Download Factors Affecting the Evolution and Behavioral Ecology of the

Factors Affecting the Evolution and Behavioral Ecology of the Modern Bears
Author(s): Ian Stirling and Andrew E. Derocher
Source: Bears: Their Biology and Management, Vol. 8, A Selection of Papers from the Eighth
International Conference on Bear Research and Management, Victoria, British Columbia,
Canada, February 1989 (1990), pp. 189-204
Published by: International Association of Bear Research and Management
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THEEVOLUTION
FACTORSAFFECTING
ANDBEHAVIORAL
ECOLOGY
OFTHE
MODERN
BEARS1
IAN STIRLING,Canadian Wildlife Service, 5320 122 St., Edmonton, Alberta, T6H 3S5, and Department of Zoology, University of Alberta, Edmonton, Alberta. T6G 2E9
ANDREW E. DEROCHER, Department of Zoology, University of Alberta, Edmonton, Alberta. T6G 2E9
Abstract: The present distribution and abundance of the ursids is but an ephemeral reflection of an evolutionary path that began with the first identifiable bear, the dawn
bear (Ursavus elmensis), 20 million years ago in the early Miocene epoch. Although the dawn bear was only the size of a fox terrier, by the Pleistocene its descendents
had evolved into some of the largest terrestrial carnivores the world has known. Most bear species evolved in the northern hemisphere although some dispersed and reached
South America,Africa and SoutheastAsia.
Each species had to cope with ecological changes that affected interspecific competition or the availability of food. Apparently the black bear was sufficiently adapted
to have survived largely unchanged from what it was like a million years ago. Numerous species went extinct, leaving only the 8 still present today. Some understand-
ing of the evolutionarypressuresthat the modem bearshave evolved throughmay help us to understandtheirbehavioralecology.
During the Pleistocene, bears at higher latitudes grew large and ecologically plastic while those closer to the equator remained small and became ecological specialists,
as predicted by Geist's (1987) dispersal theory. Adaptations of the teeth of ancestral bear species allowed them to be both herbivores and carnivores. This allowed them
to develop large size and broad ecological plasticity. Large body size enabled bears to conserve heat, capture large prey, defend carrion, travel great distances, and, as
vegetation increased in the diet, to survive on qualitatively poorer food. Quantity and quality of available food and the degree of sexual dimorphism influenced the size
of the home rangeand the evolution of social behaviorin each species.
Bears show a great deal of individual variation in behavior and may exploit different subniches as a result of learned behavior. Slight differences in phenotype may
also influence exploitation of subniches. Recent literature indicates that some terrestrial bear species are more active predators than previously thought and some evidence
suggests a degree of scaling between the size of bears and the size of their prey. Social signalling appears to have been influenced by life in forest habitats but is not well
understood. We give a preliminary interpretation of the social organization of the present day bears through the interactive framework of proximate ecological pressures,
phylogenetichistory,and learning.
There are likely few populations of bears anywhere in the world whose behavior has not been significantly influenced by man. This may confound our understanding
of their behavior and ecology. Remaining populations of bears may not be able to adapt successfully to the combined effects of human predation, disappearing habitat,
and climatic change.
Int. Conf. Bear Res. and Manage. 8:189-204
The bears are a fascinating group to analyze in an
evolutionarycontextbecauseof the remarkablevariation
in their adaptations. The largest species are about 10
times heavier than the smallest. The degree of sexual
dimorphismranges from species with none to those in
which males are twice the size of females. Bears have
evolved specializednichesas carnivores,herbivores,and
myrmecophages,while retainingthe functionalabilityto
be omnivores. Pandas,whichnormallyeat only 2 species
of bamboo will still eat meat, while polar bears, which
specialize in hunting seals, also eat berries. The home
ranges of differentspecies can range from less than 10
km2to over 100,000 km2.
The present day distributionand abundanceof the
ursids is but an ephemeralreflectionof an evolutionary
paththatbegan with the appearanceof the first identifiable bear,the dawnbear,20 millionyearsago in the early
Miocene epoch (Kurten 1976). Consequently,a brief
examinationof the evolutionaryhistoryof moder bears
may help in interpretingthe available informationon
theirpresentday behavioralecology. The dawnbearwas
only about the size of a fox terrier,but even after 10
million years, its descendentProtursus, from the midMiocene of Europe,was only modestly larger.
About 5 million years ago in the mid-Pliocene the
worldclimatebecamedrier(Kurten1976, Guthrie1984)
Invitedpaper
and, in the northernhemisphere,the biomass and diversity of plants suitable for ungulates reached its zenith
(Guthrie 1984). Savannas and steppes became widespread. At higher latitudes and altitudes the climate
amelioratedandvegetationbecameabundantduringinterglacial periods. Mammalgroups such as ungulatesdispersed,speciated,andbecamelarger(Geist 1978, 1987).
Duringthis period, the first identifiablebear species of
the genus Ursus, U. minimus,appearedin the late Pliocene deposits of Europe. It probablygave rise to the
present day members of the genus as well as several
extinct species.
Coincidentwith this largeincreasein the biomassand
diversity of prey species, there was an explosion in the
diversityof predators.Fromthe latePliocenethroughthe
Pleistocene,thebearsspeciatedandevolved into some of
the largest terrestrialcarivores known (Kurten 1968,
KurtenandAnderson1980). Duringthis period,several
species of bearsevolved, dispersed,anddisappeared.An
importantpoint is that many more species once existed
thando today. Forexample,severalspecies of the genus
Tremarctoslived in southeasternNorth America (e.g.,
the Floridacave bear,T.floridanus)and SouthAmerica.
Today only the spectacledbear (T. ornatus) survives in
the northernAndes. Similarly, the large short-faced
bears(Arctodusspp.)of NorthAmerica,thecave bearsof
Eurasia(e.g., Ursusspelaeus in Europe),variouspopula-
190
BEARS-THEIR
BIOLOGY AND MANAGEMENT
tions of brown bears (U. arctos) in Europe, Asia, and
NorthAfrica,andthe Langebaanwegbear(Agriotherium
africanum)of South Africa (Kurten 1966, 1967, 1968;
Hendey 1977) all thrivedand subsequentlywent extinct.
In generalthough,the bearswere most successful where
they originated,in northtemperateareas.
Each species had to cope with a series of climatic
changesthatin turnaffectedinterspecificcompetitionor
alteredfood availability. A few, such as the American
blackbear(U. americanus),were sufficientlyadaptedto
survive changingecological conditionsand exist today,
largely unmodifiedfrom what they were like a million
yearsago. Thebrownbearsandpolarbears(U. maritimus)
of todayaremorphologicallysimilarto, thoughsmallerin
size, thantheirPleistocene ancestors(Kurten1976).
An importantconsiderationwhen analyzing the behavioralecology of the presentday bearsis the possible
effect of man. Because of incessanthuntingand habitat
change, it is likely that few remainingpopulationsof
bearsbehave quite as their ancestorsdid. For example,
largenumbersof brownbearswereonce a commonsight
in continentalNorth America (Storerand Tevis 1955).
Canwe assumethatthe habitatpreferencesandbehavioralecology of remnantpopulationsin placeslike Yellowstone or southeasternBritish Columbiaare unchanged
from those of their predecessors? Probablynot, but it
may not be possible to evaluate the changes that have
takenplace. Similarly,the habitatoccupied by remnant
populationsof otherspecies may be significantlydifferent from that in which they originally evolved. Obviously, we can only workwith the informationat handbut
we need to remainconscious of possible biases.
Inthispaper,we havefollowed the taxonomyoutlined
by Nowak and Paradiso(1983) in which the giantpanda
(Ailuropodamelanoleuca) is considered an ursid. We
referto all races and populationsof U. arctos as brown
bears and group other bears, except for the spectacled
bear, into the genus Ursus.
Several organizationshave supportedour research
over the years, which in turngave us the experiencethat
made it possible to write this paper. In particular,we
acknowledgethe CanadianWildlife Service, PolarContinental Shelf Project, World Wildlife Fund (Canada),
NaturalSciences and EngineeringResearchCouncil of
Canada,theArcticPetroleumOperatorsAssociation,and
the Departmentof Zoology and the Boreal Institutefor
NorthernStudiesof the Universityof Alberta. We thank
M. Meagher,T. 0ritsland, B. Peyton, M. Rosenthal,D.
Ruben, and D. Weinhartfor permissionto cite unpublishedobservations.M. Ramsayand2 unidentifiedreferees providedconstructivereviews of the manuscript.
