Fauna Series No. 7

Fauna of the National Parks — No. 7 The Wolves of Isle Royale

RESULTS—THE TIMBER WOLF AND ITS ECOLOGY

Wolf-Moose Relationships

The welfare of a wolf population is related intimately to the availability of prey. In many areas wolves prey on two or more species, and if one becomes relatively unavailable, another provides sustenance. From most reports it appears that an adult moose is one of the most formidable prey animals in North America, so when another species is available, wolves tend to depend heavily on the other species. Peterson (1955:175) reported that in the St. Ignace Island area, "where moose were much more abundant than white-tailed deer," moose remains composed only 36 percent of 76 wolf scats collected, where as deer remains comprised 57 percent. In Alaska, Burkholder (1959) established that 14 of 22 wolf kills were caribou and 8 were moose. He believes that these species were killed in proportion to their availability. Nevertheless, six of the seven ageable moose were calves, and the other was a yearling, whereas several of the caribou were adults. In Mount McKinley National Park, Murie (1944:57) concluded that although caribou, Dall sheep, and moose are available, ". . . moose are not readily taken by wolves." Cowan (1947) found that, in the Rocky Mountain national parks of Canada, where several big-game species are present, moose remains occurred in 9 percent of 420 wolf scats, and most of the remains were those of calves.

MacFarlane (1905) and Stanwell-Fletcher (1942) each reported on a wolf which had been injured badly in an encounter with an adult moose, so it is evident why wolves are cautious in dealing with these animals.

Isle Royale wolves must resort to moose for their primary food, for no other big game is present. Because of this, the resident packs undoubtedly are expert at hunting and killing moose—any inept individuals probably perished long ago. Aerial observations of 66 hunts provide the basis for the following analysis of the wolves' hunting techniques. The number of hunts observed and hours of observation each year are summarized in table 15. Since the wolves hunt as they travel, the figures are based on observations of the wolves traveling, exclusive of rest periods and time spent on large bays or lakes. Moose often occur in groups, so the number of moose involved is given in each case. All observations included in the table were of the large pack or part of it.

TABLE 15.—OBSERVATIONS OF WOLVES^a HUNTING

When observing wolves hunting, we habitually flew ahead of them and spotted the next moose along their trail. This allowed observation of both wolf and moose behavior before, during, and after attacks. By refueling when the pack rested or passed through areas devoid of moose, we minimized the possibility of having to leave in the middle of a hunt. The fuel cache at Mott Island plus a 5-gallon can of fuel carried in the aircraft were advantageous in this respect. Each hunt witnessed is described in the appendix.

HUNTING HABITS OF THE WOLVES

After the wolves leave a kill, apparently any moose encountered is subject to attack. Once the animals showed interest in a moose only 35 minutes after leaving a kill. In an other case, they killed a moose within 4 to 10 hours after they abandoned their last carcass; and eight times they dispatched a moose within 26 hours after leaving a previous victim. They did almost all of their killing within 48 hours after they left their previous prey. In Alaska, a pack left a moose carcass at noon one day, and by the next morning had dispatched and eaten another moose (Burkholder, 1959). The average distances between kills made on Isle Royale are given in table 8.

The Isle Royale wolves employ a method of hunting moose which differs from methods reportedly used for other species. In Ontario, Dunne (1939) learned that in searching swamps for deer, packs split into twos and threes. Cowan (1947) and Stenlund (1955) found that wolves use a "line abreast" formation upon reaching areas to be hunted. When the predators hunt points and islands for deer in Minnesota, part of the pack drives while a few animals wait on the ice to intercept any prey flushed (Stenlund, 1955). Wolves hunt elk in the Canadian Rockies by traveling single file on ridges and rushing any quarry below them (Cowan, 1947). According to Murie (1944), the Mount McKinley wolves hunt Dall sheep by coursing through the hills hoping to surprise an animal at a disadvantage; when hunting caribou, they merely approach one of the many herds and begin chasing it.

Isle Royale wolves apparently do not have special places to hunt. Everywhere they travel is hunting ground, although more moose are killed in some areas than in others (figure 87). Areas of high moose-kill are characterized by dense moose populations and proximity to well-traveled wolf routes, as is evident by a comparison of figure 87 with figures 82 and 47.

Figure 87.—Distribution of kills.
(click on image for an enlargement in a new window)

The wolves' most common method of hunting is to travel regular routes single file until they scent a moose (figure 88). Most of the moose detected were within 300 yards upwind of the wolves. However, in one open region, the predators apparently sensed a cow and two calves about 1-1/2 miles away. After getting the weak scent of such distant moose, the pack travels toward them until it locates them more precisely.

Figure 88—The large pack hunting.

In certain cases the wolves detected moose 125 yards downwind or 200 yards crosswind. However, I once saw a moose browse undiscovered for 20 minutes, 100 yards downwind of the resting pack. In several instances wolves seemed to scent moose downwind or crosswind, but apparently could not locate them and eventually left. Commonly, when the wolves sense a moose, all stop and "point." Each stands stiffly with nose upwind and ears alert for 10 to 15 seconds, probably verifying the exact location of the moose. Often the animals assemble closely, sniff noses, and wag tails before starting toward the prey.

The wolves did not sense all the moose judged to be within range of detection. Of 160 animals in 85 groups which appeared to be within range, 29 (19 groups) went undiscovered. Some of these were within 150 yards, whereas others were as far as half a mile away. Moose were judged within range if circumstances appeared similar to those in which others had been detected during the many observed hunts. Apparently, topography, cover, local wind direction, and previous behavior of the moose all influence its "detectability."

A less-used hunting technique is tracking. When the wolves cross a fresh moose track, they follow it, single file. Sometimes a couple of animals follow downwind of the track parallel to the others but several yards ahead. On one occasion the wolves scented fresh tracks of a moose that had been working up a small valley below them. They did not actually come upon the tracks but were able to follow them from the ridge about 25 yards above. As the tracks got fresher, most of the wolves headed into the valley and began tracking. One paralleled them on the ridge and finally located the moose, which had moved up the side of the valley. An other time the wolves scented moose sign 20 minutes old from about 50 yards upwind, after the moose had left the vicinity.

From the aircraft, I could determine that a moose trail was fresh only when the moose which made it was nearby. Thus, the wolves frequently may have passed up fresh tracks without my knowledge. On February 6, 1961, the pack of 15 crossed tracks 1 minute old, but only 1 wolf followed them. It gave up after 25 yards and returned to the pack. Before and after this occurrence the wolves were chasing moose, so their unconcern apparently did not result from a lack of motivation.

After detecting a moose, the wolves head excitedly toward it single file, but they check their speed until the moose bolts. If the moose makes a stand, the wolves lunge at it from all sides, trying to force it to run. They are readily frightened by its charges, however, and seldom get close (figure 89). When a moose charges, the wolves scurry several yards away, tails between their legs. If they cannot make the moose flee, they leave. They may decide to depart within half a minute, but sometimes they harass a moose intermittently for 5 minutes before leaving. I once saw a pack harry a standing moose for 3 minutes, finally force it to run, and then kill it within about 10 minutes.

Figure 89a—Fifteen wolves approach a moose in burn area.

Figure 89b—The moose stands its ground.

Figure 89c—Wolves harass the moose from a distance.

Figure 89d—The moose holds off wolves; after 5 minutes, wolves leave.

If a moose runs at the approach of wolves, the predators suddenly spring forward with great bounds. Their gait appears exhausting, but they can maintain it for at least 20 minutes. After long runs, they rest for at least 10 minutes. During our observations, the wolves ran faster than the moose through snow less than 2 feet deep, and sometimes within 200 or 300 yards they overtook animals which had a 150-yard lead. However, on one occasion the pack took about a one-quarter of a mile to catch up to a moose with a 100-yard head start. Burkholder (1959) reported an incident in which nine wolves covered 300 yards through snow 2 to 3 feet deep while their prey, a yearling moose, ran only 100. In my experience, pursued moose do not always run at top speed; they seem to depend more on endurance.

