ALTITUDINAL LIFE-HISTORY VARIATION IN THE DUNG FLIES SCATHOPHAGA-STERCORARIA AND SEPSIS-CYNIPSEA

Field phenologies of high- (ca. 1500 m) and low- (ca. 500 m) altitude populations of the two most common European species of dung flies, Sca thophaga stercoraria and Sepsis cynipsea, differ quite markedly due to differences in climate. To differentiate genetic adaptation due to na tural selection and phenotypic plasticity, I compared standard life hi story characters of pairs of high- and low-altitude populations from t hree disjunctive sites in Switzerland in a laboratory experiment. The F1 rearing environment did not affect any of the variables of the F2 g eneration with which all experiments were conducted; hence, there were no carry-over or maternal effects. In Sc. stercoraria, high-altitude individuals were smaller but laid larger eggs; the latter may be advan tageous in the more extreme (i.e. more variable and less predictable) high-altitude climate. Higher rearing temperature strongly decreased d evelopment time, body size and the size difference between males and f emales (males are larger), produced female-biased sex ratios and led t o suboptimal adult emergence rates. Several of these variables also va ried among the three sites, producing some interactions complicating t he patterns. In Se. cynip sen, high-altitude females were marginally s maller, less long-lived and laid fewer clutches. Higher rearing temper ature strongly decreased development time and body size but tended to increase the size difference between males and females (males are smal ler); it also increased clutch size but decreased physiological longev ity. Again, interpretation is complicated by variation across sites an d some significant interactions. Overall, genetic adaptation to high-a ltitude conditions appears weak, probably prevented by substantial gen e flow, and may be swamped by the effects of other geographic variable s among populations. In contrast, phenotypic plasticity is extensive. This may be due to selection of flexible, multi-purpose genotypes. The results suggest that differences in season length between high- and l ow-altitude locations alone do not explain well the patterns of variat ion in phenology and body size.