety of factors (including plant palatability, density, and
distribution, and by herbivore population density), the PI is not a measure of
preference.
Fecal dietary measures
Because bison consume grasses and sedges almost exclusively, rarely feeding on tannin-rich browse and forbs (Shaw & Meagher, 2000), fecal nitrogen
can be reliably used to assess the diet quality of bison (Hobbs, 1987; Post et
al., 2001). At NBR, we collected fecal samples from males and females between 10 June and 22 July. Fecal samples were collected opportunistically,
usually immediately after a male or female had defecated and moved off. For
each sample replicate, we collected 4 tablespoons of dung, subsampled from
different regions of the pat, and placed inside a paper sack labeled with the
date, time, sample number, and sex class of the individual that had defecated.
We collected 100 fecal samples from males and 100 from females. Half of
the samples (50 per sex) were collected primarily in June (10 June-3 July,
when bison were sexually segregated) from males in bull groups and females in cow groups, while the other half were collected in July (4-22 July),
when bison were aggregated just prior to and during peak rut. For each sample, 2 replicates were collected: an ‘A’ sample for % fecal nitrogen and
a ‘B’ sample for diet composition by microhistology. An additional replicate was taken randomly from ∼10% of samples (11 replicates per sex) for
assessing the precision of % fecal N measurements. The fecal samples were
air dried at camp by placing the paper bags on screens raised off the ground
in the sun. Once the samples were completely dried, the ‘A’ samples were
finely ground using a Braun electric coffee grinder (vacuumed clean after
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every sample), while the ‘B’ samples were left unground. Both ‘A’ and ‘B’
samples were repackaged in new paper bags and delivered to the Wildlife
Habitat Laboratory, Department of Natural Resource Sciences, Washington
State University, Pullman in mid-August. The laboratory analyzed the samples for % fecal N (reported on a moisture-free basis) and diet composition
at the plant genus/species level by microhistological identification.
Fort Niobrara
Group activity observations were conducted on bison at Fort Niobrara National Wildlife Refuge, near Valentine, Nebraska from 13 June to 8 July,
2003. The refuge consists of 7742 ha (77 km2 ) along the Niobrara River in
the Sandhills of north-central Nebraska. In addition to bison, Fort Niobrara
supports populations of elk (Cervus elaphus), white-tailed deer (Odocoileus
virginianus), mule deer (O. hemionus), black-tailed prairie dogs (Cynomys ludovicianus), coyotes (Canus latrans), bobcats (Felis rufus), and over
260 species of birds. The bison herd is currently maintained at 350 head
after the fall roundup, and numbers ∼475 following calving. During the
summer, the main bison herd is rotated among 16 large pastures (approx.
250 ha each) in flat or gently rolling grassland. Observations were conducted
with 10× binoculars from 4-wheel drive vehicles, either from a track or
off-road. Because the terrain provided high visibility and we could go offroad when necessary, it was possible to keep a particular herd in view at all
times.
Behavioural observations
We observed the behavioural synchrony of individuals in bull and mixed
herds at Fort Niobrara (nursery herds were not observed). Because the activity of unweaned calves is not independent of the activity of their mothers, we
excluded calves from our observations. To control for the influence of time
of day and weather on activity, we observed at least one bull group and one
mixed group simultaneously for 3-5 hrs during daylight hours. We were often able to observe 3 groups simultaneously (e.g., 1 bull group and 2 mixed
groups, or 2 bull groups and 1 mixed group). Sampling sessions were equally
divided among morning (0730-1200 hrs), midday (1000-1500), and afternoon (1530-2000) shifts. We conducted group scans (Martin & Bateson,
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1993) in which the activity of all group members was recorded at 5-min intervals. Activity was dichotomized as either active (grazing, walking, standing)
or resting (lying down), as in previous studies (Conradt, 1998; Ruckstuhl,
1999; Ruckstuhl & Neuhaus, 2001). Inter-observer reliability was tested by
having all 3 observers simultaneously collect data on the same herd over
a 5-hr period. Inter-observer agreement was very high; correlation coefficients among observers for the number of active or resting animals recorded
at each 5-min scan interval ranged between 0.98-0.99. Altogether, we collected 196 h of group activity data during the pre-rut period of sexual segregation at Fort Niobrara.
The ‘activity budget hypothesis’ predicts that single-sex groups will be
more synchronised in their activity than mixed groups (containing both
sexes). For example, a group in which every member is active (or resting) would be perfectly synchronised, whereas a group in which half the
members are active and the other half are resting would be least synchronised. In an attempt to measure the synchrony of activity within and between groups, various ‘synchronisation indices’ have been suggested (Conradt, 1998; Ruckstuhl, 1999; Ruckstuhl & Neuhaus, 2001). These indices,
however, are idiosyncratic to the particular studies for which they were developed and are not easily converted to other study systems. In this study,
we used the proportion of group members performing the dominant activity (i.e., what >50% of the group was doing at a given scan) to compute
the mean within-group synchronization for each observation session. To calculate this ‘synchrony index’ (SI), the proportion of individuals active in
the group (active/active + resting) was computed for each 5-min interval
(N = 36-60 per group session); this could range between 0 and 1.0. The
proportion active was subsequently converted to the proportion of individuals performing the dominant activity during each scan (ranges between 0.5
and 1.0), and the SI was computed by taking the mean of these measures for
all scans during a group session. Analysis involved comparison of matched
pairs of bull and mixed groups observed at the same time. If the prediction
of the ‘activity budget hypothesis’ is true, single-sex bull groups should have
a higher SI compared with mixed groups.
