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Comp. by: Vpugazhenthi Stage: Proof Chapter No.: 5 Title Name: PenterianiMelletti
Date:10/6/20 Time:13:45:16 Page Number: 53
Chapter
5
Remarkable Adaptations of the American Black Bear
Help Explain Why it is the Most Common Bear:
A Long-Term Study From the Center of its Range
David L. Garshelis, Karen V. Noyce, Mark A. Ditmer, Pamela L. Coy,
Andrew N. Tri, Timothy G. Laske, and Paul A. Iaizzo
American black bears (Ursus americanus, hereafter black
bears) are by far the most abundant species of bear, numbering
more than twice that of all other bear species combined. Here
we explore the reasons for their commonness, using a long-
term case study from near the geographic center of this species’
range: Minnesota, USA.
Minnesota is a Midwestern state that borders extensive
boreal forest landscapes in Canada to the north, broad-leaved
forests of Wisconsin–Michigan to the east, and vast agricul-
tural landscapes of the Dakotas and Iowa to the west and
south, respectively. These agricultural landscapes were for-
merly prairie, and thus generally not favorable habitat to this
forest-dwelling bear. However, black bears once commonly
occurred along forested riverine corridors transecting the
prairies. In the Red River Valley of northwestern Minnesota
and eastern North Dakota, a fur trader in 1800 remarked:
“Bears make prodigious ravages in the brush and willows; the
plum trees are torn to pieces, and every tree that bears fruit has
shared the same fate ... their dung lies about in the woods as
plentiful as that of the buffalo in the meadow”(Henry &
Thompson 1897, pp. 101–102).
Forested landscapes, more typical bear habitat, were histor-
ically more extensive in Minnesota than today, stretching
continuously from the north-central to the south-eastern part
of the state. Logging and conversion to agriculture and settle-
ment caused the forest, and hence the black bear range, to
retreat northward. Compounding the loss of habitat, bears
were widely regarded as vermin, and were readily killed to
protect crops and livestock, or simply out of fear. Schorger
(1949, p. 151) noted that, for the early settlers of Wisconsin,
“the presence of the black bear ... was second only to the
approach of an Indian war party in its power to raise ... a high
pitch of excitement.”
From the 1940s through the mid-1960s, government agen-
cies in Minnesota paid a bounty to encourage the killing of
bears. As populations rapidly dwindled, concern mounted that
bears would totally disappear. In 1971 their status was elevated
to big game animal, meaning that bear hunting required a
license and was limited to a designated season. In the following
decades, hunting regulations became more restrictive, and
efforts were made to reduce the killing of bears that caused
damage or were perceived as a threat. Today the species is
highly regarded as one of the most charismatic large mammals
of Minnesota. They have recolonized north-western Minnesota
and are frequently sighted south of the forested primary range.
Many US states share this history of black bear decline and
resurgence, and today have burgeoning bear populations. To a
large extent, the comeback of this species has been a conse-
quence of restrictions on killing, and a fundamental change in
how the public perceives and reacts to black bears. However,
the success of this species is also due to its biological adaptive-
ness –its ability to live in a vast array of habitats, to adapt to
radically variable food conditions, and to tolerate the presence
of people and the changes they have imposed on the landscape.
This chapter highlights the adaptability of the black bear using
an extensive and diverse data set spanning 38 years.
Study Sites
About 35% of Minnesota is forested, and 90% of that forested
area lies north of 46°N latitude. Aspen, northern hardwoods,
and black spruce are the dominant forest types. Forested lands
are owned privately (36%), by the state (24%), counties (16%),
federal government (17%), or industries (paper, timber, 7%).
A key feature of Minnesota forests is that food production
for bears varies drastically by season and by year (Garshelis &
Noyce 2008). From collections of bear scats we learned that
bears in Minnesota consume primarily green vegetation in
spring, mainly ants and pupae in June, various sorts of berries
as they ripen during July–August, and some late-season berries
but primarily hazelnuts and oak acorns in September, until
they den in September or October. Bears must search within
different habitats to locate different foods, as they become
available. During some years, many kinds of foods are avail-
able and abundant; in other years, the variety of foods is less,
and in some years, fruiting is so poor that bears face a food
scarcity.
We chose study sites that varied in extent and type of forest
habitat, land ownership, road density, and bear hunting. We
established our first study site in 1981, within the Chippewa
National Forest (CNF) and surrounding state- and county-
owned forests. The main purpose was to better understand
the impact of hunting on the bear population. The study site
was selected because it was near the center of the bear range
53
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(Figure 5.1A) and was known to be heavily hunted due to
extensive public land and forest roads that provided easy access
to bear habitat. Also, active logging yielded a complex matrix
of variously aged forest stands, providing opportunities to
investigate bears’associations with their habitat. The forests
are a patchwork of uplands, dominated by aspen, and forested
lowlands.
In 1991 our study expanded to include Camp Ripley
Training Center, a National Guard facility at the southern
periphery of the bear range. Bear hunting is prohibited on
Camp Ripley (CR), but bears may be hunted if they range
outside. Oaks are plentiful. The 210-km
2
area is long and
narrow (6–10 km wide), and bordered by highways with fast-
moving traffic and cornfields.