THE SIZE OF BEARS
The modem bears are all medium- to large-sized
mammals,rangingfrom 27 kg for the female Malayan
sun bear(U. malayanus)(Nowak and Paradiso1983) to
over 800 kg for an exceptionallylargemale polarbearin
primecondition(DeMasterandStirling1981). The latter
representsa remarkableincreasein body size, relativeto
their small Miocene ancestor,the dawn bear,20 million
years ago. About 15 million years later, when Ursus
minimusfirstappeared,it weighed approximately50 kg,
similarto the maximumweight of the male Malayansun
bear,the smallestof thepresentdayursids. Kurten(1976)
noted thatits canines were thin and sharpbut the molars
had already become enlarged and better adapted for
feeding on vegetation, adaptationsthat together may
have madeit possibleforthebearsto evolve a largebody.
Even when the smallerspecies of bearsare included,
the ursidsaresignificantlyheavierthanall otherfamilies
of carnivores (Gittleman 1985). Clutton-Brock and
Harvey(1983) summarized4 majoradvantagesof large
body size: the abilityto producelarge neonatesor litters,
reductionof relative heat loss, the ability to catch and
handlelargerpreyor travelgreaterdistancesin searchof
food, and the ability to survive on qualitativelypoorer
food. Except for the productionof altricialoffspringin
small litters,an anomalyimposedby physiologicalconstraints(Ramsayand Dunbrach1986), all these advantages wereprobablyimportantto bearsin theevolutionof
large body size.
Reductionof the ratioof surfaceareato volume with
increasedbody size would have aided thermoregulation
in an increasinglyseasonalclimateat higherlatitudes.In
this respect,the increasedbody size of bearswas consistentwithtrendsin otherIce Age mammals.Inananalysis
of the evolutionof Ice Age mammals,Geist (1987) noted
that speciation of cervids and capridsfollowed predictions made by his dispersaltheory. In particularspecies
thatcolonizedhigherlatitudeswithincreasinglyseasonal
climates where productivitypulses became larger, became more ecologically plastic, and developed more
ornatesocial organs(e.g., hornsandantlers). Incontrast,
dispersalinto lower latitudeenvironmentswith less seasonalvariabilityled to paedomorphism(i.e., theretention
of primitiveness)and ecological specialization. As was
the case for the ungulategenerastudiedby Geist (1987),
the biggestbearswerealso northtemperatespecies of the
late Pleistocene,such as the cave bearof Europeandthe
short-facedbearsof NorthAmerica(Kurten1976). The
largestmoder ursidsarealso northtemperatespecies. In
comparison,the bearsin sub-tropicalclimates(sun bear,
slothbear[U. ursinus],andspectacledbear),andthegiant
OF BEARS * Stirling and Derocher
EVOLUTION
panda of southeasternChina, were removed from seasonal pulses in productivity. All are smaller than their
northernrelatives,less sexuallydimorphicandmosthave
become ecologically specialized (Laurie and Seidensticker 1977, Schalleret al. 1985, Lekaguland McNeely
1977).
It may be thatbearswere capableof broadecological
plasticitybecausethe morphologicaladaptationsof their
teeth allowed them to evolve into a niche combining
herbivoryand predation(Kurten1976). Their grinding
post-canineteethenabledthemto processlarge amounts
of vegetationin the seasonalpulses of high productivity.
However,the digestive tractof extantursidsis not highly
modified from that of other carnivorous mammals
(Bunnell and Hamilton 1983), which may have influenced body size. In a study of digestibilityof different
diets by 2 captive brown bears, Bunnell and Hamilton
(1983) demonstratedthatdigestiveefficiency declinedas
theproportionof vegetationincreased.Inparticular,they
noted that cellulose was poorly digested, especially in
comparisonto meat. Similarly, Schaller et al. (1985)
found that dry matterdigestibilityof bamboo leaves by
pandas varied seasonally between only 12 and 23%.
Pandas are able to obtain carbohydratesby digesting
hemicellulosefromthe cell wall of the plantsbut,as in all
bears,they cannotdigest cellulose. In comparison,Sinclair (1975) found that ungulatesliving on green grass
assimilateabout 80%of theirdiet, including40-60% of
thecelluloseandhemicellulose(VanSoest 1982). Guthrie
(1984) noted that non-ruminantscannot usually extract
all the necessaryamino acids, fatty acids, vitamins,and
other vital dietary constituentsfrom 1 or 2 species of
plants. Consequently,he speculatedthat as non-ruminantsmovedtowardincreasingproportionsof vegetation
in the diet, they probablyhadto move over largerareasto
find a varietyof plant species to provideall the requirements. The same would apply if therewas only a small
numberof digestible species but they were widely distributedat low density,in discontinuouspatches,or both.
Although Guthrie (1984) was speculating about the
evolution of groundsloths, the same could have applied
to theancestralursids.As thenon-ruminantbearsevolved
into a progressivelymoreherbivorousniche, they would
likely have needed larger home ranges to ensure an
adequatevegetativefood base andallow for seasonaland
annualvariationin its distributionand productivity.
A vegetative diet alone is not a sufficient stimulusto
producelarge size, as is illustratedby severalsuccessful
taxaof small mammals. Small animalsaremorevulnerable to predationthan are large ones so they need to
remain adjacentto escape habitatsuch as holes in the
191
ground, trees, or thick vegetation. To survive in open
habitat,a mammal must be able to defend itself from
predatorsor be fast enough to escape them. Consequently,as the body size of some bearspecies increased,
they probably occupied more open habitat for longer
periods, and were able to increase the proportionof
vegetationin the diet. Theirlargersize also made them
betterable to defend themselvesfrom predators.As the
diet became progressivelymore vegetative, therewould
havebeencontinuingpressureforbody size to increaseso
enough vegetation could be ingested and processed to
substitute for a high quality diet of animal material.
Largerbody size also made it possible to travelmore in
search of patchy food resourcesand to store and carry
morefat withwhichto surviveduringperiodsof seasonal
orunpredictablefood shortage.Withincreasedbody size
andwell-developedcanines,some bearswere able to kill
ungulates and other mammals, defend carrion from
competitors,and protect themselves from other predators. Throughthisuniquecombinationof beingableto be
predators,scavengers,andherbivores,they were able to
exploit severalfood bases.
Smallpredatorsarerestrictedto smallprey,so thatone
benefitof being largeis thatan animalcan kill bothsmall
and large prey (Gittleman1985). For example, brown
bearsarecapableof takingadvantageof relativelysmall
animalssuch as groundsquirrelsand salmon in circumstances where their abundance makes such behavior
energeticallyor nutritionallyworthwhile(e.g., Stonorov
and Stokes 1972, Murie 1981). Even so, the ratio between the size of the bearandits preymay be misleading
since the predator'slarge size may be necessaryto move
heavy stones or earthto catch groundsquirrelsor to stay
warmwhile standingin cold waterfor protractedperiods
while fishingfor salmon. In thecase of the morecarnivorous bear species, their maximum size may have been
influencedby the maximumsize of generally available
prey, as will be discussed belpw.
SEXUAL DIMORPHISM
Sexualdimorphismis stronglyassociatedwithpolygynous breedingand is classically thoughtto result from
sexual selection. Trivers(1972) proposedthatvariation
in the degree of parentalinvestment was the principal
factorthatdeterminedif an animalbredmonogamously
orpolygynously. However,in a subsequentreview,Ralls
(1977) notedthatmuchof the theoryaboutthe evolution
of sexual dimorphismwas based on avian models and
was not as applicable to mammals. In particular,she
found that althougha high degree of investmentin the
192
BEARS
THEIR
AND MANAGEMENT
BIOLOGY
offspringby bothparentswas a good predictorof monogamy, the converse was less reliable. In addition, the
degree of parental investment does not explain why
sexual dimorphismhas evolved more frequentlyin large
mammalsthan in small ones.
