Possible Causes of Population Stability
According to Young and Goldman (1944), the average wolf litter contains seven pups. In Minnesota, Stenlund (1955) found that 8 litters averaged 6.4 young. The mean size of four litters from British Columbia was five (Cowan, 1947). This information alone might lead one to suppose that wolves are prolific and that wolf populations have a high rate of turnover. However, facts do not agree with this supposition.
Cowan found that in the Rocky Mountain national parks in Canada, three packs which had been checked carefully showed no significant increase during two breeding seasons. In northern Alberta, Fuller and Novakowski (1955) poisoned 3 entire packs and found an age ratio of 3 pups to 10 adults. On Isle Royale, two wolf packs have each remained the same size for three winters; indeed, the pack of three apparently has failed to increase since at least early 1957, when Cole reported on it. The apparent inconsistency between these data and the fact that wolf litters are large probably results from one or more of the following factors: unproductive animals, prenatal losses, mortality factors, and emigration and immigration.
There appear to be at least four categories of unproductive members of wolf packs: (1) surplus males, (2) immature animals, (3) senile individuals, and (4) social subordinates. Murie (1944) found three males and two females, all at least 2 years old, at one den in Mount McKinley National Park; two more adult males joined this pack in late summer. Fuller and Novakowski found one to three extra adults in five out of six packs which they poisoned.
Young and Goldman (1944) quoted a report that the sex ratio was equal in a catch of 68 wolves from New Mexico, and Fuller and Novakowski found an even sex ratio in 58 poisoned wolves in Wood Buffalo National Park, Canada. However, males comprised 15 of 25 wolves shot in British Columbia (Cowan, 1947), and 100 of 156 animals taken in Minnesota (Stenlund, 1955).
An attempt was made to sex Isle Royale wolves on the basis of size and mating behavior. Stenlund found that, in a sample of 114 wolves, males averaged 17 pounds more than females, and Fuller and Novakowski (1955) reported an average difference of 13 pounds, although in each study, weights overlapped between the sexes. On Isle Royale, only one member of the large pack consistently appeared small, but in photographs, three and possibly four smaller animals are evident (figure 57). During all the mating activity observed, I never noticed more than three individuals being pursued amorously at one time. Perhaps other females in estrus were not noticed, or possibly some were not in estrus. Nevertheless, since three or four animals were interested in each of the three "known" females, I believe there is a substantial preponderance of males in this pack. One member of the pack of two, and one of the pack of three, also are smaller and may be females.
Since males mature in 3 years, and females in 2 (Young and Goldman, 1944), most wolf populations will contain a number of unproductive young. In an increasing population, or in one with a high turnover rate, immature animals could compose a high percentage. The only wolves on Isle Royale which were thought to have been immature were the three lanky animals in the large pack.
Wolf populations which are not hunted or trapped heavily probably include a substantial number of old animals. Young and Goldman report that old age for a wolf is 10 to 14 years. Such old wolves, according to these authors, often travel alone and subsist on old kills and carrion; presumably these would be senile. Fuller and Novakowski examined a very old male which had small testicular volume and no demonstrable spermatogenesis. There is at least one lone wolf on Isle Royale which probably is in this category, and several of the other animals also could be senile.
The most important class of unproductive animals might be the social subordinates. Mykytowycz (1960) found that in a captive population of wild European rabbits (Oryctolagus cuniculus) the dominant females bred much more effectively than the subordinates. Retzlaff (vide Christian, 1958:477) also noticed this phenomenon in a population of laboratory mice (Mus musculus). One might gather from Schenkel (1948:fig. 50a) that the same applies to wolves, for he uses the phrase "suppressed, but not entirely 'frigid' females." It would seem advantageous for the alpha individuals in a society to be the most effective reproducers, for probably they are physically the best, or at least the most aggressive. Thus, the non-reproductive members of a group could help supply the food to the reproductive members and their young. Murie (1944) believed that one of the packs he studied was organized in this manner. An extra female even helped care for the young, and stayed with them one night when the mother went hunting.
Laboratory studies of mice by Davis and Christian (1957) have shown a correlation between social status and weight of the adrenal glands, the lowest-ranking individuals (i.e., the most stressed) having the heaviest glands. A similar study by Christian (1956) demonstrated that the animals with heaviest adrenals reproduced least effectively. The same principle might apply to wild wolves, although this would be difficult to prove. It might operate both within a pack, inhibiting the reproductive ability of low-ranking members, and between packs, causing reproductive inhibition in repressed packs. In discussing territoriality, Elton (1950) stated that species having few effective natural enemies tend to be self-regulatory, and Murie (1944) and Stenlund (1955) agreed that territoriality would tend to control wolf numbers. The manner in which this might operate is unknown, but perhaps the above-mentioned relationships are involved.
