Energetic costs of the immune response and torpor use in a primate

P>1. Resource-limiting conditions impose a change in the energetic distribution between competing physiological processes. Over the past decade there has been increasing interest in trade-offs between the immune system and competing energy-consuming life-history traits. However, the trade-offs with energy saving mechanisms, such as heterothermy, have received limited attention. 2. The goal of this study is to determine how daily heterothermy expression could be adjusted to counterbalance the energetic requirements for the activation of the immune system depending on food availability (ad libitum vs. 40% calorie restriction) in a heterothermic primate, the Grey Mouse Lemur (Microcebus murinus). 3. On the day of the immune challenge, torpor was removed through the onset of fever, inducing a thermogenic cost. On the days following, food-restricted individuals returned to deep torpor (i.e. energy saving) whereas those fed ad libitum continued to skip torpor for at least three additional days. 4. The rapid return to an energy saving state in food restricted individuals raises new questions on the relationship between body temperature and immunocompetence. We suggest that (i) hyperthermia provides the first line of defence against pathogens, which is a trait common to all organisms, (ii) but that hypothermia may also protect the host by inhibiting pathogen proliferation.