Rub tree use and selection by American black bears and
grizzly bears in northern Yellowstone National Park
Authors: Bowersock, Nathaniel R., Okada, Hitomi, Litt, Andrea R.,
Gunther, Kerry A., and van Manen, Frank T.
Source: Ursus, 2022(33e7) : 1-12
Published By: International Association for Bear Research and
Management
URL: https://doi.org/10.2192/URSUS-D-21-00009.3
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Rub tree use and selection by American black bears and grizzly bears
in northern Yellowstone National Park
Nathaniel R. Bowersock1,4, Hitomi Okada1, Andrea R. Litt1, Kerry A. Gunther2, and
Frank T. van Manen3
1Department of Ecology, Montana State University, P.O. Box 173460, Bozeman, MT 59717-3460, USA
2Bear Management Office, Yellowstone Center for Resources, Yellowstone National Park, P.O. Box 168,
Yellowstone National Park, WY 82190, USA
3U.S. Geological Survey, Northern Rocky Mountain Science Center, Interagency Grizzly Bear Study Team,
2327 University Way, Suite 2, Bozeman, MT 59715, USA
Abstract: Several of the world’s bear species exhibit tree-rubbing behavior, which is thought to be a
form of scent-marking communication. Many aspects of this behavior remain unexplored, including
differences in rub tree selection between sympatric bear species. We compiled rub tree data collected
on Yellowstone National Park’s Northern Range (USA) and compared rub tree selection of sympatric
American black bears (Ursus americanus) and grizzly bears (U. arctos) at local and landscape scales.
During 2017 and 2018, we identified 217 rub trees and detected black bears at 117 rub trees and grizzly
bears at 18 rub trees, based on genetic analysis of collected hair samples. Rub trees generally were
located in areas with gentle slopes and close to existing animal trails. Trees selected by black bears were
typically in forested areas, whereas trees selected by grizzly bears were in forested and more open areas.
Use of rub trees varied seasonally and between sexes for black bears, but seasonal data were inconclusive
for grizzly bears. Black bears showed preferences for certain tree species for rubbing, but we did not
find evidence that rub tree selection by grizzly bears differed among tree species. Both bear species
selected trees that lacked branches on the lower portions of tree trunks and the maximum rub height
was consistent with the body length of the bear species that used the tree. Although the sample size for
grizzly bears was small, identifying the species and sex of bears based on genetic analysis enhanced
interpretation of rub tree use and selection by bears. Scent-marking by black bears and grizzly bears on
similar rub objects in well-traversed areas likely serves to enhance communication within and between
the 2 species.
Key words: American black bear, grizzly bear, resource selection function, rub trees, Ursus americanus, Ursus arctos,
Yellowstone National Park
DOI: 10.2192/URSUS-D-21-00009.3 Ursus 33:article e7 (2022)
Most forest-dwelling species of bears rub on trees and genetic sampling. Genetic material from bear hair can
other objects, such as rocks and utility poles, a behav- help address diverse ecological questions, ranging from
ior thought to be a form of intraspecific communication abundance and density estimation to population structure,
via chemical scent-marking (Laurie and Seidensticker reproductive fitness, and connectivity (Mowat et al. 2005;
1977, Karamanlidis et al. 2007, Latham et al. 2012, Nie Stetz et al. 2008, 2019; Proctor et al. 2012; Sawaya et al.
et al. 2012, Sato et al. 2014, Tattoni et al. 2015, Fil- 2012; Loosen et al. 2019; Morehouse et al. 2021).
ipczyková et al. 2017, Tee et al. 2020). These rubbing Bear use of rub trees is not random (Sato et al. 2014)
behaviors often involve repeated visits to trees, such that and several researchers have explored selection behav-
rubs are easily identified by their smooth, discolored bark ior of bears and characteristics associated with rub trees
and the presence of clumps of hair and bite and claw (Clapham et al. 2013, Morgan Henderson et al. 2015,
marks (Burst and Pelton 1983). Hair deposited by bears Tattoni et al. 2015). Bears tend to select larger trees
while rubbing provides a source of DNA for noninvasive with few branches on the lower portions of the trunk,
which likely facilitates rubbing (Green and Mattson 2003,
Clapham et al. 2013, Sato et al. 2014). They also tend to
4email: nathaniel.bowersock@gmail.com select living conifer trees that excrete more resin than do
1
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2 RUB TREE USE AND SELECTION BY YELLOWSTONE BEARS  Bowersock et al.