Animals from populations of high densities generally have lower reproductive rates than those in less-dense populations. Hoffman (1958) showed an inverse relationship between density and ovulation rate in Microtus montanus. Davis (1949) was able to increase pregnancy rates of brown rats (Rattus norvegicus) in Baltimore by reducing the population.
This inverse relationship between density and litter size might becaused by variations in nutrition and/or in social stress. Mason (1939) discussed the effect of nutrition on reproduction. An increase in amount and variety of nutrients just before the breeding season causes a higher ovulation rate in sheep (Clark, 1934; Stoddard and Smith, 1943; Belschner, 1951). Cheatum and Severinghaus (1950) and Longhurst et al. (1952) agree that ovulation rate in deer also is affected by the level of nutrition. According to Frank (1957), the litter size of the vole (Microtus arvalis) depends particularly on the quantity and quality of food. Lack (1946) asserted that when avian predators dependent on mice face a food shortage, they fail to breed; when mice and lemmings are excessively common, these birds may raise two broods and have clutches twice the usual size.
Stevenson-Hamilton (1937:257) writes of a similar relationship in African lions:
Whereas the effect of nutrition on reproduction has been studied for many years, the role of high-density stress is just beginning to be evaluated. Most of the significant work has been done with laboratory mice. By studying populations of mice given unlimited food and water but varying in number per cage, Christian (1956) discovered that high population density suppressed reproduction in both sexes. One of the manifestations of reproductive suppression was decreased litter size. In a later experiment, Christian and Lemunyan (1958) found that all 10 of their crowded females had bred, but that 7 of these lost all their embryos through both pre- and post-implantation mortality.
Of course, it usually is extremely difficult to separate the roles of nutrition and of social stress in wild populations of high density. However, Kalela (vide Christian, 1958:491) found that a wild population of the redback vole (Clethrionomys rufocanus) increased in density to a peak and then suddenly collapsed, despite the abundance of available food. Christian et al. (1960), studying a die-off in a herd of sika deer (Cervus nippon), ruled out malnutrition as a cause of mass mortality, and concluded that high-density stress was primarily responsible.
If high-density stress sometimes controls wild populations, conceivably this factor at least contributes to the stability of predator populations, especially in territorial species. In this connection, it may be significant that Isle Royale has one of the highest wolf densities reported (table 6).
Mortality might take one of several forms, but probably it occurs most frequently in the pup class. Cowan (1947) reported on a bitch which apparently had lost an entire litter of young. Fuller and Novakowski (1955) found a ratio of 9 pups to 36 adults in autumn and concluded that this indicated a pup-mortality rate of about 90 percent within the first 6 months. This estimate apparently is based on the questionable assumption that each pair of wolves produces six young each year. Nevertheless, the ratio found by these authors does suggest a high death rate among pups.
There are several possible causes of pup mortality. Conceivably, the bitch might obtain enough food to produce a full litter, and then because of seasonal changes, possible pack break-up, or other adverse circumstances, she might not get sufficient food to nourish all her young. This might apply especially to Isle Royale wolves. Pups should be born there in the third or fourth week of Aprilabout the time the ice goes out. Moose then can take refuge in the water so perhaps are less vulnerable at this time. New calves, which composed much of the wolves' summer diet, are not born until mid-May.
According to Speelman (1939), domestic dog pups require two or three times as much food as adults of the same weight. This should apply to wolf pups as well, so any food shortage could be crucial for them. Stevenson-Hamilton (1937) and Wright (1960) observed behavior in East African lions, which, if duplicated by wolves, would be disastrous to pups. The females and cubs feed on kills only after the males have satiated themselves, and frequently there is little left for the cubs.
Young and Goldman (1944) report that often one or two whelps are more aggressive than the others. Probably if a food shortage existed, these individuals would steal all the food, leading to the eventual death of their littermates. Lee Smits of Detroit, Mich., who has raised wolves, suggested another idea to me. He believes that, during the violent activity which occurs when wolves are fed, one pup might bite another, become excited, and end up devouring it too. In this way, only the most vigorous individuals would survive.
In a special supplement to the Kane (Pennsylvania) Republican paying tribute to Dr. E. H. McCleery and the population of captive wolves he has kept for 40 years, McCleery claims that "if a mother lobo has one outstanding pup she may keep that oneeat the others. Also she will eat an injured pup." Although these observations pertain to captive animals, it is possible that under stress, even a wild wolf would act this way.