Sexual dimorphismin body size is presentin all the
modem ursids except possibly the sloth bear, for which
therearetoo few dataavailableto be certain(Table 1). As
faras we have been able to determine,sexualdimorphism
characterizedmost of the extinct Pleistocene bears as
well.
In general, sexual dimorphismincreases as species
become larger (Clutton-Brocket al. 1977). Although
quantitativeweight dataarenot availablefor most bears,
sexual dimorphismtends to increase with size in the 3
species of North Americanbears (Fig. 1). However, in
the 3 NorthAmerica species, the developmentof sexual
dimorphismis only significantlydifferentbetweenblack
and polar bears (Tukey's test P < 0.05). It is also
particularlyinterestingto note that a similar patternof
increasing sexual dimorphismin relation to increasing
body size is present in North American brown bear
populations(Fig. 1).
The bears did not develop anything resembling the
antlersof the Ice Age ungulates(Geist 1987). However,
we suggest the significantdevelopmentof sexual dimorTable 1. Weights and approximate degree of sexual dimorphism in bears.a
Mean weight (kg)
Species
Sloth bear
Giantpanda
Malayansun bear
Males
Females
55-145
97-107
Ratio (male:female)
>1.0
86-89
27-65
Giant short-facebear 350-375
250-270
140-175
Spectacledbear
Americanblack bear 82-165
110-120
58-95
93-207
1.1 to 1.2
approximately1.2
approximately1.4
approximately1.4
1.5 (range 1.1-1.7)
1.6 (range 1.2-2.2)
Brown bear
145-389
Asiatic black bear
110-150
65-90
Polarbear
271-322
147-197
1.8 (range 1.6-2.0)
Europeancave bear
Floridacave bear
410-440
205-220
230-250
115-125
approximately2.0
approximately2.0
1.7
a Data for Americanblack bear from Bunnell and Tait (1981); polar
bearweightsbasedon a randomsampleof 100 adultmales and 100 adult
females from Svalbard and 4 different populations in the Canadian
Arctic (L0n0 1970; Canadian Wildlife Service, unpublisheddata);
brownbears from InteragencyGrizzly Bear Committee(1987); spectacledbearfromM. RosenthalandD. Weinhart(unpublisheddata)and
Nowak and Paradiso (1983); other extant species from Nowak and
Paradiso(1983); and,forextinctspecies fromKurten(1967) andKurten
(1976), based on estimates of body size.
2.5 -
0
0.
0
2-
*
O
~**"OH
*
.
o
1.5-
o
0
O
0
_0
0
0
0
*
I
o
-t-j
0.5-
Legend
x
nl
0
*
BLACKBEARS
0
BROWNBEARS
*
POLARBEARS
I
50
100 150 200 250 300
WEIGHTOF MALES(kg)
350
400
Fig. 1. Sexual dimorphism of black, brown, and polar bears as a function of body
size. Black bear weights from Bunnell and Tait (1981); brown bear weights from
Interagency Grizzly Bear Committee (1987); polar bears weights from Lone
(1970) and Canadian Wildlife Service unpublished data. Line indicates regression line for North American brown bear populations (slope of regression is significantly greater than 1 (P< 0.05)).
phismin the size of thebody andthecanineteethof males,
especially in the northtemperatefossil and extant species, functionfor display, threat,and fighting. Kadosaki
et al. (1986, 1989) comparedthe skull morphology of
brown bears and Asiatic black bears (U. thibetanus)in
Japan.Theyfoundthat,althoughsexualdimorphismwas
detectablein all measuredparts,the difference was significant only in the canine teeth. In both species, they
found the degree of sexual dimorphismwent fromgreatest to least in the following characters:canines, teeth
except canines,lengthof toothrows, andskull size. Since
the diet of males and females of each species is similar,
the significantenlargementof the canine teeth in males,
beyond that which would be expected because of their
largersize, probablyfunction for intrasexualthreatdisplay andweapons. Forexample, in male polarbears,the
canine teeth are importantin intraspecificagonistic behavior. The canines of males are often badly brokenin
intraspecificfighting; a featurethat is absent in females
(RamsayandStirling 1986). Similar,thoughmoreexaggerated,developmentof canineteethcharacterizedother
Pleistocene predators, such as the saber-toothedcats
(Smilodonspp.) (KurtenandAnderson 1980), and likely
also functionedpartlyfor social display.
Intenseintraspecificconflicts between adultmales of
several species of bears have been reported(e.g., sun
bears,LaurieandSeidensticker1977;polarbears,Ramsay
and Stirling 1986) and, although conclusive evidence
relative to mating success is absent, it seems likely that
OF BEARS * Stirling and Derocher
EVOLUTION
large body size confers reproductiveadvantage when
competing with conspecifics. Bunnell and Tait (1981)
also suggestedthatthe largersize of male ursids,relative
to females, may have evolved in partto favor the establishmentof largerhomerangesanda subsequentincrease
in potential mates (see Table 2). Ramsay and Stirling
(1986) speculated that intense intrasexualcompetition
between male polar bears for females, caused by the
somewhat unpredictabledistributionof females on the
moving sea ice and the consequentinabilityof males to
defendareasthatwouldreliablyincludeoestrusfemales,
resulted in greaterintrasexualcompetition for females
thanis found in terrestrialbears. Further,they suggested
that the markedsexual dimorphismin polar bears is a
consequence of the intense intrasexualcompetitionbetween males for breeding opportunitieswith females.
However, their hypothesis does not explain the apparently similardegree of sexual dimorphismexhibitedby
some populationsof brownbears(Fig. 1), or the extinct
terrestrialEuropeanandFloridacave bears(Kurten1966,
1967). An alternatehypothesisis thatthe largeterrestrial
Pleistocenebears,the large-bodiedpopulationsof brown
bears,and the polarbear simply reflect the trendtoward
greaterdimorphismwith increasedbody size (CluttonBrock et al. 1977).
Oneotherecological factormayhavehada significant
influence on the developmentof sexual dimorphismin
the bears. Clutton-Brockand Harvey (1978) reported
thatwithinthe primates,sexual dimorphismis greaterin
terrestrialthan in arborealspecies. They suggest this is
because increasedweight does not influenceintersexual
competition for food in ground dwelling species. In
contrast,if males of arborealspecies were larger than
Table2. Home range sizes of adult bears.
Species
Meanhomerange(km2)(S.D.)
Males
Females
Giantpandaa
8.5 (0.9)
Asiatic black bearb
12.6 (9.1)
4.6 (0.6)
Americanblack bear'
81 (55)
27 (20)
Brown beard
631 (442)
242 (186)
Polarbearf
-
96,924 (78,453)
a Schalleret al. (1985); Johnsonet al. (1988)
b Hazumiand
Maruyama(1986)
c Alt et al.
(1980), AmstrupandBeecham(1976), GarshelisandPelton
(1981), Hellgren and Vaughan(1987), LeCount(1980), Lindzey and
Meslow (1977), Manville (1983), Pelchatand Ruff (1986), Reynolds
and Beecham (1980), Rogers (1987a)
d
InteragencyGrizzly Bear Committee(1987)
e Amstrup1986
193
females it might put them at a competitivedisadvantage
whenfeeding on thinstemsor the ends of branches.This
may also be a factor influencing sexual dimorphismin
bears. Eventhoughtherearefew reliabledataon weights
of adultmale andfemale sunbears,sexualdimorphismis
reducedand both are apparentlyquite arboreal(Laurie
and Seidensticker1977). Male and female sloth bears
and giantpandasareof similarsize andboth arecapable
climbers, though they mainly feed on the ground. Finally, the least sexuallydimorphicof the 3 NorthAmericanursids(Fig. 1,Table 1), theblackbear,also makesthe
greatest use of trees. The spectacled bear and Asiatic
blackbearexhibita similardegreeof sexualdimorphism
to the Americanblack bear (Table 1), feed arboreallyin
some areas, make nests in trees to aid feeding, and
sometimesrestin the nestsas well (Schaller1968,Peyton
1980). The spectacledbearis muchless sexually dimorphic thanwas its extinctnearestrelative(Table 1), which
may have been influencedby feeding arboreally.
BEARS AS PREDATORS
The degree of active predationby the 3 bear species
found in North America varies from the carnivorous
polar bear to the less predatorybrown and black bears.