deciduous trees, which might allow scent marks to persist with trees used by grizzly bears and that the height of the
longer (Green and Mattson 2003, Clapham et al. 2013, rub area would reflect the species that used the tree. We
Sato et al. 2014, Morgan Henderson et al. 2015). Bears also predicted that the frequency of rub tree use varies sea-
usually select rub trees in areas that are easy to traverse, sonally for grizzly bears, but not for black bears. Lastly,
such as ridge tops or drainage bottoms with gentle slopes, we predicted that selection of tree species and size of rub
and areas that coincide with animal trails or human hik- trees differ between black and grizzly bears, with black
ing trails (Green and Mattson 2003, Clapham et al. 2013, bears using larger fir (Abies spp.) and spruce (Picea spp.)
Sato et al. 2014, Morgan Henderson et al. 2015). trees and grizzly bears using smaller pine (Pinus spp.)
In regions where multiple bear species occur, rub tree trees.
use and selection can vary among species, either in the
timing of when trees are used or the number and type
of marks left behind (Sawaya et al. 2012, Clapham et al. Study area
2013, Sato et al. 2014, Morgan Henderson et al. 2015). For The Northern Range covers a 1,530-km2 area that in-
example, grizzly bears (Ursus arctos) have been found cludes the northern third of Yellowstone National Park
to rub mainly on trees during the spring mating sea- and portions of southern Montana (Fig. 1), USA. We
son and in late autumn, leaving few bite or claw marks, focused on the 1,000-km2 area of the Northern Range
whereas data from American black bear (U. americanus; within the borders of Yellowstone National Park. Ele-
hereafter, black bear) studies showed they rub on trees vations range from 1,590 to 3,360 m. Whitebark pine
throughout the year and leave numerous marks (Green (Pinus albicaulis) and subalpine fir (Abies lasiocarpa)
and Mattson 2003, Sawaya et al. 2012, Clapham et al. dominated forests at elevations of 2,600–2,900 m,
2013, Sato et al. 2014). Both species of bear stand on whereas Douglas-fir (Pseudotsuga menziesii), lodgepole
their hind legs to rub their backs or chest on trees result- pine (Pinus contorta), and trembling aspen (Populus
ing in the maximum height of rub areas differing based on tremuloides) comprised lower elevation forests from
the bear species that is most common in the region (Burst 1,900 to 2,200 m. A mixture of sagebrush (Artemisia
and Pelton 1983, Clapham et al. 2013, Sato et al. 2014). spp.), grasses, and sedges (Carex spp.) occurred in
Although rub tree studies have been conducted in areas open meadows at the lowest elevations (Frank and
with multiple bear species, there is a lack of information McNaughton 1992, Singer et al. 1994). The Northern
about species-specific differences in selection of rub trees Range was used by 8 ungulate species, with elk (Cervus
(Green and Mattson 2003, Clapham et al. 2013, Morgan canadensis) and bison (Bison bison) being the most com-
Henderson et al. 2015). A better understanding of poten- mon, and hosted a suite of carnivores, including black
tial differences in rub tree selection between sympatric bears, grizzly bears, gray wolves (Canis lupus), coyotes
bear species could be important for the design of studies (C. latrans), and cougars (Puma concolor; White and
relying on genetic samples of a specific bear species from Garrott 2005, Barber-Meyer et al. 2008).
rub trees.
Rub trees were used as 1 of 2 sampling methods to
estimate the population density of bears on the Northern Methods
Range of Yellowstone National Park, with black bears oc- Identifying rub trees
curring at higher densities than grizzly bears (Bowersock Field crews searched for rub trees throughout the
2020). Both species were detected at rub trees, which pro- Northern Range from May to August in 2017. These
vided an opportunity to examine species-specific use and searches were designed to supplement collection of hair
selection at 2 spatial scales: landscape and individual rub samples via hair snares for DNA-based density estima-
tree. Based on differences in habitat selection of black tion of black bears (Bowersock 2020). Genetic sampling
and grizzly bears (Barnes and Bray 1967, Fortin 2011, occurred within a contiguous area of 26 grid cells of 5 ×
Bowersock 2020), we predicted black bears would select 5 km, comprising 650 km2. Searches for rub trees were
rub trees in forested areas, whereas grizzly bears would conducted throughout the study area, with most search
select rub trees in both forested and open areas. Based on effort occurring away from designated hiking trails. We
the findings of other rub tree studies (Sawaya et al. 2012, defined a rub tree as having 2 characteristics described
Sato et al. 2014, Kendall et al. 2015), we predicted that by Burst and Pelton (1983): smooth, discolored bark,
both species would select rub trees near roads and human presence of bear marks (bite and claw marks); and bear
hiking trails. In addition, we predicted black bears would hair on tree trunk. After locating a rub tree, we recorded
leave more bite and claw marks on rub trees compared tree species, condition (dead or alive), diameter at breast
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RUB TREE USE AND SELECTION BY YELLOWSTONE BEARS  Bowersock et al. 3
Fig. 1. American black (Ursus americanus) and grizzly (U. arctos) bear rub tree (n = 217) locations on the
Northern Range (orange shading) of Yellowstone National Park, Montana and Wyoming, USA. We located and
surveyed rub trees during 2017–2018. Each circle on the map represents a rub tree, with the color representing
the bear species detected from collected hair samples.
