Comparative hunting abilities of polar bear cubs of different ages

IAN STIRLING A N D PAUL B. LATOUR Canadian Wildlife Service, 5320-122nd Street, Edmonton, Alta., Canada T6H 3S5

Received October 24,1977

STIRLING, I., and P. B. LATOUR. 1978. Comparative hunting abilities of polar bear cubs of different ages. Can. J. Zool. 56: 1768-1772.

In mas1 areacof the Canadian Arctic polar hear (Ursus mitriti~nus) cubs apparently remain with their mothers until they are 2.5 years of age. The degree ro which cubs of each agexlass prticipate in the huntingof seals while with their mothers i s examined in this paper in order to evaluate the degree to which they might be capnble of independent hunting. should they be orphaned prior to the completion of the normal weaningperiod. Cubs of all age-classes did almosc no hunting during ihe spnng. The proportiuns of time spenr hunt~ng by yearling and 2-year-old cubs, and the d i l ~ l i o n s o f their lying 'still hunt%' were no1 signiticantiy differenrfiom each other hut they were significantly shorter than their mothers' and than adull male*' during the summer. However, t he frequency of the lying 'still hunts' of 2-year-old cubs was double thar of yearling cubs and the kill rdte or2-year-old cubs was comparable with that of adulr age-classes, despite the fact they hunted far a significantly lesser propnrtjon of their time, Thew resi~lts suegest that cubs which remain with t k i r molhers until they are weaned have a higher probability of ruw~va l than those thar do not and this ~nterpntation lends support to the managcrnent concepl of total protectiun or Family groups and the harvesting of independent hears only.

STIRLING, I., et P. B. LATOUR. 1978. Comparative hunting abilities of polar bear cubs of different ages. Can. J . Zool. 56: 1768-1772.

Dans la plupart des regions de I'Arctique canadien, les petits de I'ours polaire (Ursus muritimus) semblent rester avec leur mere jusqu'a l'ige de 2.5 ans. L'examen du degre de participation a lachasse aux phoques, chez les oursons de chaque classe d'8ge vivant encore avec leur mere, permet d'evaluer I'aptitude des oursons chasser seuls dans les cas ou ils seraient siparks de leur mkre avant la fin de In p t r i d e normale de sevrage. Les oursons, et celadans tous les groupcs d ' i g , ne chassent i peu pris pas au printemps. Les oursons de 1 an et de 2 ans chasqent B peu pres la mCme proportion de leur temps et les durees de leurs chasses 2 I'affiit ne different pas sipnificarivernent I'une de I'autre, mais ces durees sont significativement plus cuunes que cellesdesch;ts<es a I'affi~t de leur mere ou des mtles adultes durant I'etC. Cependant, la fkquence des chaweq i'afYitt e2rr cnviron deux fois plus elevee chez les oursons de 2 ans que chez ceux de 1 :in et le taux des proies tuPes par les oursons de 2 ans est comparable a celui qu'on enregistre chez les classes d'ige d'adultes, bien que ces oursons passent une moins grande partie de leur temps a la chasse. I1 semble donc que les oursons qui restent avec leur mere jusqu'au sevrage a ent une plus grande chance de survie que les autres; cette Interpretation des resultats rejoint le principe d'arnenagement qui preconise de proteger totalement les cellules familiales pour ne chasser que les ours seuls.

[Tradult par le journal]

Introduction In areas of the Canadian Arctic where polar bears

have been studied, cubs are born in December to January (Harington 1968) and apparently stay with their mothers until they are 2.5 years old (Stirling et al. 1975, 1977). which suggests. a prinri, that there is survival value in doing so. This generalization may not apply to southern Hudson Bay where it appears that cubs may be weaned at 1.5 years of age. However, most ol'the polar bears harvested in Canada are taken in the Northwest Territories (1976-1977 quota = 516). Female polar bears with cubs of the year are fully protected. However, cubs and their mothers may legally be hunted when the cubs reach a length of 137cm (54 in.), which is attained at about 1 year of age. Thus, all members of a family group may legally be hunted from the

time the cubs are about 1 year of age. A hunter with only one tag may try to kill the adult female because she has the largest and most valuable hide. Con- sequently, the orphaning of cubs 1 year of age and older occurs although the frequency of occurrence is unknown. The degree to which cubs of each age-class participate in the hunting of seals while with their mothers is examined in this paper in order to evaluate the degree to which they might be capable of independent hunting.

