S. araneus has an unusually diverse parasite fauna, which includes ectoparasites [20] [21] [22] , Bartonella and trypanosome infections [23] [24] [25] [26] [27] , Anaplasma phagocytophilum [27, 28] , and Pneumocystis carinii [29] , as well as over 20 helminth species [30] . The extent to which shrews are able to mount immune responses to these parasites is unknown: commons shrews have a fast metabolic rate even for their small body size [31] but store very little energy as fat [32] . Lack of resources may therefore limit the capacity of S. araneus to mount immunological responses, particularly as reproductive adults, and parasitism has been suggested as one of the causes of mortality of adults after breeding [12, 13, 18] . Common shrews possess a large lymphatic organ, known as the Pancreas of Aselli, which may function in defence against parasites, though its exact role is unknown, and remains the subject of discussion [33] [34] [35] . To date, there have been no studies of spleen or bone marrow function in S. araneus.