height (DBH [cm]), distance from the ground to the low- climate-controlled room. Wildlife Genetics International
est branch on the side of the tree with the rub surface (Nelson, British Columbia) conducted genotyping of hair
(distance to lowest branch: [cm]), distance to nearest an- samples collected from rub trees, using the G10J mi-
imal trail (m), and Universal Transverse Mercator coor- crosatellite marker to distinguish between samples from
dinates. We also characterized the rub area by measuring black and grizzly bears (Kendall et al. 2009). Based on
the height of the bottom and top of the rub area (cm). these genetic data, we classified each rub tree as being
In addition, we recorded the presence of bear sign (claw used by a black bear, a grizzly bear, or both species. Fol-
marks and bite marks) at each tree. We classified bear sign lowing Sato et al. (2014), we calculated the probability
as fresh (<1 yr) if bite or claw marks were white or yel- of rub tree use (frequency of bear detection/no. of times a
low in color and older (1 yr) if marks were dirty or dark tree was sampled) for 4, 30-day periods within our annual
in color. We visited all rub trees during May–September sampling seasons.
to collect hair samples, with visits occurring every 7–14
days in 2017 and 2018. Selection of rub trees
Landscape scale. To assess landscape-scale se-
Genetic analysis lection of rub trees, we applied a use–availability sam-
When present on surveyed rub trees, we collected hair pling design and developed resource selection functions.
samples for genetic analyses. We then cleared the trees We compared landscape characteristics associated with
of hair using small butane torches to ensure we collected rub trees (used) with available (random) locations within
fresh hair samples for DNA mark–recapture analysis on the study area (second-order selection; Johnson 1980,
subsequent visits and to track the frequency of use. We Boyce and McDonald 1999, Manly et al. 2002). We gen-
placed each hair sample in a paper coin envelope and erated random locations within the study area using the
stored samples in a plastic container with desiccant in a st_sample function in the sf package (Pebesma 2018) in
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4 RUB TREE USE AND SELECTION BY YELLOWSTONE BEARS  Bowersock et al.
Program R (R Development Core Team 2013), a simi- variance inflation factor (VIF) vif function in Program R
lar ratio (∼1:10) as Morgan Henderson et al. (2015) in and considered removing covariates with VIF scores >10
northwestern Montana. We limited the extent of random (Dormann et al. 2013).
locations to <13 km from roads because this was the Individual tree scale. We assessed selection at the
greatest distance from roads at which we were able to level of individual trees by pairing each used tree with 1–
search for rub trees. 5 available, but unused, trees within a 5-m radius that
To build the landscape models, we included topo- were 1.5 m tall (fourth-order selection; Thomas and
graphic, vegetation, and anthropogenic covariates com- Taylor 2006). This minimum height ensured we sampled
monly associated with bear resource selection (Belant trees that were large enough to potentially be used by
et al. 2010, Johnson et al. 2015, Morgan Henderson et al. bears for rubbing (Green and Mattson 2003, Sato et al.