Statistical analysis
We conducted a total of 352 observation h at NBR and Fort Niobrara. Data
were analyzed using the SPSS 8.0 statistical package for Windows. Because
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behavioural data are typically non-normally distributed (Martin & Bateson,
1986), thereby not meeting the assumptions of parametric tests, we used
nonparametric statistical tests for analysis of behavioural measures. Statistical tests included independent sample and paired-sample t-tests, one-sample
Analyses of Variance (ANOVA) with Scheffe multiple comparisons (on raw or
ranked data), Mann-Whitney tests, Wilcoxon ranked ANOVA, and Wilcoxon
Signed Rank tests (Siegel & Castellan, 1988). The level of significance was
set at 0.05, and all tests were two-tailed. For analysis of activity budget data,
because we did not have individually marked animals, we computed activity means for each group type or focal animal class observed. Although we
are confident that our data collection methods minimized the possibility of
sampling the same animal twice, we acknowledge that repeat sampling may
have occurred in some cases. However, had it occurred, any repeat sampling
would have been minimal and would not have altered our results, which were
robust. Because the predominant result from activity scans was no significant difference between males and females, we conducted post-hoc power
analysis using GPOWER Version 2.0 (Faul & Erdfelder, 1992). Based on the
recommended power level of 0.80 (Cohen, 1988), our analysis typically had
sufficient power to detect a medium or large effect (i.e., difference in activity
budgets between the sexes), had one existed.
To characterize space use by bull and cow groups, we used a modification
of Conradt’s Segregation Coefficient (SC) for spatial segregation (Conradt,
1998; Conradt et al., 2001):
SCspatial = 1 − (C/A · B) · a · b/c − 1
in which ‘a’ is the number of bull groups in the i th grid square, ‘b’ is the
number of cow groups in the i th grid square, ‘c’ is the total number of
groups in the i th grid square (a + b), ‘A’ is the total number of bull groups,
‘B’ is the total number of cow groups, and ‘C’ is the total number of groups
(A + B). This modification retains the mathematical integrity of the original formulation (L. Conradt, pers. comm.), while correcting the problem of
non-independence of individual males and females inherent in the original
formula as applied to bison (Conradt, 1998). We sampled every two adjacent
grids squares (at XMap zoom level 15-0) in which at least 2 groups were
sighted (1 double-grid = 8 ha). Sampling involved counting the number and
type of each group found in every double-grid fulfilling the above criteria,
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and then highlighting the double-grid with the Draw Tool to mark it as sampled. We chose to sample double-grids because single grids often had only a
single group observed, and at least 2 groups are required for a valid test of
segregation. On the other hand, larger grids would have included male and
female groups in all samples, thus we sampled at the smallest grid size for
which 2 group sightings were usually available. The Segregation Coefficient
ranges between 0 and 1. An SCspatial of 0 indicates no sexual segregation in
space use (the groups are distributed randomly on the grid squares), while
an SCspatial of 1 indicates complete sexual segregation in space use (bull and
cow groups never use the same grid squares); an intermediate SCspatial value
indicates overlap in space use by groups. Mathematically, the Segregation
Coefficient is the square of the proportion of groups that segregate from other
group types (L. Conradt, pers. comm.).
Results
Space and habitat use of groups
Bull groups were considerably smaller (x = 3.8) than cow groups (x =
96.5; t-test: Nbull = 203, Ncow = 92, t = 9.88, p = 0.0001). Based on
nearest neighbor distances from focal observations, males in bull groups
were 3 times more spread out from each other than were females in cow
groups (Males: x = 3.74 bodylengths, Females: x = 1.24; t-test: Nmale =
94, Nfemale = 144, t = 8.65, p = 0.0001). Converting nearest neighbor
distance to density (assuming a random distribution), the density of females
in cow groups (180.66 per ha) was 9 times greater than that of males in
bull groups (19.86 bison per ha). Bull groups tended to use slightly lower
elevations than cow groups (x = 48 m lower), but there was no difference in
openness, distance to water, distance to tree cover, or ruggedness of habitat
used by bull and cow groups (Table 2). The Segregation Coefficient (SCspatial )
was 0.068, which means
√ that 26.1% of groups were spatially segregated from
other group types ( 0.068 = 0.261), while 73.8% of groups associated
randomly from other group types.
Daubenmire vegetation transects identified 80 plants to the species or
genus level, including 26 species of grasses and sedges (Appendix 1) and
48 species of forbs and shrubs (Appendix 2). Comparison of vegetation transects associated with locations of bull groups versus cow groups (Figure 1)
910
Mooring et al.