Because of theirextensive use of vegetation,brownand
blackbearshave not generallybeen thoughtto be significantpredators.However,severalmorerecentandquantitative studies have forced this premise to be revised
(e.g., Cole 1972, Franzmannet al. 1980, Stewartet al.
1985,ReynoldsandGarer 1987, Boertjeet al. 1988, and
otherpapersin this volume).
Throughoutthe carnivores,there is a strongcorrelation betweenthe size of a predatorandthe size of its prey
(Rosenzweig 1966, Gittleman1985, Vezina 1985, Earle
1987)that,atfirstglance,mightnot appearto applyto the
bears. For example, the polar bear seems 'oversized'
when comparedto its majorprey,the ringedseal (Phoca
hispida)(Fig. 2). Similarly,brownbearsin severalareas
make extensive use of small prey. However, when the
weight of each species of bearis plottedin relationto the
weight of the largest prey it is now known to take
regularly,the relationshipfor solitarycarnivoresderived
by Earle (1987) fits fairly well (Fig. 2). The largest
regular,thoughless frequentlycaptured,prey species of
the polarbearis the beardedseal (Erignathusharbatus)
(StirlingandArchibald1977,Smith 1980),whichweighs
up to about360 kg (Kelly 1988). In addition,they occasionallykill whitewhales(Delphinapterusleucas)weighing upto about600 kg (Lowryet al. 1987,SmithandSjare
1990), and walruses (Odobenusrosmarus)up to about
500 kg (Kiliaanand Stirling 1978).
BEARS
194
THEIR BIOLOGY AND MANAGEMENT
1000
A
CY) 100
Legend
*
t/)
WHITE
WHALE
X WALRUS
+
>n
c.
+
v v
SEAL
O BEARDED
+
V
RINGED
SEAL
O MUSKOX
A CARIBOU
10
*
ELK
*
MOOSE
V MOOSECALVES
O DEERFAWN
.
I
O
.........
X GRNDSQUIRREL
l
........PEDI
I.S.
100
10PREDATOR
MASS (kg)
PREDATOR
I I I
K
. l X l l l.
o000
Fig. 2. Size of known prey of black, brown, and polar bears plotted in relation to
the regression line of body weight of solitary carnivores (Earle 1987) and the
weight of their largest regular prey. References by prey species as follows:
white whale, Smith (1980), Lowry et al. (1987), Smith and Sjare (1990); walrus,
Kiliaan and Stirling (1978); bearded seal, Stirling and Archibald (1977), Smith
(1980), Kelly (1988); ringed seal, Stirling and Archibald (1977); muskox (D.
Ruben, pers. commun.); caribou, Reynolds and Garner (1987), Boertje et al.
(1988); elk, Cole (1972); moose, Mysterud (1973), Franzmann et al. (1980),
Stewart et al. (1985), Boertje et al. (1988); deer fawn, Ozaga and Verme (1982),
Mathews and Porter (1988); ground squirrel, Murie (1981).
Boertje et al. (1988) reportedthat adult male brown
bearskilled an average of 3.3 to 3.9 adultmoose (Alces
alces) (400-500 kg) per year whereaslone adultfemales
killed significantlyfewer (0.6-0.8 moose/year). Female
brownbearswith cubs killed no adultmoose. All classes
of bears killed moose calves but females without cubs
killed at a higherratethandid the other2 groups. In the
area studied, brown bears were primary predatorsof
moose andkilled 4 times more animalbiomass thanthey
scavenged. Mysterud(1973) also reported2 adultmoose
killed by brownbearsin Norway but the sex of the bears
was not known.
Loneadultfemalebrownbearskilledanaverageof 0.9
to 1.0 caribou(Rangifertarandus)peryearin east central
Alaska(Boertjeet al. 1988). ReynoldsandGamer(1987)
also documentedkilling of caribouby brownbearson the
Alaskan North Slope. Most of the animals killed were
calves but some adultswere takenas well. Adultfemales
were involved in 64.7% of the chases but were only
observedfeeding on 37.3%of the carcasses,comparedto
58.5%for males. However, it is likely thatseveralof the
carcassesthatadultmales were observedfeeding on were
killed by other smaller bears (Reynolds and Gamer
1987). D. Ruben (pers. commun.) of Paulatuk,Northwest Territories,also observedan adultmale brownbear
kill an adult female muskox (Ovihos moschatus) (approximately250 kg) nearFallaise Lake, NorthwestTerritories,in 1984.
On the basis of circumstantialevidence and an observed attackby 2 subadultbrownbearsthatdrew blood
from a lone adult bison (Bison bison), brown bears are
suspected to occasionally kill young bison, although
none has been confirmed (M. Meagher 1973 and pers.
commun.). Observationshave also beenmadeof individual brownbearstesting lone bison by approachingthem
andthenleaving when the bison ignoredthem,refusedto
run, and kept on feeding (M. Meagherpers. commun.).
Healthyadultmale bisonmaybe too largeandwell armed
for a bearto attackwithoutriskof seriousinjuryto itself.
Testing likely helps a bear to identify a sick or weak
animal. Bison also have a clumped distribution,especially duringcritical periods such as calving (Calef and
Van Camp 1987), which may also make predationby
bears difficult. In a review of historical material,Roe
(1970) found evidence of brown bears scavenging on
dead bison but it appearsthatpredationwas uncommon.
Black bearsalso demonstratepredatorybehaviorand
have been found to be significant predatorsof moose
calves (Franzmannet al. 1980). The smallerbody size of
blackbearsprobablyprecludesthemfrombeing a significant predatoron adult moose. Recently, it has also
become apparentthatblackbearscanbe importantpredators of neonate white-tailed deer (Odocoileus virginianus) (Ozaga and Verme 1982, Mathews and Porter
1988). The maximum size of prey taken regularlyby
black bearsis not known but, from Figure2, it shouldbe
about 151 kg for a 100 kg bear. The vulnerabilityof prey
of differentsizes to predationby black and brown bears
probablyinfluences niche separationbetween them.
Peyton (1980) reportedseeing the hide of a woolly
tapir (Tapiruspinchaque) with blunt scars from being
attackedby a spectacledbear. Villagers had found and
killed the wounded animal. Although Tschudi (1844,
citedinPeyton 1980)reportedthatspectacledbearspreyed
on deer, guanaco (Lama guanicoe), and vicufia (Vicugna
vicugna), there are,no supportingquantitativedata.
Recent studies of the energetics of polar bears also
suggestthatbody size mayhave a significantinfluenceon
how bearshunt. Hurstet al. (1982) foundthatthe energy
requiredfor the plantigradepolar bear to move is about
double that for most other mammals. To move at the
modestspeed of 7 km/hr,uses 13 times moreenergythan
lying. This probablyexplainsthe polarbear'spreference
for lying and still-huntingfor seals (Stirling 1974, Stirling and Latour1978). Lunnand Stirling (1985) calculated that unless a 320 kg adult polar bear captureda
flightless snow goose in late summer in 12 seconds or
less, it would cost the animal more energy that it would
gain if it was successful. The lower ratioof surfacearea
OF BEARS * Stirling and Derocher
EVOLUTION
to volume,andheavierdepositsof fat on adultbears(particularlymales andpregnantfemales) meanthey can run
muchshorterdistancesthansubadultsbecausethey overheatmorequicklyandareunableto dissipateheatquickly
enoughto be able to continuehigh levels of exertion. Althoughthereare no dataavailableat presentto compare
the cost of locomotionof brownandblackbears,we suggest thatit wouldalso be easierfor smallerandleanersubadultsto runlongerdistancesthanadults.If so, thiswould
probablymean that largeranimals would make most of
theirkills aftera shortburstof runningfromambush,similarto the lion (Pantheraleo). Coincidentally,the lion is
the only other mammalknown in which the metabolic
cost of movementis aboutdoublethatof mostothermammals (Chassin et al. 1976). Not surprisingly,Schaller
(1972) foundthatfemalelions caughtmedium-sizedprey
twice as often as males. He suggested that the females
were more successful on the faster medium-sizedprey
because they were faster than the more cumbersome
males.
