ADAPTIVE PHENOTYPIC PLASTICITY IN GROWTH, DEVELOPMENT, AND BODY-SIZE IN THE YELLOW DUNG FLY

Life-history theory predicts that age and size at maturity of organism s should be influenced by time and food constraints on development. Th is study investigated phenotypic plasticity in growth, development, bo dy size, and diapause in the yellow dung fly, Scathophaga stercoraria. Full-sib families were allowed to develop under predator-free field c onditions. The time before the onset of winter was varied and each bro od was split into three environments differing in the amount of dung a set number of larvae had as a resource. When resources were abundant and competition was minimal, individuals of both sexes grew to larger body sizes, took longer time to mature, and were able to increase thei r growth rates to attain large body sizes despite shorter effective de velopment periods later in the season. In contrast, limited larval res ources and strong competition constrained individuals to mature earlie r at a smaller adult size, and growth rates could not be increased but were at least maintained. This outcome is predicted by only two life- history optimality models, which treat mortality due to long developme nt periods and mortality due to fast growth as independent. Elevated p readult mortality indicated physiological costs of fast growth indepen dent of predation. When larval resources were limited, mortality incre ased with heritable variation in development time for males, and towar d the end of the season mortality increased as larval resources became more abundant because this induced longer development periods. Sexual and fecundity selection favoring large body size in this species is t hus opposed by larval viability selection favoring slower growth in ge neral and shorter development periods when time and resources are limi ted; this overall combination of selective pressures is presumably sha ping the reaction norms obtained here. Flexible growth rates are facil itated by low genetic correlations between development time and body s ize, a possible consequence of selection for plasticity. Heritable var iation was evident in all traits investigated, as well as in phenotypi c plasticity of these traits (genotype x interactions). This is possib ly maintained by unpredictable spatiotemporal variation in dung abunda nce, competition, and hence selection.