How Long Do Rats Live? Brown Rat Lifespan in the Wild

Brown rat lifespan at a glance

All of these animals are the same species, the brown rat (Rattus norvegicus). The difference between a short wild life and a longer captive one comes almost entirely from the dangers of living outdoors.

Why do wild rats die so young?

Life is hard for a wild rat. Studies of wild populations have estimated that roughly 90–95% of brown rats die within a year. The main causes are:

• Predators: owls, hawks, foxes, weasels, stoats, mink, snakes and cats all hunt rats, and young rats leaving the nest for the first time are especially vulnerable.
• Competition with other rats: rats live in hierarchies, and when food or space runs short, low-ranking animals are the first to starve, get injured or be driven out.
• Disease and parasites: infections spread easily in crowded colonies.
• Weather: cold, wet winters and floods in burrows and sewers kill many rats, especially in the north.
• People: traffic and deliberate control remove many rats in towns and on farms.

Because so many die young, a wild population is made up mostly of animals only a few months old. A rat that survives two winters is an old animal in rat terms.

Survival also depends on the season and the place. Rats living in sheltered buildings with a steady food supply, such as farm stores or warehouses, survive winter far better than rats in hedgerows and on riverbanks. In mild climates, where breeding can continue all year and winters are less harsh, more rats survive than in cold regions.

How do rats make up for short lives?

The brown rat’s strategy is to breed early and often. Females can become pregnant again within a day of giving birth, and under good conditions a female can produce several litters a year.

• Pregnancy: about 21–24 days
• Litter size: usually 6–12 pups
• Weaning: 3–4 weeks
• Age at first breeding: about 2–3 months in the wild, earlier in well-fed captive rats

This rapid turnover means that rat populations can recover quickly after heavy losses. When many rats in an area die, the survivors often breed faster and more young survive, because there is more food and space for each one. Biologists call this a “live fast, die young” life history, typical of small mammals with many predators.

How long do pet and laboratory rats live?

In captivity, with no predators, steady food and shelter, brown rats usually live about 2–3 years. Lifespan varies between strains and individuals, and the longest-lived rats on reliable record reach around 4 years. Stories of pet rats living much longer appear from time to time but are rarely documented.

Captive rats show many signs of aging by about 2 years old, including slower movement, weight changes and a higher rate of tumors and kidney disease. Because rats age so quickly, they are widely used in research on aging, where scientists can follow a whole lifetime in just a few years.

How do rats compare with other rodents?

Rat lifespans are typical of small rodents, but rodent lifespans vary enormously:

In general, larger rodents live longer than small ones, but there are striking exceptions. The naked mole-rat, a mouse-sized burrower from East Africa, can live more than 30 years in captivity and shows few of the usual signs of aging. Tree-living and burrowing rodents, which are safer from predators, also tend to live longer than ground-dwelling species of the same size. For a tree-living example, see the eastern gray squirrel.

Myths about rat lifespans

Myth: City rats live for many years and grow huge with age. In reality most city rats are less than a year old. Rats keep growing slowly through life, but even old adults rarely exceed 500 g (18 oz), and cat-sized rats are a myth.

Myth: You can tell a rat’s age from its size. Size gives only a rough guide. Well-fed young rats can be as big as underfed older ones, so scientists estimate the age of wild rats from features such as the weight of the eye lens or tooth wear.

For more about the remarkable minds packed into these short lives, read how smart are rats?