Young and Goldman (1944) reported that a wolf was clocked at 27 m.p.h. for 200 yards, but Minnesota wardens chased a wolf on a frozen lake for 4 miles, at 35 to 40 m.p.h. (Stenlund, 1955). The latter rate correlates well with the maximum speed reported for moose (35 m.p.h., by Cottam and Williams, 1943). Thick swamps, heavy cover, blowdown, or snowdrifts slowed the wolves but did not hinder moose. Apparently wolves are aware of their limited ability under adverse conditions, for they sometimes fail to chase nearby animals which run through snowdrifts or blowdown. Hatter (19<50a) found that in British Columbia "wolves cannot prey successfully on moose in deep soft snow."

The wolves abandoned 20 observed pursuits without catching up to their intended victims. In these cases, either the moose had too great a lead, or adverse running conditions hampered the wolves. Sometimes one or two wolves got within a few yards of a moose and gave up if the rest of the pack was far behind. Usually if a moose maintained a 100-yard lead for 10 or 15 seconds, the wolves discontinued the chase unless they were gaining. Crisler (1958:106), studying wolves and caribou in Alaska, also was impressed with ". . . how quickly the wolves had judged when a chase was useless."

During extended pursuits, most of the wolves follow single file in the trail of the moose until they over take the animal, undoubtedly making travel easier. Frequently some wolves try shortcutting the moose, but if it turns, these individuals may head in the wrong direction and lose the pack.

After the wolves overtake a moose, most of them remain strung out behind (figure 89), but some stay alongside, apparently awaiting the opportunity for attack. Since pursued moose sometimes travel 2 or 3 miles through several types of cover and over varying terrain without being assailed, probably the physical condition of the moose determines its fate. Although I distinguished no behavioral difference between the three adults seen attacked and the several which fled unharmed, probably the wolves discerned a difference. Two of the moose were killed within 100 yards of where the wolves first encountered them. (The other was wounded within 100 yards but then was abandoned.)

It seems likely that wolves can detect any weakness or inferiority from the behavior of a moose. Certainly if an animal is not strong enough to outlast its pursuers, it will be killed, and this presumably is why the wolves chased some individuals so far without attacking them. Perhaps the two animals killed were debilitated enough so that the wolves immediately detected this and did not hesitate to attack. Two reports in the literature are pertinent here. In Wood Buffalo National Park, Fuller (1960) watched at least 10 wolves approach to within 25 feet of 4 bison. The only bison that showed concern was a wounded one; the others continued ruminating. On two other occasions, Fuller saw (from the air) wolves within a herd of bison which paid them no attention. In East Africa, Wright (1960) watched wild dogs pass near several groups of gazelles (Gazella thomsonii) without frightening them. Only one individual became panic-stricken and ran—it was chased and killed. In these instances, even humans detected behavioral differences between healthy or confident" animals and insecure ones.

Extensive observations of wolves hunting caribou in Alaska show that a primary technique is to chase caribou long distances until a weak or inferior individual is located (Murie, 1944; Crisler, 1956). Murie saw tracks of a chase that lasted 3 or 4 miles, and Crisler witnessed a 5-mile chase. Dall sheep may be pursued for one-half mile over rugged terrain (Murie, 1944), and even deer sometimes are chased vainly for long distances (Dunne, 1939). Thus, it appears that the long-chase technique is employed by wolves in many areas and that it probably serves as a test to distinguish vulnerable individuals.

The technique that wolves use when pursuing a cow moose and calf is to attempt to separate them. While some animals harass the cow, others remain beside the calf, and as soon as the cow charges a wolf, those guarding the calf close in. If the cow fails to keep up with the calf, if thick cover causes the two to separate, or if the cow becomes too involved with chasing off wolves, some animals immediately assail the calf. If the others keep the cow occupied for half a minute, the calf probably is doomed, for two wolves can handle a 9-month-old calf easily. It seems amazing that any cow and calf could survive such strategy. However, eight instances were observed in which cows and calves ran from wolves, and only in three of these were calves killed. This further attests to the profound respect the wolves have for the hoofs of a moose.

Since an insufficient number of hunts by smaller packs and lone wolves were observed, generalizations cannot be made regarding the hunting habits of these wolves. The reader is referred to Hunting Account 31 in the appendix, which involves the pack of three, and 42, 43, 70, and 71, concerning a lone wolf. Although no evidence was obtained that a single wolf can kill an adult moose (unless wounded), the fact that an individual was observed seriously attempting this indicates that at least the wolf thought it might succeed. Young and Goldman (1944) stated that one wolf can kill a full-grown moose, and Cowan (1947:159) reported that in the Rocky Mountain national parks of Canada, "several instances of single wolves killing moose and elk were noted."

ACCOUNTS OF WOLF ATTACKS

Information on the killing techniques of the wolf was obtained from eight hunts in which moose were killed or wounded. Complete, successful hunts of three calves and one adult were witnessed, plus parts of three hunts in which adults were killed, and one in which an adult was wounded and abandoned. In addition, certain information was obtained from an adult killed in summer. The following accounts (numbered for their chronological position in the hunting accounts included in the appendix) are edited versions of field notes. Because of the distances, speeds, and number of moose and wolves involved in each account, and because of the necessity for constant observation from the rapidly moving aircraft, notes are incomplete in some accounts. All distances are estimated.

2. (February 24, 1959. About three-quarters of a mile northeast of Siskiwit Lake Outlet, and about three-eighths of a mile inland.) At 6 p.m., 10 of the 15 wolves were traveling along the shore of Siskiwit Lake about 1 mile ahead of the others. Suddenly they stopped, and several pointed more or less crosswind for a few seconds toward three adult moose three-eighths of a mile away. Heading inland single file to an old beaver meadow, they traveled downwind a few hundred yards, veered, and continued for 250 yards until directly downwind of the moose. Then they ran straight toward the animals, which were still browsing when the wolves were within 150 yards. Two of the moose sensed the wolves 25 yards away and began running. The wolves gave chase a few yards until they spotted the third moose, which was closer and had not left. They immediately ran the 50 feet to this animal and surrounded it.

A few seconds later the moose bolted and the wolves followed in its trail (figure 90). Soon five or six animals were biting at its hind legs, back, and flanks. The moose continued on, dragging the wolves until it fell. In a few seconds the animal was up, but it fell a second time. Arising again, the moose ran through the open second-growth cover to a small stand of spruce and aspen, while the wolves continued their attacks; one wolf grabbed the quarry by the nose. Reaching the stand of trees, the moose stood, bleeding from the throat, but the wolves would not attack.

Figure 90—Wolves pursuing a moose near Malone Bay.

Within a few minutes most of the wolves were lying down, including the last five, which had caught up. Two or three continued to harass the moose without actually biting it, and the moose retaliated by kicking with its hind feet. Whenever the animal faced the wolves, they scattered. Although the moose was bleeding from the throat, it appeared strong and "confident." At 6:30 p.m. we left because of darkness.

The next morning at 11:15 a.m. the wolves were gone. The moose lay within 25 feet of where it had made the stand. After we made several low passes, it finally arose and moved on. Although walking stiffly and favoring its left front leg, the moose was not bleeding and seemed in good shape. The wolves were 16 miles away feeding on a new kill.

3. (March 1, 1959. About a quarter-mile south of Lake Desor, and about half a mile southwest of the northeast end of the lake.) From 10:10 a.m. to 5:05 p.m. the 15 wolves lay on Lake Desor, but at 5:05 they began traveling along the shore. Several seemed to point inland, but mating activity obscured this somewhat. After searching the vicinity and finding no moose nearby, we headed for Mott Island to refuel.

When we returned at 6 p.m. the wolves had a bull (as determined later) surrounded in a small stand of hardwoods. He was bleeding steadily from the throat, and had difficultly holding his head up. About 150 square feet of the surrounding snow was covered with blood. The animal's lower left hind leg was bloody, and he leaned against a tree, keeping his right hind leg centered under him. (Chances are good that this animal had been assailed the night before and then temporarily abandoned.)

