Behavioral changes associated with a population density decline in the facultatively social red fox

Understanding the causal mechanisms promoting group formation in carnivores has been widely investigated, particularly how fitness components affect group formation. Population density may affect the relative benefits of natal philopatry versus dispersal. Density effects on individual behavioral strategies have previously been studied through comparisons of different populations, where differences could be confounded by between-site effects. We used a single population of red foxes (Vulpes vulpes) in the city of Bristol, UK, that underwent a natural perturbation in density to compare key changes in 1) group structure, 2) within-group relatedness, 3) mating system, 4) dispersal, and 5) dominance attainment. At high densities (19.6-27.6 adults km(-2)), group sex ratios were equal and included related and unrelated individuals. At low densities (4.0-5.5 adults km(-2)), groups became female biased and were structured around philopatric females. However, levels of within-group relatedness were unchanged. The genetic mating patterns changed with no instances of multiple-paternity litters and a decline in the frequency of extrapair litters of cubs from <= 77% to <= 38%. However, the number of genetically monogynous groups did not differ between periods. Dispersal was male biased at both high and low densities. At high density, most dominant males in the study groups appeared to have gained dominance after dispersing, but natal philopatry was an equally successful strategy at low density; conversely, most dominant females were philopatric individuals at both high and low densities. These results illustrate how density may alter behavioral strategies such as mating patterns and how this, in turn, alters group structure in a single population.

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Body mass, territory size and life history tactics in a socially monogamous canid, the red fox Vulpes vulpes

Male-biased sexual size dimorphism is typical of polygynous mammals, where the degree of dimorphism in body mass is related to male intrasexual competition and the degree of polygyny. However, the importance of body mass in monogamous mammals is largely unknown. We investigated the effect of body mass on life-history parameters and territory size in the red fox (Vulpes vulpes), a socially monogamous canid with slight sexual dimorphism. Increased body size in males appeared to confer an advantage in territory acquisition and defense contests because heavier males held larger territories and exerted a greater boundary pressure on smaller neighbors. Heavier male foxes invested more effort in searching for extrapair matings by moving over a wider area and farther from their territories, leading to greater reproductive success. Males that sired cubs outside their own social group appeared to be heavier than males that only sired cubs within their social group or that were cuckolded, but our results should be treated with caution because sample sizes were small. Territory size, boundary pressure, and paternity success were not related to age of males. In comparison, body mass of females was not related to territory size, probability of breeding, litter size, or cub mass. Only age affected probability of breeding in females: younger females reproduced significantly less than did older females, although we did not measure individual nutritional status. Thus, body mass had a significant effect on life-history traits and territory size in a socially monogamous species comparable to that reported in polygynous males, even in the absence of large size dimorphism.

Fitness costs of dispersal in red foxes (Vulpes vulpes)

The costs of dispersal are an important factor promoting natal philopatry, thereby encouraging the formation of social groups. The red fox, Vulpes vulpes, exhibits a highly flexible social system and one that is thought to represent a possible stage in the evolution of more complex patterns of group-living. Although the potential benefits accruing to philopatric offspring have previously been studied in this species, the potential costs of dispersal have received less attention. We contrasted survival rates, nutritional status, injuries and reproductive output of dispersing and non-dispersing male and female foxes in an urban population to assess the relative costs of dispersal versus natal philopatry. Mortality rates were not significantly higher for dispersing foxes, either in the short- or long-term. There was no evidence of increased nutritional stress in dispersing individuals. Dispersing individuals did, however, exhibit greater levels of wounding, although this did not appear to affect survival. Dispersing females were more likely to miss a breeding opportunity early in their reproductive lifespan. In contrast, both dispersing and non-dispersing males were unlikely to breed in their first year. We conclude that the major fitness component in females affected by dispersing is age at first reproduction.