Maternal effects increase within-family variation in offspring survival

Maternal effects are environmental components of phenotypes that complicate relationships between natural selection and evolution because they often affect phenotypes and fitness simultaneously. We studied the effects of egg size variation on juvenile survival in a population of American coots (Fulica americana). We experimentally evaluated egg size variation at three levels: across the population, within natal nests, and within foster nests. Natal nests accounted for the most variation in population egg size. Within clutches, early-laid eggs were larger than later-laid eggs, with the exception of first-laid eggs, which were small. In the fostering experiment, posthatching survival was most strongly related to egg size relative to natal siblings and natal hatching order and less so to egg size within foster nests. These effects on survival were found even though young from natal nests were neither raised together nor raised by genetic parents. These results indicate that females allocate resources unequally among offspring such that offspring from larger, early-laid eggs have higher survival than offspring from smaller, laterlaid eggs, regardless of their size relative to foster siblings or to mean population egg size. These results suggest that egg size variation can be maintained through selection on maternal investment strategies and not on egg size per se.