Most of the wolves were yards away, resting and playing, but a few were licking the bloody snow. One wolf in particular, whose legs were covered with blood, was harassing the moose. It stayed near the bull most of the time, often nipping at the injured leg. However, each time the moose faced it or any nearby wolves, they scrambled away. At 6:30 p.m. we left because of darkness.

Unfavorable weather prevented a check on the situation until March 4. At 10:45 a.m. the bones of the moose were scattered around the spot where we had seen him last. The wolves had just left and were 10 miles away. An examination of the remains showed this to be a bull in wear-class VI (Passmore et al., 1955).

7. (February 5, 1960. About 200 yards south of Siskiwit Lake and 1 mile west of Wood Lake.) At 3:50 p.m. we left the 16 wolves heading across Siskiwit Lake near Ryan Island. After refueling, we tracked them to the southeast shore of the lake, up the first ridge, and along it northeastward. At 4:35 p.m. we saw them running upwind on the open ridge toward a cow and two calves about three-quarters of a mile away. The way the wolves had veered upon reaching the ridge about 1-1/2 miles from the moose suggests they had smelled the animals at that distance.

When still three-quarters of a mile from them, several of the wolves stood on a 100-foot ridge and pointed toward the moose, which now faced them. The first few animals charged off the ridge and ran toward the moose but a little north of them. Two wolves were far ahead, and two others ran south of the trail left by the moose.

The cow and calves eventually headed toward Wood Lake, but upon encountering a steep drop-off, they turned southward. The first two wolves sped after the moose, gained rapidly, and overtook them within a quarter mile. As the moose ran through open second-growth birch, one wolf remained on each side.

The cow was immediately behind the calves, and twice she feinted toward the wolves, which leaped out of the way. Most of the pack began catching up, and as the moose entered a small cedar swamp (the nearest conifer cover), four or five animals tore at the rump and sides of a calf and clung to it. Within 50 feet, the calf went down in a thick clump of cedars. The cow and the other calf continued through the cover with two wolves still following for 20 yards. When these wolves gave up, the moose stopped and returned 50 yards toward the wounded calf. Gradually, however, the moose drifted back toward where they originally had started. Most of the wolves concentrated on the wounded calf, which remained where it had fallen. The cedars obscured our vision, but the calf appeared dead within 5 minutes after it fell.

The snow in the area was only a foot deep, but the wolves were sinking in about 6 inches.

8. (February 7, 1960. About half a mile southeast of the south west end of Angleworm Lake.) At 4:10 p.m. the pack of 16 headed north from Moskey Basin for a quarter-mile on an old wolf trail. At 5 o'clock the animals suddenly veered upwind and became alert, often stopping and pointing or scenting the wind. All wolves stayed close together and did not dally. They traveled three-quarters of a mile to within 250 yards of a cow and calf which were browsing directly upwind (5:30 p.m.).

The wolves gave no indication of scenting the moose. Instead, they turned through a thick spruce swamp; but when a third of the way across, they suddenly headed toward the moose. As the pack approached to within 100 yards, the moose started running, the cow behind the calf. The wolves gave chase and soon were racing alongside and behind them.

Throughout the chase, the cow defended the calf, charging the wolves frequently. One animal managed to bite the calf's rump once but did not hinder the animal. The pursuit continued for 200 to 300 yards (through many types of cover and over varying terrain) without an attack, but eventually the wolves separated the moose. Most of the pack pursued the calf, while two animals followed the cow. After a chase of several hundred yards more, a few wolves attacked the rump and flanks of the calf; one grabbed it by the left hind leg. The cow caught up with the group and managed to stamp on one wolf, which arose instantly and appeared unhurt. The others released the calf and continued pursuing it for another hundred yards before attacking again. They finally pulled the animal down and tore at it, but it arose and the cow rushed in. Some of the wolves fled, but others chased the cow. Then the wolves assailed the calf once more. One grabbed it by the nose, and three or four tore at its neck and throat; others ripped at its rump. The calf's hind quarters went down, but the animal continued on, dragging its hind legs and the wolves that were attached to its body. It managed to stand once more, and the cow started to charge again, but one wolf chased her away.

The wolves made a final attack on the calf, and it was unable to arise. Then they lined up side-by-side around the carcass and began feeding. The cow gradually wandered back toward where she had been jumped.

13. (February 12, 1960. About 1-1/2 miles southwest of Halloran Lake and about 200 yards northwest of the Isle Royale shore.) The 16 wolves were traveling along the shore when suddenly they veered inland about 2:30 p.m. toward a lone cow (sex determined later) standing on a ridge 200 yards upwind. The animal ran when the pack was 100 yards away, and the wolves charged up the ridge and continued on her trail. The cow ran slowly and stopped to look back at the approaching pack, which caught up within 100 yards. She stood next to a bushy spruce for protection, and as the wolves lunged, she charged and kicked at them with all four feet. Although she seemed to connect with her hind feet, apparently no animals were injured.

Meanwhile, the whole pack caught up. The moose defended herself for about 3 minutes while backed against the spruce, but suddenly she bolted and fled toward the end of the ridge. The wolves attacked her rump and flanks but released their holds as she brushed through some thick spruces. They pursued the animal for 25 yards to the end of the ridge, where all plunged down the steep slope.

When the moose landed at the base of the ridge, the wolves were attached to her back and flanks, and one held her by the nose. The downed animal attempted to rise, but the sheer weight of the wolves seemed to anchor her. The wolf grasping her nose held on firmly while she violently shook her head. Most of the animals continued working on her rump and flanks, while two tore at her shoulders.

The moose struggled for more than 5 minutes while the wolves, packed solidly around her, tugged away. Two individuals had to wait at one side, for there was no room around the moose. The "nose-wolf" continued its hold for at least 10 minutes, while the others pulled from all sides. After about 10 minutes, the moose appeared dead. This cow was in wear-class VI.

14. (February 15, 1960. About half a mile downstream of the junction of the Grace Creek Trail and Grace Creek.) At 2:10 p.m. the 16 wolves were heading down Grace Creek. Suddenly, they pointed toward a cow (as determined later) 200 yards to their left. Then they continued down the creek to where it wound closer to the moose. Heading inland over a knoll, the wolves surprised the cow 25 yards away.

The animal fled, but the wolves caught up almost immediately. One grabbed her right hind leg just above the hoof. However, as the cow trotted through some spruces, she shook the wolf loose. She then ran in a semi-circle toward the creek (figure 91), and several times the wolves overtook her but failed to attack. Once when she ran through a snowdrift, the wolves lost ground, but they quickly caught up again.

Figure 91—Wolves pursuing a moose near Grace Creek.

As the moose started down a shallow valley, the wolves attacked her rump. She soon shook them, however, and proceeded to the frozen creek bed, where the wolves attacked again. One animal kept jumping at her nose and finally grabbed it; others fastened onto her rump and flanks. The cow fought hard and dragged the wolves about 100 yards downstream (figure 92). Three or four times, she lifted the "nose-wolf" off the ground and swung it for several seconds before lowering her head. This wolf maintained its grip for over a minute. The moose continued fighting hard and finally shook the wolves and ran back upstream, with the whole pack following.

Figure 92—Wolves attacking a moose. Note that one wolf has the moose by the nose (arrow).

The cow started into the woods and the wolves lunged again. The moose kicked constantly and trampled two individuals into the snow. One of them crawled away but later seemed unhurt. The moose then stood next to a small balsam along the creek shore and continued to fight off the wolves, which soon gave up temporarily and lay on the ice. At 2:35 p.m. they went 200 yards downstream and assembled. They returned to the animal three times but found her belligerent, although blood from her wounded rump covered several square yards of snow. Nevertheless, there appeared to be no mortal wound.