The data were collected as part of a detailed long-term study of the behavior of wild polar bears, initiated in 1973 (Stirling 1974).

Materials and Methods Observations of undisturbed free-ranging polar bears were

made on the annual ice of Radstock Bay, southwest Devon Island, Northwest Territories, from the top of Caswall Tower, a

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STIRLING A1 VD LATOUR 1769

natural rock formation 234 m above sea level (Stirling 1974, Fig. 1) using 10 X 50Buschnell binoculars and 15 to 60 x Bausch and Lomb zoom balscopes. Except during inclement weather, 24-h daylight permitted continuous observation of those polar bears present.

Radstock Bay is approximately 30 km long and 12 km wide at the mouth. During the winter (December to March) and spring (April to early June), both the bay and Barrow Strait are com- pletely frozen over and the ice is covered with drifted snow. The floe edge is usually 30 to 50 km to the east. The mean April and May temperatures at Resolute, about 95 km to the west of Radstock Bay, are -22.7 and -10.3"C respectively (Meteorological Branch 1970). During that period, the bears mainly hunt ringed seals (Phoca hispida), and to a lesser degree bearded seals (Erignuthus barbutus), around the mouths of the bays along the south coast of Devon Island, to some extent into the bays themselves, and offshore in Barrow Strait (Stirling and Archibald 1977). The bears mainly use their sense of smell to locate seal breathing holes and subnivean lairs. When a hole or lair is located, the bear attempts to smash in or dig through the snow cover immediately if it senses that a seal is present. Otherwise, the bear waits beside the undisturbed snow above the hole until it senses a seal surfacing to breathe and then breaks through and attempts to capture it.

By June, the mean temperature reaches 0.6"C, the snow cover on the ice melts, and the fresh water runs through cracks and breathing holes in the ice into the sea below. By late June or early July, Lancaster Sound and eastern Barrow Strait are open water and the bears concentrate on the ice in the bays of south- e m Devon Island, such as Radstock Bay. Mean temperatures in July and August are 4.6 and 3.0°C respectively. By mid-August most of the bays are completely free of ice and the bears either move onto land until freeze-up in the fall or move further north to areas where there is still some ice cover. Mark and recapture studies have shown that individual bears, especially females, show a high degree of fidelity to Radstock Bay between years (Stirling et al. 1977).

For the purposes of this paper, all hunting behaviors were pooled by age and sex category and by season to facilitate the analyses required. Details of the behaviors recorded, their regu- larity of occurrence, and their ecological importance will be published later.

Individual bears were watched continuously and the begin- ning and end of all behavioral activities were recorded to the nearest minute. Although several polar bears were captured and tagged for other phases of this study, only data from polar bears that were not disturbed during the periods of observation were included in the analyses presented herein because we have not yet evaluated whether or not drugging and tagging has any significant effect on the subsequent behavior of the bears. Whenever possible, individual records were kept on each member of a family group. Frequently, however, two siblings paralleled one another so closely that it was impossible to keep their records separate. In these cases, the behavioral record for each cub was identical. The term 'still hunting' means that a bear stands, sits, or lies motionless beside an exposed or subnivean breathing hole waiting for a seal that might surface to breathe there.

Observations were made during the following periods in the spring (April to early June) and summer (late June to mid- August): 25 July to 7 August 1973; 13 July to 10 August 1974; 12 May to 7 June and 3 July to 29 July 1975; and 16 April to 27 May and 30 June to 21 July 1976.

