HABITAT USE AND BEHAVIOR OF MALE MOUNTAIN SHEEP IN FORAGING ASSOCIATIONS WITH WILD HORSES

5 Pages·1994·2.8 MB·English

by  Kevin P Coates

GreatBasin Naturalist54(1), © 1994,pp.S(i-9()
HABITAT USE AND BEHAVIOR OF MALE MOUNTAIN SHEEP
IN FORAGING ASSOCIATIONS WITH WILD HORSES
Kevin F C.'oatesl-and Sanford I). Schemnitz^
Keywords: mountain slwep, Ovisc. canack-nsis, uiUlhorses. Juihitat use, licharior. Bifjunn Cauyott NationalRecre-
ationArea. Montana, Wijoininfi.
R()ck\ Mountain bighorn sheep {Ovi.s protection from predators (Berger 1978, 1986,
canadensis canadensis) maximize survival by Festa-Bianchet 1991). The purpose of this
foraging in secure habitats that afford high note is to present unique observations which
visibiht>' and have good interspersion of pre- suggest that male mountain sheep maybenefit
ferred forage plants with escape cover (Risen- from close foraging relationships with wild
hoover and Bailey 1985). Good visibilit\ and horses. Few data exist on resource competi-
precipitous escape cover are structiual habitat tion between mountain sheep and feral horses
elements that provide mountain sheep with (Berger 1986), and though not statistically
security from predators (Buechner 1960, Geist quantifiable, these limited observations sup-
1971, Wishart 197S, Risenhoover and Bailey port Berger's (1986) hypotheses regarding for-
1985). age facilitation ofnative species by exotics.
Wild horses {Eqmis cabaUus) also maximize
survival by foraging in secure habitats with Study Area and Methods
good interspersion ofpreferred forage plants.
However, different structural elements of the The study was conducted at Bighorn
habitat provide security for mountain sheep Canyon National Recreation Area (BICA), a
and horses; mountain sheep select foraging 48,679-ha National Park Service unit that has
areas near precipitous escape terrain while as its focal point a 114-km-long reservoir in
horses select foraging areas near open, flat ter- southeastern Montana and north central
rain. This is due to basic differences in preda- Wyoming. Moimtain sheep recolonized BICA
tor escape tactics for the species: mountain in 1975 because of dispersal of 4-6 animals
sheep climb to avoid predation and horses from a nearby transplant. By 1986 the popula-
iim. tion had increased to over 60 animals (Coates
Although grasses dominate the diets of and Schemnitz 1986).
both horses and mountain sheep, each Portions of BICA are federally designated
species' predator-avoidance strategy selects as the Prvor Mountain Wild Horse Range
for structurally different habitats. However, (PMWHR). The 17,402-ha PMWHR supports
when spatial distributions overlap, a competi- approximately 120 wild horses and is located
tive situation may occur, with mountain sheep 80 km south of Billings, Montana (Bureau of
being negatively impacted. In several Land \hmagement 1984).
instances such competition with feral equids The area is characterized as a desert-shrub
has resulted in mountain sheep declines woodland (Lichvaret al. 1985), and dominants
(McMichael 1964, Weaver 1973, Seegmiller include a sparse overstory of curlleaf moun-
and Ohmart 1981). tain mahogany {Cercocarpus ledifolius var.
A growing body of literature supports the intercedens), Utah juniper ijiiniperus osteo-
hypothesis that horses and other exotics may, sperma), sagebrush {Artemisia spp.), and
in some respects, facilitate the foraging effec- greasewood {Sarcobatus spp.), with a poorly
tiveness of some native ungidate species developed understory of bunchgrasses (Lich-
either bv habitat modification or increased var et al. 1985). Annual precipitation averages
'DcpartilU'litIIII'-islici-NanilWikllilcSciences.N, Mc'xieoSlatel'niversit\,LasGraces,NewMexico8800.3.
^Presentaddress;Box271. Tios.Montana.5993.5.
86
1994] Notes 87
15-20 cm. Soils present include limestone categories: foraging, social, alert (Risenhoover
and sandstone in the precipitous canyonland and Bailey 1985). An animal was engaged in
and dolomite in the nonprecipitous areas foraging when it actively ingested forage and
(Knight et al. 1987). Elevations vary from a when it moved about with animals that were
mean pool level of 1109 m at the reservoir to actively ingesting forage.
