Wolf Ecology and Prey Relationships on Isle Royale
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Study Area
     Geology and Physiography
     Mammalian Fauna
Methods of Study
     Winter Wolf Study
     Winter Moose Census
        Accuracy of aerial moose census
     Summer Ecology of the Wolf
     Moose Mortality Patterns
     Moose Population Structure
     Statistical Analysis


Numbers and Organization
     A Brief History
     Annual Fluctuations, 1971-74
Winter Movements
     Travel Routes
        Effect of shoreline ice
        Effect of snow conditions
     Distance Traveled by Wolf Packs
Food Economy of Isle Royale Wolves
     The Wolf as a Predator of Big Game
     Nonwinter Food Resources
        Increased predation on beaver
     Winter Predation Patterns
        Hunting success
        Effect of snow conditions
        Winter food availability
     Long-term Changes in Food Resources
Intraspecific Behavior
     Social Hierarchy within Packs
        Expression of dominance and subordination
        Pack leadership
     Courtship and Breeding
        Mate preferences
        Restriction of sexual behavior among subordinate wolves
        Implications of socially controlled mating
        Exclusive territories
        Territory size
        Expansion of East Pack territory
        Maintenance of territory
        Small packs and "loners"
     Communication Among Wolves
Summer Ecology
     Rendezvous Areas
        Observations at a midsummer rendezvous
        Movements between rendezvous sites
     Pup Production
Relations with Nonprey Species
     Red Fox


An Historical Perspective
Recent moose numbers
     Population Size, 1964-70
     Population Size, 1970-74
Sex Ratio and Productivity
     Sex Ratio
        Calf ratios, 1970-74
        Net productivity and calf mortality
        Long-term trends in productivity
Moose Distribution and Habitat Relationships
     Winter Distribution of Moose
     Moose Distribution During Other Seasons
     Moose-Habitat Relationships
Direct Mortality Factors
     Wolf Predation on Adults
        Seasonal aspects
        Age distribution of kills
        Selection related to sex
        Predation compared to hunting
     Wolf Predation on Calves
        Calf losses in summer
        Calf losses in winter
     Drowning and Other Accidents
     Mortality from Unknown Causes
Indirect Mortality Factors
     Residual Effects of Severe Winters
     Factors Affecting Adult Vulnerability
        Jaw Necrosis
Life Table for Isle Royale Moose
     Possible Biases
     The Life Table
Sex Difference in Survival


Moose Population Regulation
Wolf Population Adjustments



INDEX (omitted from the online edition)

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Library of Congress Cataloging in Publication Data

Peterson, Rolf Olin.

Wolf ecology and prey relationships on Isle Royale.
National Park Service scientific monograph; no. 11)
Bibliography: p.
Includes index.
Supt. of Docs. no.: I 29.80:11
1. Wolves. 2. Predation (Biology) 3. Mammals—Ecology.
4. Mammals—Michigan—Isle Royale National Park. 5. Isle
Royale National Park. I. Title. II. Series: United States.
National Park Service. Scientific monograph; no. 11.

QL737.C22P47 599'.74442 77-608019


1 Isle Royale has supported wolves since the late 1940s.

2 Moose colonized isle Royale in the early 1900s.

3 Map showing location of Isle Royale.

4 Map of Isle Royale, showing locations mentioned in text.

5 Cross-section through Lake Superior, Isle Royale, and Keweenaw Peninsula (Huber 1975).

6 Characteristic ridge and valley topography of Isle Royale.

7 Lake Superior is responsible for high humidity and frequent fog.

8 Winter temperature records for Windigo, Isle Royale National Park.

9 Snowfall and index of snow depth at Grand Marais, Minnesota, 1959-74.

10 Snow depth at three sites on Isle Royale, 1972-74, illustrating annual variations in snow levels and the effect of overhead canopy.

11 Penetrability (determined by compaction gauge) of snow at three sites on Isle Royale, 1972-73.

12 Ram hardness number (R) (Appendix B) of snow at three sites on Isle Royale, 1974.

13 Spruce-fir forest is typical of Isle Royale shorelines.

14 Yellow birch—sugar maple forest in island's interior.

15 Vegetation map for Isle Royale, modified principally from Krefting et al. 1970

16 Fires and subsequent erosion create thin soils on ridgetops.

17 Woodland deermouse, the smallest rodent species on Isle Royale.

18 Snowshoe hare, primary prey of the red fox on Isle Royale.

19 A unique subspecies of red squirrel inhabits Isle Royale.

20 Pilot Donald E. Murray.

21 East Pack alpha male (1974) urinating in posture unique to males; alpha female stands behind him.

22 Pups may appear scraggly, as three wolves on left, or may be indistinguishable from adults.

23 Aerial counts were used to determine moose population size and composition.

24 Summer field headquarters on Rock Harbor.

25 Extensive carcass examinations revealed moose mortality patterns.

26 Skeletal remains of moose may remain for decades.

27 Annual cementum deposits in moose teeth indicate their age. Cross-section of molar appears in (a), while magnified view of cementum annulations is shown in (b).

