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Dispersal Patterns of Red Foxes Relative to Population Density

Discussion


Our recovery of 43% of all foxes tagged as pups is within the range of 40-54% reported from other areas of North America (Storm et al. 1976, Pils and Martin 1978, Tullar and Berchielli 1980, Voigt 1987). Those workers also found recovery rates were greater for males than for females, and greater during the first year of life than later.

Interest in fox trapping and hunting was high throughout our study period because fur prices were relatively high (Sargeant 1982); most of our recoveries came from trappers and hunters. Nevertheless, we found higher survival of foxes tagged as pups than reported by others. Storm et al. (1976), Pils and Martin (1978), and Tullar and Berchielli (1980) tagged a total of 2,692 red fox pups, but in each study only 3-4% were recovered >2 years after tagging; 1 fox was recovered >7 years after tagging (an 8.5-yr-old female reported by Tullar [1983]). Seven percent of males and 10% of females we tagged were not recovered until >2 years after tagging, including 1 male and 3 females that survived > 7 years. Female foxes surviving > 2 years are especially important to populations because of age-related increases in reproductive performance (Allen 1984).

We found both the mean recovery distance and the percentage of foxes recovered that dispersed increased with age-class, and were greater for males than females, similar to findings of others (Storm et al. 1976, Jensen 1973, Pils and Martin 1978, Tullar and Berchielli 1980, Voigt 1987). Storm et al. (1976) found monthly increases in mean dispersal distances generally occurred in both sexes. The increases we observed likely reflected many first-year foxes being captured before or during dispersal, as well as additional dispersal by some adults after their first year (Storm et al. 1976). However, harvesting may select a different part of the population than nonreportable mortality such as disease and old age.

Our findings differ from those of Storm et al. (1976) for red foxes in Iowa-Illinois and Pils and Martin (1978) for red foxes in Wisconsin, the only 2 data sets from midcontinent North America suitable for comparison with our findings. The percentage of North Dakota foxes recovered that dispersed was 15-47% lower by sex and age-class than in the Iowa-Illinois and Wisconsin studies. Mean straight-line dispersal distances for males by age-class were similar in all 3 studies, but the dispersal distances for females by age-class were about one-half as great in Iowa-Illinois as in North Dakota and Wisconsin.

Storm et al. (1976) noted that onset of dispersal may be related to litter age. North Dakota red foxes breed about 2-3 weeks later (Sargeant et al. 1981) than red foxes in Iowa-Illinois (Storm et al. 1976) and Wisconsin (Pils and Martin 1978). Thus, we hypothesize dispersal by some North Dakota red foxes may be delayed, thereby affecting the proportion of recovered foxes that dispersed by age-class. Statistical analysis was not possible because of differences in data presentation among studies. However, in our study, as in the Iowa-Illinois and Wisconsin studies, >95% of males recovered in the oldest age-class examined had dispersed. In the Iowa-Illinois and Wisconsin studies, ≥95% of males recovered during the second recovery year had dispersed (assumed for the Wisconsin study because 88% of first-year recoveries had dispersed), whereas in our study 77% of males recovered during the second recovery year had dispersed. In the Wisconsin study 58% of females recovered as adults during their first recovery year had dispersed and in the Iowa-Illinois study 37% of those recovered the first year and 58% of those recovered the second year had dispersed. We found 29% of females recovered as adults their first recovery year had dispersed; this increased to 36% for the second year and to 52% for the third recovery year age-classes. Thus, comparable dispersal percentages appear to occur 1 year later in North Dakota red foxes than in those from Iowa-Illinois and Wisconsin.

We found greater mean straight-line recovery distances for males than females, as did other studies (e.g., Storm et al. 1976, Pils and Martin 1978, Lloyd 1980, Tullar and Berchielli 1980, Trewhella et al. 1988). Much of the disparity in recovery distances between males and females probably occurred because a greater percentage of males than females had dispersed when recovered (Storm et al. 1976). However, Harris and Trewhella (1988) and Storm et al. (1976) showed that males disperse farther than females.

