Ann. Zool. Fennici 44: 415–424 ISSN 0003-455X (print), ISSN 1797-2450 (online) Helsinki 19 December 2007 © Finnish Zoological and Botanical Publishing Board 2007 Relaxed competition during winter may explain the coexistence of two sympatric Microtus species Katrine S. Hoset 1 & Harald Steen 1,2 1) Program for Experimental Behavioral and Population Ecological Research, Department of Biology, University of Oslo, PB 1050 Blindern, N-0316 Oslo, Norway (e-mail: katrine.hoset@utu.fi) 2) Norwegian Polar Institute, Hjalmar Johannesens gt 14, N-9296 Tromsø, Norway Received 13 Feb. 2006, revised version received 30 Oct. 2007, accepted 30 Oct. 2007 Hoset, K. S. & Steen, H. 2007: Relaxed competition during winter may explain the coexistence of two sympatric Microtus species. — Ann. Zool. Fennici 44: 415–424. Root voles (Microtus oeconomus) are competitively superior during summer and locally displace ield voles (M. agrestis). Since the two species co-exist in many areas, competition may be relaxed or reversed during winter. To assess the competitive interaction during winter, we used PIT tags to study activity patterns and home range overlap of the two species in two enclosures with food-supplemented and non-supple- mented plots during the winter of 2002–2003. Additionally, we established single spe- cies systems of root-vole and ield-vole populations in the winter of 2004. Home-range size was smaller and activity lower in ield voles than in root voles, and food supple- mentation caused an increase in activity in both species. Activity and home-range size in either species was not affected by the presence of the other. Despite the different activity patterns of the two species, home range overlap between ield voles and root voles did not differ from random in the food-supplemented plots. In the non-supple- mented plots daily overlap was higher than expected. As predicted, we were not able to detect any interference competition between ield and root voles during winter. This supports our hypothesis that coexistence between the two may be caused by relaxed contest competition during winter. Introduction In northern habitats with distinct breeding and non-breeding seasons, seasonal variation in competitive intensity between species may be of importance. Studies of bank voles (Myodes glareolus, previously Clethrionomys glareolus) and ield voles have shown that aggression levels in individual encounters are low during the non- breeding season both in mixed and single spe- cies communities (reviewed in Eccard & Ylönen 2003). The non-breeding season in the north is energetically demanding and may not allow maintenance of intensive competitive interac- tions. A prerequisite for competition between the two species is that there is a shortage of some resource that both depend upon. Korslund and Steen (2006) found that winter survival of root voles increased with increased food availability and we can therefore assume that there are food shortages during the winter. A study of grey- sided voles (Myodes rufocanus) supports this assumption since over-winter rate of population change was negatively affected by the size of the This article has been scientifically edited by: Johan Kotze