Comparative Weight Loss in Three Species of Ursids under Simulated Denning Conditions
Author(s): P. D. Watts
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. 139-141
Published by: International Association of Bear Research and Management
Stable URL: http://www.jstor.org/stable/3872913
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COMPARATIVE
WEIGHT
LOSSINTHREESPECIESOF URSIDSUNDER
SIMULATED
DENNING
CONDITIONS
P.D.WATTS,Instituteof ArcticEcophysiology,Box 1028, Churchill,
Manitoba.ROBOEO/Centre
forNorthernStudies,LakeheadUniversity,
ThunderBay,Ontario,Canada
P7B 5E1
Abstract: Captive animals were used to conduct 3 simulateddenning experimentson each North AmericanUrsid species. One polar bear was studiedin its natural
maternityden. Initialweight of the study animalsrangedfrom 70 to 285 kg (?1 kg). Althoughthe initial weight between black and grizzly bearsand between grizzly
andpolarbearsoverlapped,the resultsprovideevidence of a species-specificrateof weight loss. The averagebody weight loss perday of all of the studyanimalsranged
from0.18 to 0.91 kg. The presentdatacombinedwith the previouslyreportedinformationon metabolicratesareused to calculatethe caloricequivalentof weight loss.
These calculationsindicatethatpolarbearsmay have preferentialproteincatabolismand/orelevated waterloss duringdenning.
Int. Conf. Bear Res. and Manage. 8:139-141
The denning of Ursids is generally interpretedas a
strategyto avoid unfavourableenvironmentalconditions
and reduceenergy expenditures.The black bear (Ursus
americanus) and the brown bear (Ursus arctos) are
characterizedby denningduringthe winterby bothsexes
andall age classes. Comparatively,the polarbear(Ursus
maritimus) is limited primarily to maternity denning
(Harington1968, Jonkelet al. 1972, Lentfer1976). The
use of winter dens by bears is an example of seasonal
starvationand results in dramaticfluctuationsin body
weight. The present work compares weight loss on
captiveanimalsduring3 simulateddenningexperiments
on each of the aforementionedspecies and 1 natural
denningexperimenton a polarbear.
Weight loss during starvationis determinedby the
ratio of proteinand fat catabolizedand waterloss. The
combinationof these factorsdeterminesthe specific caloric equivalentof weight loss. The presentwork uses
weight loss informationandmetabolicdatafrom3 black
bears, 3 polar bears and from 2 grizzly bear denning
experimentsto estimatethe caloric equivalentof weight
loss overwinter. A female polar bear in its naturalden
was also studied.
To survive throughthe denning period, energy demandsmust be balancedby energy reserves. Although
minimumbody compositionhas not been establishedfor
bears, there is evidence that polar bears, in part, also
dependon adiposetissuedepositsforinsulation(Oritsland
1970). The insulativesignificanceof fatdepositsin polar
bears is probably an adaptationto their unique semiaquaticlifestyle. The semi-aquaticadaptationof adipose
retentionmay resultin a catabolicpreferencefor protein
in polar bears. If maintenancerequirementsare minimized duringdenning,the lower caloric value of protein
would resultin a higherrate of weight loss.
MATERIALSAND METHODS
All of the polar bears used in the study were adult
females; black and grizzly bearswere adultmales. One
grizzly bear was studiedduring2 winters. Polar bears
that were used in the study came from the region of
Churchill,Manitoba,Canada. These female polarbears
were selected for the simulated denning experiments
from the animals capturedeach year in the provincial
tagging project. Bears capturedfor the simulateddenning experimentswere given food and snow ad libitum
until 2 to 3 weeks before measurements.
On Cape Churchill,polarbeardenningoccurs on the
high peat banksof the lakes and riversin the area. Den
locations have been observedbetween HudsonBay and
the Canadian National rail line with a northernand
southernboundarygenerally correspondingto the tree
line. DuringSeptemberand October1977, aerialfixedwing reconnaissancewas carriedout to locate bearsand
potentialmaternitydens. Once the lakes were frozen, a
Bell helicopterwas taken into the area and bears were
immobilizedwith the drugSernylanin conjunctionwith
the relaxant,Sparine. While the animals were unconscious,a radiocollarwas placedaroundtheirneck. These
radiocollars were in the 40MHz range(F. Anderkapers.
commun.). One adultfemalepolarbearwas successfully
trackedto its denandits weightestimatedbothbeforeand
after denning. Weight of the animal in the field was
estimated by chest girth measurementand the cattle
weight tape method(Stirlinget al. 1977).
