| Summary: | This study of breeding arctic foxes (Alopex lagopus) and their major prey, the brown lemming (Lemmus trimucronatus) and the Greenland collared lemming (Dicrostonyx groenlandicus), was conducted at Aberdeen Lake, District of Keewatin, N.W.T. in the summers of 1968, 1969 and 1970. Lemming trapping showed that the density of both species was low in 1968 and 1969, and a high occurred in 1970 with maximum density of 50 lemmings/ha. Poor fox reproduction occurred in 1968, no whelps were raised in 1969 and high reproductive success was recorded in 1970. Analysis of the frequency of dens occupied in late spring during this study and previous years suggested that a low proportion of the fox population bred when lemming densities were low the previous winter and spring. Literature on the relationship between reproductive success and nutritional planes and reinterpretation of earlier data on foxes in the area, suggested that insufficient food during fall and winter and spring when mating occurs could inhibit reproduction in a large proportion of the foxes. An hypothesis explaining the increase of fox densities which sometimes occur one year after lemming peaks as well as increases of foxes during lemming population increases is presented. During 1970 observations of hunting foxes showed that the mean hunting range was 2.9 km2 and all hunting was conducted between 4:00 p.m. and 10:00 a.m. Prior to parturition and during the suckling period male foxes do most of the hunting whereas both adults at all dens hunted when the whelps consumed lemmings. Although both adults at one den hunted most frequently within 600 m of the den, the female did more hunting within 600 m of the den and made shorter hunts than the male. Her behaviour suggested that she hunted more efficiently and had greater concern for the whelps' food supply than the male, whose behaviour indicated that he spent a large part of his time on territorial behaviour during his hunts. During early June 1970 rapid flooding of low habitats forced Lemmus into higher habitats where they were caught by foxes utilizing stalking and dashing capture techniques. As the lower habitats dried and thawed during the latter half of June, Lemmus reoccupied them and initiated breeding. The foxes responded by increasing the frequency of hunts and captures in lower habitats by digging into peaty hummocks where Lemmus burrows. Upon emergence of the new Lemmus cohort in the latter part of July the foxes conducted more stalking and dashing behaviour in the lower habitats in response to the higher prey density there. Dicrostonyx occupied the higher habitats throughout the summer and were mostly captured by stalking or dashing as their burrows are located deep in sandy soil or under rocks, and are difficult for foxes to excavate. The ratio of Lemmus to Dicrostonyx consumed by whelps at nine breeding dens during July 1970 was 6.9 to 3.1 whereas the weight ratio of the two species was 5.7 to 4.3, respectively. The difference between the quality and quantity of the two prey consumed was related to the selection of Lemmus habitats by foxes for hunting and their failure to excavate Dicrostonyx nestlings from their burrows. Food consumption of two captive whelps averaged 363 g/whelp per day or 11,975 g/whelp during the second half of the denning period. Food consumption of a mean litter of 10.6 whelps was estimated to be 127 kg equivalent to about 2,400 lemmings. The impact of a pair of breeding foxes on lemmings during an entire denning period was estimated to be 15 lemmings/ha within the hunting area.
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