The relationshipsbetweenbrownandblackbearsand
theirpreymay not be as simple as the regressionin Earle
(1987) or Figure2. We suggest thatbody size and energetics aresignificantfactorsin determininghow bearsof
differentbody sizes huntungulates.Learningis probably
also important.We predictthatmost kills madeby adult
males will be after a short rush from ambush or after
stalkingto close range and thatmost kills thatrequirea
longerchase will be madeby adultfemales withcubs and
by subadults. Thereis some supportfor this hypothesis.
Cole (1972) noted that subadultor subordinateyoung
bearsappearedto be the most effective predatorson elk
(Cervuscanadensis). He also reportedseparateincidents
of elk beingkilledby 2 subadultbearshuntingtogetherin
chases that lasted 15 and 20 minutes each. On the
AlaskanNorthSlope, only 1 of 17 brownbearsof known
age and sex observedchasingcaribouwas an adultmale
(Reynolds and Garer 1987). The remaining16 chases
were made by adultfemales and subadults.
For the omnivorous bears, sexual dimorphismdoes
notappearto conferbenefitsin the areaof resourcesegregation (Bunnell and Tait 1981). However, in the North
Americanbears, sexual dimorphismappearsto become
more pronounced with increasing predatorybehavior
(Fig. 1). Rails (1976) suggestedthatsexual dimorphism
could be relatedto niche exploitationas well as to a polygynous breedingsocial system. Sexual dimorphismin
birdsof prey is thoughtto functionpartiallyto separate
niches and reduce intersexual competition (Selander
1972). We suggest partialniche separationof males and
females could be importantin the predatorybears and
195
may also have influencedthe developmentof sexual dimorphism. As alreadynoted above, adult male brown
bears kill far more adult moose than do lone females,
whereasthereverseis trueforthekillingof moose calves.
In the case of the sexually dimorphicand polygynous
polar bear (Ramsayand Stirling 1986), the adult males
appearto prey most frequentlyon beardedseals and occasionallykill whitewhales. Fromdatacollectedthroughout the CanadianArctic,93%(14/15) of the beardedseal
kills thatwerefoundwhile still in thepossessionof a polar
bearwere being consumedby adultmales. Femalespredominantlykill the smaller(60 kg) ringedseal. Thereis
some segregationof differentage andsex classes of polar
bearsby habitattype (RamsayandStirling 1986). Much
of this appearsto resultfromfemales withcubs minimizing the riskof infanticideby avoidingadultmales. Consideringthe large degree of sexual dimorphismin polar
bears, segregationmight also be influenced by differences in the vulnerabilityof different sized species of
prey.
Surpluskillinghasbeendocumentedin wolves (Canis
lupus) (Miller et al. 1985) and other carnivores(Kruuk
1972). Similarly, surpluskilling of domestic sheep by
brown bears has been reportedin Norway (Mysterud
1975) and of harpseal pups (Phoca groenlandicus)off
the coast of Newfoundlandby polarbears(T. 0ritsland,
pers.commun.). Lowryet al. (1987) reportedpolarbears
killing several white whales at 1 location and leaving
some uneaten.In the CanadianArctic,we once observed
a polarbearkill a ringedseal, feed extensively on it and
then kill 2 more within the next hour but leave them
uneaten.It is notclearwhy animalsdo this. Althoughthe
benefitto otherbears,particularlysubadults,of scavenging on theremainsis probablyconsiderable,thisseems an
unlikely explanation.
Anotherdifferencebetweenbearspecies relatesto the
lengthof time spentprocessinga kill. Polarbearsusually
feed once on a seal kill and then abandonthe remains,
which sometimesare substantial(Stirling 1974, Stirling
andArchibald1977). The only exceptionsto this pattern
of carcassuse area few adultmales thatwe have seen kill
an adult bearded seal and apparentlyremain with it,
possibly for several days. In contrast,brown bears are
well knownforremainingfor severaldays withananimal
they havekilled, or 1 they arescavenging(e.g., Boertjeet
al. 1988). This differencein how long polarand brown
bearsremainwith theirkills may also relateto the rateat
whichpreycan be capturedin each ecosystem, preysize,
and gross energy value in relation to alternate food
sources,andthenumberof potentialinter-andintraspecific scavengersthatmight attemptto contest ownership.
196
BEARS-THEIR
BIOLOGY AND MANAGEMENT
INTER-AND INTRASPECIFICCOMPETITION
Interspecificand intraspecificcompetition between
bears,includingpredation(e.g., Miller 1985, Ross et al.
1988),has been significantin theirevolution. Cubsof all
the predatoryspecies of bears are sometimes killed by
adult male conspecifics (e.g., Jonkel and Cowan 1971,
Tayloret al. 1985, Dean et al. 1986, LeCount 1987) as
well as by othermammalianpredators(e.g., Rogers and
Mech 1981, Ramsayand Stirling 1985, Paquetand Carbyn 1986). Intraspecificpredationhas not been reported
for any of the subtropicalspecies of bears, althoughit
may occur.
In a comparisonof black andbrownbearsin different
biogeoclimaticzones in NorthAmerica,Herrero(1978)
found thatbrownbearsare consistently 1.5 to 2.0 times
the size of black bears. He furtherconcludedthatblack
bears had retained the forest dwelling niche of their
ancestorswhile the brown bear moved to exploit more
open habitattypes as well as forests. When faced with a
serious threat,the strategyof a female black bear is to
send hercubs up a tree safe fromdanger,flee, andreturn
for her cubs when the threathas passed. In contrastto
black bears,the largerbrownbearsprotecttheircubs by
standingtheir groundor attackingthe perceived threat
directly(Herrero1978). Althoughthecubsarecapableof
climbing trees, they apparentlydo so much less frequentlyinresponseto dangerthando blackbears.Female
polarbears,which live in open habitat,also defendtheir
cubs by directattack,even if threatenedby an adultmale
twice her size or a helicopter hovering over her head.
Along the southwesterncoast of Hudson Bay, where
polar bear family groups spend significant amountsof
time on land duringthe late summerand fall, we have
neverseen cubs attemptto climb trees,even thoughlarge
trees arepresentin some areas. In lieu of trees as escape
habitatin the High Arctic,female polarbearswith cubs,
andlone subadultsoftenclimb 100m ormoreupa hillside
above the sea ice anddig a pit to lie in wherethey can see
aroundthembefore sleeping. This precautionappearsto
be in responseto the threatof intraspecificpredation.
A plausiblehypothesisto explain why the black bear
did not move out of the forestmay be thatit was not large
enough to protect its young, or possibly itself, on the
groundfrom largercarnivores(Herrero1978). During
the Pleistocene, therewas an abundanceof large terrestrialpredatorsin the opencountry,includingthecursorial
giant short-facedbear(Arctodussimus),one of the most
powerful terrestrial mammalian predators to evolve
anywhere (Kurten 1967, Kurten and Anderson 1980).
Withthe passingof the giantshort-facedbearduringthe
Pleistocene extinctions, its open country habitat was
occupied by the brown bear. Brown bears crossed the
Bering land bridge in the late Pleistocene and invaded
western North America via Alaska, and presumably
continuedto exclude the black bearfrom the plains.
No brown bears occurredin eastern North America
duringthe Pleistocene. In this situation,the now extinct
black bear(Ursus americanusamplidans)often became
as large as brownbears and theirremainsare still occasionally mis-identified as such (Kurtenand Anderson
1980). Presentday blackbearsin easternNorthAmerica
are still largerthantheirwesterncounterpartsthat share
habitatwith brownbears. Kurten(1963) has speculated
thattheextinctionof theFloridacave bearandtheeastern
short-facedbear (A. pristinus) may have been brought
aboutby competitionfromthe largeeasternraceof black
bears and invadingbrownbearsfrom the north. In this
context, althoughblack and brown bears first appeared
together in the fossil record in Europe about the late
Miocene or early Pliocene, only the brown bear exists
there today, possibly because it displaced its smaller
competitor(Kurten1976).
Another open country habitat occupied by brown
bearsin NorthAmericais thetundra,northof thetreeline.
In parts of northernCanada, Haringtonet al. (1962)
suggestedthatbrownbearpopulationshave diminished
as a result of overhunting. Coincidentally,Jonkel and
Miller(1970) noteda possible increasein the numbersof
black bears recordedin various tundraareas and suggested this might be because of reducedcompetitionor
threatfrom brownbears.
