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Wild Rats. 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.
The brown rat is a rather large true murid and can weigh twice as much as a black rat and many times more than a house mouse. The length is commonly in the range of 20 to 25 cm (8 to 10 in), with the tail a further 18 to 25 cm (7 to 10 in), thus being roughly the same length as the body.Adult body weight averages 350 g (12 oz) in males and about 250 g (9 oz) in females.he brown rat can breed throughout the year if conditions are suitable, with a female producing up to five litters a year. The gestation period is only 21 days, and litters can number up to 14, although seven is common.