28 Bone marrow is one of the last fat reserves to be utilized by malnourished moose. This is a fat-depleted example.

29 A cow showing white vulva patch characteristic of females, plus mounting marks of bulls during rut.

30 Alpha pair (female facing right) and six probable pups in East Pack, 1972.

31 West Pack alpha pair (male is black), 1972. Note crooked front leg of alpha female.

32 East Pack alpha pair in 1972 (a), 1973 (b), and 1974 (c). Alpha female was the same wolf all 3 years; alpha male the same in 1972 and 1973.

33 Travels and kills of the East and West packs, January—March 1972.

34 Travels and kills of the East and West packs, January—March 1973.

35 Travels and kills of the East and West packs, January—March 1974.

36 Wolves frequently use shelf ice while traveling.

37 Wolves usually travel single-file through snow.

38 Scat analysis was used to determine summer wolf diet.

39 Beavers are an important prey for wolves in summer.

40 Wolves investigating beaver lodge.

41 East Pack in successful chase. Victim was 16-year-old cow with arthritic hip and vertebrae and was pulled down within 2 minutes. Alpha wolves indicated.

42 Results of observed moose—wolf encounters on Isle Royale, 1959-61 (Mech 1970:210) and 1972-74 (this study)

43 Carcasses of wolf kills were often underutilized in the early 1970s.

44 East Pack (1973) in unsuccessful chase. Note cow at rear of calf.

45 Wolves feeding on carcass.

46 Wolves bedded near kill.

47 Distribution of wolf kills located during winter study before (1971) and after (1972) the establishment of the East Pack.

48 Alpha female and male lead West Pack, 1972. Note raised tails of dominant wolves.

49 Alpha female (Brookfield Zoo) follows subordinate female. Note posture and head and tail position.

50 Alpha male (East Pack, 1974) stands in dominant display next to subordinate wolf.

51 Changes in leadership of the large packs on Isle Royale.

52 Incidence of courtship behavior between wolves of different social rank in the East and West packs on Isle Royale, 1972-74.

53 Play behavior is an integral part of wolf courtship (Brookfield Zoo).

54 Female accepting a courting male (Brookfield Zoo).

55 Female rejecting a courting male (Brookfield Zoo).

56 Alpha male attacking (with inhibited bites) a subordinate male mounting a female (Brookfield Zoo).

57 Black alpha male stands between alpha female (behind him) and subordinate male (head turned away), West Pack, 1972.

58 Winter territories of Isle Royale wolf packs, 1971-74.

59 Movements of the East Pack into traditional West Pack territory in 1974.

60 Author recovering carcass of female pup killed by East Pack during trespass in 1974.

61 East Pack (a) gathered around West Pack scent post while trespassing in 1974, (b) standing on shore after scattering from scent post.

62 Avoidance behavior of the West Pack in response to an East Pack scent post, 1973.

63 Travel routes and carcasses utilized by small groups of wolves on Isle Royale in midwinter, 1972-74.

64 Frequency of howling near East Pack rendezvous sites in 1973 (4 July—6 October).

65 Group howl (Brookfield Zoo).

66 Pilot D. Murray examines wolf scent post.

67 Scent post and scratch mark of alpha pair, West Pack, 1972.

68 Rare look at wolves in summer. Second wolf running through brush in upper right.

69 Wolves dug out this old den site (a beaver lodge) in March.

70 West Pack den in hollow log.

71 Aerial view of wolves at rendezvous site in October.

72 Well-utilized carcass of adult moose killed by wolves in summer.

73 Probable pup in East Pack, 1974. Note lack of characteristically stiff guard hairs.

74 Wolf kills provide food for scavengers such as red foxes.

75 Ravens are found at virtually all wolf kills.

76 Wolves playing with carcass of raven they caught near a kill.

77 Young bull moose in summer.

78 Relative distribution of moose in February 1972.

79 Relative distribution of moose in February 1974.

80 One-month-old moose calf.

81 Calf production declined by the early 1970s.

82 Even in winter moose may seek shade.

83 Moose utilize old burns on Isle Royale only with low snow depths.

84 Cow in beaver pond in late June.

85 Cow lying in Lake Superior on hot summer day.

86 White spruce, unbrowsed by moose, grows unimpeded in old burns.

87 Variations in balsam fir growth form are attributable to moose browsing.

88 Most adult moose on Isle Royale are eventually killed by wolves.

89 Age-distribution of wolf-killed adult moose from Isle Royale compared to the hypothetical age-distribution of the population.