2015, Duquette et al. 2017). We used digital elevation 2014, Morgan Henderson et al. 2015). At each available
models (U.S. Geological Survey 2017; 10-m resolution) tree, we recorded the same tree characteristics as the
to derive aspect (°), elevation (m), and slope (%). We used rub trees (i.e., tree species, tree condition, DBH,
converted aspect data to a ratio, with values ranging distance to lowest branch). For the distance to lowest
from 1 (northern aspects) to −1 (southern aspects; Deng branch measurement, we randomly chose a side of the
et al. 2007). We used net primary productivity (NPP [kg tree based on one of the 4 cardinal directions. Available
carbon/m2]), a measure of digestible energy from plant trees had not been used for rubbing, so we could not mea-
matter, during 2017 and 2018 to assess whether availabil- sure features associated with the rub area, such as bear
ity of herbaceous vegetation was associated with rub tree marks or rub height. We excluded rub trees from analysis
selection (250-m resolution; Xu et al. 2012, Numerical for which no available trees were present within a 5-m
Terradynamic Simulation Group 2019). Additionally, we radius.
assessed potential differences in rub tree selection among We used conditional logistic regression to assess se-
vegetation communities using a Geographic Information lection at the individual tree scale, comparing charac-
System layer based on climatic overstory and understory teristics of rub trees used by black bears or grizzly
plants (50-m resolution; Despain 1990, Yellowstone Spa- bears with paired available trees that were not used by
tial Analysis Center 2010). Human activities associated bears (coxme package in Program R; Therneau and Lum-
with hiking trails and roads can influence bear move- ley 2018). By explicitly pairing observations to match
ments (Northrup et al. 2012, Morgan Henderson et al. our data collection, selection was conditional on what
2015, Ladle et al. 2018), so we again used the st_distance was available to an individual bear at a specific loca-
function to measure distances to nearest hiking trails and tion. We categorized tree species into 4 groups based
roads using a geospatial layer of trails and roads (Yel- on species or tree type (Douglas-fir, lodgepole or lim-
lowstone Spatial Analysis Center 2010, Pebesma 2018). ber [Pinus flexilis] pine, Engelmann spruce [Picea en-
We extracted these landscape data for the rub tree and gelmannii], and deciduous [trembling aspen]) to ensure
random locations using the raster::extract function in sf sufficient sample sizes for inference. We again explored
package. whether we needed to exclude parameters from our mod-
Before fitting models, we explored whether there was els by checking for potential collinearity among covari-
any correlation among pairs of covariates using the cor ate pairs before running models. We then developed sep-
function in Program R and considered retaining only one arate selection models for black and grizzly bears and
of the pair of covariates for further analysis if the r value began with a global model including 4 covariates: tree
exceeded 0.7 (Dormann et al. 2013). To fit models at the species group, tree condition, DBH, and distance to low-
landscape scale, we used generalized linear models with est branch. We selected these covariates based on pre-
a binomial distribution and logit-link function using the vious research and hypothesized relationships, namely
lmer4 package (Bates et al. 2019) and developed sepa- that bears typically rub on living coniferous trees with
rate selection models for black and grizzly bears. We first large circumference and with few to no branches on the
built a global model that included all landscape covariates lower portion (Clapham et al. 2013, Sato et al. 2014,
as additive effects, followed by backward variable selec- Morgan Henderson et al. 2015). We then used back-
tion to identify the most parsimonious model by remov- ward variable selection to remove variables that showed
ing variables that showed little association with rub tree little association with rub tree selection (P > 0.10),
selection (P > 0.10). We based our inference on effect assessed multicollinearity using VIF scores, and again
sizes and associated confidence intervals. We checked based inferences on effect sizes and associated confidence
for multicollinearity among model covariates using the intervals.
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RUB TREE USE AND SELECTION BY YELLOWSTONE BEARS  Bowersock et al. 5
Table 1. Characteristics of rub trees used by American black bears (Ursus americanus) and grizzly bears (U.
arctos), Northern Range, Yellowstone National Park, Montana and Wyoming, USA, 2017–2018. Four of 18 rub
trees were used exclusively by grizzly bears (all were Engelmann spruce [Picea engelmannii], mean diameter
at breast height [DBH] = 100 cm, maximum rub height = 171 cm, 3 of the 4 trees had bear marks).
