Combined effects of cold exposure and sub-lethal intestinal parasites on host morphology and physiology.

Multiple, simultaneous demands elicit physiological and morphological responses that may jeopardize an animal's ability to respond to future challenges, especially when resources are limited. Laboratory mice (Mus musculus) experimentally infected with an intestinal nematode (Heligmosomoides polygyrus) and then exposed to cold showed phenotypic plasticity of morphological and physiological responses. The parasitized mice maintained a similar body mass to the unparasitized mice but had less body fat and showed changes in some organ masses, a greater resting metabolic rate (RMR) and a diminished glucose uptake capacity both at the site of infection and in regions of the small intestine not occupied by parasites. Cold-exposed mice had a greater RMR, less body fat, a greater glucose transport capacity and showed changes in organ masses compared with mice maintained at room temperature. The responses to cold exposure were not affected by parasitism for any dependent variable. The costs of having parasites during simultaneous cold exposure included decreased energy reserves and greater maintenance requirements, which may then decrease the energy available for future expenditures, such as reproduction.