Table 2. Mean habitat parameters and Palatability Index of vegetation associated with bull groups, cow groups, and mean availability (mean of 16
vegetation transects) at National Bison Range during the period of sexual
segregation, and the t-statistic and p-values from intergroup comparisons
with 2-sample t-tests. Group values that departed significantly from mean
availability in one-sample t-tests are indicated by an asterisk and either a
plus (greater-than-average) or minus (less-than-average) sign in brackets.
∗
N = 203 for bull groups, N = 85 for cow groups.
Measure
Habitat parameter
Elevation (m)
Openness score
Distance to water (m)
Distance to trees (m)
Ruggedness (# contours)
Palatability Index
All vegetation
Grasses
Bull
groups∗
Cow
groups
Mean
availability
t
p-value
1093.2
2.66
231.4
315.8∗(−)
5.55∗(+)
1140.9∗(+)
2.72
231.8
337.2
5.50∗(+)
1101.5
211.2
367.4
5.13
3.06
0.78
0.02
0.72
0.22
0.003
0.43
0.99
0.47
0.82
60.3
70.8
2.58
1.65
0.011
0.10
57.8∗(−)
65.8∗(−)
62.8
69.7
Figure 1. Percent cover of different vegetation types from sites utilized by bison males
in bull groups (black bars), females in cow groups (cross-hatched bars), and the mean of
all vegetation transects (open bars) at National Bison Range during the period of sexual
segregation. An asterisk indicates a significant difference between males and females, or
between males and the mean of all transects (p < 0.05). Bull groups utilized sites with
less grasses, native plants, and palouse prairie grasses, and more non-natives grasses, than
cow groups and the mean percent cover.
911
Sexual segregation in bison
Table 3. Mean percent cover of vegetation associated with bull groups, cow
groups, and mean availability (mean of 16 vegetation transects) at National
Bison Range during sexual segregation, and the t-statistic and p-values from
intergroup comparisons with 2-sample t-tests. Group values that departed
significantly from mean availability in one-sample t-tests are indicated by an
asterisk and either a plus (greater-than-average) or minus (less-than-average)
sign in brackets. ∗ N = 203 for bull groups, N = 85 for cow groups.
Percent vegetation cover
Grasses
Per capita/ha
Forbs
Shrubs
Natives
Weeds
Palouse prairie species
Bluebunch wheatgrass
Idaho fescue
Rough fescue
Junegrass
Needle-and-thread
Other native grasses
Baker wheatgrass
Buffalograss
Slender hairgrass
Mountain muhly
Invasive non-native grasses
Red threeawn
Japanese brome
Bulbous bluegrass
Kentucky bluegrass
Bull
groups∗
34.2∗(−)
172.2 m2
12.6
2.0
36.5∗(−)
16.9
10.9∗(−)
2.7∗(−)
6.5∗(−)
0.8∗(−)
0.9∗(−)
0.01∗(−)
Cow
groups
Mean
availability
t
p-value
37.0
20.5 m2
13.1
1.7
40.7
16.2
16.6
3.8
10.3
1.2
1.4
0.03
36.0
2.23
0.027
13.2
2.0
40.6
15.9
16.5
3.2
10.6
1.2
1.4
0.03
0.52
0.68
2.04
0.58
3.48
1.82
3.33
1.08
1.92
1.23
0.61
0.50
0.043
0.56
0.001
0.07
0.001
0.28
0.06
0.22
0.04∗(−)
0.04∗(+)
3.8∗(+)
0.5∗(−)
0.21
0.03
2.0
0.8
0.13
0.02
1.6
0.7
2.46
0.70
2.41
2.44
0.015
0.48
0.017
0.016
3.8∗(+)
2.7∗(+)
0.02∗(−)
4.3∗(+)
1.1∗(−)
1.3∗(−)
0.01∗(−)
4.3
1.9
1.8
0.05
3.6
3.73
3.84
2.46
0.003
0.0001
0.0001
0.014
0.99
revealed that bull groups used habitat containing fewer native bunchgrasses
and more introduced (invasive weed) grass species than that found at cow
sites (Table 3). Specifically, bull group locations were associated with less
palouse prairie grass species (Idaho fescue, junegrass) and more non-native
invasive grasses (Japanese brome, red three-awn, bulbous bluegrass) than
cow group locations (Table 3). Bull sites also had more slender hairgrass and
less mountain muhly than cow sites. In addition, a number of invasive forbs
912
Mooring et al.
and shrubs were more abundant at bull group locations compared with cow
locations (western yarrow, silver sagebrush, golden aster, filaree, gumweed,
snakeweed, yellow pepperweed, skunkbrush sumac, tumblemustard). Overall, there was no difference in cover of forbs or shrubs between the groups,
but grass cover was slightly less for bull group sites (34.2%) than for cow
group sites (37.0%). Taking into consideration mean group density, the per
capita grass availability was far greater for males than for females. Based
on a hypothetical 1 ha grazing unit, the average male in a bull group would
have had 172.2 m2 of grass available compared to only 20.5 m2 available
for the average female in a cow group (ratio of grass available for males vs.
females was 8.4:1.0). The Palatability Index (PI) for all vegetation (grasses,
forbs, shrubs) was significantly less for locations associated with bull groups
compared with groups of females (Table 2). However, bison eat primarily
grasses, and there was no significant difference in the PI for grasses between
group locations.