From 2:50 to 3:25 p.m. the wolves lay on the nearby ice. Meanwhile, at 3:20 the moose walked about 10 yards and lay down. At 3:25, the pack approached and she arose again. Although appearing stiff, she charged the wolves effectively. Many of them were eating the bloody snow where she had stood first (figure 93). At 3:40 the wolves lay down again and at 3:50 so did the moose. About a minute later, a wolf approached the moose and she arose again. At 4:12 this occurred once more. Then the wolves entered some spruces 25 yards south of the moose and curled up. From 4:20 to 4:40 we were refueling, but when we returned, the wolves were still there.

Figure 93—The large pack waiting for the wounded moose to weaken. See Hunting Account 14. (c) National Geographic Society, courtesy National Geographic Magazine.

At 5 o'clock, they arose, tested the moose, and found her quite pugnacious. Ten minutes later, 14 of the animals left and headed southward while 2 remained curled up within 25 yards of the wounded moose, which was also lying down.

From 5:35 to 6:05, the pack visited an old kill half a mile south of the creek; the animals then traveled back along a ridge until half a mile from the wounded cow. Meanwhile, the two "guards" arose and stood near the moose. The pack headed almost directly toward them, and at 6:40 p.m. when we had to leave, the pack was within a quarter-mile and still heading toward the wounded moose. The next morning at 10:50 a.m. the wolves were feeding on the carcass, which was where we had seen the live animal. Ground observation later showed that this cow was in wear-class VI.

40. (March 17, 1960. About 50 yards southwest of the Island Mine Trail and about half a mile from Siskiwit Bay.) At 11:10 a.m. the large wolf pack was resting along the trail, and at 11:25 the animals slowly headed 150 yards farther up the trail. At 11:35 they suddenly turned upwind and ran about 50 yards into a thick spruce stand. Two moose ran through the stand and split up. Because of thick cover, we could not see the wolves.

Suddenly, however, a calf, pursued closely by two wolves, headed out of the stand, down the trail 100 yards, and into a spruce swamp on the other side of the trail. Within 100 yards, the wolves began nipping at the hind legs of the moose. After another 50 yards, one wolf was clinging to the animal's rump and the other to its throat. The moose stopped and trampled the front wolf, but the wolf would not let go. It clung to the calf's throat for about 2 minutes while the calf continued to pound it and drag it about.

Finally this wolf released its throat-hold, but the other still stuck to the rump. The first wolf then stood on its hind legs, and placing its front paws on the left side of the moose, started chewing the side of its neck for several seconds. The calf soon brushed this animal against a tree, but the wolf then dived under the moose and fastened to its throat. As the running moose straddled the wolf, the wolf ran along with it for about a minute.

Meanwhile, two other wolves caught up. One bit the calf around the head and finally grasped its nose. The other grabbed the right flank and then changed to the rump where it clug for about a minute while the moose continued on. This, one wolf had the calf by the nose, one by the throat, and two by the rump. The animal soon stopped and was pulled down under a small clump of trees. In about 3 minutes, it ceased struggling (11:45 a.m.).

When the two wolves first attacked the calf, the cow tried to catch up with it but was too far behind the swamp to find it. Total distance of the chase was about one quarter of a mile. Apparently, the rest of the animals had been chasing the cow; eventually they found their way to the calf also and joined in the feed.

41. (August 26, 1960. West shore of the north arm of Chickenbone Lake about 75 yards south of the outlet.) At 6 p.m. on August 23, four campers noticed a cow moose about 30 feet inland of the above location with a large open wound on her left hind flank. She seemed reluctant to move. Two days later, Chief Ranger B. J. Zerbey reported that the moose appeared sore and short of breath and would not arise. Two nearby bloody beds indicated that she had arisen a few times but had been hesitant, or unable, to leave the area. At 6 p.m. the same day, two fishery biologists saw her in the mud at the edge of the lake and thought she acted lively. However, the next morning at 8 o'clock, as these men approached the area, they heard wolves barking. Rowing by in their boat, they saw that the moose was dead.

At noon I examined the carcass. There was a surface wound about half an inch wide on the left cheek, and several long gashes on the throat, but none of these had bled much. Horizontally across the upper left hind leg was a wound about 2-1/2 inches deep, 4 inches wide, and 8 inches long. The exposed muscle hung ragged, appearing well chewed; undoubtedly, this was the wound observed by the campers, Zerbey, and the biologists. The only other exposed area was the pelvic region. The flesh there had been eaten through to the coelom, and a few loops of intestine were pulled from the body. Probably most of this damage resulted from feeding, for none of the observers mentioned wounds in this area although they did notice the less conspicuous upper-leg injury.

This cow was in wear-class V and harbored 57 hydatid cysts in her lungs.

During the next 2 days, I saw (from an aircraft) at least three, and possibly four, wolves on a nearby ridge, and heard at least four. These may have represented both small packs or part of the big pack.

71. (March 12, 1961. About 1-1/2 miles southeast of the northeast end of Lake Desor.) From March 6 to 11, the aircraft underwent its 100-hour check, so no aerial work was done until the 12th. About 10 a.m. the large pack was found at a fresh kill southwest of Halloran Lake. Tracks showed that the last kill had been near Fisherman's Home. While backtracking the wolves from there, we saw a lone wolf also backtracking them along the northwest shore of Siskiwit Bay at 10:30 a.m. We continued following the trail to the above location, where a badly wounded moose lay on an open hillside. Tracks showed that the large pack had wounded this animal, stayed around for at least several hours and then abandoned it. This probably happened about March 8.

At 12:20 p.m. we saw the lone wolf 1-1/2 miles from the moose, still backtracking the pack. From the animal's attitude when approaching I had little doubt that it knew the wounded moose was there. When 250 yards away the wolf ran excitedly up the trail but became cautious when 50 yards away and circled to the west to approach. I learned later that the moose was a bull. He was lying in a blood-soaked patch of snow about 15 feet in diameter which he had not left since the attack. As the wolf came within 30 yards, he arose (12:55 p.m.). The wolf approached to within 10 feet, circled the bull a few minutes, and went off 30 feet and lay down (figure 94). After 5 minutes, the moose lay down; immediately the wolf ran to him, so the moose stood (figure 95). The wolf lay down about 20 feet away, and 10 minutes later the bull lay down. Again the wolf threatened him, tail wagging excitedly, and seemed to try for his nose but failed. The moose just stood without moving quickly or threatening the wolf. A few minutes later the wolf lay down again. The bull continued standing at least from 1:20 until 1:45 p.m. when we left to refuel.

Figure 94——A lone wolf waiting for a wounded moose to weaken.

Figure 95—The wounded moose stands upon approach of the wolf.

From 2:45 to 3:30 the wolf lay sprawled on its side about 20 feet from the moose, which continued standing.

At 4:45 the bull was alive but lying down, and the wolf was tugging at his rump. Intermittently the moose watched the wolf but made no threats. He seemed to have no feeling in the rump, or more probably, he was too weak to stop the wolf. At 5:55 the moose was still alive, but by 6:30 p.m. he was dead, lying on his right side.

The next morning at 11 o'clock, I examined the carcass. The only apparent wounded areas were the rump and thighs, but the pelvic region had been fed upon, so it also may have been wounded. The animal was a bull in wear-class VIII and was heavily parasitized with ticks and hydatid cysts, although the femur marrow was normal.

DISCUSSION OF KILLING TECHNIQUES

In all kills witnessed, the first point of attack was the rump. In fact, this region was the only site of severe wounds on the two adults examined from the ground (figure 96). During 1956 and 1957, Cole (1957) observed adult moose on Isle Royale wounded in the thighs. In Alaska, Burkholder (1959:9) chased off a pack of wolves attacking a moose. The animal died that night and was examined from the ground the next day. "The only injuries observed consisted of deep bites and tears on the hams above the hocks. The animal was not hamstrung, the tendons being still intact. No other wounds were noted." Cowan (1947:159) examined several elk kills and in each instance the attack had been from the rear and side with the wolf seizing the flank at the point where the leg joins the abdomen." In Minnesota, a moose was reported wounded on the hind flanks by wolves (Stenlund, 1955).

