Flea infestation does not cause a long-term increase in energy metabolism in Gerbillus nanus

Fleas can increase the metabolic rate of their hosts. It has been suggested that a constitutive response, in which the host constantly maintains a relatively high level of energy metabolism to combat the parasite, is advantageous for hosts with high parasite infestation, while an induced response, in which the host increases energy metabolism in response to a parasite attack, is advantageous with low parasite infestation. As free-living Gerbillus nanus show a relatively low flea infestation, we hypothesized that this host uses an induced strategy and, consequently, flea infestation would not impose a long-term effect on energy metabolism. In a previous study in spring, higher field metabolic rate (FMR) was found in free-living parasitized than in non-parasitized G. nanus. In this study, G. nanus were captured at Hazeva in spring; some had fleas (N=14) and some did not (N=10). We brought them to the laboratory, removed the fleas from those that were infested and, after 3weeks, measured average daily metabolic rate (ADMR) of all rodents. ADMR averaged 8.68±0.95kJg^-0.54day^-1 for all rodents and was similar between previously parasitized and non-parasitized G. nanus while free living. Thus, the hypothesis that flea infestation does not have a long-term effect on energy metabolism was supported, as was the idea of an induced over a constitutive immune response by G. nanus in combating parasites.