The otherforestdwellingbearsareable to climb trees
althoughthe extent to which they do so varies. Spectacledbearsandsunbearsareexcellentclimbersandfeed
arboreally.Sun bearslack hairon the soles of theirfeet,
which has been suggested to be an adaptationto aid
climbing (Bunnell 1987). Although pandas feed predominantlyon the ground, unattendedyoung are still
vulnerableto predation(Schalleret al. 1985). They use
trees for escape habitatunder some circumstances,including when harassedby courtingmales, but the presence of leopards(Pantherapardus), which areknownto
kill pandasoccasionally,may reducethe overallvalue of
thatescape strategy.
The subtropicalslothbearis also anexpertclimberbut
thecubsremainon thegroundwhenthefemaleis in a tree.
If threatened,the female leaves the treeimmediatelyand
flees on the ground,hercubs sometimesdepartingahead
of her (Laurieand Seidensticker1977). They attribute
this behaviorto the presenceof the leopard,a significant
arboreal predator,which sometimes kills sloth bears
(Kurtand Jayasuriya1968). Consequently,climbing a
EVOLUTION
OF BEARS * Stirling and Derocher
tree is ineffective for escape and might make the bear
more vulnerableto predation. They also note thatlarge
ungulatesaremorelikely to standandfight predators,or
attackthem,thanrunaway. They suggestthepresenceof
elephants (Elaphus maximus)and rhinoceros(Rhinoceros unicornis), which are capable of pulling down or
pushing over even medium-sized trees, furtherreduce
theirvalue as escape habitatfor sloth bears. Despite its
small size, the sloth bear is known to attackhumans if
surprisedand is regardedas dangerousby local people
(Laurieand Seidensticker1977). Like the much larger
brownbears,aggressivebehaviormay be a consequence
of not being able to rely on trees for escape.
Interspecificcompetitionfrom mammalsother than
bears may have played a significant role in preventing
bears from invading some areas. Alternatively,an absence of competitors for a particularniche may have
facilitated evolution and dispersal. For example, the
sloth bearapparentlywas able to enterthe myrmecophagic niche in southernAsia because areas of suitable
habitatwere unoccupiedby the 3 otherallopatricmammalianmyrmecophagespresent(LaurieandSeidensticker
1977). In South America,thereare 5 sympatricspecies
of myrmecophages,which appearto occupy all available
habitat,and there are no bears that eat ants or termites
(LaurieandSeidensticker1977). However,in temperate
North America and northernAsia, where there are no
mammalianmyrmecophages,ants may form a significantpartof the diet of blackandbrownbears(e.g., Hatler
1972, Vereschagin 1976).
Pruittand Burghardt(1977) note that the mountain
gorilla (Gorilla gorilla beringei) appears to occupy a
similar niche in central Africa to that occupied by the
blackbearin NorthAmerica. Similarly,therearenumerous species of primatesin northernSouth America and
other tropicalregions. What role, if any, primatesmay
have had on distributionpatternsof the moder bearsis
unknown.
Althoughthereareno bearsin southernAfricatoday,
bears of the genus Agriotheriumhave been found in
Pliocene deposits dating back between 4 and 5 million
years (Hendey 1977). The Langebaanwegbear is the
largestterrestrialpredatorrecordedfromsouthernAfrica
and,fromits largejaws andthe degreeof developmentof
its incisors and canines, appearsto have evolved into a
largelycarnivorousniche. Itsmuzzlewas shortandbroad
and the palate was even shorterthan that of the shortfaced bearsof NorthAmerica. The sagittalcrestwas high
and the zygomaticarchwas wide, which arealso indicative of strong musculatureindicative of a carnivore.
ApparentlyAgriotheriumwas a short-lived, but wide-
197
spread, genus that first evolved in Europe during the
Miocene but duringthe Pliocene spreadto NorthAmerica, Asia, and Africa (Hendey 1977). They were apparently extinct by the end of the Pliocene. It is possible,
though unknown,that in the northernhemispherethey
were out competedby otherspecies of bears. However,
the present day lack of ursid successors in central or
southernAfrica is puzzling, considering the large and
variablefood base, unless the rapid evolution of other
carnivoresand primates(includinghumans)into available niches excluded them after the passing of the
Langebaanwegbear.
It is likely thatinterspecific,and possibly intraspecific, predationinfluencedthe behaviorof temperatebears
duringwinterdenning. Bearsdifferfromotherhibernating mammalsin thattheirbody temperaturedropsonly a
few degreesandthey arecapableof rapidarousal(Folket
al. 1972). This may be necessarybecause females give
birthto altricialyoung and probablyhave to give them
continuousattentionto ensuretheir survival. However,
fromthe numberof reportsin the literatureon the killing
of bearsin theirdens by otherpredators,includingursids
(e.g., PaquetandCarbyn1986,Ross et al. 1988), it is clear
thatbearsin dens mustalso be able to arousethemselves
quickly,if necessary,to protectthemselvesandtheircubs
from being killed.
INDIVIDUALVARIATION
In general, biologists who have worked with bears
have been impressedwith how variablethe behaviorof
individualsappearsto be. Similarly,most of us have the
distinct,if unquantifiedimpressionthatbearsremember
a greatdeal aboutparticularareasand how to do things.
The success that circuses have had with trainingbears
also suggests they are good at learningnew tasks. Yet,
withthe exceptionof a very few quantitativestudiessuch
as Bacon and Burghardt(1976a, b), there is little more
than an anecdotalappreciationin the literatureof their
ability to learn or remember things. As long-lived
mammalsthat spend most of their lives within a home
range and show strong seasonal fidelity to particular
locations, bears probably learn much about the area,
includingwhereandhow to findfood orothernecessities
undera varietyof circumstances. The variabilityin the
ways bears from the same populationbehave within a
particularareamay be influencedby bothgenetic factors
and learning. If there is a lot of variabilitywithin the
habitat(e.g., in food types and availability,cover, and
topography),over time therewill be differencesin some
of the experiencesindividualbears have while feeding.
198
BEARS-THEIR
BIOLOGY AND MANAGEMENT
Thus,throughlearning,some bearsmaydevelopindividual differencesin food preferences,vary in the degreeto
which they prey on live animals, or respond to disturbances. Individualswill developbehavioralpatternsthat
are molded by their own experiences. Similarly, some
behaviorswill be learnedby cubs while accompanying
their mothers during the long period before weaning.
However, there has been no attemptin the literatureto
review or quantifythe aspects of learningnoted above.
An unexploredbut possibly fertile areafor investigation is thatof individualvariationin nicheexploitationby
different individualswithin a populationas a result of
variations in their morphology. In a recent review,
Lomnicki(1988) arguedfor the adaptivesignificanceof
individual variation within populations. Similarly,
Wallace (1982) suggestedthatthe higherdegree of phenotypic variationthat sometimes occurs among the offspring from the same parents, compared to those of
unrelatedanimals,could be adaptivebecause,in the case
of a shortageof resources,the survivalof at least some of
the progeny may be possible. Kurten and Anderson
(1980) noted thatErdbrink(1953) listed 232 recent and
39 fossil species and subspeciesof brownbearsin what
they describeas "... a waste of taxonomiceffort which ...
is unparalleled". Ignoring the unrealistic amount of
taxonomicsplittingthat was done, the fact remainsthat
thereis substantialmorphologicalvariationin the skulls
of these mammals.This was broughthome to us recently
when we examineda small sample of skulls from male
brown bears collected from the southern Caucasus
Mountainsin the U.S.S.R. In broad terms, the shapes
variedfromwhatwe thinkof as normalfor a brownbear
to one with a shortenedface and high foreheadreminiscent of a pandato anotherthatwas elongatedratherlike
a wolf skull. A similaramountof variationwas reported
andillustratedfor brownbearsin Hokkaidoby Kadosaki
et al. (1989:27, Fig. 2). The consequences of such
variationin phenotypicexpressionon thedevelopmentof
different feeding strategies by individual bears is unknown. Possibly the variabilityin skull morphologyis
greaterin a species like thebrownbearbecauseits habitat
and the food resources are highly variable. Does this
allow differentindividualswithina populationto specialize in particularsubniches? The retentionof such variability in skull morphologyin brown bears from fossil
throughrecent species (Erdbrink1953) suggests there
may be some benefit we do not understand.In comparison, based upon a superficialcomparisonof polar bear
skulls, the same amountof obvious variabilitywas not
presenteitherwithinpopulationsor between them. The
diet and habitatof polarbearsare probablyless variable
than those of brown bears but we do not know if that
accountsfor a lackof comparablevariabilityin the skulls.