Figure 96—Freshly killed moose showing wounded area.

Young and Goldman (1944) reported that wolves hamstring their prey and eat out the hams. However, no evidence of hamstringing was found during the present study; and Burkholder, Cowan, and Stenlund each stated specifically that they observed no instance in which wolves hamstrung their prey. During caribou studies in the Northwest Territories, Banfield (1954:47) found that hamstringing was seldom done. "The method that the wolf generally uses for killing a caribou is to race alongside of it and pull it down by grasping the flank, shoulder, or throat, with the jaws." Stenlund (1955:31) gave the following description of killing techniques used on deer:

No evidence of hamstringing of deer was found on freshly killed carcasses although the possibility does exist. Usually deer are run down from behind, the wolf or wolves biting at the hind flanks and abdomen, or at the hind flanks and head region simultaneously. Often the deer is knocked to the ground two or three times before it is killed. It is possible that some deer might even die from a combination of shock, fright, and exhaustion rather from actual wounds since in some cases it did not appear that animals were wounded badly enough to cause death.

Five of the six times I watched wolves wound moose, several animals slowed down the prey and occupied its attention by pulling at its rump, and then one wolf grabbed the nose (figure 92). During the sixth hunt, trees obscured my view after the wolves attacked the animal's rump. Although the nose hold is not mortal, it stops the prey and distracts it from the wolves on the rump. In some hunting accounts furnished by Young and Goldman, mention is made of part of the pack distracting the prey at the front while others inflict significant damage to the rump.

Although the rump region contains no especially vital parts, it seems to be the least dangerous and most advantageous attack point on such a large animal as a moose. After the wolves rip through the 5-inch hair and thick hide, every injury to the upper-leg musculature would hinder the movement of the prey and render the animal more susceptible to intensified attack. Once downed and besieged by several wolves, the moose succumbs quickly. Each of three calves and one adult was killed within 10 minutes.

Apparently many adults are wounded, left to stiffen and weaken from their rump wounds, and then killed (figure 93). Three instances were observed in which this tactic was employed, and Cole (1957) reported two. Probably bulls attacked by the large pack, and any adults tackled by the smaller packs, are likely to be victims of this tactic. This probably is not done purposely; rather it appears to result from the wolves' failure to wound a moose sufficiently to incapacitate it.

Sometimes when the wolves wound a moose, they cannot complete the kill, so they abandon the animal. The moose probably dies within a week, and there are indications that the wolves return and feed on the carcass. The animal involved in Hunting Account 2 was abandoned on February 24, 1959, but on March 13 the pack was feeding on a kill within a quarter-mile of where the animal had been left. Of course, there is no certainty that this moose was the one abandoned. An observation on March 15, 1960, also may be significant. After we followed the large pack all day, the animals began exploring a swamp southwest of Halloran Lake. They did not appear to be hunting but seemed to be searching for something. Eventually they discovered a calf carcass beneath a clump of cedars and began feeding. The femur marrow of the animal was fat-depleted and the stomach was full of cedar. Since this area is heavily hunted by the wolves, it seems unlikely that a moose would get a chance to starve to death. It is more likely that the wolves had wounded and abandoned the animal in this area, and that the only available browse was the small amount of cedar from surrounding trees.

On another occasion, when the wolves had not eaten for 4 days, we followed them all day until they excitedly entered a small clump of trees. Although there was no chase, the animals raced to the clump and gathered there. After about 45 minutes, a few left the clump and lay in the open for the rest of the day.

The next day we found that they had fed on a carcass under the trees. Either they killed the moose while we circled above (doubtful, since their actions did not indicate this), or they found the carcass. The femur marrow was normal, so the animal was not in a state of extreme emaciation. Perhaps it had been wounded and abandoned weeks before.

The moose involved in Hunting Account 72 undoubtedly would have died within a few days if the lone wolf had not finished it off, for it remained for days in a small area, which soon was completely browsed out.

MOOSE DEFENSE

Regarding the senses of moose, Peterson (1955:102) found that ". . . the ears often serve to alert the animal, the eyes to investigate, while the final stimulus, causing immediate reaction, is transmitted by smell." Moose behavior during the present study indicated the same. Since wolves generally traveled upwind to their intended prey, they often approached to within 100 yards before discovery by the moose. Under certain conditions, moose sensed wolves when a quarter of a mile away, but on one occasion the pack came within about 5 feet of two moose in their beds. Differences in wind direction and velocity, cover, terrain, and other factors probably accounted for the varying abilities of moose to sense wolves.

No observations were made on the summer defense of moose against wolves, but two reports indicated that the animals seek refuge in water when threatened. In 1958, campers related to D. L. Allen that they saw a cow and calf enter Tobin Harbor at its head. Then three wolves emerged from the woods, paced the shore about three times, and returned into the brush. A similar occurrence was noticed by Mrs. Alfreda Gale of Tobin Harbor and H. T. Orsborn of Rock Harbor. Mrs. Gale reported that, in July 1959, she and Orsborn spotted a moose "prancing and snorting" about 20 feet from shore in Gutt Bay, Tobin Harbor. Then they spied a wolf on shore near the moose and were convinced that the moose was taking refuge from it. After a few seconds, the wolf ran off, but the moose remained in the water for several minutes.

Sign around remains of a calf killed by wolves about August 5, 1960, indicated that the moose had been killed in the shallow water of an old beaver pond. Apparently, it sought protection in the pond but did not reach deep water. Cowan (1947:160) reported that "on several occasions single deer, elk, and moose have been seen making use of this defensive behavior . . .," and Peterson (1955:104) agreed that ". . . moose regularly make for the nearest water when seeking protection from predators."

In seven hunts (11 animals) witnessed in winter, moose detected wolves before being discovered, and in each case, the moose immediately left the area. Since it will be shown that a low percentage of moose tested by wolves are killed, it is safe to assume that most of the above-mentioned moose were not highly vulnerable and that, nevertheless, they chose to avoid an encounter with the predators.

During 36 hunts involving 73 moose, the animals fled without being attacked (table 16). They all were pursued, but either obtained adequate headstarts, outdistanced the wolves, or outlasted them. Soon after the wolves discontinued a chase, the pursued animal stopped and watched its backtrail. Several times moose in such situations appeared to be saving their strength until hard pressed. Murie (1944) noticed that caribou frequently stood around watching nearby wolves ". . . when they could have been moving away to a more secure position." Perhaps this was for the same reason, for caribou also depend upon speed and endurance to escape wolves.

TABLE 16.—DEFENSIVE BEHAVIOR OF MOOSE ENCOUNTERED BY LARGE PACK OF WOLVES OR PART OF IT^a

[Parentheses indicate the number of hunts involved]

Even when the wolves overtook their quarry, in many cases the moose continued running until their pursuers tired. In seven instances (nine animals) moose ran at least half a mile before the wolves abandoned chase. Three animals, including a cow and calf, traveled about 2-1/2 miles, and another cow and calf ran approximately 3 miles before the wolves gave up. During long pursuits, even after the pack stops, the moose usually continue running for at least a quarter of a mile.

The gait of a moose chased by wolves appears effortless; the animal takes long, deliberate, trotter-like strides. The longest distance I saw moose maintain this gait was an estimated 3-1/4 miles. This was accomplished by a cow and calf closely pursued by the large pack on March 11, 1960, and it helped them escape. Although healthy moose apparently can outlast wolves, they cannot out-man them. Once wolves catch up to a moose, they usually remain even with the animal unless deep snow or tangled blowdown interferes. However, the maximum speeds of the two species appear to be similar, for if a moose maintains about a 100-yard lead for 15 or 20 seconds, the wolves soon give up. Reported speeds for the moose are: 19 m.p.h. (Peterson, 1955), 22 m.p.h. (Findley, 1951), 27 m.p.h. (Cowan, 1947), and 35 m.p.h. (Cottam and Williams, 1943); a moose maintained the latter speed for a quarter of a mile. Peterson reported that moose gallop occasionally when frightened, but I did not observe this gait used by any moose pursued by wolves.