Max. rub height
DBH (cm) (cm) Bite marks Claw marks
Tree species n Mean SE Mean SE No marks Fresh Old Fresh Old
Black bear
Douglas-fir 22 83.8 5.5 158.9 2.6 15 1 1 1 6
Engelmann spruce 45 143.9 5.4 168.2 1.5 22 3 11 3 16
Limber pine 1 210.0 — 145.0 — 1 0 0 0 0
Lodgepole pine 41 127.6 3.8 165.7 1.8 5 5 14 9 25
Rocky Mountain juniper 4 69.1 2.2 167.1 2.0 0 1 1 1 3
Subalpine fir 1 95.0 — 112.0 — 0 0 0 0 1
Trembling aspen 1 110.0 — 142.0 — 0 0 0 1 0
White spruce 2 119.3 24.3 144.3 5.7 1 0 0 0 1
Total or mean 117 119.8 5.9 150.4 1.9 44 10 27 15 52
Grizzly bear
Douglas-fir 3 109.3 21.1 191.7 7.2 1 0 1 0 2
Engelmann spruce 14 147.0 8.3 179.2 3.4 6 2 4 2 4
White spruce 1 168.0 – 133.0 – 0 0 0 0 1
Total or mean 18 141.4 9.8 168.0 3.5 7 2 5 2 7
Results marks (Table 1). Based on genetic samples, black bears
Identifying rub trees and genetic results showed seasonal patterns in rub tree use that varied by
Field personnel spent >900 survey hours searching for sex, with male bears using rub trees more frequently dur-
rub trees in 2017 and documented 217 rub trees. Between ing spring and early summer, whereas females used rub
2017 and 2018, hair samples were collected from 186 trees more frequently in mid- to late summer (Fig. 2).
different rub trees, but bear genotyping was only suc- Probability of rub tree use was low among female griz-
cessful for samples collected from 121 of those trees. We zly bears and showed no seasonal variation, whereas male
detected black bears (46 females, 54 males) at 117 rub probabilities were highest in spring and early summer but
trees and grizzly bears (3 females, 15 males) at 18 trees had substantial uncertainty (Fig. 2).
(Table 1). Of these, we detected both species at 14 trees:
we documented an average of 2.5 (range = 1–6) black
bear detections and 1.6 (range = 1–5) grizzly bear de- Selection of rub trees
tections per tree. Only 4 trees had genetic samples exclu- Landscape scale. Based on the 117 rub trees that
sively from grizzly bears. black bears used (and the available [random] locations
withing the study area), we found that aspect, eleva-
tion, slope, distance to nearest road, distance to nearest
Rub tree use hiking trail, and vegetation community were important
Black bears used trees with smaller DBH (mean = landscape features for selection (Table 2, Fig. 3) and did
119.8 cm, standard error [SE] = 5.9) compared with griz- not find any evidence of collinearity or multicollinearity
zly bears (mean = 141.4 cm, SE = 9.8). The maximum among model parameters. Black bears selected rub trees
rub height at trees used by grizzly bears (mean = 168.0 that were at lower elevations, on more gentle slopes, with
cm, SE = 3.5) was greater than the rub height of trees used southern aspects, and closer to roads and hiking trails
by black bears (mean = 150.4 cm, SE = 1.9; Table 1). (Table 2). Black bears selected rub trees found in mostly
Both bear species used rub trees that were in proximity to forested vegetation communities (except for subalpine
animal trails (black bear: mean = 57.1 m, SE = 4.9; griz- fir–grass sedge [Carex spp.]) and were less likely to select
zly bear: mean = 40.2 m, SE = 13.7). Overall, we found rub trees found in big sagebrush (Artemisia tridentata)–
hair on 85.7% of rub trees, and rub trees used by black grass sedge and Idaho fescue (Festuca idahoensis)–grass
bears showed a higher frequency of claw marks than bite sedge communities, compared with big sagebrush–sticky
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6 RUB TREE USE AND SELECTION BY YELLOWSTONE BEARS  Bowersock et al.
Fig. 2. Probability (means and standard errors) of rub tree use by American black (Ursus americanus)
and grizzly (U. arctos) bears by season and sex, Northern Range, Yellowstone National Park, Montana and
Wyoming, USA, 2017–2018. Probability of use was calculated as the frequency of bear detections divided
by the number of times a tree was sampled within a 30-day period, which fell into 1 of 4 seasons: spring
(15 May–14 Jun), early summer (15 Jun–14 Jul), late summer (15 Jul–14 Aug), and autumn (15 Aug–14 Sep).
Table 2. Parameter estimates from models characterizing selection of rub trees by American black (Ursus
americanus) and grizzly (U. arctos) bears at the landscape scale, Northern Range, Yellowstone National Park,
Montana and Wyoming, USA, 2017–2018. For the black bear model, we compared 117 used rub trees with 1,170
paired available trees. For the grizzly bear model, we compared 18 used rub trees with 180 paired available
trees. Elevation, slope, distance to nearest road, and distance to nearest hiking trail were continuous variables.
