Summary1. Home range size, spatial organization and territoriality of reproductive Arctic foxes were studied during the summer. The influence of spatial distribution and availability of the main prey was investigated in order to evaluate whether the spatial organization of Arctic foxes was coherent with key predictions of the resource dispersion hypotheses (RDH). The RDH includes the spatial characteristics of resource abundance, while there is also growing attention to the importance of the temporal characteristics of resource abundance. Hence the role of temporal and spatio-temporal predictability of prey explaining carnivore spatial organization was also investigated in this study. 2. The study was conducted on Svalbard; a simple High-Arctic terrestrial ecosystem which allowed unique estimates of prey abundance. The main prey of the Arctic fox (Svalbard reindeer Rangifer tarandus platyrhynchus , seabirds Alcidae and Procellariidae and geese Anseridae ) was surveyed systematically. These surveys revealed highly contrasting patterns in prey abundance within the terrestrial ecosystem. 3. Arctic fox summer home ranges varied in size (4-60 km 2 ), as well as in overlap (17-76%). The diverse spatial organization covaried with spatial and temporal patterns in prey abundance. Small home ranges (10 ± 5·6 km 2 ) with large overlap (76 ± 19·6%) were characteristic for coastal areas where prey was concentrated in small patches and predictable both in space and time. Medium home ranges (23 ± 4·2 km 2 ) and overlap (50 ± 6·6%) occurred inland where prey was clumped in larger patches and less predictable. Large home ranges (52 ± 8·4 km 2 ) with little overlap (17 ± 3·5%) occurred inland where prey was widely scattered and unpredictable. 4. Spatial dispersion and richness of prey resources explained most of the variation in Arctic fox spatial organization. The RDH framework could be used to explain the presence of relaxed territoriality found in this study. We suggest that the observed absence of more permanent social groups is due to the extremely severe winter conditions which force juvenile individuals to disperse from the natal area during the first winter. 5. Predictability of resources was an additional significant factor affecting both home-range size and overlap. Resource predictability captures the degree to which an animal can depend on its environment to offer suitable and secure living conditions over time. 6. This study emphasize the need to incorporate both spatial and temporal characteristics of resource distribution in order to fully understand the diversity of spatial arrangements among carnivores.

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In this article physiological, behavioural and morphological adaptations by the arctic fox to low temperatures and food scarcity in winter are discussed. The arctic fox (Alopex lagopus) adapts to the low polar winter temperatures as a result of the excellent insulative properties of its fur. Among mammals, the arctic fox has the best insulative fur of all. The lower critical temperature is below-WC, and consequently increased metabolic rate to maintain homeothermy is not needed under natural temperature conditions. Short muzzle, ears and legs, a short, rounded body and probably a counter-current vascular heat exchange in the legs contribute to reduce heat loss. A capillary rete in the skin of the pads prevents freezing when standing on a cold substratum. By seeking shelter in snow lairs or in dens below the snow cover and by curling up in a rounded position, exposing only the best-insulated parts of the body, the arctic fox reduces heat loss. The arctic fox copes with seasonal fluctuations in food supply by storing fat and caching food items during summer and fall. Saving energy through decreased activity and decreased basal metabolic rate might also be an adaptation to food scarcity in winter.

Arctic fox (Alopex lagopus) numbers vary greatly, with cyclic fluctuations often associated with fluctuations in microtine rodents. However, in areas where small prey mammals are absent, such as Iceland and Svalbard, such cyclic fluctuations are lacking. Annual fluctuations in the density of the arctic fox population on the Brøggerhalvøya peninsula and Kongsfjorden region on Svalbard, Norway, were studied from 1990 to 2001 by using indices of fox abundance. All indices showed similar trends; fox numbers were low in 1990, increased until 1995 whereupon they decreased sharply, before increasing again and levelling off in 2001. Increasing numbers of foxes during the first part of the study paralleled increasing numbers of Svalbard reindeer (Rangifer tarandus platyrhynchus) carcasses in winter and increasing numbers of nesting barnacle geese (Branta leucopsis) in summer. This study shows that the number of arctic foxes varies greatly even in areas without fluctuating microtine rodents.

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