2682 m at East Piyor Mountain. An animal was engaged in social behavior
Gray limestone cliffs rise >250 m vertical- for all intraspecific and interspecific interac-
ly from the lakeshore. Clifffaces, ledges, and tions. Social interactions included looking at
eroded limestone soils (karst topography) pro- another animal, moving toward/away from
vide abundant escape terrain for mountain another animal, and mother/young interac-
sheep. Escape terrain predominates the tions. Alert behavior was recorded ifthe focal
entire study area, from East Pryor Mountain animal stopped foragingto lookup in the typi-
to the reservoir. Other than an alluvial fan cal alert posture for mountain sheep (i.e., ears
located at the northern extreme of the study up and neck outstretched; Geist 1971), if it
m
area, virtually all habitat is within 300 of looked at a disturbance (e.g., a vehicle on the
cliffs, ledges, or karst topography (Coates highway, or a person approaching on foot), or
1988). when it ran to avoid a disturbance (e.g., aper-
Three adult ewes (>18 months old) and a son approaching on foot). Foraging efficiency
6-year-old ram were captured and equipped was calculated as percentage oftime devoted
with radio collars manufactured by Telonics to foraging behavior during the 15-min peri-
(Mesa, Arizona). Systematic radio relocation od. Percentage oftime spent in alert or social
of these animals provided the opportimit\' to interactions provided a measure of the rela-
locate and observe 328 groups of mountain tive security of moimtain sheep in different
sheep between June 1986 and November habitats.
1987.
Group size and age/sex composition were Results and Discussion
recorded for each observation. Additionally,
three habitat parameters were analyzed: horse Four vegetation types (Knight et al. 1987)
use (Yes/No), distance to precipitous terrain, occur within the obseived range ofmountain
and vegetation type. A preference ratio (per- sheep: Utah juniper/mountain mahogany
cent use/percent availability) was used to ana- woodland (JU/CE), Utah juniper woodland
lyze preference and/or avoidance of vegeta- (JUOS), mountain mahogany woodland
tion types (Risenhoover and Bailey 1985). (CELE), and Douglas fir woodland (PSME).
Because escape terrain was nearly continuous Distribution ofJUOS was limited to an allu-
throughout the southern portion of the study vial fan at the north end ofthe study area and
area (distance rarely >300 m), all habitat tvpes narrow fingers interspersed within the JU/CE
were considered available to nioimtain sheep. habitat type. Horse use was always "No" for
The alluvial fan was considered available to karst topography and "Yes" for the alluvial fan
mountain sheep, primarily to investigate dif- at the northern extreme of the study area,
ferences in habitat selection between male based on the presence/absence ofhorse feces
and female cohorts ofmountain sheep, and to observed during fieldwork. Horse use was
analyze the influence ofdistance to escape ter- also obsei'ved along fingers ofnonprecipitous
rain on foragingbehavior habitat interspersed throughout the JU/CE
The foraging behavior of adult mountain type. Distribution of PSME was restricted to
sheep was analyzed to determine the effects a deeply incised drainage present in the core
of habitat security on foraging efficiency useareaoccupied by rams.
(Risenhoover and Bailey 1985). Once a group Overall, 85.7% of male mountain sheep
ofmoimtain sheep was located, a focal animal observations involving mixed age/sex groups
was selected for analysis offoraging behavior. occurred in JU/CE woodland. JUOS, CELE,
Recognition of focal animals was aided by and PSME woodlands were used in 13.6, <1,
identifying marks on pelage or scars. Foraging and <1% of the observations, respectively
behavior was obsei'ved for five consecutive 3- (Table 1). The preference ratio forJU/CE is 4.5,
min periods to detemiine the amoimt oftime indicating that mountain sheep foraging with
the focal animal devoted to three behavioral conspecifics prefer this type (Risenhooverand
Great Basin Naturalist [Volume54
Table 1. Percent lial)itat utilization 1)\ male mountain wild horses may prefer JUOS. Conversely,
sheep in foraging associations with conspecifics com- male mountain sheep foraging with con-
paredwith associations with wild horses. Habitat prefer-
ence ratios are expressed as + or - and are given in specifics avoided JUOS, but male mountain
parenthesesbeloweachappropriatecategor\'. sheep foraging with wild horses avoided
JU/CE.