In 1997 we added Voyageurs National Park (VNP), located
along the northern edge of the Minnesota bear range
(bordering Canada). The study site is a 300-km
2
roadless
peninsula bounded by three large lakes. Like CR, hunting is
prohibited, but bears are exposed to hunting if they leave
the park.
In 2007, we initiated work in a fourth site, at the north-
western edge of Minnesota’s bear range (NW). This area is
largely agricultural, although only 2% of the land area is
planted with crops consumed by bears. Forested land, which
comprises less than 20% of the area, is patchily distributed in
small, privately owned woodlots and state-owned Wildlife
Management Areas, which are open to hunting. The area is
heavily transected by roads. A low density of bears occupied
this area until 1995, when poor natural food in neighboring
areas spurred an influx of bears attracted to corn and sun-
flowers. Unexpectedly, this wave of bears stayed to reside
permanently.
We hypothesized that bears on these four study sites would
have different rates of reproduction and different sources of
mortality. CNF had the most diverse foods, due to the mixture
of forest types and ages, so we thought it would support high
reproduction, including large litters. VNP had the poorest and
shallowest soils, no timber cutting, and a largely coniferous
forest, so provided the least food for bears; we therefore sus-
pected reproduction would be lowest. Although CR had plen-
tiful oaks, and NW had abundant crops, we thought
reproduction would be somewhat limited by lack of forest
diversity in CR and patchiness of forests in NW. We thought
that hunting would be the primary source of mortality in the
CNF, collisions with vehicles the main mortality in CR,
depredation-related killing the main cause in the NW, and
natural mortality dominant in VNP. We theorized that bears
in the NW would be most stressed by people, due to the
extensive system of farm roads, paucity of forest cover, and
their reliance on crops for food.
Research Methods
We captured bears in baited traps made of 55-gallon barrels.
We measured and weighed them, took temperature and blood
NW VNP
CNF
CR
Minneapolis-St. Paul
Manitoba
Ontario
Michigan
Wisconsin
N
North
Dakota
Iowa
South
Dakota
Legend
Minnesota bear range
Primary
Bear Range
Secondary
Bear Range
0 50 100 150 20025
Km
(A)
CR
NW
CNF
VNP
25
50
75
100
125
0 2 4 6 8 10 12 14 16 18 20
Age (Years)
Mass (kg)
(B)
Figure 5.1 (A) American black bears were studied in four areas, representing
different parts of their range in Minnesota. Minnesota bear range was
differentiated as primary or secondary, based mainly on continuity of forested
habitat and overall bear density. Bears near the western and southern fringes of
the bear range (NW and CR) had access to more abundant foods, especially in
fall, which enabled them to grow more quickly and reach higher maximum
weights than bears near the center of the primary range (CNF). Bears along the
northern edge of their range in Minnesota (VNP) had the poorest foods, and
were smallest. (B) Graphs are nonlinear regressions (±95% confidence interval) of
age-specific weight data from denning female bears during February–March.
Differences in rates of growth lead to earlier maturation of CR and NW females,
and very delayed maturation in VNP.
Systematics, Ecology, and Behavior
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samples, and attached a radiocollar. For the first 25 years we
used VHF-transmitting collars, and tracked them from an
airplane. We switched to GPS collars as they became available
and affordable. Recent GPS collars provide near real-time data
by sending locations via a link with a satellite. Collared bears
could be killed legally by hunters, but beginning in 2001, we
asked hunters to voluntarily avoid shooting them.
We visited all bears in their winter dens each year to
reassess their weight and condition, and to quantify the cubs
born and the previous year’s cubs that survived to yearlings
(denned with their mother). We fit collars to yearling bears,
and readjusted the fit of all other radiocollars at each den visit.
Beginning in 2015, we set trail cameras in front of dens to gain
information about when bears exited, and possibly what
caused them to do so.
From 1981 to 2019 we radiocollared nearly 700 individuals.
We monitored most of them until they died. These study
subjects provided the data reported below.
In 2009, we began using cardiac monitors, which were
implanted subcutaneously near the heart (Laske et al. 2018).
We used these to better understand hibernation physiology,
and also to measure potential stress in different circum-
stances, which we discerned by matching heart rates to GPS
locations.
Physical Growth, a Function of Food
Bears on the four study sites grew at different rates and reached
different maximum weights (Figure 5.1B), in accordance with
abundance of food, especially fall food. Females in CR and NW
grew most quickly: on average they reached 100 kg by 8–10
years old, and 90% of this weight by age 5–6 years. In contrast,
average maximum weight for CNF females was 85 kg and VNP
females only 75 kg, which they reached by their early teens
(90% by age 8–9 years). Males also grew most quickly and to
heavier maximum weights in CR and NW (one individual
reaching 250 kg), but male sizes were more variable within
each area than those of females.
Habitat-related differences in the growth of bears were also
evident on a finer scale. In the NW, bears of both sexes were
heavier and fatter (more prepared for hibernation) if they
consumed crops. Frequent crop use also increased the physical
stature of males, which likely enhanced their social dominance
(Ditmer et al. 2016).