Habitat parameters associated with bull or cow groups were compared
with mean availability as estimated by the mean of all transects (Tables 2
and 3). In general, locations used by bull groups tended to depart from
average habitat parameters, whereas cow group locations tended to be no
different from mean availability. Bull groups were found in habitat with less
native plant cover, less palouse prairie grass species, and lower palatability
of grasses compared with mean availability. Specifically, bull group sites had
less-than-average cover of all palouse prairie species (bluebunch wheatgrass,
Idaho fescue, rough fescue, junegrass, needle-and-thread), and greater-thanaverage cover of introduced invasive grasses (red threeawn, Japanese brome,
Kentucky bluegrass). In contrast, cow groups had less-than-average cover of
some introduced invasive grasses (red threeawn, Japanese brome, bulbous
bluegrass), but otherwise did not differ from mean vegetation cover. Cow
groups were found at slightly higher-than-average elevations, whereas the
elevation of bull group locations did not differ from mean availability.
Fecal dietary analysis
Matched-pair comparisons of original and replicate samples for % fecal
nitrogen analysis indicated a satisfactory degree of precision. There was
no significant difference between the paired replicates (Matched-pair t-test:
Sexual segregation in bison
913
N = 21, t = 0.12, p = 0.91), which were highly positively correlated (Pearson correlation: N = 21, r 2 = 0.94, p = 0.0001). The mean difference between paired replicates was 0.001, which was less than the mean difference
between all male and female % fecal nitrogen values (N = 200) of 0.060,
and less than the mean differences for the analyses that follow (0.004-0.255).
Results from the nutritional analysis are illustrated in Figure 2. Percent
fecal nitrogen of males (x ± SEM = 1.63 ± 0.03) was significantly lower
than that of females (1.75 ± 0.03) during the June sampling period when
males and females were sexually segregated (t-test: Nmale = Nfemale = 50,
t = 2.98, p = 0.004). However, there was no difference in % fecal N
between the sexes in July, when males and females aggregated for the rut
(Females: x = 1.50 ± 0.02, Males: x = 1.50 ± 0.03; Nmale = Nfemale = 50,
t = 0.131, p = 0.90). Percent fecal N declined significantly from June to
July for both males (N = 100, t = 3.37, p = 0.001) and females (N = 100,
t = 6.75, p = 0.0001), indicating a deterioration of diet quality in most
forage components.
Diet composition by microhistology analysis confirmed that bison ate primarily grasses (x = 97.8% of total diet), with forbs and shrubs contributing
only 1.9% and 0.3% of the diet, respectively. Because most grasses could
only be identified to the genus level, it was not possible to compare the use
of native and introduced species between males and females, thus we pooled
Figure 2. Mean percent fecal nitrogen of bison males (black bars) and females (open bars)
at National Bison Range during sexual segregation (June pre-rut) and aggregation (July rut).
An asterisk indicates a significant difference between males and females (p < 0.004). Males
consumed lower-quality forage than females during sexual segregation in June, but not during
aggregation in July.
914
Mooring et al.
all the samples for reporting purposes (Appendix 3). The major grasses identified in the fecal samples corresponded to the genera of grasses identified
in the vegetation transects, with the exception of pinegrass (Calamagrostis
spp.), which was not seen in any of the transects.
Activity budgets
Prerut behavioural observations measured activity budgets when bison were
segregated into bull and cow groups. Activity scan data failed to reveal any
significant difference between males and females in most activities (Figure 3), including feed (Mann-Whitney: N = 144 females, 99 males = 243,
z = 1.20, p = 0.23), ruminate (z = 0.95, p = 0.34), lie (z = 0.69,
p = 0.49), move (z = 1.13, p = 0.26), and vigilance (z = 1.42, p = 0.16).
(Because not all behaviours were performed in every 20-min focal sample,
the percent of activities sums to >100%.) Females spent more time standing compared with males (z = 2.73, p = 0.01). There was also no significant difference in the percentage of scans devoted to foraging and ruminating among males, females, and subadult males (Ranked ANOVA: Feed,
F2,327 = 1.65, p = 0.19; Ruminate, F2,311 = 1.88, p = 0.15). Foraging selectivity observations indicated that the number of steps taken while foraging
Figure 3. Mean (± SEM) percentage scans engaged in different activities for bison males
(black bars) and females (open bars) at National Bison Range during sexual segregation. An
asterisk indicates a significant difference between males and females (p < 0.01). Except
for standing, there was no difference in activity budgets between the sexes. Abbreviations:
Feed = feeding, Rum = ruminating, Std = Standing, Lie = lying down, Move = moving,
Vig = vigilance.
Sexual segregation in bison
915
did not differ significantly between males (x±SEM = 7.9±0.4) and females
(7.2 ± 0.2; Mann-Whitney: Nmale = 74, Nfemale = 143, z = 1.56, p = 0.12).