The proportions of time spent hunting by polar bears of differ- ent sex and age categories were tested against each other by comparing the two regressions of time hunted on time observed, using the following modified F test, developed by M. C. S.

Kingsley, Canadian Wildlife Service. A normal x2 could not be used because the measurements were in minutes and not fre- quencies of occurrence of events.

Data for bears of each sex and age category were tabulated for testing as follows:

Bear 1 Bear 2 Bear 3 Bear n Total

Total No. minutes observed (0 ) 0, 0, 0, 0, ZO

Total No. minutes hunted (H) HI HZ H3 H, Z H

Hi= ZO; 'x2' can be expressed as x-. - - ZH; 0; CHi

ZH. H~~ ( Z H ~ ) ~ and hence2. xZ = x-- CO; i Oi

where p = the average proportion of time spent hunting. Also aZ = the variance of q; = sampling error variable of the ith observation; Hi = minutes spent hunting in the ith observation; and Oi = minutes observed in the ith observation.

Ifwe take as amodel, Hi=pOi+ ern i , resultsfor the several sex and age-classes of bear are combined to give

wherepj, xZj, and nj represent the values for thejth age-class.

Summary data for the several classes are tabulated as

Class 1 Class 2 Class j Total

Total minutes observed ( Z0)t (Z0)z (ZO), C ZO Total minutes hunted ( ~ H ) I (ZH)Z (ZH) , C C H

We can similarly obtain a '2' for the variation between clas- ses which has the form

- px2 = $ (ZH)jZl(ZQ)j - (7 (ZH)j)2/5

An independent estimate of a2 is then given by p?/(j - 1) and comparability of classes is tested by testing

.F?/o' - 1111[Z pjxSII: (nj - 1)l asFj-,.mcni-,,. 1 I

Results and Discussion A total of 682.15 h of observations were made of

polar bears in family groups during the spring and 1714.58 h during the summer. The number of hours of observation made of polar bears of each age and sex category is detailed by season in Table 1.

The duration of the observation period for differ- ent family groups varied. One female and her two yearling cubs were observed for almost 4 succes- sive days. However, most family groups were ob- served continuously for 8 to 15 h before they moved out of the field of view.

During the spring, cubs of the year and yearlings spent less than 1% of their time hunting compared

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1770 CAN. J. ZOOL. VOL. 56. 1978

TABLE 2. Mean lengths of lying 'still hunts' done by polar bears of different sex and age categories during summer

Sample Standard Category of polar bear size Mean deviation

t 1) Yearling 44 25.07 39.93 (2) 2 year oid 49 20.20 25.11 (3) Adult 9 with COY* 61 36.52 53.72 (4 ) Adult P with yearling 162 40.36 70.68 (5) Adult E with 2 year old 1 15 33.52 ?9.9S (61 Adult d 137 68.22 91.86

Categories tested t d f P

1 vs. 2 0.71 9 1 >0.05 1 + 2 v s . 3 + 4 + 5 4.31 429 < 0.01

'COY, cub of year.

with about 18% by their mothers. The sample sizes for Zyear-old cubs and their mothers were too small (Table 1) to permit a meaningful comparison but even so, the amount of hunting behavior ob- served was surprisingly low (0.0% and 4.2%) re- spectively). During the summer. cubsof the year still did very

little hunting. Yearling and 2-y ear-old cuhs hunted only 4.3 and 7.3% of their time respectively, values which were not significantly different (Table 1). In contrast with these low values, the adult females accompanying the cubs of different age-classes hunted between 35.1 and 53.1% of their time, dif- ferences which were not significantly different (Table 1). Females accompanied by cuhs of any age during summer hunted 38.2% of their time, which was significantly greater than the yearlings and 2-year-olds combined (5.3%) (Table 1).