90 Temporal variations in age-distribution of 200 adult moose killed by wolves in late January, February, and March on Isle Royale, 1959-74 (see Appendix L for data sources).

91 Bull moose following cow during rut.

92 Comparison between age distribution of wolf-killed adult moose from Isle Royale and hunter-killed moose from Ontario (Addison and Timmerman 1974).

93 A calf depends on its mother for protection in the first year of life.

94 Calves are more vulnerable to wolves when snow is belly-deep.

95 A cow moves to the rear end of her calf when confronted by wolves.

96 Calves may die of nonpredatory causes soon afterbirth.

97 Injuries, such as this broken leg, may predispose moose to predation.

98 These two bulls drowned in a beaver pond after locking antlers, a rare occurrence during the rut.

99 Abnormal antlers of 2.5-year-old moose killed by wolves in 1972.

100 Normal vs. arthritic pelvic joint of moose.

101 Incidence of arthritis in the coxofemoral (hip) joints of moose dying on Isle Royale.

102 Jaw necrosis in Isle Royale moose, apparently resulting from (a) excessive wear, and (b) food impaction.

103 Incidence of jaw necrosis among moose dying on Isle Royale.

104 Male tick (second from left) and three female ticks in varying stages of engorgement with moose blood (scale in centimeters).

105 Hydatid cysts in moose lung.

106 Survivorship curves for male and female adult moose on Isle Royale.

107 Age-specific mortality rates for Isle Royale moose.

108 Calculated proportion of males alive in each age interval, based on a hypothetical age distribution for adult moose on Isle Royale.

109 Wolf predation is essential to the healthy maintenance of a high moose population on Isle Royale.


1 Mean annual precipitation in mainland areas in Minnesota and Michigan.

2 Winter precipitation (cm) on Isle Royale and nearby mainland weather stations.

3 Mammals currently present on Isle Royale.

4 Classification of fat content of moose bone marrow.

5 Estimated number of wolves in midwinter, 1959-76.

6 Travel estimates for wolf packs, 1971-74

7 Occurrence of prey remains in wolf scats, 1973.

8 Beaver occurrence in summer wolf scats, and beaver population trends.

9 Incidence of beaver and moose remains in wolf scats from various homesites and associated trails, 1973.

10 Estimates of food availability for West and East packs, 1971-74.

11 Minimum number of breeding pairs present, 1971-74.

12 Numbers, movements, range, and prey availability for the West Pack, 1971-74.

13 Numbers, movements, range, and prey availability for the East Pack, 1972-74.

14 Scent-marking frequency in traveling wolves.

15 Successive rendezvous areas of the East Pack, 1973.

16 Pup production on Isle Royale, 1970-73.

17 Population estimates for the Isle Royale moose population, 1960-74.

18 Results of aerial moose censuses on Isle Royale, 1972 and 1974.

19 Fall aerial classification counts of Isle Royale moose, 1972-74.

20 Moose herd composition, summer and fall, 1970-74.

21 Proportion of moose calves in successive fall and winter aerial composition counts.

22 Twinning rates for Isle Royale moose from summer ground observations.

23 Average occurrence of twins in summer and fall, 1959-73.

24 Effect of snow depth on location of wolf-killed moose.

25 Cause of death for 836 moose, dying of natural causes, examined on Isle Royale, 1958-74.

26 Incidence of antlers and antler growth in skeletal remains of male moose.

27 Age-distribution of 307 wolf-killed adult moose compared to hypothetical occurrence in the Isle Royale moose herd.

28 Percentage of males (±95% confidence interval) in total sexed remains of carcasses and skeletons of adult moose.

29 Effect of snow depth on occurrence of calves in wolf kills.

30 Mean metatarsal length (in mm) for 6- to 12-month-old moose calves according to year of birth.

31 Mean metatarsal length (mm) of Isle Royale moose (primarily wolf kills).

32 Site of arthritic lesions in Isle Royale moose.

33 Incidence of fat-depleted bone marrow in moose killed by wolves, 1958-74.

34 Incidence of hydatid cysts in lungs of moose.

35 Life table for Isle Royale moose.

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Last Updated: 06-Nov-2007