HabitatT\pe
Grasses accounted for <1% of the vegeta-
JLOS CELK JU/CE PSME tive cover in the JU/CE type but approxi-
Malemountainsheep: mately 6% of the JUOS type (Knight et al.
1987). Although grasses were present in low
Withconspecifics 85.'J 13.6 <1 <1 composition in both vegetation types, moun-
Habitatpreference (+) tain sheep foraging in JUOS had a higher
Witliwildhorsi's 16.7 83.3 availabilityofgrasses.
Habitatpreference + Average distance to escape terrain was
( )
determined for male mountain sheep that for-
aged with conspecifics and compared to the
Bailey 1985). Preference for JU/CE habitat distance formale mountain sheep that foraged
probably resulted more from the intersper- with wild horses (Table 2). Male mountain
sion ofescape terrain than from differences in sheep foraging with conspecifics remained
visibility between habitats. Juniper was within an average of 47 m (SD 69.5 m) from
sparsely distributed throughout both JU/CE escape terrain, partially because of the ewes'
and JUOS types. Distinction between types reluctance to venture farther than 50 m from
was based on occurrence ofcurlleafmountain secure habitat. However, male mountain
mahogany rather than on increasing frequency sheep foraging with wild horses were an aver-
ofUtahjuniper (Lichvar et al. 1985). Visibility age of217 m (SD 310 m) from escape terrain.
obstruction was low in both JU/CE and JUOS These limited data suggest that male moun-
habitats. Ewes never occupied the PSME tain sheep foraged farther from escape terrain
type, even though it was located on rocky (in less secure habitat) when associated with
slopes, because visual obstruction was much wildhorses thanwith conspecifics.
higher than in JU/CE orJUOS. Foraging efficiency ofmountain sheep with
Male mountain sheep were observed for- wild horses was 100% for all 12 locations (no
aging with wild horses 22 times on 20 differ- alert or social interactions). Male mountain
ent days, and habitat parameters were record-
sheep that foraged with wild horses ignored
ed for 12 observations. Foraging associations
disturbance (e.g., they could be approached
usually involved 2 specific male horse/harem
readily, and they rarely looked up to scan
groups with bachelor ram groups. Ram group
their surroundings even when horses were
size ranged from 3 to 7animals, 3 to 10years of
fighting in their vicinity). Group size ranged
age. Female mountain sheep were never ob-
from 9 to 16 animals, including rams and
served in association with wild horses. Horse
horses. Foraging efficiency of male moimtain
galrloyupinsviozelvweads 1dyonfam2ics,pebcuitfiacssmocailaetiohnorusseus- sheep with conspecifics was only 66% [n =
accompanied by 5 to 8 mares and subadults. 67) and was characterized by high levels of
Of the 12 observations, 83.3% (n = 10) aggressive or social interaction (Table 3).
occurred in the JUOS vegetative type, and Aggressive interactions were exhibited
16.7% (n = 2) occurred in JU/CE (Table 1). between rams when two or more followed a
The preference ratio for JUOS is 1.2. The ewe, and when they established dominance
preference ratio for JUOS by male mountain rank in the male cohort. Social interactions
sheep foraging with wild horses is noteworthy between rams occurred when they attended
because habitat utilization patterns forJU/CE ewes. Aggressive or social interactions were
and JUOS were reversed when male moun- never observed when male mountain sheep
tain sheepassociatedwitliwildhorses (Table 1). foraged with wild horses. This may have been
These limited observations suggest that due to size-related dominance in mountain
male mountain sheep foraging with con- sheep (Geist 1971) and subordinate behavior
specifics may prefer the JU/CE vegetation ofmale mountain sheep in the presence ofthe
type, but male moimtain sheep foraging with relatively large wild horse (Berger 1986).
1994] Notes 89
Table 2. Average distance to escape terrain (m) of Table 3. Averageforagingefficiencyofmale mountain
male nionntain sheep in association with conspecifics sheep in foraging associations with conspecifics com-
compared to distance when associated with wild horses. paredwithassociationswithwildhorses.
stanaaraaeviationssno\
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