SOCIAL ORGANIZATION
In general,the presentday bearstend to live most of
their lives as solitary individuals, with social groups
being limitedto females withcubs andmale-femalepairs
duringthe breedingseason. Thereis no indicationfrom
the fossil recordthat the ancestorsof the modernbears
were any moregregarious(Kurten1976). As with other
species, their social organizationappearsto have been
influencedby the risk of predation,intraspecificcompetition, and the distributionof resources(Wranghamand
Rubenstein1986).
It is possible that some of the ancestralursidsmight
have been vulnerableto significantpredationbecauseof
their small size. However, it appearsthat as body size,
teeth, and claws increasedin size throughthe Pliocene
and Pleistocene epochs, bears were probably not as
vulnerableto predationfrom non-ursids. As a result,
therewas probablylittle significantpressureto aggregate
forthepurposeof groupdefenceorvigilance. We suspect
that, similarto the moder bears,the threatof predation
by otherspecies of bears,or males of theirown species,
might have had a significantinfluenceon theirbehavior
andecology. The smallerbearsmighthave used treesas
escape habitatwhile largerspecies could standandfight
if necessary.
Intraspecificcompetition,includingpredationby large
males, has been suggestedto be importantin population
regulation of North American black and brown bears
(e.g., Kemp 1972, Stringham1983). The importanceof
this factor to other species is unknown. Intraspecific
predationalso occurs in polarbears (Tayloret al. 1985)
but in the open habitatof the arcticsea ice it may be less
importantthanin the forest (Ramsayand Stirling 1986).
Cubs develop rapidlyand are able to outrunadultmales
by the time they are 6 months old. Nevertheless, 1
consequenceof intraspecificcompetition,includingthe
threatof predation,is differentialdistributionof bearsby
age andsex class. Forexample,adultfemale polarbears
with cubs on the sea ice duringthe breedingseason, and
on land in HudsonBay duringthe non-breedingseason,
tend to segregatethemselves from habitatswhere adult
males are most abundant(Stirling et al. 1977, Ramsay
and Stirling 1986, Derocherand Stirling 1990a).
Homerangesof bears,forwhichdataareavailable,are
extremelyvariable(Table 2) and have a high degree of
overlap with conspecifics. In most cases, this probably
indicates that resources are widely distributedand not
EVOLUTION
OF BEARS* Stirling and Derocher
defendable. Group defence of a home range would
probablynotbe anymoreenergeticallybeneficialsince it
is unlikelythe defendableareawould increaseenoughto
supporta groupof bears.
Intraspecificcompetition for patchy and ephemeral
resources,and the evolutionof a largelyvegetariandiet,
which necessitatesa large food intakebecause of a nonruminantdigestive tract,likely acted againstthe formation of large social groups of bears. Groups of bears
would be likely to exhaust food patches more quickly
than individuals so they would have to move more
frequentlyin searchof new feeding areas. The effect of
additionaltravelcould be negativebecausethe largesize
and massive build of bearsappearsto reducetheirenergetic efficiency of movement(Hurstet al. 1982). If there
is a similartrendin otherspecies of bears,it is likely that
intraspecificcompetitionwouldnegatefeedingin groups
except in exceptionalcircumstanceswherethereis a high
density of high qualityfood, such as at a salmon stream
or a garbagedump.
In consideringwhy bearsarenot foundin sleuths,it is
insightfulto comparetheirsocial organizationto thatof
lions. Bearsandlions bothgive birthto altricialoffspring
so thatsubstantialpostpartumcare is required. In lions,
the mother-offspringbondis thebasisof groupformation
(Packer 1986). Also like bears, the ancestors of lions
were solitary carnivores, as most other felids still are
today (Packer1986). Most studiesattributegroupliving
in lions to the benefits associatedwith the advantagesof
killing large prey, but Packer(1986) arguedthat group
living in lions appearsto have evolved despite possible
disadvantagesof grouphunting.The kin-basedstructure
of femalepridesresultsfromnatalareaphilopatry(Packer
1986) and may representthe rudimentarymechanism
from which higherlevels of sociality develop. A major
advantageof group living in lions may be protectionof
the cubs from infanticidalmales, resultingin increased
reproductivesuccess thatnegatespossibledisadvantages
of grouphunting(Packer 1986, Packeret al. 1988).
Both infanticideand natalareaphilopatryare closely
paralleledin ursidecology, yet all ursidsremainsolitary.
However,thereare2 importantdifferences.Femalelions
can come into oestrus at any time of year comparedto
bears, which are seasonal breeders. Thus, a male lion
may be able to mate with a lioness if he kills her cubs,
regardlessof the season, somethingunlikely with bears.
Consequently,infanticideis a less importantstimulusfor
groupliving in bearsthanin lions. We suggest food size
and density may constitute the critical difference. By
hunting together, lions are able to obtain large enough
units of high quality food, often enough, that all the
199
membersof the group can be fed within their territory.
Exceptforbriefperiodsin specialcircumstances,such as
at salmonstreams,bearscannotdo this becausethe prey
availableto them are distributedat densities lower than
those in lion habitat.Forbrownbears,killing anungulate
is a rareenoughevent thatit may be advantageousnot to
share it. Additionally,in temperateregions, the cooler
climate may act to preserve a carcass, making it more
beneficialforan animalto guardit forcontinueduse. The
warmerclimate in lion habitat,plus the abundanceof
scavengers,may makeit advantageousto sharea kill and
ensure consumption by kin. Although brown bears,
especially subadults,are occasionally cursorialhunters
of mammals as large as elk or caribou (Cole 1972,
ReynoldsandGamer 1987), andmay huntin pairs(Cole
1972, M. Meagherpers.commun.),therateat whichthey
are able to kill does not approachthat of lions. If bears
huntedin sleuths,they would have to travelextensively
betweenkills, as do packsof wolves, because of the low
density of prey species. Each bear would receive a
reducedcaloricreturnfromeach kill becauseof the need
to share. However, comparedto wolves, long distance
travelby bearsis slow andlikely muchmorecostly in the
energeticsense, so thatsolitaryhuntingis the best overall
strategy.
While feeding on a vegetarianor omnivorous diet,
individualbearsarecapableof sustainingandprotecting
both themselves and their cubs in most circumstances
without the assistance of conspecifics. Consequently,
there is little apparentnutritionalbenefit in becoming
more socially developed. Even in the completely carnivorous polar bears, the small size of their majorprey
item, ringedseals, probablynegatesany potentialadvantage from huntingas a group or sharinga kill with any
conspecific except offspring. Like brownbearshunting
ungulates,the rate at which polar bears are able to kill
large prey such as beardedseals or odontocete whales
does not appearto be frequentenough to stimulatethe
development of group hunting. Nevertheless, large
numbersof polar bears will scavenge togetherat whale
carcassesor dumps,as do brownor black bears,though
in those circumstanceseach animal still behaves independently.
Lastly,in contrastto lions, manypastandpresentursid
species hibernateand this may restrictthe development
of sociality. By way of analogy, there is a fairly wide
rangein the developmentof socialityin groundsquirrels.
Apparentlywhatcharacterizesthe most social species is
a prolonged association between adults and subadults
that can only occur with short, or absent, periods of
hibernation(Armitage 1981). Even though some pres-
200
BEARS-THEIR
BIOLOGY AND MANAGEMENT
ent-daybearsmay not hibernatein warmerregions,their
herbivorousdiet in a forest environmentdoes not lend
itself to the developmentof sociality as discussedabove.
In contrastto the varied diet of omnivores such as
blackbears,morethan99%of the diet of the giantpanda
consists of stems,branches,andleaves of only 2 bamboo
species (Schalleret al. 1985). Thereis seasonalvariation
in the use of each species and the partsof the plantsthat
are eaten but, since bamboo grows year-roundat high
density, pandascan normallyobtain all their metabolic
requirementswithin much smallerareasthanother species of bears(Table 2). This is probablystronglyinfluenced by the additionalfact that there is no seasonal
variationin bambooquality(Schalleret al. 1985). Thus,
there is no need for the pandato either change diet or
hibernate. Even so, they show little social organization.