By way of conlparison, adult males hunted 24.7% of their time in the spring compared with 19.3% by females accompanied by cubs of the year and yearl- ings, which was not significantly different (F = 1.00; df = 1,81; p > 0.03. In the summer, adult males hunted 39.9% of their time which was also not significantly different from adult females ac- companied by cubs of any age (F = 0.08; df = 139; p > 0.05). Thus, adult males hunted for about the same proportion of their time as adult females with cubs.

Most hunting during the summer was done by 'still hunting' in the lying position (Stirling 1974). The mean lengths of lying 'still hunts' of yearling and 2-year-old cubs were not significantly different from each other. but the mean of the pooled sample from yearling and 2-year-old cubs was significantly shorter than that of females accompanied by cubs of any age (Table 2).

The frequencies with which polar bears of all categories killed seals during the summer are given

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STIRLING AND LATOUR 1771

in Table 1. Spring data were not included because cubs of all ages did almost no hunting then (Table 1). Yearling cubs were clearly not effective hunters in comparison with other categories of bears (Table I), excepting cubs of the year. In the summer, however, 2-year-old cubs killed seals at a rate which compared favorably with adult bears despite the fact that the former hunted for a significantly smaller percentage of their time. Although the per- centage of the total observed time that yearling and 2-year-old cubs spent hunting and the duration of their lying 'still hunts' were not significantly differ- ent (Table I), the frequency of 'still hunting' by 2-year-old cubs was double that of yearlings (49 in 15 733 rnin = 1 per 321 rnin vs. 44 in 31 133 rnin = 1 per 707 min). This may have influenced thegreater hunting success of 2-year-old bears. Another factor which probably affects the kill rate of cubs of dif- ferent ages is the way in which they choose places to hunt relative to their mothers. Cubs of the year and yearlings tend to stay close behind their mothers, watch their mothers' movements, and irn- itate them. If these younger cubs hunt when the female lies down to 'still hunt,' they hunt more or less where they were when the female stopped moving. Thus, there is less independent choice in- volved in selection of a site to hunt than with 2-year-old cubs which also move along in the same direction as the female but which may range up to 1 to 2 km away from her and independently choose their own sites to hunt. Thus, it appears that al- though cubs have learned to hunt fairly proficiently by the time they are 2 years of age, they are appar- ently not motivated to spend a large proportion of their time hunting, possibly because of the con- tinued food supply being provided by the adult female.

The almost total lack of hunting by cubs of any age during the spring was unexpected. However, it may be related to the fact that in fast ice areas during the spring, most seal breathing holes and lairs are buried under hard compacted snow drifts. In the Western Arctic, Smith and Stirling (1975) found that there was an average of 30 cm of com- pacted snow over ringed seal birth lairs and haul- out lairs and 60-70cm of snow over breathing holes. We have observed adult female polar bears weighing 135 to 200 kg, and sometimes even adult males up to about 400 kg of body weight, which often had to pound on the snow with their forepaws several times and dig as well, to gain access to these lairs and breathing holes. It is likely that yearling and 2-year-old cubs are simply not heavy enough to break through the snow cover in time to catch a seal before it escapes. Snow conditions in the spring may be critical to determining the size (and con-

sequently the age) that cubs may reach before they can survive independently through the winter.

Subjectively at least, winter and spring hunting appears to require a higher level of proficiency than hunting in summer. In spring, the subnivean seal lairs and breathing holes must be located accurately by smell from a distance, as well as detection of the arrival of a seal at its hole beneath the snow. For example, we have frequently observed adult bears detect a lair, pause for varying periods of time, and then suddenly rush 5-15 m and smash through the lair with the forelegs, all in one motion. Examina- tion of seal lairs that have been broken into by polar bears show that the entry was almost always made right over the breathing hole. Such accuracy is probably essential to success. Skill at this type of hunting must take a longer time to develop, espe- cially considering the fact that cubs of all ages showed very little hunting behavior of any kind during the spring.

