WOOD MOUSE APODEMUS SYLVATICUS WINTER FOOD SUPPLY: DENSITY, CONDITION, BREEDING, AND PARASITES

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Abstract

We determined the individual and population responses to food availability of wood mice Apodemus sylvaticus wintering in small forest woodlots by means of a food addition experiment. The allocation of the extra provided energy to the maintenance of body condition and reproduction was expected to vary between sexes due to their different energy investment in reproduction and to the different indirect costs of reproduction associated with different hormone-mediated susceptibilities to parasite infections. There were no effects of food supplementation on the density of adult males or on the overall density of mice. However, adult females were marginally more abundant and the sex ratio was more balanced in supplemented woodlots. Body condition (as estimated by body mass, body mass relative to body size, and amount of fat reserves), did not differ among treatments for either adult males or females. The prevalence and intensity of endoparasites of short life cycle (oxyurid nematodes) were lower in food-supplemented woodlots than in controls and lower in females than in males. No differences were found for endoparasites with long life cycles (telacid nematodes and cestodes). Males with intermediate parasite loads were in better body condition than either nonparasitized or highly parasitized individuals, and the body condition of each group of males was better in food-supplemented woodlots than in controls. This nonlinear relationship between parasite loads and body condition was apparently due to indirect costs of reproduction in control woodlots, where there was a direct relationship between male reproductive effort (relative testis size) and parasite loads. This relationship was not found in supplemented woodlots. Most individuals were breeding actively at the end of winter. Males had larger testicles, and females showed an advanced breeding, and probably enhanced survival and larger litter sizes, in supplemented woodlots. Individual mice wintering in Mediterranean fragmented forests are thus food limited, and directed most of the extra provided food resources to reproduction without losing body condition or antiparasite abilities. This enhanced reproduction, rather than improved body condition, may translate into larger population densities and recruitment.

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