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The Wild World of Wolves
Selection of Wolf Dens in Relation to Winter Territories in Northeastern Minnesota
P. Ciucci and L. David Mech

Wolf dens have been studied extensively, and much emphasis has been placed on the den structures and den site characteristics. Although factors involved in den selection have been reviewed, no one has attempted to study wolf-den position in the territory. A few observers suggest that dens tend to be located toward the territory center, although some are close to the edges. We believe that, in addition to the characteristics of the den site, larger-scale factors could play a significant role in selection of the den site. These factors could involve a cognitive knowledge of territory (e.g., size, configuration, hunting areas, travel routes, proximity to adjacent packs), and could affect the denning strategy of wolves.
The possibility that wolves have cognitive maps supports the idea that they might be able to select the den location in response to, among other factors, area, shape, and boundaries of the territory. A tendency to locate a den toward the center of the territory might represent an effort to minimize interference from adjacent packs, or to optimize travel and hunting efficiency to different parts of the territory.
We hypothesized that wolf dens tend to be located disproportionately toward the center of the territories. To test this, we examined data from radiocollared wolves in the Superior National Forest, Minnesota, from 1968 through 1988....Although we made no effort to quantify abundance of potential den sites, such sites appear to be numerous. Sandy soils, rock cavities, hollow logs, and old beaver lodges are abundant throughout the area, and neither water nor hills seem to be limiting....
METHODS
Wolves were captured, radiocollared and, after release, located once or twice a week from aircraft and from the ground from 1968 through 1988. Active dens were located from the air in the denning season, and by reviewing March-June locations of alpha females known to have produced young. Only dens where young were born were considered....
To test our hypothesis that dens primarily are located toward the center of territories, we analyzed den locations in relation to previous winter territories (October through March) for each denning female. Winter territories were chosen because presumably they best represent the breeding territory of the alpha pair and are not affected by the den location itself. In spring and summer, wolves tend to travel less extensively because they must return regularly to the den, which represents the focal point of pack activities....
To determine whether den locations were random or center-clumped, we calculated for each territory the mean radius as the average of the radii from the center to the vertices of the polygon. Thus, den distances from the center could be expressed as percentages of their mean radius of the territory, allowing comparisons among territories of different sizes and shapes. Then the observed distribution of percent-mean-radius values was compared with an expected uniform-probability distribution to test if the difference was statistically significant....
RESULTS
From 1969 through 1988, 29 active dens of radiocollared, alpha-female wolves were found. Of these dens, 20 (69%) were new denning sites, and nine (31%) were dens reused either in consecutive or nonconsecutive years....
Of the original 29 cases, only 19 were considered for the spatial analysis. Of the other 10, nine involved reused dens, and one involved a female whose winter territory could not be defined precisely due to winter migration.
....Fifteen (79%) of the den locations were within 60% of the mean radius of their territories. However, for these den locations, the maximum difference between the distribution of observed percent-mean-radius values and an expected uniform-probability distribution was not significant, contrary to the hypothesis. The remaining four (21%) dens were found in the outer portion of their mean-territory radius and, as mentioned, their distribution could not be statistically tested against an expected uniform distribution. Nevertheless, the percent-mean-radius for these locations...show that wolves choose dens toward the edges of their territories, also contrary to our hypothesis. Only two dens, however, were located within a 1-km-wide strip inside the territory boundaries....
DISCUSSION
....Chapman (1977) wrote that there generally are several potential den locations within the pack territory, and that no one has demonstrated that such sites are in short supply. On Isle Royale, where there are few opportunities to dig dens, wolves utilized structures such as beaver lodges and hollow logs. Also, even in the subarctic, where the ground remains frozen longer than in Minnesota and wolves cannot excavate new dens before parturition, there generally are several denning sites available. This seems to support our assumption that den sites are not limited in our area, as also shown by our radiocollared wolves that denned in different locations within the same territory.
Our attempt to quantify and test the idea that wolf dens tend to be located toward the center of territories led us to reject our hypothesis. No significant pattern of den distribution was found and, at least within 60% mean radius, dens tended to be located randomly relative to the territory center. In addition, 21% (4) of the dens were located in the outer portion of the territories, further contradicting our hypothesis.
Our regression analysis shows that den position and territory size are related, with dens in larger territories tending to be more central....Our analysis also suggests that in large territories, minimizing traveling distances from and to the den could influence selection of den location. This would be consistent with observations of wolves traveling almost on a straight line for long distances when bringing food to young in the den. However, the low coefficienct of determination indicates that territory size alone is a poor predictor of den position in Minnesota....
Traditionality of den use accounted for den selection in 85.7% (6) of our denning females studied for [more than] 1 year. Traditionality of den use often has been reported both for consecutive and nonconsecutive years. Clearly, a function of the longevity of the den itself, traditionality of use could be related to familiarity and individual preferences for certain site characteristics, previous success in raising young, habituation, or lack of alternatives. We found an indication that the longest tradition of den use was where food supply was most stable. In a 15-year period (1969-1988), one den near a garbage dump has been used for at least 13 years. In our study area, wolves that used the same den over several years did so regardless of its position relative to the winter territory.
Mech (1977c) postulated that along the edges of wolf territories there is greater probability of interpack contact, and Fritts and Mech (1981) found that neighboring packs had a major influence on the pattern of territory use and distribution of kills. For the same reason, one would expect that adjacent packs also could affect the distribution of den locations. We found that only two (10.5%) of 19 dens were located within a 1-km-wide strip inside the boundaries of each territory, this being the portion of the territory edge that Fritts and Mech (1981), Mech (1977c), and Peters and Mech (1975) considered to be under the influence of neighboring packs. In addition, one of these two dens was located near one of the territory edges that was adjacent to a large lake, where no contacts with other packs could be expected. This fact leads us to believe that denning wolves tend to avoid areas where influence by adjacent packs is more likely.
We conclude that, at least in relatively small territories, wolves do not tend to locate the den centrally, although they may tend to do so in large territories. In our study area, the process of selecting a den location within the territory is likely a function of tradition, availability and distribution of food resources, influence of neighboring packs, and the interactions of these factors.
INCLUDES
Table 1: Observed and expected...frequencies of den locations within different classes of radius length, up to 60% of the mean radius....
Fig. 1: Cumulative observed distribution of mean-radius values for wolf dens in relation to territory centers, and cumulative expected uniform-probability distribution; the differences are nonsignificant....
Fig 2: Relationship between natural logarithm of territory size (square km) of wolf and distance of wolf den from territory center expressed as percentage of the mean radius for a sample of 19 locations of wolf dens in northeastern Minnesota....
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