Behav Ecol Sociobiol (2005) 59: 198–206 DOI 10.1007/s00265-005-0025-8 ORIGINAL ARTICLE Mikhail Goltsman · Elena P. Kruchenkova · Sergei Sergeev · Paul J. Johnson · David W. Macdonald Effects of food availability on dispersal and cub sex ratio in the Mednyi Arctic fox Received: 2 March 2005 / Revised: 1 June 2005 / Accepted: 2 June 2005 / Published online: 23 August 2005 C  Springer-Verlag 2005 Abstract Since the Pleistocene, Arctic foxes, Alopex lago- pus, on Mednyi Island in the North Pacific have been iso- lated in a small area with rich food resources and no other terrestrial carnivores. This situation provides an unusually simple system within which the effect of food dispersion on demography and social organisation was examined. We studied the composition, location and dispersal of 67 Arctic fox groups and mapped their major food resources (seabird colonies) during 1994–2000 on Mednyi. We compared our observations with the predictions of models of sex-ratio determination. Our observations are most consistent with the predictions of Julliard’s (2000) model, where mothers are expected to produce more offspring of the most dispers- ing sex in low-quality habitats, and more offspring of the most philopatric sex in high-quality habitats. The polyg- ynous foxes on Mednyi Island lived where the principal food resources were patchily distributed (present on 11% of the shoreline), and cub survival to dispersal age or re- productive adult was higher in rich (25/45) than in poor (24/79) home ranges. Furthermore, dispersal was strongly sex-biased: most females (60%) remained on their na- tal ranges, whereas very few males (9%) did so. Signifi- cantly more female than male cubs (54 compared with 24) emerged from dens in resource rich ranges, whereas the sex ratio on poor ranges was approximately equal (51 females and 56 males). While our observations are also to some ex- tent consistent with the local resource enhancement (LRE) hypothesis (which predicts a bias towards the sex most likely to cooperate with parents), this does not account for the observed spatial variability. Communicated by M. Festa-Bianchet M. Goltsman · E. P. Kruchenkova · S. Sergeev Department of Vertebrate Zoology, Faculty of Biology, Moscow State University, Moscow 119899, Russia E. P. Kruchenkova · P. J. Johnson · D. W. Macdonald () Wildlife Conservation Research Unit, University of Oxford, Tubney House, Abingdon Road, Tubney, Abingdon OX13 5QL, UK e-mail: David.Macdonald@zoo.ox.ac.uk Tel.: +44-01865-393100 Keywords Food availability . Dispersal . Cub sex ratio . Mednyi . Arctic fox . Alopex lagopus semenovi Introduction The influence of the dispersion and availability of resources on the social organisation of territorial carnivores has been a focus for theoretical models (e.g. Macdonald 1983; Carr and Macdonald 1986; Bacon et al. 1991) and empirical studies (e.g. Kruuk 1978; Kruuk and Parish 1982; Mills 1989; Mills and Gorman 1997; Geffen et al. 1992; Patterson and Messier 2001; Valenzuela and Macdonald 2002). Patterns in the dispersion of food resources in space and time may affect population parameters such as survival, litter size, sex ratios and dispersal (e.g. Tannerfeldt and Angerbj¨ orn 1998; Geffen et al. 1996), and may facilitate the formation of groups and constrain their sizes (see reviews in Macdonald and Carr 1989; Woodroffe and Macdonald 1993; Johnson et al. 2003). We have gathered 7 years of data on an unusual popu- lation of Arctic foxes, and we use these to compare the predictions of models connecting offspring sex ratio and sex-biased dispersal with the heterogeneity of resource dis- persion. The Arctic fox population in question has been isolated since the end of the Pleistocene on Mednyi Island, a small island in the ice-free part of the North Pacific in the Commander Islands chain. The population is protected both from predators and immigrating Arctic foxes. Within this small insular population, under conditions where the rate of evolutionary change may be high (Mayr 1963; Gould and Eldredge 1977), selection appears to have caused changes in the biology of the Arctic fox. Indeed, on the basis of morphological (mainly craniological) parameters, the Mednyi population of Arctic foxes was distinguished as a separate species by Ognev (1931), and is now considered as an endemic subspecies, Alopex lagopus semenovi (Ognev 1931; Geptner and Naumov 1967) with ancestral cranial attributes (Zalkin 1944).