There was also no difference in the forage efficiency (percent of active time
spent foraging) between males (87.2 ± 1.7) and females (86.4 ± 1.1; MannWhitney: Nmale = 74, Nfemale = 143, z = 0.66, p = 0.51).
Although we could detect no difference between individual males and
females in activity budgets relevant to the predictions, it is possible that
single-sex bull groups and mixed groups composed of both sexes differed
in activity synchrony, resulting in segregation. To test this possibility, we
compared the ‘synchrony index’ (SI) of 38 matched pairs of bull and mixed
groups at Fort Niobrara. Group size ranged from 2 (for the smallest bull
group) to 327 (for the largest mixed group). There was no difference in
activity synchronisation between bull and mixed groups (Wilcoxon Signed
Ranks: N = 38, z = 0.03, p = 0.97). The mean (±SEM) SI for bull
groups was 0.718 (±0.013), compared with 0.728 (±0.012) for mixed
groups.
During rut, when males and females were aggregated in mixed herds,
tending males spent less time feeding compared with tended females (MannWhitney: Nfemale = 119, Nmale = 99; z = 3.96, p = 0.0001). This was
because tending males spent more time engaged in rutting behaviors, standing, and movement than did the females they tended (Rutting behaviors:
z = 9.06, p = 0.0001; Stand: z = 4.86, p = 0.0001; Move: z = 2.66,
p = 0.008). Comparing activity rates (percentage of scans) of males during the pre-rut period of sexual segregation and the rut period of aggregation, tending males during the rut spent less time feeding, ruminating, and
lying down (Mann-Whitney: Npre-rut = 99, Nrut = 99; Feed: z = 4.57,
p = 0.0001; Ruminate: z = 2.02, p = 0.04; Lie: z = 6.38, p = 0.0001),
and more time standing, moving, and engaged in rutting behavior than did
pre-rut males (Stand: z = 9.74, p = 0.0001; Move: z = 7.18, p = 0.0001;
Rutting: z = 7.41, p = 0.0001).
Discussion
Habitat selection and spatial segregation
Males and females in this study utilized the same open grassland habitat, but
used within-habitat space differently. There was considerable spatial overlap
916
Mooring et al.
by bull and cow groups. The Segregation Coefficient revealed that about 74%
of the 8-ha double-grids we surveyed were used by both groups, while 26%
were used by one group but not the other. This indicates that spatial segregation between bull and cow groups was modest. Given the overlap in space
use, much of the sexual segregation observed in this study was temporal
segregation, whereby bull and cow groups used the same grassland habitat at
different times. Other studies have failed to find any clear differences in habitat selection between male and female bison (Larter & Gates, 1991; Komers
et al., 1993). That sexual segregation may occur on the within-habitat level
is suggested by studies that failed to detect large-scale habitat segregation
by deer but did detect spatial segregation at a finer scale (Bowyer et al.,
1996; Lesage et al., 2002). We acknowledge that the the moderate to low
degree of segregation reported here was influenced by the scale of the grids
that we sampled. For example, a smaller scale would have created many
grids with only one group (indicating more segregation at that level), while
a larger scale would have produced most grids with all groups present (indicating less segregation). However, both smaller and larger scales would have
produced spurious results; smaller grids would usually not contain enough
groups for a valid test of segregation, while larger grids would typically contain all groups. We sampled at the smallest level at which segregation could
be compared.
Daubenmire vegetation transects revealed that bull groups used areas that
contained fewer native palouse prairie bunchgrasses and more invasive nonnative grasses compared with cow groups. Furthermore, cow groups for the
most part utilized areas with the mean available forage, whereas bull groups
were found in areas that were poorer-than-average because they contained
fewer natives and more non-natives than mean availability. These results concur with previous studies which found no clear difference in habitat selection
between male and female wood bison, but found that diet composition differed between the sexes (Larter, 1988; Larter & Gates, 1991; Komers et al.,
1993).
Fecal nitrogen results indicated lower dietary quality for males than females during sexual segregation in June compared with aggregation in July.
This implies that the non-native grasses found in greater abundance at bull
sites contained less nitrogen than the native bunchgrasses, indicating that
males did not select the highest-quality forage. These nutritional results
are supported by previous findings that bison males utilize lower-quality
Sexual segregation in bison
917
grasses than females, apparently to maximize intake rates, while females select forage based upon nutritional quality (Larter, 1988; Coppedge & Shaw,
1998a,b; Post et al., 2001). The preference of cow groups for feeding on recovering burn sites (high in forage quality), and the avoidance of such sites
by bull groups (Coppedge & Shaw, 1998a), reflects this pattern. The rapid
decline of % fecal nitrogen in bison diets at NBR between June and July was
documented in other bison studies in which a steep decline in dietary nitrogen was recorded between May and October (Larter & Gates, 1991; Post et
al., 2001).
