Local variation in endoparasite intensities of bank voles (Clethrionomys glareolus ) from ecologically similar sites: morphometric and endocrine correlates C.J. Barnard 1 *, J.M. Behnke 2 , A. Bajer 3 , D. Bray 4 , T. Race 2 , K. Frake 2 , J. Osmond 2 , J. Dinmore 2 and E. Sinski 3 1 Animal Behaviour and Ecology Research Group, 2 Infection and Immunity Research Group, School of Life and Environmental Sciences, University of Nottingham, University Park, Nottingham, NG7 2RD, UK: 3 Department of Parasitology, Institute of Zoology, University of Warszawa, ul. Miecznikowa 1, 02-096 Warszawa, Poland: 4 Animal Behaviour Group, Department of Veterinary Clinical Science and Animal Husbandry, University of Liverpool, Leahurst, Neston, CH64 7TE, UK Abstract Much interest has centred recently on the role of adaptive trade-offs between the immune system and other components of life history in determining resistance and parasite intensities among hosts. Steroid hormones, particularly glucocorticoids and sex steroids, provide a plausible mechanism for mediating such trade-offs. A basic assumption behind the hypothesis, however, is that steroid activity will generally correlate with reduced resistance and thus greater parasite intensities. Here, we present some findings from a field study of bank voles (Clethrionomys glareolus ) in which we have looked at associations between parasite intensities, anatomical and morphometric measures relating to endocrine function and life history variation in three local populations inhabiting similar but mutually isolated woodland habitats. In general, sites with greater parasite intensities were those in which male C. glareolus had significantly larger adrenal glands, testes and seminal vesicles for their age and body size. Females also showed a site difference in adrenal gland weight. Some aspects of site-related parasite intensity were associated with asymmetry in adrenal gland weight and hind foot length, which may have reflected developmental effects on glucocorticoid activity. Introduction Parasite intensities are known to vary considerably between the individuals in host populations. The basis of this variation is multifactorial, but the consequence is aggregation of parasites in particular hosts and a non- random distribution across the whole population (Crof- ton, 1971; Shaw & Dobson, 1995). The processes underlying these phenomena are of central interest in the study of host – parasite community ecology and aggregated distributions are almost an axiom of the field (Gregory & Woolhouse, 1993; Haukisalmi & Henttonen, 1999). Much is now understood about the ecological, immunological and demographic factors contributing to different patterns of prevalence and to aggregated abundance of parasitic infections (Anderson & May, 1978, 1991; Anderson & Gordon, 1982; Quinnell & Keymer, 1990; Wakelin & Blackwell, 1993). Recently, however, interest has focused on the possible role of *Fax: +44 (0) 115 9513251 E-mail: christopher.barnard@nottingham.ac.uk Journal of Helminthology (2002) 76, 103–112 DOI: 10.1079/JOH2001109