Frequently, the longest dispersal distances are by males (Storm et al. 1976, Tullar and Berchielli 1980, Trewhella et al. 1988), but we found the longest dispersal distances in each age-class were by females. While some females apparently did not disperse (e.g., 2 females recovered ≥7 years after tagging were recovered <8 km from their release sites), other females were recovered long distances (up to 302 km) from their release sites. The retention of at least some females in or near natal areas helps explain the frequent instances of communal denning and helper foxes (Pils and Martin 1978, MacDonald 1979, Tullar and Berchielli 1980). Such foxes have social ties with mothers and siblings that would contribute to an extended family and to possible maintenance of the parental territory after parents die.

Several investigators have shown that dispersal directions of recovered foxes were not uniform. For midcontinent North America, Arnold and Schofield (1956), Storm et al. (1976), and Pils and Martin (1978) reported a tendency for more foxes to be recovered north of release sites than expected by chance. Our findings also show that dispersal directions of both sexes were not uniform. Although these findings suggest a tendency for midcontinent foxes to disperse in certain directions, they could be biased by unequal harvest rates in surrounding areas (Storm et al. 1976, Pils and Martin 1978). Major rivers (Storm et al. 1976) and interstate highways (Pils and Martin 1978) may block or alter direction of dispersing foxes. Our data indicate that an interstate highway deflected dispersal directions and provide further evidence of the importance of major physical barriers and terrain features in altering dispersal directions. Pils and Martin (1978) suggested that littermates sometimes disperse together or use the same routes. Our findings support that suggestion and indicate the need for more data to determine the nature, magnitude, and biological importance of the above aspects of fox dispersal.

We found a negative relationship between fox family density in spring and the percentage of recovered male foxes that dispersed, but no such relationship for females. Thus, at least for males, the proportion of recovered foxes that dispersed was highest where there appeared to be the least reason for them to disperse (e.g., presumably largest home ranges [Andelt 1985 for coyotes, Trewhella et al. 1988 for red foxes] resulting in lowest probability of interactions with neighbors, and greatest probability of vacant or lightly occupied habitat near parental territories). Increased contact with conspecifics, which would occur as population density increases, may inhibit male dispersal. Other factors that may influence the probability of fox dispersal include family size (primarily litter size), survivorship of pups to autumn, and food supply. However, Allen (1984) found no relationship between estimates of fox family density in North Dakota in spring and embryonic litter sizes. Also, there was no evidence of any epizootics or food shortages affecting foxes during our study. Such population influences are generally noticed and reported by the populace in North Dakota.

Our data provided a unique opportunity to examine effects of fox density on dispersal distances because the population in the study area nearly tripled during the study period, a regional phenomenon (Allen and Sargeant 1975). Although our 1973 population was relatively high compared with other times and areas (Storm et al. 1976, Pils and Martin 1978, Tullar and Berchielli 1980, Sargeant et al. 1984, Voigt 1987), our population was much lower than most populations studied in Europe (e.g., Lloyd 1980, Harris 1981, MacDonald 1981, Harris and Rayner 1986, Trewhella et al. 1988). We found no effect of population density on straight-line dispersal distances of foxes of either sex. Thus, within the range of fox densities we studied, once dispersal was initiated, the abundance of foxes in surrounding areas appeared to have no effect on straight-line distances traveled before death occurred. This suggests that dispersal distance was strongly influenced by innate species traits (Howard 1960) that prompted the foxes to travel a set distance before stopping, or by habitat features (Tullar and Berchielli 1980), or both. In contrast, studies of red fox dispersal in Great Britain generally suggest an inverse relation between population density and dispersal distance (Lloyd 1980, Harris 1981, MacDonald and Bacon 1982, Trewhella et al. 1988).


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