Territorial aggression increases with reproductive activity. Much individual, seasonal, and geographic variation in aggressive behavior. Breeds year-round, weather permitting, in southern part of the state. In northern counties, reproduction is confined to spring and summer, peaking from April to June. Fewer litters and smaller litter sizes are typical in marginal habitat.
Sexual maturity is reached by most individuals the spring after birth.
Well adapted for aquatic existence, as evinced by partially webbed feet, laterally-flattened tail, and pelage with dense underfur. The muskrat is the most important aquatic furbearing mammal in North America. Populations fluctuate markedly, and may be cyclic. Predators, in addition to humans, include minks, raccoons, and other large bird and mammal predators. Burrowing activities can result in extensive damage to water impoundments and agricultural lowlands.