The 2 male grizzlies were capturedas nuisance or
problembearsin AlbertaandMontana.The 3 maleblack
bears were capturednear The Pas, Manitoba, also as
problembears. I would have preferredto have females
for comparisonwith the polar bears, but unfortunately
none were available for the study. Grizzly and black
bears were shippedto Churchillby rail.
Simulateddenningexperimentswere conductedduring the naturalUrsid denning season beginning in late
Novemberor earlyDecemberandlasting27 to 121 days.
Furtherdetails on individualdenning experimentsand
140
BEARS-THEIR
BIOLOGY AND MANAGEMENT
studyanimalsareavailableelsewhere (Wattset al. 1987,
Watts and Cuyler 1988, Watts and Jonkel 1988). Food
was withheldandambienttemperaturesin the laboratory
were similarto those in nature(-10 to -25 C). Polarbears
were given an opportunity to use the simulated den
voluntarily. Black and grizzly bears were shut into the
dens at the beginning of the experimentalperiod.
Weightof thebears(+ 1kg) was determinedbeforeand
aftersimulateddenning by use of a balance beam scale,
calibratedwithknownweights. The simulateddens were
constructedfrom steel and covered with insulation. To
minimize vibrationsresulting from direct contact with
the floor, dens were elevated on a wooden platformor
rubbertires.
The polarbearsimulatedden was rectangular,122 cm
x 185 cm x 193 cm, with a volume of approximately
3,000 L. The interiorof the den was designed to have a
sleeping platformin the rearand a tunnel leading to the
adjoiningroom, with a ventilationhole in the roof. Polar
bears were given the choice of using the den or the
adjoiningroom,althoughnone of the polarbearsused the
den voluntarily. After several weeks in the adjoining
room,the bearswere placed in the simulatedden andthe
entrancewas sealed.
The simulatedden for the grizzly bearshad a volume
of 2,000 L, and was twice the size of the black bearden.
300
* U. maritimus, natural conditions
n=1
O U. maritimus, simulated conditions
n=3
O U. arctos, simulated conditions
n=3
L
U. americanus, simulated conditions
n=3
280 260
240220 .
The dens were fitted with a door at 1 end and a barred
openingattheother.The barredend was sealedby a sheet
of plywood andventilationholes wereestablishedatboth
ends.
RESULTS
The rate of weight loss was calculatedfor 9 bears in
simulated den experiments and for 1 bear in a natural
maternityden. All animals appearedhealthy when removed fromthe dens andtherewas no indicationof feces
or urine in the simulateddens or the naturalmaternity
den. Polarbearshad the highest rate of weight loss and
blackbearshadthe lowest rate (Table 1, Fig. 1). The time
between weight determinationsin 7 of the 9 simulated
den experimentswas shorterthan 1 in the wild (Table 1).
DISCUSSION
Conclusionsthatcan be basedon the presentworkare
limited by the use of male black and grizzly bears for
comparison with female polar bears. Weight loss in
denningUrsids will be limitedby the fasting potentialof
individual animals. Morrison(1960) estimated fasting
potentialby dividing the caloric value of maximumfat
contentby theoreticalbasalmetabolicrate. Maximumfat
content was considered to be 50% of body weight.
Morrison'smethoddoes not, however, take into account
the use of proteinor the retentionof fat for thermoregulation. The relatively poor insulation of polar bear fur
(Scholanderet al. 1950) may necessitatethe use of fat for
thermoregulation(Oritsland1970). Haywardand Keat-
Table 1. Weight loss of Ursids in dens.