In the Western Arctic between 1970 and 1975,38 yearling cubs and twenty-nine 2-year-old cubs were tagged while still with their mothers (Stirling et al. 1975). Up to the summer of 1977, significantly more of the 2 year olds (8 of 29 = 27.6%) than the year- lings (3 of 38 = 7.9%) had been recaptured as inde- pendent bears (xZ = 4.65, df = 1, p < 0.05). During the same period in the Western Arctic, two lone yearling bears were tagged, neither of which have been seen since. In the High Arctic, three lone yearlings have been tagged, none of which has been recaptured in a subsequent year although one bear was recaptured 4 months later in the same summer. Vibe (personal communication) tagged four lone yearling bears in East Greenland. One was later found starved to death, one was shot in a camp as a starving problem bear, and the other two have not been seen since. The situation may be different in southern Hudson Bay where it appears that cubs weaned at 1.5 years are capable of surviving. Be- tween 1966 and 1975,46 yearlings and thirty-seven 2-year-old cubs were tagged as apparently inde- pendent cubs on the Manitoba coast of Hudson Bay. Nineteen and 20 bears of each group respec- tively were recovered 1 or more years later, a dif- ference which was not significantly different (xZ = 1.34, p < 0.05). The ecological reasons for this apparent difference would be worthy of further study.

The benefits of harvesting polar bears in the spring only (when the hides are most valuable) with complete protection of family groups were demon- strated in a management context, with a simplistic model by Stirling et al. (1976). Now, our behavioral observation of polar bear hunting behavior and the few quantitative and anecdotal data available from

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1772 CAN. J . ZOOL. VOL. 56, 1978

long-term mark and recapture studies suggest that cubs that remain with their mothers until weaning, which in most areas of the Canadian Arctic appears to be at about 2.5 years of age, have a higher prob- ability of survival than cubs which remain with their mothers for shorter periods of time. This in- terpretation lends support to the concept of total protection of family groups and harvesting of inde- pendent bears only.

Acknowledgements This study was supported by the Canadian

Wildlife Service, Department of the Environment. We also gratefully acknowledge the following: The Polar Continental Shelf Project for logistic support; L. Allison, D. S. Andriashek, W. Calvert, H. Kiliaan, P. Smith, and S. Stirling for assistance in the field; M. C. S. Kingsley for statistical advice; and F. L. Bunnell, F. L. Miller, and P. Smith for constructive criticism of the manuscript.

HARINGTON, C. R. 1968. Denning habits of the polar bear (Ur- sus maritimus Phipps). Can. Wildl. Serv. Rep. Ser. No. 5.

METEOROLOGICAL BRANCH. 1970. Climate of the Canadian Arc- tic. Department of Transport, Canada.

SMITH, T. G., andI. STIRLING. 1975. The breeding habitat of the ringed seal (Phoca hispida). The birth lair and associated structures. Can. J . Zool. 53: 1297-1305.

STIRLING, I. 1974. Mid-summer observations on the behavior of wild polar bears (Ursus maritimus). Can. J . Zool. 52: 1191-1198.

STIRLING, I., D. ANDRIASHEK, P. LATOUR, and W. CALVERT. 1975. The distribution and abundance of polar bears in the eastern Beaufort Sea. A final report to the Beaufort Sea Pro- ject. Fisheries and Marine Service, Department of the Envi- ronment, Victoria, B.C.

STIRLING, I.,and W. R. ARCHIBALD. 1977. Aspectsofpredation of seals by polar bears. J. Fish. Res. Board Can. 34: 1126-1129.

STIRLING, I., A. M. PEARSON, and F. L. BUNNELL. 1976. Popu- lation ecology studies of polar and grizzly bears in northern Canada. Trans. 41st North Am. Wildl. Nat. Res. Conf. 41: 421-430.

STIRLING, I., R. E. SCHWEINSBURG, W. CALVERT, and H. P. L. KILIAAN. 1977. Population ecology of the polar bear along the proposed Arctic islands gas pipeline route. Progress Report to the Environmental Management Service, Department of the Environment, Edmonton, Alta.

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