Species which are closely related and share many char- acteristics while differing in others, can provide useful comparisons for ecological or evolutionary studies and for management purposes. Three of the smaller foxes make strong candidates for such a comparison, viz. the swift fox (Vulpes velox) of the shortgrass prairies, the kit fox (V. macrotis) of the desert, and the arctic fox (Alopex lagopus) of the tundra. The former 2 are restricted to North America, while the arctic fox has a circumpolar distribu- tion. Despite differences in geographic range, habitat and physiology, there are striking similarities between the 3 species in genetics, behavior and ecology. For example, swift, kit and arctic foxes share a strong dependence on good denning sites for breeding. They all live in open habitats and are subject to harsh climatic con- ditions. Furthermore, they live sympatrically with larger mammalian carnivores that often act as predators or com- petitors. Underground dens provide shelter for these small foxes, especially during breeding. The young are born blind and are dependent on their parents for approximate- ly 2 months. Their den dependence has become a useful tool for ecologists, as den surveys can give good estimates of reproductive success in a population. For arctic foxes, it is a widely used method for population estimates (e.g., Elton 1924, Macpherson 1969, Angerbjörn et al. 1995, Tannerfeldt 1997). This den dependence has also been used in extermination campaigns (Hersteinsson 1984, Bailey 1992). For swift and kit foxes, den surveys lead to assessments of breeding frequency and litter sizes, which serve as indicators of reproductive success between years and regions (Egoscue 1975, Covell 1992, White and Ralls 1993, White and Garrott 1997). This paper reviews and compares the den ecology of swift, kit and arctic foxes. It is essential to have an under- standing of how availability and distribution of dens affect fox life histories and population dynamics. For example, increasing agricultural and industrial land use in sensitive areas might interfere with fox management and conserva- tion goals. We will thus examine the den as a resource for these small foxes. Our interest was in factors that deter- mine the distribution of dens, their structure and how they are used. How important are den sites to these foxes and what are the relationships between dens, reproductive out- put and other ecological parameters? Finally, we will try to determine the implications of our results for the manage- ment of these species. The Species Recent research suggests that the arctic fox should be included in the genus Vulpes to form a monophyletic group, and that arctic, kit and swift foxes are very closely related (Martin 1989, Geffen et al. 1992, Mercure et al. 1994). This genetic similarity is the closest that exists between any of the Vulpes-like species (Wayne and O’Brien 1987). They share an ancestor possibly adapted to an open desert and prairie habitat. From this, the arctic fox evolved into an exclusively tundra-dwelling species. Recent studies recog- nize only 3 subspecies of the arctic fox, 2 of which are indigenous to the isolated Commander Islands (Ginsberg and Macdonald 1990). The taxonomy of swift and kit foxes has been intensively debated. Based on protein-elec- trophoretic methods, Dragoo et al. (1990) concluded that swift and kit foxes were the same species but they argued that morphological differences warranted classification into separate subspecies. More recent mitochondrial DNA Part IV – Population Ecology 167 Den Ecology of Swift, Kit and Arctic Foxes: A Review + + Magnus Tannerfeldt, Axel Moehrenschlager and Anders Angerbjörn Abstract: The availability and use of denning sites are important aspects of the ecology of most canids. Swift, kit and arctic foxes are closely related genetically, are similar in size, and share a number of behavioral and ecological traits. Yet, there are many differences between the species, which can be used in comparative studies. In this review, we examine differences and similarities in the den ecol- ogy of these species, in order to analyze the relationship between den use and other ecological parameters. We also discuss implica- tions for the management of these foxes. We have found 2 different den ecology strategies, where swift and kit foxes have small litters and regularly change dens during the breeding season, while arctic foxes have large litters in large dens. The primary function of a breeding den is most likely to provide protection against predators. Sufficient escape routes can be achieved either by having several small satellite dens within each home range or by having large dens with many openings. These different den ecology strategies also involve territoriality, and are related to differences in a number of ecological parameters, such as predation rates, availability of dens, food resources and litter sizes. Identification and classification of den sites is a means of making surveys and population estimates more effective, especially for the arctic fox. An analysis of den sites is important for habitat protection and as a preparatory task for re-intro- duction programs for all 3 species.