Although female wood bison and cattle took more steps per min while
foraging than males (Alfonso & Ramon, 1993; Komers et al., 1993), our behavioural data indicated no difference between bison females and males in
foraging selectivity based on steps. Furthermore, foraging efficiency (percentage of active time spent foraging) did not differ between the sexes. Our
failure to find any difference in forage selectivity or efficiency between the
sexes may reflect the bulk and roughage feeding strategy of bison (Van Soest,
1994), in which selection is thought to occur at the level of the patch rather
than the species or plant part (Jarman, 1974; Langvatn & Hanley, 1993).
Bison in South Dakota, for example, selected for warm-season grasses and
against forbs during the summer (Plumb & Dodd, 1993). Therefore, greater
forage selectivity might not require increased step rate in bison.
Sexual segregation
Our results support the ‘sexual dimorphism-body size hypothesis’ (Table 1), which predicts that males will utilize areas with abundant, highfiber grasses, while females will forage selectively on less abundant, higherquality grasses. At NBR, our results revealed that bison females had a higherquality diet (higher % fecal nitrogen) than males during sexual segregation,
while vegetation transects showed that bull sites had a greater abundance
of lower-quality, non-native grasses. Bull sites had over 8 times greater per
capita availability of grass compared with cow sites due to the lower density of animals in bull groups. These results are supported by other studies of bison (Coppedge & Shaw, 1998b; Post et al., 2001; Schuler, 2002).
At the Konza Prairie in Kansas, males consumed a higher proportion of C4
grasses (less digestible and lower in energy than C3 grasses) compared with
females, subadults, and calves (Post et al., 2001). The % fecal nitrogen of
918
Mooring et al.
males at Konza Prairie was also lower than that of females and young during
the month of July, although not in June (Post et al., 2001).
One possible mechanism for the ‘sexual dimorphism-body size hypothesis’ is articulated by the ‘indirect (or ‘scramble’) competition hypothesis’
(Clutton-Brock et al., 1987; Conradt et al., 1999, 2001). This hypothesis proposes that males are forced into habitats of lower forage quality but higher
biomass through indirect competition with females, whereby females graze
preferred high-quality food down below the minimum required for males.
Although this sort of intersexual competitive exclusion might possibly explain the foraging and spatial segregation patterns observed in this study
(we cannot disprove indirect competition, as the predictions over a single
season are the same as for the ‘sexual dimorphism-body size hypothesis’),
this mechanism has been rejected as a cause of spatial separation between
the sexes for various ruminants (Miquelle et al., 1992; du Toit, 1995; Bleich et al., 1997; Kie & Bowyer, 1999; Spaeth et al., 2004), including red deer
Cervus elaphus, for which the hypothesis was modeled (Conradt et al., 1999,
2001).
A more likely mechanism for the ‘sexual dimorphism-body size hypothesis’ is provided by the ‘gastrocentric hypothesis’ (Barboza & Bowyer,
2000; Bowyer, 2004), which can explain the observed pattern of sexual
segregation based on digestive physiology alone, without reference to indirect competition or predation. The gastrocentric hypothesis is an allometric model that explores the nutritional consequences of sexual dimorphism and provides a mechanism for the pattern of sexual segregation predicted by the ‘sexual dimorphism-body size hypothesis’ based on allometry, minimal food quality, digestive retention, and differing reproductive
requirements of the sexes (Barboza & Bowyer, 2000, 2001). The gastrocentric model predicts that larger, sexually-dimorphic males will consume
abundant forages high in fiber because the larger volume of their digestive
tract and longer ruminal retention are better equipped than females to maximize intake by fermenting fiber for energy and recycling urea for protein
(Bowyer, 2004). Due to low density of segregated males, high forage abundance, and adaptations of rumen microflora, the gastrocentric model predicts that males should use fibrous forages until forage abundance declines.
In fact, males are not expected to compete with females for higher quality forages because the digestive morphology and physiology of males is
not equipped to digest forages of too high a quality (Barboza & Bowyer,
Sexual segregation in bison
919
2000). Switching to high-quality forage would disrupt ruminal fermentation, risk excess production of gases, and result in bloat, malabsorption,
or scouring of male digestion (Barboza & Bowyer, 2000). Furthermore,
large males would gain no advantage from sharing less abundant but higherquality forage with females because they must meet greater absolute requirements for energy and protein. Barboza and Bowyer (2000) point out
that tolerance and retention of fiber would be of less consequence for very
large species, because ruminal retention of females may already provide
nearly maximum degradation of fiber. Because dimorphism confers greater
energetic demands on males of very large species, sexual segregation in
large species may be more influenced by abundance than by quality of forage (Barboza & Bowyer, 2000). For these reasons, a gastrocentric interpretation may not predict as sharply defined spatial segregation between
the sexes for bison (ranging from 450-900 kg) compared with smallerbodied, sexually-dimorphic species (e.g., cervids of 30-550 kg). Our results support such a pattern, given the modest degree of spatial segregation (26%) and broad overlap in space use by bull and cow groups observed
at NBR.