Within the CNF, a sharp habitat division existed between
predominantly uplands on the east side versus lowlands in the
west; bear foods were more diverse and abundant in the
uplands. Accordingly, the skull length (an easily measured
index of physical stature) of bears that lived mainly in upland
habitats reached maximum size 2 years earlier than for bears
that lived principally in lowlands; lowland bears eventually
caught up in skull size, but not weight.
Reproductive Variability
Since our first den visits in 1982, we observed 315 litters with
819 cubs. Litters contained 1–5 cubs, with three being most
common (54% overall; Figure 5.2; average litter size = 2.6).
First-time mothers tended to have smaller-than-average litters,
as did bears in VNP, but otherwise, litter size did not increase
with mother’s weight (Noyce & Garshelis 1994).
Mother’s weight also did not affect the normal 2-year
birthing interval. Bears typically bred in June or July, gave
birth in January, and remained with their litter for about 17
70%
60%
50%
40%
30%
20%
10%
0%
123
Litter size
Litter size frequency
45
CNF (n=192)
CR excl. 3 yr-olds (n=55)
VNP (n=27)
NW (n=35)
CR (n=61)
Figure 5.2 Litter size of black bears in Minnesota
ranges from one to five cubs, but 90% contain two
or three. Three cub litters are most common in CNF
and NW study sites. In CR, 3-year-old mothers
have smaller litters than older bears. In VNP, two cub
litters are most common (n= number of observed
litters).
Remarkable Adaptations
55
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months, denning together through the cubs’first birthday.
Family break-up was prompted by the next breeding activity.
Of 195 birthing intervals, all were 2 years, except seven where a
whole litter died and the mother reproduced again the next
year, and seven with a 3-year gap between births (although
there may have been an undetected litter that died soon after
being born). We observed no relationship between non-
surviving or missed litters and the mother’s body condition.
The age that females had their first litter (age of primipar-
ity) was the most variable reproductive parameter, ranging
from 3 to 10 years. Typically it was 4 or 5 years, but production
of a litter with at least one surviving cub by age 4 varied from
0% of females in VNP to 38% in CNF, to 86% in CR and NW,
in accordance with body weight. Only females that weighed at
least 41 kg in March (during hibernation) produced their first
cubs the following year. Within the CNF, bears living in
lowland habitats with less food began reproducing about
2 years later than their heavier counterparts in the adjacent
uplands. The weight and growth of cubs during their first year
corresponded with the weight of their mother (Noyce & Gar-
shelis 1994) and strongly affected when they matured. Bears
that were small as yearlings tended to stay small, and thus
reproduced later (Garshelis & Noyce 2008). One anecdote
from VNP exemplifies this point: two sisters were the same
size as 8-week old cubs, but one was 20% heavier by the time
we saw them again as yearlings, still with their mother. This
gap continued to widen with age, and ultimately the heavier
one produced cubs a year earlier than her sister.
In our study, only two females lived long enough to reach
reproductive senescence. One was 25 years old when first cap-
tured, and did not produce cubs that year or the next. The other
produced 11 litters from age 5 to 25, but none for the next 14
years. If most females lived this long, a year or two difference in
when they produced their first litter would have little effect on
their lifetime reproductive output. However, because of
hunting, females rarely reach reproductive senescence, and in
fact, many never reach reproductive age; of the females who
survive their cub year, about half die before reaching age 4.
Therefore, even a 1-year delay in the age of primiparity might
determine whether a bear produces cubs in her lifetime. The
downside of maturing early is that young mothers are less
experienced in all it takes to be a bear, and so are more apt to
lose a whole litter than older first-time mothers.
Male reproduction also is affected by age and weight,
because males compete for access to females. Through radio-
telemetry we observed male–female pairs during the mating
season, and also measured serum testosterone of captured
bears. Males that were 6–8 years old were heaviest and had
the highest testosterone (Garshelis & Hellgren 1994). Three-
year-old males were capable of breeding (and had frequent
facial wounds from fighting during the breeding season), but
rarely paired with estrus females; when they did, it tended to be
late in the season. We speculate that prime breeding males may
tire, as we have evidence that they fast during the breeding
season (Coy & Garshelis 1992).
Primary Causes of Mortality
Mortality during a cub’sfirst year of life is relatively low in
Minnesota (19%, but 25% in VNP). The mortality of male cubs
(23%) exceeds that of females (15%). We do not know what
causes cubs to die, as we did not radiocollar them until they were
1 year old. However, on a few occasions we observed cubs that
fell out of trees while playing or sleeping, scampered carelessly
across roads while trailing behind their mother, and in three
cases became separated from their mother and never rejoined
her. We suspect that male cubs were more adventurous,
accounting for their higher mortality. The sex ratio of cubs at
8 weeks old (when we first observed them in the den) was 51%
male. By yearling age, it shifted to 52% female.
Bears in Minnesota almost never died in their dens, and
never of starvation. Even very skinny yearlings made it
through the winter, though some (all weighing <10 kg) died
of malnutrition after den emergence in spring. By 15 months
of age, natural mortality was a rare event in this population.
No bears died of disease. We knew of only two that were killed
by wolves, even though wolves were common in all study sites.
