Host age is a key factor in host-parasite relationships because it potentially affects infestation levels, parasite-induced mortality, and parasite distribution. Researchers tested two hypotheses about infestation pattern and survival under parasitism by arachnids (ticks and mites) in relation to the age of host animals (gerbils). The parasite recruitment hypothesis assumes that parasites are recruited more quickly than they die and thus suggests that adult hosts will show higher infestation levels than juveniles because adults have more time to accumulate parasites. The parasite mortality hypothesis assumes that parasites die more quickly than they are recruited and thus suggests that adults will show lower infestation levels because of an acquired immune response. As the negative effects of parasites on hosts are often density-dependent, the researchers expected that, in testing respectively the first and the second hypothesis, age-related differences in infestation might translate into lower or higher survival under parasitism of adults. However, they found no age-related differences in infestation and distribution among host individuals after adjusting the parasite numbers to the host's surface area: parasites were more abundant on adults than on smaller juveniles, but not more dense. Nevertheless, the survival probability for parasitized juveniles was lower than for nonparasitized juveniles, whereas the survival probability for adults was not affected by parasites. Researchers also found no effect of parasite infestation on host body mass. This may be due to food compensation by parasitized juveniles through foraging for longer periods, suggesting that the lower survival of parasitized juveniles may result from higher exposure to predators. Perhaps the processes of parasite recruitment and parasite mortality depend on host age but operate in opposite directions and thus the two processes cancel each other. For example, the tendency of adult wood mice to accumulate more nematodes than juvenile wood mice counters the better ability of adults to mount an immune response. Or perhaps factors other than the level of host exposure, immune response, and parasite-induced host mortality influence processes of specific parasite recruitment and mortality. For instance, host-age-biased flea levels fluctuate between juvenile-biased and adult-biased levels, perhaps as a function of soil temperature.