the increase in size of northern american red squirrels is a great example of an adaptive maternal effect producing a phenotype that resulted in an increased fitness. the adaptive maternal effect was induced by the mothers sensing the high population density and correlating it to low food availability per individual. her offspring were on average larger than other squirrels of the same species; they also grew faster. ultimately, the squirrels born during this period of high population density showed an increased survival rate (and therefore fitness) during their first winter.