Why are the offspring of older mothers less fit to live long and prosper?

In a new study in rotifers (microscopic invertebrates), scientists tested the evolutionary fitness of older-mother offspring in several real and simulated environments, including laboratory culture, under threat of predation in the wild, or with reduced food supply. They confirmed that this effect of older maternal age, called maternal effect senescence, does reduce evolutionary fitness of the offspring in all environments, primarily through reduced fertility during their peak reproductive period. They also suggest an evolutionary mechanism for why this may occur.