Running moose were not hindered by blowdown, thick swamps, or 3 feet of snow. Peterson (1955) found that 30 inches of encrusted snow presented little hindrance to moose, but cited findings by Wright et al. that crusted snow lacerates their legs, seriously hampering travel. Murie (1944) also mentioned this hazard. Since strong crusts apparently did not form on Isle Royale until early spring, no observations were made on this subject. However, in late March and early April, crusty snow might be a significant factor in wolf-moose relationships. In British Columbia, changing snow conditions were found to affect predation on moose profoundly. Stanwell-Fletcher (1942) reported that wolves did not hunt moose "in earnest" until the end of January, when 6 to 8 feet of snow impeded travel by the latter.

Since so many moose escaped by running (even those which the wolves overtook and chased for more than a mile), running appears to be a successful defense. However, each of the five mortal attacks witnessed from the beginning involved running animals. Perhaps this is because a moose does not employ the most effective weapons, its hoofs, so easily when running as when standing still, and because weak or inferior animals, which may be afraid to defy wolves, probably are among those moose that flee. Since any weakness probably affects the ability of a moose both to run and utilize its hoofs effectively, the wolves soon might sense the animal's debility.

Twelve moose, involved in 11 hunts, fled as wolves approached but stopped before, or as soon as, the pack caught up. The moose then stood at bay and held off the wolves. In 17 other instances, 24 moose stood their ground immediately, including 4 cows with calves. In addition, a cow and calf stood at first and then defensively strode off with wolves following for about a quarter of a mile.

None of the 24 moose which stood at bay throughout an attack was killed or wounded. I do not know whether this is because only strong, healthy, "confident" animals defy their attackers or because defiant moose are so formidable. Probably the reasons are of equal importance. It is obvious that moose which stand their ground when confronted by wolves enjoy full command of the situation (figure 1). On February 28, 1961, I saw a moose stride boldly for about 70 yards to meet seven approaching wolves which turned and left when the moose was about 30 yards away.

It is easy to see why wolves fear an enraged moose. With mane erect, ears flattened back, neck extended and head held low, the surrounded animal lashes out at the nearest wolf, and wheels quickly to chase any individual which dares close in behind. Both fore and hind hoofs are used with great facility and exactness. Sometimes a harassed moose backs against a conifer or other protection for its rear. Because my observations were from an aircraft, only the conspicuous manifestations of rage were seen, but Denniston (1956:111) provided a more complete description of the rage pattern:

In this pattern the mane is erected, the ears flattened back against the neck, the lips retracted, the tongue protruded and curled up over the upper lip and nose and repeatedly darted in and out licking the upper lip. The animal usually rears on its hind legs pawing the air with the forefeet, if the stimulus object is at a distance of less than about forty feet.

An informative close-up account of defensive behavior of an Isle Royale moose against a dog is furnished by Hickie (n.d.:28):

The cow . . . charged Togo, ears back, head close to forelegs with outstretched neck and bristling mane, and both fore and hind feet stamping at every jump—and how she came! We barely had time to get around behind a cabin before she charged past. Togo tried to duck to the side, but the heavy snow was too much for him and he floundered. I thought it was all up with him, for she caught him and pounded him with her front feet; but as luck would have it, her aim was poor and he ran toward me. She wheeled and caught him, again rolling him in the snow. This time he managed to roll back under her front legs, scramble out to one side and run down towards the house. After this Togo stayed his distance and as soon she laid back her her ears and moved towards him, he was gone like a flash down the trail.

I once watched a moose beat two wolves into the snow, but these escaped unharmed; apparently the blows were softened by the snow. In another instance a 9-month-old calf pounded a wolf clinging to its throat and finally persuaded the animal to seek a less-hazardous hold; but the wolf was not hurt. In British Columbia, a large male wolf was found barely alive, with broken ribs and legs. "Surrounded by moose tracks, blood patches and moose hair, the wolf had been cripped in a great battle" (Stanwell-Fletcher, 1942:138). In the Canadian Northwest Territories, MacFarlane (1905) found a live adult wolf with a hind leg shattered by a kick from a bull moose.

When a moose stands defiantly at bay, the wolves try to force it to run, but unless it does, they abandon it. Once when a moose stood its ground, the wolves left within 30 seconds, and the longest the pack harrassed an uninjured standing moose was 5 minutes. Cowan (1947) reported instances of a cow elk standing off seven wolves and of a moose discouraging three wolves by standing and striking with its forefeet. Additional evidence that Isle Royale wolves fear moose at bay was afforded by five observed hunts in which the pack chased one of a group of moose until it stopped, and then immediately pursued one of the other fleeing animals. Even when a moose is wounded but throws the wolves and stands its ground, the wolves fail to continue the attack. In each of four instances observed in which a moose was only wounded, the animal finally stood at bay. The wolves then waited for it to weaken considerably before attacking again. In two of the cases they eventually left the wounded moose. This evidence emphasizes that when a moose stands its ground, the wolves are reluctant to attack.

Defense of the calf in winter is quite strong and sterotyped. If wolves are detected soon enough, the cow leads her calf quickly away. If the wolves pursue in such a situation, or if they surprise the moose, the cow immediately rushes to the rear of the calf. In four observations, the two moose stood at bay, and the wolves left within a minute. While standing off a pack of wolves, the cow makes short charges at the animals and instantly returns to the calf's rump. If necessary, the calf employs its front feet for defense, but these probably do not afford adequate protection. In each of the observed cases, the cow provided the effective defense.

In eight hunts the cow and calf ran from the wolves, and in six of these the wolves pursued closely for long distances, once for about 3 miles. During such chases, the moose run slowly and deliberately, with the cow staying close to the calf's rump. If any wolf threatens the calf, the cow charges it but immediately returns to the calf. When the two animals are together they constitute an invulnerable team. The calf charges wolves in front of it, and the cow protects the rear and flanks of the calf and kicks any wolves at her rear. However, while traveling through varying cover and over widely dissimilar terrain, the two cannot always remain together. If a calf gets a few yards from the cow, the wolves close in; if the cow does not rush in promptly, the calf is doomed. Each of the three calves which I saw killed was separated from its parent while running.

Little is known about the summer defense of the calf. Murie (1944) described a cow standing and protecting her newborn calf from two wolf-size huskies. Peterson (1955) discussed the close relationship between cow and calf, and related personal observations of the strong protective instinct of the cow. (For a discussion and analysis of the strong cow-calf bond, see Altmann, 1958.) During the first week or two of a calf's life, the cow would have to stand off wolves instead of leading the calf away, for the calf could not run fast or far enough. On May 22, 1961, I startled a cow and very young calf about 100 feet away. The calf failed to keep up with the cow, so the cow had to wait for it. At times, the calf dropped about 25 feet behind. If the cow were to behave in this manner with a wolf in pursuit, the wolf probably could catch it before the cow could interfere. Undoubtedly, a single bite from a wolf would incapacitate a young calf. Had I been a wolf, however, the cow probably would have stood her ground, or at least remained close to her calf.

An encounter on June 13, 1961, demonstrated the attitude probably assumed by a cow and calves retreating from wolves. While I stood on a moose trail photographing moose in Ojibway Lake, a cow and twin calves ambled down the trail behind me, about 30 feet away. Since there were no nearby trees to climb, I motioned to alert the animals to my presence before they got too close. The cow stopped, grunted, and laid back her ears, and the calves huddled around her hind legs. She acted quite "confident" while I fumbled with my camera. Then she turned and headed slowly and deliberately away, the calves remaining at her heels.

