ShuiEro ... from vague up to sometimes ;) crystal clear drop by drop ...

Given a trade-off between offspring size and number and an advantage to large size in competition, theory predicts that the offspring size that maximizes maternal fitness will vary with the level of competition that offspring experience. Where the strength of competition varies, selection should favor females that can adjust their offspring size to match the offspring's expected competitive environment. We looked for such phenotypically plastic maternal effects in the least killifish, Heterandria formosa, a livebearing, matrotrophic species. Long-term field observations on this species have revealed that some populations experience relatively constant, low densities, whereas other populations experience more variable, higher densities. We compared sizes of offspring born to females exposed during brood development to either low or high experimental densities, keeping the per capita food ration constant. We examined plastic responses to density for females from one population that experiences high and variable densities and another that experiences low and less-variable densities. We found that, as predicted, female H. formosa produced larger offspring at the higher density. Unexpectedly, we found similar patterns of plasticity in response to density for females from both populations, suggesting that this response is evolutionarily conserved in this species. K E Y W O R D S : Competition, life-history strategies, maternal effects, optimal offspring size, phenotypic plasticity. Environmental conditions experienced by offspring during pre-and postnatal development can have dramatic effects on their fitness (e.g., Clutton-Brock 1991; Mousseau and Fox 1998; Holbrook and Schal 2004). An important source of these effects is the maternal environment; stressful conditions experienced by the female parent can reduce offspring fitness (McCormick 1998; Jann and Ward 1999; Naguib et al. 2006) but, in certain cases, All authors contributed equally in this article. mothers respond to environmental challenges by manipulating the phenotype of individual offspring to enhance their fitness in that environment. These adaptive maternal effects include adjust-ments of nutrient provisioning, hormones (Benton et al. 2005; Meylan and Clobert 2005), and agents that enhance resistance to disease (Spitzer 2004). The initial size of free-living offspring has been a focal trait for studies of adaptive maternal effects because, in a broad range of taxa, the environment experienced by the dam affects size of 1 3 4 1