200
Initialweight
180 -
IDa
(?lkg)
kg
%/day
kg/day
N77
110
285
100
0.32
0.91
140
M78
27
214
18
0.30
0.67
120
M77
59
198
43
0.37
0.73
M76
45
182
32
0.40
0.71
G78
40
145
18
0.30
0.45
80
G77b
121
223
51
0.17
0.42
60
G76b
160 .
-
Weight loss
Days
0a
w
100
0
20
40
60
80
100
Number of days
Fig. 1. Weight loss of denning Ursids
120
140
76
182
27
0.20
0.36
A78
110
70
20
0.26
0.18
A77
103
122
24
0.19
0.23
A76
45
145
11
0.18
0.24
a A = U. americanus,G = U. arctos, M = U. maritimus,N77 = U. maritimus
in naturalmaternitydens.
bSame animal in differentyears.
DENS * Watts
WEIGHTLoss OF URSIDSPECIESIN SIMULATED
HAYWARD
M.G., ANDW.R. KEATINGE.1981. Roles of subcu-
Table2. Caloricequivalentof weight loss for denning Ursids (kcal/kg)'.
A76
A77
A78
G77
G78
M76
M77
8,955 4,687
6,836
7,654
4,335
3,217
3,530
141
M78
4,100
a See Table 1 for legend
reflexesin determining
taneousfatandthermoregulatory
in water.J. Physiol.
abilityto stabilizebodytemperature
320:229-251.
G.B. KOLENOSKY, R.J. ROBERTSON, AND R.R.
RUSSELL.1972. Furthernotes on polar bear denning
JONKEL, C.J.,
habits.Int.Conf.BearRes.andManage.2:142-158.
J.W. 1976. Polar bear reproductivebiology and
LENTFER,
inge (1981) workingon humans,foundthatsubcutaneous
fatthicknesshada negativecorrelationon metabolicrate.
A similarmechanismmay occur in polarbears.
The results on weight loss can be combined with
concurrentmetabolicmeasurementsreportedelsewhere
(Watts et al. 1987, Watts and Cuyler 1988, Watts and
Jonkel 1988) to calculatethe caloricequivalentof weight
loss (Watts 1983). Althoughgrizzly bearshave a higher
rateof weight loss thanblack bears(Fig. 1, Table 1), the
present results do not indicate a similar patternin the
caloricequivalentof weight loss (Table2). The calculations do indicate that polar bears have a lower caloric
equivalentof weight loss thanthe other2 species studied
(Table2). The reducedcaloricequivalentof weight loss
calculatedfor the polarbearsimplies an elevatedrateof
waterloss and/orpreferentialcatabolismof proteinduring denning.
LITERATURECITED
C.R. 1968. Denning habits of the polar bear.
HARINGTON,
(Ursus maritimus). Can. Wildl. Rep. Ser. 5.
denning.Alas.Dep.FishandGame.22pp.
P. 1960. Some interrelationsbetween weight and
MORRISON,
function.Bull.Mus.Comp.Zool. 124:75-91.
hibernating
ORITSLAND,N.A.
1970. Temperatureregulationof the polar
bear.Comp.Biochem.Physiol.37:225-233.
P.P., V. WALTERS,R. HOCK,AND L. IRVING.
SCHOLANDER,
1950. Bodyinsulationof somearcticandtropicalmammalsandbirds.Biol.Bull. 99:225-250.
AND D.
STIRLING,I., C. JONKEL,P. SMITH,R. ROBERSTON,
CROSS.1977. The ecology of the polarbearUrsus
maritimus
alongthewesterncoastof HudsonBay. Can.
Wildl.Serv.Occas.Pap.33. 64pp.
P.D. 1983. Ecologicalenergeticsof denningpolar
WATTS,
bearsandrelatedspecies. Ph.D.Thesis. Univ.of Oslo,
Norway.
, ANDC. CUYLER.1988. Metabolismof the black bear
undersimulated
denningconditions.ActaPhysiol.Scand.
134:149-152.
, ANDC. JONKEL.1988. Energetic cost of winter
ingrizzlybear.J.Wildl.Manage.52:654-656.
dormancy
ANDR.J. HURST.1987. Standard
, N.A. ORITSLAND,
metabolicrateof polarbearsundersimulateddenning
conditions.Physiol.Zool. 60:(6)687-691.