Aspect was scaled between 1 (north) and −1 (south). Vegetation community also was important for selection
at the landscape scale (Fig. 3).
Black bears Grizzly bears
Landscape characteristic Estimate SE P Estimate SE P
Elevation (m) −0.0037 <0.001 <0.001 — — —
Aspect −0.8876 0.108 <0.001 − 0.5895 0.259 0.023
Slope (%) −0.0831 0.010 <0.001 − 0.1605 0.042 <0.001
Distance to road (m) −0.0003 <0.001 <0.001 − 0.0002 <0.001 0.042
Distance to trail (m) −0.0003 <0.001 <0.001 — — —
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RUB TREE USE AND SELECTION BY YELLOWSTONE BEARS  Bowersock et al. 7
Fig. 3. Parameter estimates (circles) and standard errors (vertical lines) based on models of characterizing
selection of rub trees by American black (Ursus americanus) and grizzly (U. arctos) bears, specifically compar-
ing vegetation communities at the landscape scale, Northern Range, Yellowstone National Park, Montana and
Wyoming, USA, 2017–2018. The model for black bears included 117 rub trees (compared with 1,170 random
locations) and the model for grizzly bears included 18 rub trees (compared with 180 random locations). Big
sagebrush–sticky geranium (Artemisia tridentata–Geranium viscosissimum) was the reference level for both
models. Elevation, aspect, slope, and distances to roads and trails also were important for rub tree selection
at the landscape scale (Table 2).
geranium (Geranium viscosissimum; the reference com- genotype, leaving 98 rub trees for which we were able
munity) or Idaho fescue–sticky geranium (Fig. 3). to identify the bear species, which were paired with 307
Aspect, slope, distance to road, and vegetation com- available trees.
munity were the most important landscape features as- For black bears, we compared characteristics of 94
sociated with the selection of rub trees by grizzly bears used trees with 293 paired available trees. Black bears
(Table 2, Fig. 3); and again, we did not detect any cor- were more likely to select pine than Douglas-fir trees
relation or multicollinearity between model parameters. and were more likely to select Douglas-fir than decid-
Similar to black bears, grizzly bears selected rub trees on uous trees (Table 3). However, we did not detect a dif-
more gentle slopes, with southern aspects, and closer to ference in selection between Douglas-fir and spruce trees
roads (Table 2). Grizzly bears selected rub trees in vege- (Table 3). Black bears were more likely to select trees
tation communities dominated by Douglas-fir and Idaho without branches on the lower portions of the trunk. We
fescue over big sagebrush–sticky geranium (the reference found little evidence that rub tree selected by black bears
community) or subalpine fir–grouse whortleberry com- differed with tree diameter.
munities (Fig. 3). For grizzly bears, we compared characteristics of 13
Individual tree scale. We collected data on 169 used trees with 47 paired available trees. Similar to black
used rub trees and 570 paired available trees. Some hair bears, grizzly bears also were more likely to select trees
samples collected from these trees failed to produce a without branches on the lower portions of the trunk
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8 RUB TREE USE AND SELECTION BY YELLOWSTONE BEARS  Bowersock et al.
Table 3. Parameter estimates from models characterizing selection of rub trees by American black (Ursus
americanus) and grizzly bears (U. arctos) at the individual tree scale based on conditional logistic regres-
sion, Northern Range, Yellowstone National Park, Montana and Wyoming, USA, 2017–2018. For the black bear
model, we compared 94 used rub trees with 293 paired available trees. For the grizzly bear model, we com-
pared 13 used rub trees with 47 paired available trees. Tree species was a categorical variable, with Douglas-
fir (Pseudotsuga menziesii) as the reference group. The pine category included lodgepole (Pinus contorta) or
limber pine (Pinus flexilis), spruce included Engelmann spruce (Picea engelmannii), and deciduous included
trembling aspen (Populus tremuloides).