W. Leslie Robinette reported an incident to me which showed what might be in store for wolves which attempt to bother a calf. While studying moose browse in the park, on May 19, 1961, Robinette happened upon a cow and newborn calf (with umbilicus still apparent) south of Siskiwit Lake. After observing the pair from one tree for 45 minutes, he climbed another for better visibility. Each time he changed position, the cow snorted and her mane bristled. When he descended the second time, the cow charged. Robinette ducked behind the nearest tree just as the animal straddled the tree and pounded the ground on each side of him; her nose was but a few inches from his. Immediately after the initial attack, the cow returned to her calf.

PREDATION EFFICIENCY

Many biologists have suspected that predators have a low rate of hunting success, but quantitative evidence for this belief is not easily obtained. One of the few studies comparing figures on successful and unsuccessful hunts by any predator is Rudebeck's (1950, 1951). By recording observations of all hunts by four species of European raptores for 5 years, Rudebeck found that only 7.6 percent of 688 attempts to secure prey were successful. Success percentages for individual species varied from 4.5 percent to 10.8 percent.

The present study provided the opportunity to obtain figures on the rate of success, or "predation efficiency," of the large pack of wolves (table 17). A total of 160 moose were estimated to be within range of the hunting wolves while under observation, but only 131 were detected. Of these, 77 were tested by the wolves; i.e., the wolves chased them or held them at bay, so those which escaped did so because of their superior condition or ability. Those which were detected but not tested also may have escaped on this basis, but circumstance probably was more important. Therefore, predation efficiency is considered here as the percentage of animals tested that are killed. Since 6 moose were dispatched out of 77 tested, the predation efficiency is 7.8 percent. The almost-exact agreement with Rudebeck's figure undoubtedly is coincidental, but the fact that both percentages are of the same order of magnitude is notable.

TABLE 17.—RESULTS OF HUNTS BY THE LARGE PACK^a
[Figures in parentheses indicate number of hunts involved]

PATTERNS OF SELECTION IN THE MOOSE KILL

Several other authors have reported that wolves make many vain attempts to secure prey (Murie, 1944; Cowan, 1947; Harper, Ruttan, and Benson, 1955; Crisler, 1956), although Burkholder (1959:9) stated that "there was no evidence during this period of my study to indicate that wolves, even singly, had a difficult time catching adult caribou." After extensive study of wolf-Dall sheep relations, Murie (1944:109) concluded that

Many bands seem to be chased, given a trial, and if no advantage is gained or weak animals discovered, the wolves travel on to chase other bands until an advantage can be seized. The sheep may be vulnerable because of their poor physical condition, due to old age, disease, or winter hardships. Sheep in their first year also seem to be specially susceptible to the rigors of winter. The animals may be vulnerable because of the situation in which they are surprised. . . . My general observations indicate that weak animals are the ones most likely to be found in such situations.

Murie also witnessed several caribou hunts in which wolves appeared to be testing a herd for weak animals. Regarding predation on moose, Murie (1944:186) stated

Wolves perhaps worry many moose which fight them off with such vigor that they are unwilling to expose themselves to the deadly hoofs. However, if any sign of faltering is shown, due to old age, food shortage, or disease, the wolves would no doubt become aware of it, and one would expect them to become more persistent in their attack in hope of wearing down the animal. Moose which are actually known to have been killed by wolves should be closely examined to determine their condition. Unfortunately in many cases the evidence is destroyed.

Since such a low percentage of moose tested are killed, wolves probably are selecting certain types of individuals. These could be weak or inferior moose, or merely animals in unfavorable situations. The latter possibility seems unlikely, for observations have been made of unsuccessful hunts occurring on several types of terrain, in various cover, and in diverse situations.

Information on sex, age, and condition of wolf-killed moose was obtained from remains found both from the aircraft in winter and from ground search in summer, as described on page 115.

Sex and Age Distribution. When possible, sex and age data were secured from all moose remains. Sex determination was based on the presence or absence of antlers or antler pedicels. The sex ratio of winter wolf kills was 22 females to 11 males, indicating a strong selection for cows, at least during February and March (assuming an even sex ratio in the population). However, the sexable remains found by ground search in spring and summer, which should include year-round mortality, showed a ratio of 18 females to 27 males. If there has been an even sex ratio in the population for the last few years, the number of males and females in this category of remains should be even. The preponderance of males might be caused by the probability that the more massive male skulls remained intact longer than female skulls; thus, more females than males would be classified "unknown." Since many of the remains found by ground search consisted only of old, bleached bones, a high proportion of the sample includes animals dead several years. If data from animals which probably died before 1955 are eliminated, the sex ratio is 8 females to 7 males.

Age estimates were based upon mandibles, toothrows, or molariform teeth collected from the remains. Specimens of like molar-wear patterns were segregated, and 10 classes corresponding to those described by Passmore et al. (1955) resulted. These authors estimated that classes I to IV correspond to the specimen's actual age but that classes V to IXA contain specimens varying 2, 3, or more years in age; e.g., class VII might contain specimens from 8-1/2 to 10-1/2 years old. However, Sergeant and Pimlott (1959) aged moose on the basis of annuli found in sectioned incisors, apparently a more precise and accurate method. They compared age estimates provided by Passmore et al. with their own for the same wear classes, and concluded that both variation in age, and maximum age, were greater in most classes than formerly thought; e.g., class VII included specimens 10 to 17 years old. In the present study, the ages furnished by Sergeant and Pimlott for each wear class were accepted as more representative of actual ages of the specimens.

The similarity in age composition between remains representing year-round mortality and those of winter wolf kills can be seen in table 18. The greatest bias undoubtedly occurs in the calf class of the year-round sample, for calf remains would be hardest to find and would disintegrate sooner. Nevertheless, both samples indicate that calves bear more losses than any other class. This is expected merely because calves form the largest single class. To demonstrate that wolves select calves, one must prove they kill a larger proportion of calves than exists in the population. Since calves composed a 3-year average of 15 percent of the sampled winter population but 36 percent of the 3-year sample of winter kills, a definite selection for calves is indicated, at least in that season.

TABLE 18.—AGE DISTRIBUTION OF DEAD MOOSE

In Alaska, Burkholder (1959) found that six of seven ageable, winter wolf kills were calves, and one was a yearling.

During summer, calves seem to compose even a higher proportion of the kill, if scat-analysis figures are valid indicators (table 10). In scats from 3 summers, there were 162 occurrences of moose remains identifiable as calf or adult; of these, 75 percent were calf remains. (These statistics might be biased toward calf remains because calves have a higher proportion of hair—the primary identifiable remain in scats—than do adults. Also see page 164.) In addition, three of the four moose found fed upon by wolves in summer were calves. Cowan (1947:167) reported that, in British Columbia, all remains of moose in the summer wolf scats he examined were calf remains. However, he cautioned that moose calves are more prone to accidents than are the young of other big game, and that "carrion may well make up a fair part of the calf moose item."

Calves probably are especially vulnerable to wolf predation because, like other growing animals, they are smaller, weaker, less experienced, and less independent than adults. In winter, calves may be heavily infested with ticks (see page 107), which probably would predispose them to predation. Fenstermacher (1937) reported that ticks are particularly debilitating, and Peterson (1955:186) believes that ". . . the most serious effects of tick parasitism are manifested in the reduction of the vitality of the moose, making them more vulnerable to other factors, such as diseases, predation, abortion, and malnutrition." In British Columbia, Hatter (1950a) discovered that calves constituted two-thirds of 161 moose found dead from tick-malnutrition complex early in 1947.

Since calves depend so completely on their mothers for protection from wolves, the condition of cows also is all-important to calf survival.

The most significant information in table 18 is that most moose in wear-classes I to IV are invulnerable to wolf predation. This is obvious in the winter wolf-kill sample, but the year-round category also strongly indicates the same. The few specimens in classes I to III of the latter sample could be results of accidents, for animals of any age are susceptible to accidental deaths.