Probablybecause of their solitary existence, female
bears appearto be inducedovulators(Bunnell and Tait
1981). There is little advantageto ovulating spontaneously if the chance is good there would not be a male
nearbyat the time. Fromobservationsin zoos andin the
field (Meyer-Holtzapfel 1957, Horocker 1962, Sparrowe 1968, Herreroand Hamer 1977), it appearsthat
male and female pairs may remaintogetherfor up to 2
weeks. Males may try to sequesterfemales to isolated
areaswhere they are less likely to encountercompeting
males (Herreroand Hamer 1977, Ramsay and Stirling
1986) and copulation usually occurs many times. Intrasexualcompetitionbetween males for mating privileges can be intense and some females may mate with
more than 1 male (Sparrowe1968). Since female bears
mayhavemultipleovulations(RamsayandStirling1986),
it is possible for members of the same litter to have
differentfathers(Bunnelland Tait 1981). For example,
Hornocker(1962) observed a female brown bear that
mated 10 times with 4 males in 2 hours. How frequent
polyandrousmatingis, andwhateffect it mayhavehadon
the evolution of the social organizationof bears is unknown.
Nevertheless, some sociality does exist although its
function may not necessarily be fully understood. For
example, duringthe ice-free period in western Hudson
Bay, 2 to 14 or more adultmales may aggregateclosely
and show extremetoleranceof conspecifics (Kolenosky
1987, Derocher and Stirling 1990b). In this circumstance, considerable ritualized fighting may also take
place in which combatantsarenot injured(Latour1981).
Two adultfemale polarbears,each with cubs, were observed interactingnon-aggressively,and even attending
each other's cubs on occasion, over a periodof 6 weeks
while feeding togetherat a garbagedump(Lunn 1986).
It is also possible that the apparentlysolitaryforestdwelling bearsare more social thanpreviouslythought.
Reassociationof family groups after breakuphas been
reported(Reynolds and Beecham 1980, Garshelis and
Pelton 1981). In black bears, female offspring tend to
settle in or adjacentto the home rangesof theirmothers
(Rogers1987a)whilemalecubsdisperse.Rogers(1987b)
also suggested that female black bears can recognize
independentoffspringandbehavein a bereficial manner
dictatedby genetic relatedness.
COMMUNICATION
Because bears are solitary animals and are usually
difficultto observe,theirmethodsof communicationare
not well understood.The degreeto whichbearsvocalize
varies between species. Herrero(1978) suggested that
blackbearsaremorevocal thanbrownbearsbecausethey
aremoreof a forestanimalandhave restrictedvisibility.
Otherforestspecies suchas spectacledbears,slothbears,
and sun bears also seem to be quite vocal (Laurieand
Seidensticker1977,Peyton 1980,Bunnell 1987). Pandas
in zoos vocalize during oestrus (Kleiman et al. 1979)
while thosein thewild havea repertoireof approximately
11 identifiablesounds(Schalleret al. 1985). In contrast,
in ourexperience,polarbearsvocalize little. Femalesand
cubs may call to each otherif they become separated,or
if the female is leavinga site andwantsthe cub to follow.
Males snort and chuff (Wemmer et al. 1976) during
intrasexualagonisticbehaviorbutotherwisetheylackthe
distinctivecalls thatcharacterizeothercarnivores.
Several species of bearsmarktrees. The functionof
this behaviorhas not been confirmedexperimentallybut
the information conveyed likely includes territorial
markingand possibly the statusof the individuals.
The use of scents, including pheromones,have not
been documentedin bearsthoughthey arelikely present.
For example, we have trackedindividualadultmales on
the sea ice duringthebreedingseasonin the spring,when
theirpathsmay cross the tracksof dozens of otherbears.
Thereis no discernibleresponseuntila male crosses the
pathof a lone adultfemale. Recognitionis instantandhe
then follows the track until he catches up to her or is
distractedby anotherfemale. We do not know how a
male polar bear recognizes the track of a lone, and
presumablyoestrus, adult female but it is clear that he
can. It seems likely a chemical stimulusis involved.
In general, the presence of color patternsin bears is
fairly conservative,especially when comparedto some
othercarnivores(Nowak andParadiso1983). However,
contrastingcolor patternson the body may help commu-
EVOLUTION
OF BEARS * Stirling and Derocher
nicatean animal'spresence,especiallyin a foresthabitat.
The existence of color patternsandcontrastis greatestin
lower latitudespecies such as the spectacled bears and
pandasin particular,butalso to a lesserdegreein the sloth
bear. Sun bears, Asiatic black bears, spectacledbears,
and usuallyAmericanblackbears,have a patchof white
on the chest. The muzzle of these smallerforest bears,
andeven partof the face of the sun bear,aremuchlighter
in color than the rest of the head or upper body (see
Nowak andParadiso1983, Bunnell 1987). By comparison, there is less contrastin the body color patternsof
brownbears. In the polar bear, which lives in the most
open habitatof any species of bear,thereis no contrastin
body color at all, withthe exceptionof the blacknose and
eyes. Schalleret al. (1985) suggestedthe complex body
markingsof the pandasentimportantsignalsto conspecifics and Ewer (1973) thoughtthatthe chest markingsin
bearspossibly servedto accentuatea threatposturewhen
an animalstood on its hindlegs. Bearsin generaltendto
approachconspecifics in a head-onmannerso thatconcentrationof the markingpatternsin the chest and facial
areaswould enhanceidentificationandcommunication.
Headand neck orientation,ear position, andthe orientation of the mouth and teeth are probablyall important
thoughthey have been little studied.
Tails in the ursidsareconspicuousby theirsmall size.
In manycarnivorespecies, tails serve an importantfunction in communicationand in many instancesare highly
patterned(Ewer 1973). Even in otherspecies with short
tails such as the lynx (Lynxcanadensis), the tail is held
uprightand is used in maintainingsocial contact and
communication(Ewer 1973). The absence of a functional tail may have been influenced by the relatively
non-social natureof the bears and the fact thatthe most
importantdisplays seem to involve frontaland bipedal
display.
MANAGEMENTIMPLICATIONS
It is notpossible to predictthedirectionin whichbears
may evolve but it is obvious from the generationtime
involved thatchange will be slow. Largespecies evolve
more slowly because of low rates of reproductionand
high rates of extinction (Stanley 1979). This may not
bode well in a world which appearsto be changingever
morequickly,especially when, as Peters(1983:196) has
noted,"Evolutionhas favoredsmallerspecies, andlarge
size tends to be an evolutionarydead end".
All extant populations of bears have been hunted,
displacedfromhabitat,or impactedin some otherway by
the presenceof man. It is difficultto assess the effect this
201
may have hadon the evolutionof the moder bearsbutit
has probablyinfluencedaspectsof theirleaned behavior
such as movementpatternsand diet preferences. As the
habitat available for bears continues to decrease, we
reducethegeneticvariabilitythatremainsavailableto the
species. This may alter the patternof naturalselection
because genetic drift increases in smaller populations
(Allendorf 1983).
Largecarnivoresaresensitiveindicatorsof ecosystem
health and could be used to define the minimum area
necessaryto preserveintactecosystems(Eisenberg1980).
However, it is estimatedthat areasin excess of 106km2
are requiredfor mammals 50 kg and larger to persist
throughevolutionarytime (i.e., 105to 106 years) (Belovsky 1987). This estimate assumes a scenario of no
majorclimatic change. It is clear that bears will never
regain the vast expanses in which they evolved, so it is
vital for us to ensurethatthe remainingpopulationshave
large enough areasof appropriatehabitat. Yet, it is also
apparentfromthe size of thehomerangesof most species
of bears (Table 2) and the minimum population sizes
requiredto maintainan adequategenetic pool thatlarge
geographicareaswill be requiredif bearsare to survive.
Solution of this dilemma will requirecreative thinking.
Somehow, we will need to delineate and manage large
areasof the most optimalremainingwild and semi-wild
habitatfor a varietyof equallyimportantpriorities,one of
which is the conservationof large carnivores,including
bears.
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