Cats reach a maturity level that allows breeding sometime between seven and twelve months of age. Unlike their wild brethren, cats can have multiple litters in a single year. Since the average number of kittens in a litter is four, cat populations can increase quite rapidly if efforts are not made to counteract the occurrence of pregnancy. Historically, selective killing was the primary means of keeping newborn numbers manageable, but sterilization has become preferential in present day society. Interestingly, sterilized cars have a longer life expectancy than those that are reproductively unaltered.