While there have been a number of studies of ecological immunology in birds and insects [4] [5] [6] , there has been little effort to understand immune function of small mammals in this context outside of the laboratory. Here, we examined the unique haemolymphatic system of the European common shrew (Sorex araneus) to investigate the capacity of this short-lived mammal, restricted by a fast metabolism and extremely limited fat reserves, to defend against its unusually diverse parasite fauna, both as a young animal and an adult.