Parasite infections in a social carnivore: Evidence of their fitness consequences and factors modulating infection load.

There are substantial individual differences in parasite composition and infection load in wildlife populations. Few studies have investigated the factors shaping this heterogeneity in large wild mammals or the impact of parasite infections on Darwinian fitness, particularly in juveniles. A host's parasite composition and infection load can be shaped by factors that determine contact with infective parasite stages and those that determine the host's resistance to infection, such as abiotic and social environmental factors, and age. Host-parasite interactions and synergies between coinfecting parasites may also be important. We test predictions derived from these different processes to investigate factors shaping infection loads (fecal egg/oocyte load) of two energetically costly gastrointestinal parasites: the hookworm and the intracellular , in juvenile spotted hyenas () in the Serengeti National Park, in Tanzania. We also assess whether parasite infections curtail survival to adulthood and longevity. infection loads declined as the number of adult clan members increased, a result consistent with an encounter-reduction effect whereby adults reduced encounters between juveniles and infective larvae, but were not affected by the number of juveniles in a clan. Infection loads decreased with age, possibly because active immune responses to infection improved with age. Differences in parasite load between clans possibly indicate variation in abiotic environmental factors between clan den sites. The survival of juveniles (<365 days old) to adulthood decreased with load, increased with age, and was modulated by maternal social status. High-ranking individuals with low loads had a higher survivorship during the first 4 years of life than high-ranking individuals with high loads. These findings suggest that high infection loads with energetically costly parasites such as hookworms during early life can have negative fitness consequences.