Ecological Studies of the Timber Wolf in Northeastern Minnesota Part 7

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AN a.n.a.lYSIS OF THE AGE, s.e.x, AND CONDITION OF DEER KILLED BY WOLVES IN NORTHEASTERN MINNESOTA

L. David Mech and L. D. Frenzel, Jr.

The selective effect of predation on prey populations is of significance in studies of evolution and population dynamics. Selective predation can be an important agent in the process of natural selection, and it influences the extent to which predators limit the numbers of their prey.

One of the predators most commonly chosen for investigating the selective effect upon prey is the wolf (_Canis lupus_). Because animals preyed upon by wolves generally are large, their remains can be more easily located and examined. It already has been established that in most areas wolves kill primarily young, old, and other inferior members of such prey populations as Dall sheep (_Ovis dalli_), moose (_Alces alces_), caribou (_Rangifer tarandus_), bison (_Bison bison_), and musk-oxen (_Ovibos moschatus_); evidence for this generalization has been summarized by Mech (1970).

However, only recently has it been shown that this generalization may extend to predation on the smallest hoofed prey of the wolf in North America, the white-tailed deer (_Odocoileus virginia.n.u.s_). Pimlott _et al._ (1969) demonstrated a difference between the age structure of 331 deer killed by wolves during winter in Algonquin Park, Ontario, and 275 deer a.s.sumed to represent the actual population in the same area.

Whereas only 13 percent of the deer from the population at large were estimated to be more than 5 years old, 58 percent of the wolf-kills were in this age category.

We employed a similar a.n.a.lysis for deer killed by wolves in northeastern Minnesota, but used a more refined aging technique and included comparisons of the age and s.e.x structures of various subsamples of wolf-kills. Whereas the Ontario research involved a prey population unlimited by man, our work was carried out on both a hunted population and on one relatively unhunted. Further comparisons were made between deer killed during periods of normal snow conditions and those taken during unusually high snow acc.u.mulations. The incidence of various abnormalities in wolf-killed deer was also compared with that in hunter-killed animals.

The study was carried out in the Superior National Forest in northern St. Louis, Lake, and Cook Counties of northeastern Minnesota (fig. 1), in conjunction with other aspects of wolf research (see Mech _et al._ p. 1).

[Ill.u.s.tration: _Figure 1.--The study area showing locations where wolf-killed and hunter-killed deer were taken. Line arbitrarily separates the hunted area from the wilderness area._]

METHODS

The investigation began in February 1966 and continued through March 1969; the basic objective was to examine as many wolf-killed deer as possible and compare their ages, s.e.x, and condition with a large sample of deer from the population at large in the same area. Wolf-kills were examined only during December through March when they could be found from the air. Aircraft ranging in size from an Aeronca Champ to a Cessna 206 were used to fly over frozen lakes at alt.i.tudes up to 2,000 feet to locate wolves (fig. 2), wolf tracks, or kills (fig. 3). We often discovered kills by tracking a wolf pack.

[Ill.u.s.tration: _Figure 2.--Wolves were located from the air, usually on frozen lakes. (Photo courtesy of L. D. Mech.)_]

[Ill.u.s.tration: _Figure 3.--Wolf-kills were easily spotted from aircraft. (Photo courtesy of L. D. Mech.)_]

During the winter of 1968-69 this method of finding kills was supplemented by radiotracking five wolves and their a.s.sociates via aircraft (see Mech _et al._, p. 1). The latter technique resulted in increased discovery of inland kills.

A deer carca.s.s was judged killed by wolves if the death had been recent, if tracks or other sign indicated that wolves had fed upon it, and if no other possible cause of death was discovered. Carca.s.ses fed on by wolves but not clearly identifiable as kills were labeled "probable" wolf-kills. Although the cause of death of the specimens in this latter category could not be determined with certainty, there was no reason to believe other agents were involved.

In addition to the wolf-kills examined by project personnel, data and lower jaws from deer judged killed by wolves were contributed by other biologists, game wardens, forest rangers, and others whose competence was known. Nevertheless, if certain identification of carca.s.ses as wolf-kills was not possible, the data were relegated to the "probable"

wolf-kill category.

Whenever possible, kills discovered from the air were examined on the ground (fig. 4). Often only skeletal parts remained, but soft parts were also examined when available. Femur marrow, heart, lungs, liver, kidneys, reproductive tracts, and omenta were usually inspected in the field for fat, parasites, and abnormalities, and the degree of subcutaneous back fat was also noted. Hoofs and lower legs were checked, and those showing pathological conditions or abnormalities were collected and examined by the Veterinary Diagnostic Laboratory of the University of Minnesota. All lower jaws found were collected, aged, and examined for dental abnormalities and pathological conditions.

[Ill.u.s.tration: _Figure 4.--As many wolf-killed deer as possible were examined from the ground. (Photo courtesy of L. D. Mech.)_]

In November 1967 and 1968 hunter-check stations were operated on the study area (fig. 5), and deer bagged by hunters were field-checked for age (Severinghaus 1949) and hoof abnormalities. As many lower jaws as possible were collected from field-checked deer and other deer killed in the area for age determination and examination for abnormal dent.i.tion.

[Ill.u.s.tration: _Figure 5.--Information about hunter-killed deer in the study area was obtained through hunter-check stations. (Photo courtesy of L. D. Frenzel.)_]

An a.s.sumption was made that the age structure and incidence of abnormalities in the sample of hunter-killed deer would be _reasonably representative_ of those in the population at large, an a.s.sumption also implicit in a similar comparison made by Pimlott _et al._ (1969). In this respect, the following statements by Maguire and Severinghaus (1954, p. 109) about deer in New York State are pertinent: "It may be concluded that, considering the open season as a whole, wariness does not significantly distort the age composition of the [deer] kill in relation to that of the corresponding wild population, except possibly for buck seasons of only 1 or 2 days duration.... A reliable appraisal of the age composition of the kill by hunting may be obtained through the operation of roadside checking stations." However, in critically reviewing the present paper Severinghaus stated that in States such as Minnesota, with fewer hunters and higher hunter success rates, age compositions of deer from checking stations may not be the same as those of wild populations. Reviewers Peek and Downing also made similar comments.

Nevertheless, for our comparison with wolf-killed deer it is not necessary that the hunter-kill age structure be exactly representative of the age structure of the actual deer population. All that is required is that there be reasonable agreement between the two. The hunting regulations in our study area allow a 9-day period of taking deer of any age or s.e.x, and a single hunter may legally shoot as many deer as he and his party or a.s.sociates have permits for. Thus there is no reason for selective hunting, and we feel confident that the age structure of the hunter-kill in our study area does basically represent that of the deer herd at large.