The mating system of R. norvegicus is best described as polygynandrous. Social animals, Norway rats tend to breed in large groups. Once a female enters her six-hour estrus period, she may mate as many as five-hundred times with competing males.
Mating System: polygynandrous (promiscuous) ; cooperative breeder
Although not technically a seasonal breeder, a mating increase occurs in the warmer months of the year. An average female is capable of giving birth approximately seven times per year. Around 18 hours after giving birth, females experience postpartum estrus, and mate again. This reproductive function is responsible for the huge birthrates of Norway rats, which can reach 60 young each year per female. After a short gestation period of 22 to 24 days, the litter of approximately 8 pups is born. The young are very small and underdeveloped. It takes 14 to 17 days for the young's eyes to open. Newborns weigh an average of 5 grams and are milk-fed until weaning occurs at 3 to 4 weeks, and the young then leave the nest.
Often, the litters of numerous females will occupy the same nest, and all the young are cared for by the adults, regardless of who the true mothers are. This communal care makes the species something of a cooperative breeder.
Males usually reach sexual maturity at 3 months and females at 4. However, it is usually the female who mates first because competition for mates among males prevents the smaller, less-dominant individuals from succeeding immediately. Rattus norvegicus is capable of mating for up to two years.
Breeding interval: Norway rats may breed up to 7 times per year
Breeding season: Breeding occurs year round, but is less pronounced during colder months.
Range number of offspring: 2 to 14.
Range gestation period: 22 to 24 days.
Range weaning age: 3 to 4 weeks.
Range time to independence: 4 to 5 weeks.
Range age at sexual or reproductive maturity (female): 3 to 4 months.
Range age at sexual or reproductive maturity (male): 3 to 4 months.
Key Reproductive Features: iteroparous ; year-round breeding ; gonochoric/gonochoristic/dioecious (sexes separate); fertilization ; viviparous ; post-partum estrous
Average birth mass: 5.81 g.
Average number of offspring: 9.9.
Parental care is provided by females. Because these animals often nest communally, the litters of several different females often occupy the same nest. In nesting groups of more than one female, if a mother is killed, the other females will take over nursing the newborns. Males do not particupate in parental care.
Parental Investment: no parental involvement; altricial ; female parental care ; pre-fertilization (Protecting: Female); pre-hatching/birth (Provisioning: Female, Protecting: Female); pre-weaning/fledging (Provisioning: Female, Protecting: Female); pre-independence (Protecting: Female)