A few bears also were killed by other bears, and one under-
nourished 2-year-old starved after biting a porcupine. Of
387 deaths that we documented, only one bear died of natural
causes related to old age. She was 39.5 years old, the oldest
known wild individual of any species of bear. The average
lifespan for a bear in Minnesota is about 4 years for males
and 6 years for females (although females in CR and VNP
lived, on average, to 7 and 9 years, respectively).
The highest proportion of natural deaths occurred, as we
had hypothesized, in the two unhunted populations, VNP and
CR. However, VNP and CR bears were hunted when they left
the park or military reserve to seek better foods; consequently,
even in these two areas, legal hunting was the primary source
of mortality, as it was in NW and CNF (Figure 5.3). In CNF we
used black-colored radiocollars for most years, purposefully so
hunters could not distinguish collared bears, and found that
hunting constituted 81% of deaths (excluding cubs, which were
not legally hunted). We observed that upland bears were sub-
ject to higher hunting mortality because few people hunted the
lowland forests, which had poor road access.
In Minnesota, most hunters use bait to lure a bear close
enough to shoot. Because bears are so food-driven in fall,
baiting is a highly successful hunting technique. Hunting suc-
cess varies inversely with natural food conditions: in years
when fall foods are plentiful, bears are less apt to come to baits
than when foods are scarce. Females, which are more wary of
baits than males, are disproportionately less inclined to feed at
baits than males when natural foods are adequate (Noyce &
Garshelis 1997). The greater wariness of females is an adaptive
trait that, in a hunted population like Minnesota, yields per-
petually male-biased harvests and hence greater potential for
population growth.
Hunters’baits often include attractive and nutritious foods
like fruits and nuts (as well as sweets), which may enhance
Systematics, Ecology, and Behavior
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reproduction by hastening growth and maturity; on the other
hand, bait consumption is a mortality risk. Individual bears
must weigh the attraction of the concentrated but foreign food
against the risks they discern from human smells at the site.
One of our study bears with a heart monitor showed an
elevated heart rate for 3 hours before being shot at a hunter’s
bait (Laske et al. 2011). Wary or shy bears that avoid baits or
only feed at bait sites nocturnally are more likely to survive,
thus potentially altering bear behavior (e.g. attraction to
human foods) on a population level.
The two other main sources of mortality for bears in
Minnesota are collisions with a vehicle and being shot as a
nuisance. As we had postulated, road kills were most common
for CR, which is bordered by highways, and depredation kills
were most common in NW (Figure 5.3), where bears are
surrounded by agriculture, including fields of corn and sun-
flowers. Even so, hunting was still the dominant source of
mortality in these areas. This was despite our requests since
2001 (i.e. the entire span of our study in NW) for hunters not
to shoot collared bears, which we marked with large, colorful
ear tags. Hence, collared bears likely underrepresented hunting
as a source of mortality.
Home Ranges and Migrations Relate to Food
During the 1980s, when our study was limited to the CNF and
all bears were tracked from an airplane, we were struck by the
number of bears that made excursions well outside their
normal summer home range during August–September, most
returning to den about 6 weeks later. Due to the speed and
apparent directness in their travel, and the long distances
covered, it was nearly impossible to track these bears enroute.
Nevertheless, significant trends emerged from these data
(Noyce & Garshelis 2011, 2014), which were later corroborated
with more detailed data from GPS-collared bears (Figure 5.4).
Bears generally travelled south and/or west to areas with better
food resources (oaks and agriculture). Such travel was
common (~40% of bear-years) and included both sexes and
all ages, although males tended to travel further (median = 26
km) than females (median = 10 km). Sometimes bears from
different parts of the study area ended up together at distant
locations; and they seemed to use similar travel routes. Taken
together, these are characteristics of a migration, albeit not the
familiar en-masse migration that occurs in birds or ungulates.
Early accounts of naturalists in this region mention dis-
tinct, well-used trails that “could easily be followed for 20 or 30
miles”and “droves of bears”travelling together on a migration
(Schorger 1949). Minnesota hunters in the mid-1800s waited
for migrating bears to cross a certain predictable “bear pass”
along a major river. These anecdotes have been largely dis-
missed by scientists as exaggerations. Possibly the old migra-
tion routes became less predictable with changing landscapes
and declining bear populations. Nevertheless, our data clearly
showed that many bears followed similar routes and ended up
close together. It seems that a network of migratory travelways
really does exist. We found that these were not governed by
environmental features, nor were they learned from their
mother. We surmised that bears cue offthe scent trails of each
other, with routes varying somewhat year to year.
We expected that bears that migrated in fall would be
particularly inclined to visit hunters’baits along the way,
because the anticipation of finding a concentrated food source
is what motivated their migration. However, we observed just
the reverse: hunting-related mortality of migrating bears was
less than for bears that remained in their home ranges, sug-
gesting that bears were more wary of people while traveling
outside their home ranges (Noyce & Garshelis 2011).