In contradiction to the predictions of the ‘reproductive strategy-predation
risk hypothesis’ (Table 1), male bison at NBR did not choose the most nutritious forage available (Main et al., 1996; Bleich et al., 1997; Ruckstuhl &
Neuhaus, 2000, 2002; Mooring et al., 2003). Failure to support the ‘reproductive strategy-predation risk’ model might reflect the absence of a predator fauna (e.g., wolves, Canis lupus) capable of killing bison in most areas
where populations exist. Field studies of predation on bison and wisent (European bison, Bison bonasus) in the few reserves where wolves are present
indicate that bison are not preferred prey, and when attacked, wolves kill
primarily calves and females (Carbyn & Trottier, 1987; Jedrezejewski et al.,
2000; Smith et al., 2000). Over a 4-year period at Yellowstone National Park,
only 1 male was killed by wolves (Smith et al., 2000), and it had a broken leg and was therefore unusually vulnerable. Thus, females and young in
mixed herds would be more at risk from predation, if predators were present.
However, unlike mountain sheep (whose main antipredator defense is escape
into rough terrain) or deer (which hide in thick vegetation), an important antipredator strategy for bison is the formation of large groups of high density,
in which young and other vulnerable individuals can take advantage of the
920
Mooring et al.
antipredator benefits of group living through the detection, encounter, dilution, and selfish-herd effects (Hamilton, 1971; Vine, 1971; Pulliam, 1973;
Turner & Pitcher, 1986; Mooring & Hart, 1992). Therefore, if predators are
present and females are more vulnerable to predation than males, grouping
allows females the ability to optimize forage selection at the same time that
they minimize predation risk. Although bison females formed larger groups
and selected higher-quality forage compared with male groups (supporting
prediction 2b), the absence of predators and failure to support prediction 3
(females were not closer to water than males) makes it less likely that differential predation risk plays a role in bison sexual segregation. However,
we cannot rule out the possibility that bison are responding to the “ghosts of
predators past” (Byers, 1997).
In contradiction to the predictions of the ‘activity budget hypothesis’ (Table 1; Conradt, 1998; Ruckstuhl, 1998; Ruckstuhl & Neuhaus, 2000), intensive observations to characterize individual and group activity budgets
failed to show any differences between males and females or between bull
groups and mixed groups during sexual segregation. Focal animal observations revealed no differences in any of the activities predicted to differ between males and females, including feeding, ruminating, lying down, moving, foraging selectivity (measured by the number of steps taken per minute
while foraging), or foraging efficiency (percent of active time spent foraging). Nor did group scans reveal more activity synchronization in singlesex bull groups compared with mixed herds of females and young. Tending
males during the rut (period of sexual aggregation) spent less time feeding than tended females, but this was because of increased time spent in
reproductive activities (rutting behaviours, standing, and moving). Recent
field studies have also failed to support the predictions of the activity budget hypothesis. No difference in foraging and ruminating activity was found
between male and female desert bighorns (Ovis canadensis mexicana) and
mule deer (Odocoileus hemionus), and activity synchrony was similar in segregated groups of merino sheep (Ovis aries) (Mooring et al., 2003; Bowyer
& Kie, 2004; Michelena et al., 2004; for an exchange, see Neuhaus & Ruckstuhl, 2004; Mooring & Rominger, 2004).
The ‘activity budget hypothesis’ has been proposed as a driving force for
sexual segregation (Ruckstuhl & Neuhaus, 2002), but if sexual segregation
occurs in the absence of activity budget differences between the sexes, then
activity budgets cannot explain all cases of sexual segregation (Mooring &
Sexual segregation in bison
921
Rominger, 2004). Indeed, a recent model based upon data from 144 ungulate
species failed to support the activity budget hypothesis as the main factor
explaining sexual segregation (Yearsley & Perez-Barberia, 2005). Neuhaus
and Ruckstuhl (2004) recently clarified that the ‘activity budget hypothesis’
mainly explains social segregation rather than habitat segregation. However,
bull and cow groups at NBR were spatially and temporally segregated within
the same grassland habitat. We conclude that activity budgets cannot explain
social segregation in bison. Because of the inherent flexibility of activity
budgets, we suggest that activity differences (where they occur) may be a
consequence or correlate rather than a cause of sexual segregation (Bowyer,
2004; Mooring & Rominger, 2004).
In our view, the ultimate factor driving sexual segregation in ungulates is
sexual dimorphism and resultant differences in digestive physiology, predation risk, and reproductive strategies between the sexes. Sexual segregation
enables males and females to use different strategies to maximize their fitness, with the result that sexual segregation in ungulates may have multiple
causes (Bonenfant et al., 2004). In bison, sexual segregation can primarily
be explained by the influence of sexual dimorphism on digestive physiology,
and its impact on forage selection resulting in spatial and temporal segregation.
Acknowledgements
We thank National Bison Range and Fort Niobrara National Wildlife Refuge for permission to
study their bison herds and for providing refuge housing and use of vehicles during our study.