It is surprising that yearlings are so secure from wolf predation. In February and March, short-yearlings are killed despite their mother's protection, but apparently by June, when they become independent, they seldom are taken. Many yearlings in June seem no larger than they do in March, so size probably does not make the difference in early summer. Possibly the primary reason that few yearlings are killed is that wolves change their hunting habits. Just after yearlings are forced away by their parents, a new crop of calves is produced, and wolves undoubtedly prey on these throughout the summer. Meanwhile, the yearlings grow quickly and by August probably are large enough to protect themselves. Murie (1944) found that wolves in Mount McKinley National Park preyed heavily upon caribou calves and took only a few adults during the summer.

Moose in wear-class VI, estimated at 8 to 15 years old, seem to be preyed upon most. Of course, individuals older than these would be as vulnerable, or probably more so, but since most kills are in the 8- to 15-year-old class, few older animals remain. Even if a moose population were not hunted by man or wolves, one would expect most adult mortality to occur in the oldest age classes.

No figures are available on the age distribution of moose mortality in other areas for comparison with the Isle Royale statistics. However, Peterson (1955:176), without furnishing figures, stated that "a great majority of the skeletal remains of moose found in the St. Ignace area were of old animals with well-worn teeth." Sources of mortality included wolf predation.

In Mount McKinley National Park, Murie (1944) established that over 50 percent of adult Dall sheep mortality occurs in ages 9 to 14 years. Tener (1954) found that, in the Canadian Northwest Territories, 21 of 24 adult musk-ox remains were from animals at least 6 years old. He believes that many of these had been killed by wolves. During a study of wolves and deer in Minnesota, Stenlund (1955:43) concluded that "there is no indication that wolves tend to take old animals in preference to those in the prime of life." However, his figures seem to indicate that the predators were selecting older animals, for of 29 wolf-killed adults, only 3 were yearlings or 2-year-olds. Burkholder (1959:7) found in his Alaskan studies that four of five ageable wolf-killed caribou adults were over 4 years old, although he states that "all of the wolf kills I could check were in excellent condition and of the 'age of primeness'."

Probably most workers would consider moose 6 to 12 years old in their prime. Indeed, the growth curve for bulls, provided by Skuncke (vide Peterson, 1955:77) shows that bulls gain weight until at least 12 years of age. Nevertheless, many of the wolf-killed moose on Isle Royale were in this category. Probably the significant point about wolf predation on any big game is not so much the selection for old animals, but rather selection against young animals (other than calves). In the present study, the invulnerability of adults less than 6 years old is striking. In addition, Murie's (1944) sample of remains of 829 Dall sheep shows that a total of only 3.4 percent of the mortality involved animals aged 2, 3, or 4 years. The figures from Tener and Stenlund, and even the limited data from Burkholder, also support the thesis that most big-game animals in their first few years are safe from wolf predation.

Young animals may be relatively invulnerable because they are more alert or they may be faster, stronger, and more agile. Perhaps they should be considered "prime," instead of the more mature individuals usually regarded as such, for security from wolves certainly is a realistic criterion. Of course, young animals could be less susceptible only because they might inhabit areas less frequented by wolves, but, at least in the case of Isle Royale moose, this seems improbable.

Incidence of Debilitating Conditions. One explanation for the vulnerability of apparently prime moose is that they are infected heavily with hydatid cysts (Echinococcus). Cowan (1947:17) proposed that the high proportion of elk kills which he found in the "prime"ages might have resulted from the heavy hydatid infections. On Isle Royale, cysts were found in the lungs of one moose each in wear-classes IV, V, and VIII. The youngest animal had only 5, but the others harbored 57 and 35, respectively. Because the parasite is contracted as moose feed, older animals should have heavier infections. Ritcey and Edwards (1958) found this to be true in British Columbia moose. In 6 or 7 years a moose might ingest enough eggs to cause a heavy infection. A very heavy infection in a moose about 4 years old was found in the British Columbia studies.

It has been well demonstrated that a higher proportion of older animals are infected with hydatid cysts than are younger ones. In Saskatchewan, Harper et al. (1955) found that only 1 of 12 immature moose examined harbored hydatid cysts whereas 13 of 31 adults were infected. The same trend was noted in deer and caribou. Rausch (1959) showed Alaska harbored cysts, but 15 percent of animals in wear-classes I, II, and III were infected, and 62 percent of those in classes VII, VIII, and IX. R. O. Skoog found cysts in only 4 of 67 caribou examined, and all 4 were over 7 years old (Rausch and Williamson, 1959).

Undoubtedly, heavily infected animals are more susceptible to predation. Fenstermacher (1937) believes that hydatid cysts are especially debilitating, and Cowan (1951:52) stated: "Infected animals are usually impoverished and of low vitality. They almost certainly have an impaired ability to survive adverse circumstances." However, Rausch (1952) disagrees, for he saw 18 cysts (80 to 90 mm. in diameter) in an Alaskan moose in apparently excellent condition. Nevertheless, it seems that such an animal would have little power to run far, or to fight off wolves. The observation by Ritcey and Edwards (quoted on page 111) is especially significant in this respect.

In Ontario, a fresh wolf-killed moose with diseased lungs was found, and signs showed that it had not fought much (Peterson, 1955). A lung was removed and sent to the Royal Ontario Museum of Zoology. Peterson (1955:176) described it as follows:

When the lung was examined in the museum laboratory it was found to be so completely filled with hydatid cysts of tapeworms that it seemed incredible that it could have functioned sufficiently to keep the animal alive during normal activity, much less allow it to ward off an attack by timber wolves. Well over 50 percent of the volume of the lung was occupied by large cysts up to one inch in diameter.

Figure 97—Most important relationships in wolf ecology. (Solid arrows indicate primary relationships.)

Crisler (1956, 1958) described a "tired" caribou running from wolves, faltering, lying down, and being killed. The lungs contained eight cysts, some as big as ping-pong balls. One of the caribou kills examined by Burkholder (1959) also harbored hydatid cysts, but the author did not elaborate.

Philosophically, it seems logical that a heavy hydatid infection should predispose a moose to predation, for the parasite depends for its perpetuation on the moose being eaten by a wolf. The greater the intermediate host's infection, the more beneficial it would be to the parasite to render the host susceptible to predation.

Old moose even without cysts or other debilitating organisms probably would be susceptible to predation because of their general infirmity and lack of agility. However, many old moose undoubtedly are infected with parasites and/or disease, which probably would increase their chances of succumbing to predation. Only two relatively intact carcasses of wolf-killed adults were examined, but each had a heavy hydatid infection. The one checked in winter also was heavily infested with ticks, and had an abnormal liver and badly congested lungs.

All that remained of most kills investigated were bones, but these also showed that many of the moose had been in poor condition. The marrow was fat-depleted or nearly so in the femurs of 5 (14 percent) of 33 adults checked; and 9 (29 percent) of 31 adult skulls or mandibles were necrotic, probably from actinomycosis (figure 86). In the total of 35 adult kills from winter plus 1 from summer, 14 (39 percent) had depleted marrow, jaw necrosis, or hydatid cysts. Some of the animals showed more than one of these conditions and probably also supported heavy tick populations (figure 84). Undoubtedly, a thorough examination of the intact carcasses of these moose would have revealed a much higher percentage of debilitating conditions.

Other studies indicate that adults of big game killed by wolves generally are old or diseased. Murie (1944) discovered that although most Dall sheep mortality occurs in the old-age category, mortality in the prime class includes a much higher percentage of diseased individuals. In the Rocky Mountain national parks of Canada, Cowan (1947) found that 17 (37 percent) of the remains of 46 adult-elk kills were either senile or diseased; and he cautioned that a condition such as hydatid infection is not evident from bones, so many others also may have been diseased. Crisler (1956:346) reported that "at least half of the kills that we observed involved crippled or sick caribou." However, all 5 fresh adult kills (4 caribou and 1 long-yearling moose) examined by Burkholder (1959) were judged to be in excellent condition, although at least one of these was infected with Echinococcus.

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