A fungal parasite selects against body size but not fluctuating asymmetry in Swiss subalpine yellow dung flies

Evidence for selective disadvantages of large body size remains scarce in general. Previous studies the yellow dung fly Scathophaga stercoraria have demonstrated strong positive sexual and fecundity selection on male female size. Nevertheless, flies from a Swiss study population has declined by ~10% 1993–2009. Given substantial heritability size, this negative evolutionary response an evidently positively selected trait suggests important factors being missed. An episodic epidemic outbreak fungus Entomophthora scatophagae permitted assessment natural exerted fatal parasite. Fungal infection varied over season ~50% cooler more humid spring autumn to almost 0% summer. The probability dying fungal increased with adult Females never laid any eggs after infection, so there was no effect beyond its impact mortality. Large males showed their typical mating advantage field, but nullified infection. Mean fluctuating asymmetry paired appendages (legs, wings) did not affect viability, or success field. This documents rare parasite-mediated large-sized Reduced ability combat parasites such as may be immunity cost flies, although hypothesized trade-off between asymmetry, presumed indicator developmental instability environmental stress, immunocompetence found here.