1 Food resources and foraging habits of the common shrew, Sorex araneus: does winter food shortage explain Dehnel’s phenomenon? Sara Churchfield, Leszek Rychlik and Jan R. E. Taylor S. Churchfeld (sara.churchfeld@kcl.ac.uk), Dept of Anatomy and Human Sciences, King’s College London, Guy’s Campus, London, SE1 1UL, UK. – L. Rychlik, Dept of Systematic Zoology, Inst. of Environmental Biology, Adam Mickiewicz Univ., Umultowska 89, PL-61-614 Poznań, Poland. – J. R. E. Taylor, Inst. of Biology, Univ. of Białystok, ul. Swierkowa 20 B, PL-15-950 Białystok, Poland. It is widely assumed that winter is a critical time for homeotherms because of decreased ambient temperatures coupled with reduced food supply. Shrews are excellent models for investigating overwintering strategies, not only because of their partic- ularly small size, high energy requirements relative to their size and short fasting endurance, but also the dramatic reduction in body size (Dehnel’s phenomenon) exhibited by soricine shrews in northern temperate winters. Te cause of Dehnel’s phenomenon is poorly understood but food supply is implicated. To test the hypothesis that winter at higher latitudes is a period of food shortage for small homeotherms, we compared feeding habits of common shrews, Sorex araneus, and abundance and biomass of their prey in winters and summers in northeastern Poland using scat analysis combined with pitfall and ground core sampling for invertebrates. Ground-surface activity and numbers of invertebrates in pitfall traps were greatly reduced in winter but, contrary to prediction, no signifcant diferences between winter and summer were found in total numbers and biomass of prey invertebrates in ground core samples. However, certain prey types changed seasonally with respect to numbers, biomass and distribution in the soil profle, which was refected in shrews’ food com- position and foraging behaviour. Dehnel’s phenomenon appears not to be caused by reduction in total prey numbers and biomass, at least in our study area. Smaller body mass coupled with lowering of absolute food requirements may have important survival value in winter with its reduced numbers of certain major prey coupled with increased difculty of locating and extracting invertebrates within the soil profle resulting in higher energetic costs of foraging. It is widely assumed that winter is a critical time for homeo- therms because of decreased ambient temperatures coupled with reduced food supply. Te occurrence of very small mammals, such as shrews, in regions of severe winter cli- mate presents a particular paradox because they cannot migrate or hibernate. With their large surface-area to volume ratios and high metabolic rates relative to their size, shrews approach a critical mass for maintaining endothermy at low temperatures (McNab 1983, Genoud 1988, Taylor 1998). Shrews are renowned for their small size, high energy require- ments relative to their size, short fasting endurance and low capacity for fat deposition and so are excellent models for investigating overwintering strategies and testing hypo- theses about winter food availability and foraging behaviour. An interesting but poorly understood aspect of the winter biology of soricine shrews in northern temperate regions is their dramatic reduction in body size in winter, known as Dehnel’s phenomenon (Dehnel 1949, Mezhzherin 1964, Pucek 1970). Although a winter size decrease in small murine and arvicoline rodents has been reported, it is less evident (Iverson and Turner 1974, Merritt and Zegers 1991, Gliwicz 1996). An autumn–winter decline in lean body mass of 12–15% in small insectivorous marsupials (Antechinus spp.) inhabiting the Snowy Mountains, Australia has also Oikos 000: 001–010, 2012 doi: 10.1111/j.1600-0706.2011.20462.x © 2011 Te Authors. Oikos © 2012 Nordic Society Oikos Subject Editor: Stan Boutin. Accepted 8 November 2011 been recorded (Dickman et al. 1983, Green 2001). Te average winter weight loss of soricine shrews varies from 10 to 30% of their summer body mass (Borowski and Dehnel 1952, Hyvärinen 1984, Churchfeld et al. 1995, Ochocińska and Taylor 2003). It is manifested uniquely in these small mammals by reduction in length of the vertebral column, reduction in brain mass accompanied by decrease in depth of braincase, decline in mass of many of the internal organs and decrease in body fat and water content (Bielak and Pucek 1960, Pucek 1963, 1965, Hyvärinen 1969, Churchfeld 1981). Reduced prey abundance has been implicated as a cause of decreased body size of shrews in winter. Dehnel’s phenomenon has been interpreted as an adaptation permitting a reduction in absolute food require- ments at a time of food shortage in winter since a small animal eats less than a large one (Mezhzherin 1964, Pucek 1970, Hyvärinen 1984, McNab 1991, 1999, Churchfeld 2002, Gliwicz and Taylor 2002). Te cause and adaptive value of Dehnel’s phenomenon pre-supposes that food availability (here defned as the presence and abundance of potential prey in sites frequented by shrews) is much reduced in winter. Indeed, the autumn– winter body mass decline in small insectivorous Antechinus marsupials is clearly associated with reduction in surface-