Wolf U. BlanckenhornPlease use the format "First name initials family name" as in "Marie S. Curie, Niels H. D. Bohr, Albert Einstein, John R. R. Tolkien, Donna T. Strickland"
Evidence for selective disadvantages of large body size remains scarce in general. Previous phenomenological studies of the yellow dung fly *Scathophaga stercoraria* have demonstrated strong positive sexual and fecundity selection on male and female size. Nevertheless, the body size of flies from a Swiss study population has declined by almost 10% from 1993 to 2009. Given substantial heritability of body size, this negative evolutionary response of a trait that is measurably positively selected suggests important selective factors being missed. We took advantage of a periodic epidemic outbreak of the fungus Entomophthora scatophagae to assess selection exerted by this fatal parasite. Fungal infection varied over the season from ca. 50% in the cooler and more humid spring and autumn to almost 0% in summer. The probability of dying from fungal infection increased with adult body size. Females never laid any eggs after infection, so there was no fungus effect on female fecundity beyond its impact on mortality. Large males showed the typical mating advantage in the field, but this pattern of positive sexual selection was nullified by fungal infection. Mean fluctuating asymmetry of paired appendages (legs, wings) did not affect the viability, fecundity or mating success of yellow dung flies in the field. Our study demonstrates rare size-selective parasite-mediated disadvantages of large adult body size in the field. Reduced ability to combat parasites such as Entomophthora may be an immunity cost of large body size in dung flies, although the hypothesized trade-off between fluctuating asymmetry, a presumed indicator of developmental instability and environmental stress, and immunocompetence was not found here.
body size, developmental stability, Entomophthora, fecundity selection, fluctuating asymmetry, fungal parasite, insect immunity, Scathophaga stercoraria, sexual selection, trade-off, viability selection.
Behavior & Social Evolution, Evolutionary Ecology, Life History, Sexual Selection
