Age, intensity of infestation by flea parasites and body mass loss in a rodent host

Parasitism by the flea Synosternus cleopatrae does not affect the body mass of its principal rodent host, Gerbillus andersoni under natural infestation levels. We hypothesized that the lack of negative effects of flea parasitism on rodent body mass could be related either to the low level of natural infestation or to the differential susceptibility of rodent age cohorts to flea parasitism. We tested these hypotheses by measuring body mass change under flea parasitism in (a) adult rodents infested with fleas above the natural infestation level (the first hypothesis) and (b) juvenile rodents infested with fleas at natural infestation levels (the second hypothesis). Adult individuals parasitized by a number of fleas higher than in nature lost body mass at higher rates than non-parasitized control individuals. Parasitism significantly affected daily body mass change of juvenile gerbils. Juvenile rodents parasitized by fleas at the natural level of infestation lost body mass faster and gained body mass slower than control animals. We suggest that some regulating mechanisms may limit natural flea densities at a point at which the negative effect on hosts is below the accuracy of our measurements. However, natural flea densities are sufficiently high to harm the more susceptible, juvenile cohort.