Species Tree characteristic β SE P
Black bear Tree species—pine 3.08 0.78 <0.001
Tree species—spruce 1.05 0.58 0.920
Tree species—deciduous − 1.59 0.73 0.028
Distance to lowest branch 0.004 0.001 0.004
Grizzly bear Distance to lowest branch 0.015 0.005 0.007
(Table 3). We did not detect differences in rub tree se- grizzly bears, which selected rub trees in both forested
lected by grizzly bears based on tree species group, tree and nonforested vegetation communities, reflecting gen-
condition, or DBH. Furthermore, we did not detect any eral patterns of habitat selection in the region (Barnes and
collinearity or multicollinearity before or after running Bray 1967, Fortin 2011, Bowersock 2020). Additionally,
models for both bear species. rub trees used by black bears had a maximum rub height
similar to the average body length of black bears captured
in this area (males: 164 cm, 95% CI = 150–177; females:
Discussion 136 cm, 95% CI = 127–145; National Park Service, un-
Using genetic data, we were able to characterize and published data). In comparison, the maximum rub height
compare use and selection of rub trees by 2 sympatric bear on trees used by grizzly bears matched the larger average
species. At the landscape level, black and grizzly bears body length of grizzly bears captured in Yellowstone Na-
showed similar patterns of selection. Both bear species on tional Park (males: 188 cm, 95% CI = 168–208; females:
the Northern Range selected rub trees that were located 170 cm, 95% CI = 155–185: Green and Mattson 2003).
mainly at lower elevations with low to modest slopes, In contrast to our prediction that black bears would use
consistent with the findings of other studies (Green and rub trees throughout the year, we found that the frequency
Mattson 2003, Clapham et al. 2013, Sato et al. 2014, Mor- of rub tree use varied seasonally for both female and
gan Henderson et al. 2015). These areas represent typical male black bears. The probability of rub tree use among
travel corridors for bears because the terrain is easier to male black bears was highest in spring and early summer,
traverse (Carnahan et al. 2021) and may enhance detec- which may reflect scent communication among compet-
tion of scent left at rub trees (Clapham et al. 2013, Mor- ing males during the mating season (Taylor et al. 2015).
gan Henderson et al. 2015, Revilla et al. 2021). Similar However, contrary to other black bear studies that found
to other studies (Sawaya et al. 2012, Sato et al. 2014, female black bears use rub trees less often than do males
Kendall et al. 2015), we found that both species of bears (Sawaya et al. 2012, Taylor et al. 2015), we found that
selected for rub trees closer to hiking trails and roads. Al- both sexes had similar probabilities of use during spring
though we did not find plant productivity (NPP) to be a and early summer and females had higher probabilities of
good predictor of rub tree selection, rub trees were asso- use in midsummer. In some cases, previous studies have
ciated with vegetation communities containing food and relied on visual observation of bears using rub trees to
cover resources that both bear species select (Barnes and make comparison between sex and age classes (Taylor
Bray 1967, Fortin 2011, Schwartz et al. 2014, Bower- et al. 2015, Revilla et al. 2021), which potentially under-
sock 2020). In coastal British Columbia, Clapham et al. represented the frequency of female use of rub trees. For
(2013) also found rub trees in areas associated with food male grizzly bears, we found some evidence of greater use
resources, such as salmon (Oncorhynchus spp.) or fruit- of rub trees during the spring mating season, consistent
ing vegetation. with other studies (Clapham et al. 2012, Sato et al. 2014,
We observed some differences in rub tree selection be- Lamb et al. 2017). However, we found little evidence of
tween the 2 bear species. Black bears selected rub trees seasonal changes in rub tree use for female grizzly bears.
in forested vegetation communities more often than did In either case, our inference regarding seasonal use of
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RUB TREE USE AND SELECTION BY YELLOWSTONE BEARS  Bowersock et al. 9
Fig. 4. Remote camera pictures showing American black bears (Ursus americanus; A, C) rubbing on the
same rub trees as grizzly bears (U. arctos; B, D) during July 2018, Northern Range, Yellowstone National Park,
Montana and Wyoming, USA.
rub trees by grizzly bears were limited because of small the lower branches in dense, mature forest (Anderson
sample sizes. 2003), possibly making this tree species more desirable
At the individual tree scale, both bear species selected compared with other tree species. We did not detect differ-
trees lacking branches on the lower portions of tree trunks. ences in selection of tree species by grizzly bears, but the
Whether a rub tree lacked branches on the lower por- small sample size may have limited our ability to detect
tion of the trunk because of the trees’ self-pruning or patterns. We also note that grizzly bears rubbed on utility
from bears and other animals breaking the branches off, poles more often than did black bears. Thus, the selection
these branch-free sections likely facilitate tree-rubbing of lodgepole pine, which are less common on the North-
(Shaffer 1971, Sato et al. 2014, Seryodkin 2014). We did ern Range compared with other tree species, and utility
not find evidence that tree condition or diameter were im- poles may reflect that both bear species select conspic-
portant factors for rub tree selection. Black bears selected uous objects for rubbing, which is a common behavior
coniferous over deciduous trees, which is consistent with among bears and other mammals for scent communica-
other studies (Green and Mattson 2003, Clapham et al. tion (Karamanlidis et al. 2007, Alberts 2011, Clapham
2013, Sato et al. 2014). However, contrary to our predic- et al. 2013, González-Bernardo et al. 2021).