Social stress during the period that young are being raised could be an important factor. A laboratory experiment with crowded mice produced the following conclusion (Christian and Lemunyan, 1958:517) :
Again, interaction of the nutrition and stress factors probably would be more important that the action of either alone.
No information is available on the incidence of pup mortality from injuries by prey, but it might be quite significant, particularly on Isle Royale. Even the large pack of adult wolves must chase many moose before killing one, and the animals obviously are afraid of a threatening moose. MacFarlane (1905) and Stanwell-Fletcher (1942) reported instances in which a wolf was found badly injured by blows from a moose. Naive and inexperienced puppies might not respect moose as adults do, and in the excitement of a chase might be especially vulnerable to the deft kicks of their intended prey. Even experienced adults might perish in this manner, although apparently this has not happened on Isle Royale during the study.
The diseases and parasites discussed previously might be important in controlling wolf numbers in certain locations. It is doubtful that these are directly significant on Isle Royale, because no evidence of adult mortality was found. If pathogenic organisms were primarily responsible for pup mortality, they probably would cause death to a few adults, too. Indirectly, such parasites as Taenia hydatigena and Echinococcus granulosus in heavy infections might add to any general stress affecting the wolves, and, therefore, might contribute to whatever reproductive inhibitions there may be.
Old age eventually may be a significant mortality factor on Isle Royale. Seton (1937) writes of a male wolf in the National Zoological Park, which lived over 16 years. If any of the original Isle Royale immigrants remains, it would be at least 14 years old. Wolves over 10 years of age generally are considered old; they usually have worn and broken teeth and find it difficult to obtain food (Young and Goldman, 1944). Such individuals visit old carcasses more often than other wolves. At least one Isle Royale animal, the lone wolf, spent much time at old kills. Since this individual was not fully accepted by the pack, it probably was inferior in some respect, perhaps in age.
Because wolves did immigrate to Isle Royale, the possibility exists that other movements to or from the island have occurred since, or will occur. Significant immigration probably depends upon the following: (1) a high wolf population or a food shortage on the nearby mainland, causing "pressure" for animals to seek new territory; (2) a solid, snow-covered "ice bridge" to the island; (3) the type of reception given the newcomers by the residents; and (4) the ability of the immigrants to kill moose. Since wolves apparently did not populate Isle Royale during this century until about 1949, it seems reasonable to assume that the necessary combination of circumstances does not occur often.
Continuous ice does not connect Isle Royale with the mainland every year, so the ice-bridge factor probably is most critical in determining whether movement in either direction occurs. Cole (1957) collected several reports of years when ice connected the island with the mainland. In two of the three winters during the present study, extensive sheets of thin, drifting ice spanned the lake to Canada several times. Reports, apparently originating from pilots of high-flying aircraft, once claimed that the entire lake was frozen over.
However, after each high wind, the ice piled onto the north shore of Isle Royale, leaving the lake open. Thus, reports of ice bridges to the island should be viewed cautiously. Nevertheless, from February 15 until at least March 21, 1961, the ice with stood several severe winds, and appeared safe for even a vehicle to cross.
Besides depending on suitable ice, movements of wolves from Isle Royale also probably would depend on the animals having a strong reason to leave their home range, or at least a great desire to travel (perhaps only temporarily) to new territory. We did witness one apparent attempt by the large pack to leave the island. On March 1, 1960, the 16 wolves were heading northeastward in Rock Harbor at 2:25 p.m., after traveling about 29 miles from their last kill. They reached Blake's Point, the northeast tip of the island, at 4:40 p.m., and by 4:45 were about a mile due north of the point.
The wolves continued toward Canada another half-mile, gradually curved eastward, and then headed toward Passage Island. The ice was smooth in places, but elsewhere it consisted of older chunks frozen together with new ice. The animals were cautious about crossing from one type of ice to another. Although most of the wolves appeared reluctant to proceed, the leader seemed determined. Several times this animal returned to the hesitant pack and apparently tried to urge the members on. They continued about one-half a mile until encountering ice composed of many small, sharp pieces frozen together. After testing this, the pack returned to Isle Royale (figure 53).
I do not believe the wolves were heading to Passage Island, for they could have taken a more direct route. They might have been able to reach Canada, since there had been little wind the previous week. However, they probably would not have returned, even if they had wanted to, for a few days later the wind had shifted the ice, leaving large cracks.
During the 1961 study period, when a substantial ice bridge existed, the pack certainly could have emigrated, but no sign of an attempt was observed. Nevertheless, it always is conceivable that someday one of the packs might wander off and never return. Future investigators should attempt to watch the wolves very closely during periods when continuous ice extends to Canada.