Genetics, care and the origin of your pet rat will determine life expectancy more than anything else. In general, rats are expected to live around two years, although this varies. Life expectancy is not one-size-fits-all -- not all store-bought rats are doomed to die an early death, and not all fancy rats come from reputable breeders.
Store-Bought and Feeder Rats
Store-bought rats, also commonly referred to as "feeder" rats, are sold both as pets and as food for predator reptiles such as snakes. In general, store-bought rats come from weak genetic lines, are susceptible to illness and cancers and are not expected to live as long as fancy rats. Most store-bought rats will live 12 to 24 months. Some pet stores will receive lines from local fancy rat breeders, and your store-bought rat could live up to 36 months with great care, a stimulating environment and nutritious food.
Fancy rats are those that have traced genetic lines, and are bred in "ratteries" by knowledgeable and experienced rat owners. Fancy rats typically live to 2 or 3 years of age, and are less prone to upper respiratory diseases or malignant tumors. Research your prospective breeder before purchasing your rat as some breeders are not responsible; over-breeding, breeding from feeder rat origins or neglecting to breed for health and strength of stock can lower life expectancy.
In the wild, the life expectancy for a rat is much lower: 95 percent of wild rats die before they reach their first birthday. A 1-year-old rat would be around 30 years old in human terms. The most common cause of death in wild rats comes from predators, such as owls, hawks and foxes. Perhaps equally as dangerous for wild rats are humans; cars, rat traps and poison all affect wild rat life expectancy.
The Oldest Rat
According to the Guinness Book of World Records of 1995, the oldest documented rat was a domestic rat named Rodney. Rodney lived from January 1983 to the May 25, 1990; he was 7 years, 4 months old when he died. Rodney belonged to the Mitchell family in Tulsa, Okla., and it's unknown as to what may have contributed to his longevity. While no studies can demonstrably show concrete proof of longer life expectancy, factors such as new and stimulating environments, a free-feeding diet or exercise all offer possibilities.