tions, black bears selected pine (mainly lodgepole pine) Few studies have been conducted on rub tree use and
over spruce or fir trees, despite lodgepole pine–dominated selection by sympatric black bears and grizzly bears. Our
forests being relatively uncommon in our study area. This findings are somewhat limited because of small sample
selection could be because lodgepole pine trees self-prune sizes for grizzly bears, but broadly confirm those of other
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10 RUB TREE USE AND SELECTION BY YELLOWSTONE BEARS  Bowersock et al.
rub tree studies (Green and Mattson 2003, Clapham et al. BATES, D., M. MAECHLER, B. BOLKER, S. WALKER, R.H.B.
2013, Sato et al. 2014, Morgan Henderson et al. 2015). CHRISTENSEN, H. SINGMANN, B. DAI, F. SCHEIPL, G.
Furthermore, there are marked differences in black and GROTHENDIECK, P. GREEN, AND J. FOX. 2019. Linear mixed-
grizzly bear densities on the Northern Range, reflecting effects models using “Eigen” and S4. R package lme4
differences in rub tree preferences we observed (Bower- version 1.1-27.1. https://cran.r-project.org/web/packages/
sock 2020,). Despite small sample sizes for grizzly bears, lme4/lme4.pdf. Accessed 18 May 2021.
BELANT, J.L., B. GRIFFITH, Y. ZHANG, E.H. FOLLMANN, AND
we detected multiple instances of both species using the L.G. ADAMS. 2010. Population-level resource selection by
same rub trees (Fig. 4). That finding was contrary to other sympatric brown and American black bears in Alaska. Polar
studies (Mattson et al. 2005, Sawaya et al. 2012, Stetz Biology 33:31–40.
et al. 2019) and suggests that interspecific communication BOWERSOCK, N.R. 2020. Spatiotemporal patterns of resource
may play a role, but further research is needed. Our find- use and density of American black bears on Yellowstone’s
ings also demonstrate that future rub-tree studies would Northern Range. Thesis, Montana State University, Boze-
benefit from including genetic sampling to enhance es- man, Montana, USA.
timating the frequency of rub tree use and selection by BOYCE, M.S., AND L.L. MCDONALD. 1999. Relating popula-
different bear species and sexes. Additionally, bears of tions to habitats using resource selection functions. Trends
both species might frequently scent mark on similar rub in Ecology and Evolution 14:268–272.
objects in areas that are well-traversed by many animals BURST, T.L., AND M.R. PELTON. 1983. Black bear mark trees
in the Smoky Mountains. International Conference on Bear
to enhance communication within and between species. Research and Management 5:45–53.
CARNAHAN, A.M., F.T. VAN MANEN, M.A. HAROLDSON,
Acknowledgments G.B. STENHOUSE, AND C.T. ROBBINS. 2021. Quantify-
ing energetic costs and defining energy landscapes expe-
This research was supported by funding from Yellow- rienced by grizzly bears. Journal of Experimental Biology
stone Forever and the Undergraduate Scholars Program 224:jeb241083
at Montana State University. We thank E. Loggers, M. CLAPHAM, M., O.T. NEVIN, A.D. RAMSEY, AND F. ROSELL.
Wright, L. Bryant, S. Schmidt, K. Piecora, N. Tatton, N. 2012. A hypothetico-deductive approach to assessing the
Hurst, L. Hayward, A. Hiorns, and the many volunteers social function of chemical signalling in a non-territorial
that helped locate and collect data on rub trees in our solitary carnivore. PLoS ONE 7:e35404.
study. We thank A. Carlson of the Yellowstone Research ———, ———, ———, AND ———. 2013. The function of
Permit Office for help securing our research permits and strategic tree selectivity in the chemical signaling of brown
M. Sawaya and J. Stetz for field and technical support. bears. Animal Behavior 85:1351–1357.
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