Northern Prairie Wildlife Research Center
Because the Minnesota wolf population has saturated most of the wilderness and continues to increase, more wolves are currently inhabiting agricultural lands. Thus depredation control costs and compensation costs continue to rise. For example, when wolf numbers increased from about 1,600 in 1988 (Fuller et al. 1992) to probably 2,000 in 1994, the number of depredation complaints, the amount of compensation paid, and number of wolves trapped for depredation control increased disproportionately (Fig. 1). Presumably in Minnesota, Spain, Italy, and elsewhere, the public eventually will object to the cost of subsidizing wolves and demand wolf population reduction in certain areas.
The question remains, however, as to whether sterilized pairs lacking pups would continue to hold territories. Intact pairs, of course, hold territories long enough to produce pups (Rothman and Mech 1979, Fritts and Mech 1981). However, some wolf pairs that do not produce pups sometimes break up and leave their territory (Mech 1987, Mech and Seal 1987). If vasectomized pairs did so, then sterilization would have little lasting effect, for other lone wolves would soon fill the vacant territories (Rothman and Mech 1979, Fritts and Mech 1981).
To determine whether vasectomized wolf pairs would continue to hold territories, we conducted an experiment in the Superior National Forest of northeastern Minnesota from 1987 through 1994. During summers 1987 and 1988, five adult male wolves, weighing 32-38 kg and with testis lengths 3.0-3.9 cm were live-trapped from four packs, transported to a veterinary lab, and surgically vasectomized. Veterinarians removed large portions of the vas deferens of each. The animals were then radio-collared, released in their territories, and followed by aerial radio-tracking approximately weekly and their presence in packs, pairs and territories was observed. We observed post-vasectomy pack sizes of the four packs for 1,3, 4, and 7 years respectively (Table 1).
|Table 1. Pack sizes of vasectomized wolves in the Superior National Forest, Minnesota after vasectomies.1|
|Maximum Sizes of Packs with Vasectomized Wolves|
|Year||Wolf 35||Wolf 77||Wolf 79||Wolf 119||Wolf 129|
|1||Wolves 35, 77, and 79 were vasectomized in summer 1987; 119 and 129, in summer 1988.|
|2||Killed by other wolves in July 1988; the only observation of this pack after the 1988 whelping season was of 4 adult wolves in July.|
|3||Members of the same pack; one possible observation of pups was made with this pack on 2 November 1988.|
|4||Dispersed 18 km, paired with female and held new territory until killed.|
The effectiveness of the vasectomies in sterilizing males cannot be fully documented because there were other pack members present which could have been breeding males. However, the sterilization technique used is standard for dogs, and the veterinarians who performed the vasectomies were practiced at the technique. Only one possible observation of pups was made, and that observation was questionable. In all years, the pack size either remained the same or decreased after the males were vasectomized (Table 1).
All of the vasectomized wolves for all the years observed remained in territories until they died or their transmitters expired. One wolf spent two years in one pack territory after being vasectomized, then dispersed about 18 km and bonded with a female; the pair held its new territory for 2 years until the male was killed. Another vasectomized wolf remained in his territory for 7 years after being vasectomized (Table 1). The evidence obtained in this study demonstrates that vasectomized wolves do not necessarily lose pair-bonds or territories; one wolf even formed a new pair bond.
The greatest disadvantage of the vasectomy technique is that individual wolves must be captured and handled. However, chemical vasectomy involving the non-surgical injection of sclerosing agents directly into the epididymides (Freeman and Coffey 1973, Pineda and Hepler 1981), which the authors learned about after this study, could be applied by wildlife biologists in the field. This technique would make vasectomy far more practical.
Thus it may be possible to vasectomize wolves around local livestock herds sustaining chronic losses from wolves and reduce the local wolf population by as much as 2/3 given that wolves usually produce an average of 4-6 pups per litter (Mech 1970). This should also reduce depredations considerably because a litter of pups would quadruple a wolf pair's need for food. Situations may also arise in which it would be preferable to vasectomize members of non-depredating pairs rather than remove them and risk their territories being filled by others who might.
Furthermore, as a general wolf-population-control measure, vasectomizing a certain percentage of a population would tend to reduce its biotic potential and its size for periods of several years. If each year, a certain percentage of the wolf population was sterilized, population growth and recolonization of new areas could be curtailed while a reservoir wolf population was maintained. Field studies and wolf-population modeling would give reasonable insights into the approximate percentage of each population that might need vasectomizing so that a given wolf population might be adjusted to a desired level.