We are particularly grateful to Lindy Garner and Lynn Verlanic at National Bison Range, and
Kathy McPeak and Bernie Petersen at Fort Niobrara for their consistent support and assistance. Bruce Davitt of the Wildlife Nutrition Lab was unfailingly helpful with the nutritional
analyses. Larissa Conradt generously spent time to explain the Segregation Coefficient and
how it should be modified for bison. Funding for this study was provided by the Research
Associates, and by a Research and Special Projects grant and a Dean’s Discretionary Grant
from PLNU.
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Appendix 1. Scientific and common names of grass and sedge species identified from 16
Daubenmire vegetation transects conducted at National Bison Range, summer 2002.
Scientific name
Common name
Scientific name
Common name
Agropyron bakeri
Agropyron cristatum
Agropyron spicatum
Agropyron
trachycaulum
Aristida longiseta
Bromus
breviaristatus
Bromus japonicus
Bromus mollis
Bromus tectorum
Buchloe dactyloides
Danthonia californica
Deschampsia
elongata
Baker wheatgrass
crested wheatgrass
bluebunch wheatgrass
slender wheatgrass
Eleocharis palustris
Elymus cinereus
Festuca idahoensis
Festuca scabrella
Juncus tenuis
Koeleria cristata
Muhlenbergia
montana
Phleum pratense
Poa bulbosa
Poa compressa
Poa pratensis
Poa secunda
Stipa columbiana
Stipa comata
creeping spikerush
basin wildrye
Idaho fescue
rough fescue
poverty rush
junegrass
mountain muhly
red threeawn
mountain brome
Japanese brome
soft brome
Cheatgrass
buffalograss
California oatgrass
slender hairgrass
Timothy grass
bulbous bluegrass
Canada bluegrass
Kentucky bluegrass
Sandberg bluegrass
Columbia needlegrass
needle-and-thread
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Appendix 2. Scientific and common names of forb and shrub species identified from 16
Daubenmire vegetation transects conducted at National Bison Range during summer 2002.
Scientific name
Common name
Scientific name
Common name
Achillea lanulosa
Achillea
millefolium
Agoseris glauca
wooly yarrow
western yarrow
Grindelia sp.
Gutierrezia
sarothrae
Hypericum perforatum
Lepidium perfoliatum
Linaria dalmatica
Lomatium triternata
Lupinus sericeus
Monarda fistulosa
Oenothera biennis
gumweed
broom snakeweed
Ambrosia
psilostachya
Arenaria sp.
Artemisia cana
Artemisia frigida
Aster sp.
Astragulus sp.
Balsamorhiza
sagittata
Centaurea
maculosa
Chrysopsis
villosa
Cirsium
undulatum
Descurainia sp.
Dianthus armeria
Erigeron sp.
Eriogonum
ovalifolium
Erodium
cicutarium
Fragaria
virginiana
Galium boreale
Geranium
viscosissimum
Geum triflorum
short-beaked
agoseris
western ragweed
sandwort
silver sagebrush
fringed sagebrush
aster
milkvetch
arrowleaf
balsamroot
spotted knapweed
golden aster
wavyleaf thistle
tansy mustard
pink
fleabane
silver plant
filaree
wild strawberry
northern bedstraw
sticky geranium
prairiesmoke
Orthocarpus luteus
Orthocarpus
tenuifolius
Penstemon procerus
Phacelia sp.
Phlox caespitosa
Plantago patagonica
Polygonum bistortoides
Potentilla diversifolia
Potentilla gracilis
Rhinanthus minor
Rhus aromatica
Rosa woodsii
Sisymbrium altissimum
Solidago missouriensis
Symphoricarpos
occidentalis
Taraxacum officinale
Tragopogon dubius
common St. Johnswort
yellow pepperweed
Dalmatian toadflax
nineleaf biscuitroot
silky lupine
bee balm
common evening
primrose
yellow owl clover
owl clover
slender blue penstemon
phacelia
tufted phlox
wooly plantain
American bistort
diverse-leaved
cinquefoil
graceful cinquefoil
rattlebox
skunkbrush sumac
prairie rose
common
tumblemustard
Missouri goldenrod
western snowberry
dandelion
yellow salsify
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Sexual segregation in bison
Appendix 3. Diet composition from microhistology analysis of bison fecal samples
(N = 200), with bull samples (N = 100) and cow samples (N = 100) pooled.
Scientific name
Grass
Agropyron spp.
Agrostis spp.
Aristida longiseta
Bromus spp.
Calamagrostis spp.
Deschampsia elongata
Festuca spp.
Koeleria cristata
Muhlenbergia montana
Poa spp.
Stipa spp.
Unidentified grasses
TOTAL GRASSES
Forbs
Achillea lanulosa
Eriogonum ovalifolium
Unidentified forbs
TOTAL FORBS
Shrubs
Artemisia spp.
TOTAL SHRUBS
Common name
% of total diet
wheatgrass
redtop
red threeawn
brome
Pinegrass
slender hairgrass
fescue
junegrass
mountain muhly
bluegrass
needlegrass
–
–
24.00
0.60
0.30
6.15
18.30
2.65
9.90
1.65
0.20
13.90
15.90
4.25
97.80
wooly yarrow
silver plant
–
–
0.60
0.20
1.10
1.90
sagebrush
–
0.30
0.30