Also unexpectedly, we found that bears migrated most
often not when food supplies in their home range were scarce,
but when food was plentiful. In years of food scarcity, it could
be risky to expend energy migrating, as point sources of more
abundant food might be difficult to find. Conversely, in good
food years, there would likely be many patches of super-
abundance, which could significantly bolster weight gain. This
Unknown cause
Natural mortality
Other human-caused
Vehicle collision
Shot as nuisance
Shot by hunter
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
CNF
(n=304)
CR
(n=32)
VNP
(n=23)
NW
(n=28)
11%
11%
22%
4%
4%
6%
31%
9%
5%
7%
Relative cause of mortality
81%
64%
70%
44%
Figure 5.3 Legal hunting is the leading cause of
death for bears across Minnesota. CR and VNP are
unhunted, but bears are vulnerable to hunting
when they leave on foraging forays. Vehicle
collisions just outside CR are also relatively
common. Natural mortality is common only in VNP.
(n= number of collared bears monitored
until death).
Remarkable Adaptations
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could make the difference in whether females produced their
first litter, the weight of their cubs, and when these cubs
reached maturity. Lowland females on the CNF, whose growth
and reproduction were suppressed, migrated at twice the rate
of upland females.
VNP bears also migrated southward. This area typically
had low food abundance in fall. In some years bears swam 3–5
km across a large lake to reach an area where both natural and
human-related foods were more abundant. In one particular
year, when wild plums along this south shore were especially
plentiful, most of the VNP bears migrated outside the park.
The bears had either detected the scent of these fruits wafting
across the lake, or they simply left the park because they were
hungry and happened upon this bounty of food just outside.
CR and NW bears did not show the same migratory pat-
terns. Most CR bears left the reserve at some time, but we
could not discern a particular pattern of directional move-
ment, possibly because CR is already at the southern fringe
of the range. NW bears often expanded their fall home ranges
to include crops or oak stands, but as these were scattered
about the landscape, they also showed no consistent migratory
pattern.
The patchiness of the NW landscape required bears to
cross agricultural fields that lacked food or cover. During these
crossings, their heart rates accelerated beyond what was
expected for their hastened movement across the field, indicat-
ing a stress response (Ditmer et al. 2015b). Traversing this
landscape, which was mainly non-habitat embedded with
patches of forest, inflated their home range size. Whereas
weekly home ranges of males averaged about 130 km
2
, their
weekly ranges shifted spatially during the year, creating yearly
home ranges that encompassed over 800 km
2
–the largest
Aitkin
Grand Rapids
Bena
Warba
Remer
Emily
Bovey
Zemple
Cuyuna
Ironton
Calumet
Trommald
Riverton
Palisade
McGregor
Deerwood
Cohasset
Longville Hill City
Crosslake
Boy River
La Prairie
Deer River
Fifty Lakes
Federal Dam
Breezy Point
Aitkin
Grand Rapids
Bena
Warba
Remer
Emily
Bovey
Zemple
Cuyuna
Ironton
Calumet
Trommald
Riverton
Palisade
McGregor
N
Deerwood
Cohasset
Longville Hill City
Crosslake
Boy River
La Prairie
Deer River
Fifty Lakes
Federal Dam
Breezy Point
Legend
Summer home range
Leave 2016
Return
Cities
Lakes
Legend
Summer home range
Leave 2017
Return
Cities
Lakes
25 0 25 Km12.5
Figure 5.4 Bears from the CNF study site, near the center of Minnesota’s bear range, frequently migrate south in late summer or early fall to locate areas of
richer food (typically an oak stand or cornfield). This was discovered initially with VHF-collared bears, located weekly from an airplane. GPS collars, taking locations
every 2 hours, highlighted the directness of the travel routes, there and back. Shown here is an adult male with a GPS collar who travelled virtually the same migration
route in consecutive years (left 2016, right 2017).
Systematics, Ecology, and Behavior
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known for this species (Ditmer et al. 2018a). These expansive
home ranges were partly the result of males searching for a
suitable crop field, where they would feast for some time,
apparently (based on their slow heart rates) feeling unthreat-
ened. This was similar to the CNF bears in terms of long
travels to find concentrated sources of food, but differed in
not being directional on a population level. Also unlike CNF,
females in NW travelled outside their home ranges to use
crops mainly as a fallback when natural food conditions were
poor (Ditmer et al. 2016). By studying captive bears that had
never eaten corn or sunflowers, we found that females were
inherently less apt than males to try these unfamiliar foods
(Ditmer et al. 2015a). Females with cubs were especially hesi-
tant to visit crop fields, likely trying to avoid infanticide by
males (one female that ventured into a cornfield had one of her
cubs attacked by a male). The sprawling wanderings of males
and their use of crops led us to predict that some will continue
to expand their range further west, into even more treeless
habitat; however, it is unlikely that females, which are more
bound to forests and natural foods, will follow (Ditmer et al.
2018a).
Roads seem to have little impact on the movements of
Minnesota bears. Heart monitors showed that bears were wary
when they approached and crossed roads, which occurred
about every 3 or 4 days (but sometimes multiple times per
day); however, their slightly elevated heart rates indicated that
it was a low-stress activity (Ditmer et al. 2018b). They crossed
high-traffic roads mainly at night, but otherwise showed no
avoidance of roads.
Denning and Hibernation –Surviving Half the
Year Without Food
Minnesota bears typically entered dens during September–
October. Pregnant females denned the earliest: following
summer breeding, the blastocyst (not yet an embryo) does
not implant until November, but by denning time, progester-
one is noticeably elevated, so females are physiologically aware
of their impending pregnancy.
Bears used a wide variety of dens, some of which seem (to a
human observer) unappealing for a 6–7-month hibernaculum
in one of the coldest places within the range of this species.
Many individuals used different types of dens year to year, but
they did not become choosier in their den selection as they got
older. Also, they rarely reused a den in a subsequent year. The
seeming indifference to the quality of their den was one of the
most surprising findings of our study, as we tend to think of
hibernation as a critical and vulnerable time for bears.
Bears commonly dug dens into the ground. Females excav-
ated dens more than males. Both sexes also used existing
structures, like brush piles from former logging operations
or natural hollows under a root mass of a tipped-over tree
(Figure 5.5). Classic rock caves were only available in VNP.
Dens became more insulated as snow accumulated; sometimes
just a small hole permitted air exchange (and some had no air
hole) (Figure 5.6). On the opposite end of the spectrum were
nest dens, comprised of a pile of branches, boughs, or grass
collected from the immediate vicinity (Figure 5.5D). Some
nests were situated under thick conifer cover, whereas others
had no overhead cover, exposing the bear to snow and wind,
and providing no protection against radiative cooling on clear
winter nights [temperatures dropped below –18°C (0°F) for an
average of 50 days each winter]. Nest dens tended to be more
common among males, but also were used by females, includ-
ing some giving birth.
Not uncommonly, we observed wolf tracks around dens.
A bear in a nest, especially a female with newborn cubs, would
seem like an easy target, yet only once did we find a denning
bear (a young male) injured by wolves (and once observed sign
of a scuffle between a female with cubs and wolves, but no
injuries to the bear). Oddly, whereas some bears in nests
remained vigilant to sounds, with some leaving as we
approached, others, with their head tucked under their chest,
exhibited no overt reaction to our presence, even to within 2 m.
Many nest dens were in relatively remote areas. One quar-
ter of CNF males made a northward fall migration averaging
50 km (some having just returned from a southward migration
to forage) to den in a vast roadless bog (Noyce & Garshelis
2011). In NW, many bears denned in marshes where they
created a huge woven nest of cattails, open to the sky. It is
difficult to explain the paradox of bears moving to remote
areas to den, possibly to avoid wolves or people, yet then often
denning in an open nest. It is also difficult to explain move-
ments to remote habitats by some bears while others denned
just tens of meters from roads, railroads, snowmobile trails, or
houses, seemingly unconcerned about noise and human
proximity.
We surmise that bears rely more on their remarkable
physiological adaptations than choosing an optimal den for
their well-being during winter. One of their most notable
adaptations is for “fight or flight”during hibernation. Rectal
temperature of Minnesota bears drops only a few degrees from
37°C in summer to typically 32–36°C while hibernating, enab-
ling them to awaken and respond quickly, if needed. This
distinguishes them from other hibernators, which are small
enough to fully escape predation risks by hibernating in deep
burrows or crevices; this security allows them to drop body
temperature drastically, thus saving energy but losing the
ability to respond quickly.
We learned that part of the bear’s suite of special adapta-
tions includes modulation of their heart rate. Implanted heart
monitors revealed heart rates slowing from daytime averages
of 85–90 bpm during summer to 20–25 bpm in winter, and
becoming synchronized with their breathing, which slows to
2–4 per minute. This respiratory sinus arrhythmia allows the
heart to pause and rest (for 5–30 seconds), then pump 3–5
times after an inhalation to circulate oxygenated blood (Laske
et al. 2010, 2011). Because the blood can be stagnant between
breaths, bears avoid strokes or heart attacks by tripling clotting
time during hibernation (Iles et al. 2017). However, this
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adaptation involves only clotting within their vessels. Hiberna-
ting bears retain or enhance their ability to heal external
injuries that might be incurred just before or during denning
(Iaizzo et al. 2012).
Respiratory sinus arrhythmia is so characteristic of hiber-
nation that we could tell, by disruptions in this pattern, when
females gave birth (early–mid January), and when physio-
logical hibernation ended in spring (Laske et al. 2017). The
trail cameras we set at dens revealed a transitional process
from denning to activity. Bears spent days to weeks exiting
and re-entering their den (generally beginning mid-March)
before leaving the site (late March or early April). Females
with cubs delayed their departure from den sites by an average
of 3 weeks, allowing cubs time to gain strength and skills for
walking and tree climbing. Many bears remained at their den
site after they first emerged to collect more bedding material,
especially if it had become saturated by melting snow. One
benefit of nest dens may be that they stayed drier, because no
snow dripped from above, and they were typically thicker than
the beds bears made inside more structured dens (Figure 5.5d).
Adaptability, the Key to Success
The black bear has rebounded across North America to
become one of the continent’s true conservation success stor-
ies. This success relied on people’s growing tolerance for a
species that might sometimes cause property damage, but
otherwise presents little danger to human safety. Restrictions
on hunting were also paramount to this success. We learned in
this study that natural mortality is very low, and legal hunting
Figure 5.5 Bears in Minnesota use various types of dens: (A) excavated (here dug into a mound along the edge of a farm field); (B) under the roots of a tipped-over
tree; (C) under a brush pile (from logging or storms); and (D) in an open nest that they construct. Individuals often switch den types year to year, and rarely reuse dens.
Type of den does not affect reproduction or survival; even cubs born in open nests survive normally. Other animals (e.g. squirrels, rabbits, wolves) commonly
approach dens, as in panel (A). (Photos by D. Garshelis.) (A black and white version of this figure will appear in some formats. For the color version, please refer to the
plate section.)
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the driving cause of mortality; even in unhunted areas, bears
commonly venture out and are eventually killed by hunters.
Thus, management agencies have direct control over the size
of black bear populations, unlike species whose mortality is
affected largely by environmental factors.
Food conditions, though, had significant effects on repro-
duction. Baseline food conditions within our four study sites
differed greatly, producing different-sized bears with different
reproductive rates, due mostly to different rates of maturity.
Reproduction is constrained to what the land will support.
Poor fruiting years reduce the chance that young females reach
the threshold weight for first giving birth, causing lower
population-wide cub production some years (Garshelis &
Noyce 2008). However, once females began reproducing, poor
food years did not disrupt their normal 2-year cycle or cause
higher cub mortality. Experienced mothers found ways of
caring for cubs in the face of periodic adversity.
Black bears are masters of contending with food variability.
They are able to take advantage of far-flung hotspots of boun-
tiful food; when normal foods are lacking, they use substitute
foods, sometimes from human-related sources. Meanwhile,
their tolerance and resilience to undernutrition is profound.
The smallest hibernating yearlings (5–6 kg) weighed about the
same as the largest 2-month-old cubs; the heaviest hibernating
yearlings (52–65 kg) were 10 times larger, and were more than
double the weight of the smallest hibernating 2-year-olds
(19–22 kg). Some juveniles barely gained weight one year,
but doubled or nearly tripled their weight the next.
Minnesota was an ideal place to study the adaptiveness of
this species. The food supply here is particularly variable
spatially and temporally, yielding natural experiments in how
bears cope. We were fortunate to have been able to conduct
our studies over a very long time frame (approaching four
decades), enabling us to witness large swings in food abun-
dance. Also, Minnesota is noted for its long, cold winters: food
is entirely absent for half the year. Certainly, the most remark-
able adaptation of this species is its ability to withstand such a
long period without food or water, and extreme cold (–40°C);
moreover, during this period, females give birth, lactate, and
nurture altricial cubs.
The bear’s ability to store fat is key to their ability to
survive a half-year without eating. Thus, fall is often con-
sidered a critical time for bears, as fall foods dictate how much
fat they can amass. Extensive fall migrations by Minnesota
bears is on par with hibernation as an adaptation for surviving
prolonged food deprivation. Unexpectedly, this otherwise soli-
tary animal is not only generally tolerant of multiple individ-
uals sharing a rich fall feeding area, but also reliant on social
cues to navigate to such sites.
Often neglected in considering the seasonal adaptations of
bears is the springtime. In Minnesota, bears emerge from dens
having lost 15–50% of their body weight. Particularly for
young, skinny bears, this is the toughest time of year. The
ground is still frozen and partly snow-covered; late spring
snowstorms are not unusual. Shoots of green vegetation begin
to poke up about 2 weeks after den emergence. Fruits are not
available for 3.5 months. Bears are relatively inactive during
this period, becoming increasingly active as the spring pro-
gresses (Laske et al. 2011, 2017). It was previously thought that,
before the fruiting season, bears continued to lose weight
feeding exclusively on green vegetation, and later insects. How-
ever, we discovered that most bears not only gained weight,
but grew structurally during spring –another surprising find-
ing (Noyce & Garshelis 1998).
Despite the many studies of this species across its range,
each year we learn more. Many of our early well-reasoned
hypotheses turned out to be incorrect. Long-term field studies
employing constantly improving techniques and technology
remain essential for gaining a deeper understanding of how
the black bear has been able to thrive in a landscape increas-
ingly dominated by people. While it is appealing for nature
conservation to promote the image of bears, generally, as icons
of the wilderness, that characterization distracts from the real
reason why this species has been so successful: its adaptability
to change.
Acknowledgments
This work was funded by the Minnesota Department of Nat-
ural Resources with additional support from the Minnesota
Army National Guard, Voyageurs National Park, Agassiz
National Wildlife Refuge, and University of Minnesota’s Insti-
tute for Engineering in Medicine and Department of Surgery.
Medtronic plc donated the cardiac devices. We graciously
thank the many people who assisted with data collection.
Figure 5.6 American black bears in northern Minnesota –one of the coldest
places within their range –commonly spent the winter in unprotected dens
that they construct. Even some females that gave birth in mid-winter, like the
bears pictured here, used dens with little or no cover. Due to the insulatory
properties of their thick layer of fat and dense winter coat, and reduced blood
supply to the skin, snow that fell on their backs sublimated or was shaken off,
rather than melting into their fur. (Photo by Dave Garshelis.) (A black and white
version of this figure will appear in some formats. For the color version, please
refer to the plate section.)
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References
Coy, P. L. & Garshelis, D. L. (1992).
Reconstructing reproductive histories of
black bears from the incremental layering
in dental cementum. Canadian Journal of
Zoology 70: 2150–2160.
Ditmer, M. A., Burk, T. E. & Garshelis, D. L.
(2015a). Do innate food preferences and
learning affect crop raiding by American
black bears? Ursus 26: 40–52.
Ditmer, M. A., Garshelis, D. L., Noyce, K. V.,
et al. (2015b). Behavioral and
physiological responses of American
black bears to landscape features in an
agricultural region. Ecosphere 6(3): 28.
http://dx.doi.org/10.1890/ES14–00199.1
Ditmer, M. A., Garshelis, D. L., Noyce, K. V.,
Haveles, A. W. & Fieberg, J. R. (2016).
Are American black bears in an
agricultural landscape being sustained by
crops? Journal of Mammalogy 97: 54–67.
Ditmer M. A., Garshelis, D. L., Noyce, K. V.
& Fieberg, J. R. (2018a). Delineating the
ecological and geographic edge of an
opportunist: the American black bear
exploiting an agricultural landscape.
Ecological Modelling 387: 205–219.
Ditmer, M. A., Rettler, S. J., Fieberg, J. R.,
et al. (2018b). American black bears
perceive the risks of crossing roads.
Behavioral Ecology 29: 667–675.
Garshelis, D. L. & Hellgren, E. C. (1994).
Variation in reproductive biology of male
black bears. Journal of Mammalogy 75:
175–188.
Garshelis, D. L. & Noyce, K. V. (2008).
Seeing the world through the nose of a
bear –diversity of foods fosters
behavioral and demographic stability. In
Fulbright T. & Hewitt, D. (Eds.), Frontiers
in wildlife science: Linking ecological
theory and management applications (pp.
139–163). Boca Raton, FL: CRC Press.
Henry, A. & Thompson, D. (1897). New light
on the early history of the greater
Northwest. The manuscript journals of
Alexander Henry and of David Thompson
1799–1814. Exploration and adventure
among the Indians on the Red,
Saskatchewan, Missouri, and Columbia
Rivers. Coues, E. (Ed.), vol. 1. New York,
NY: Harper.
Iaizzo, P. A., Laske, T. G., Harlow, H. J.,
McClay, C. B. & Garshelis, D. L. (2012).
Wound healing during hibernation by
black bears (Ursus americanus) in the
wild: elicitation of reduced scar
formation. Integrative Zoology 7: 77–89.
Iles, T. L., Laske, T. G., Garshelis, D. L. &
Iaizzo, P. A. (2017). Blood clotting
behavior is innately modulated in Ursus
americanus during early and late denning
relative to summer months. Journal of
Experimental Biology 220: 455–459.
Laske, T. G., Harlow, H. J., Garshelis, D. L. &
Iaizzo, P. A. (2010). Extreme respiratory
sinus arrhythmia enables hibernating
black bear survival –physiological
insights and applications to
human medicine. Journal of
Cardiovascular Translational
Research 3: 559–569.
Laske, T. G., Garshelis, D. L. & Iaizzo, P. A.
(2011). Monitoring the wild black bear’s
reaction to human and environmental
stressors. BMC Physiology 11: 13.
doi:10.1186/1472-6793-11-13
Laske, T. G., Iaizzo, P. A. & Garshelis, D. L.
(2017). Six years in the life of a mother
bear –the longest continuous heart rate
recordings from a free-ranging mammal.
Scientific Reports 7:40732. doi: 10.1038/
srep40732.
Laske, T. G., Evans, A. L., Arnemo, J. M.,
et al. (2018). Development and utilization
of implantable cardiac monitors in
free-ranging American black and
Eurasian brown bears: system
evolution and lessons learned. Animal
Biotelemetry 6: 13. doi.org/10.1186/
s40317–018-0157-z
Noyce, K. V. & Garshelis, D. L. (1994). Body
size and blood characteristics as
indicators of condition and reproductive
performance in black bears. International
Conference on Bear Research and
Management 9: 481–496.
Noyce, K. V. & Garshelis, D. L. (1997).
Influence of natural food abundance
on black bear harvests in Minnesota.
Journal of Wildlife Management 61:
1067–1074.
Noyce, K. V. & Garshelis, D. L. (1998).
Spring weight changes in black bears in
northcentral Minnesota: the negative
foraging period revisited. Ursus 10:
521–531.
Noyce, K. V. & Garshelis, D. L. (2011).
Seasonal migrations of black bears (Ursus
americanus): causes and consequences.
Behavioral Ecology and Sociobiology 65:
823–835.
Noyce, K. V. & Garshelis, D. L. (2014).
Follow the leader: social cues help guide
landscape-level movements of American
black bears (Ursus americanus).
Canadian Journal of Zoology 92:
1005–1017.
Schorger, A. W. (1949). The black bear in
early Wisconsin. Transactions of the
Wisconsin Academy of Sciences, Arts and
Letters 39: 151–194.
Systematics, Ecology, and Behavior
62
