Social groups of R. rattus are often formed of multiple males and multiple females. One male is dominant and a linear male hierarchy may form. Two to three females are often dominant to all other group members except the dominant male. Females are generally more aggressive than males. The species is polygynous, and generally, the dominant male is the most successful breeder. Territories and mates are defended through aggressive behavior. If environmental conditions allow it, successful breeding may occur all year.
Mating System: polygynous
Rattus rattus is able to breed throughout the year if conditions allow. The peak breeding seasons are summer and autumn. Females can produce up to 5 litters in one year. The gestation period ranges between 21 and 29 days, and young rats are able to reproduce within 3 to 5 months of their birth. Neonates are altricial, like most rodents, and their eyes do not open until 15 days of age. Young remain hairless for much of their nursing period. Weaning and independence from the mother occur at about 3 to 4 weeks of age.
Breeding interval: R. rattus breeds year-round producing up to five litters in that time.
Breeding season: R. rattus mates throughout the year if environmental conditions permit, however peak times are summer and autumn seasons.
Range number of offspring: 6 to 12.
Average number of offspring: 8.
Range gestation period: 21 to 29 days.
Range weaning age: 3 to 4 weeks.
Range time to independence: 3 to 4 weeks.
Range age at sexual or reproductive maturity (female): 3 to 5 months.
Range age at sexual or reproductive maturity (male): 3 to 5 months.
Key Reproductive Features: iteroparous ; year-round breeding ; gonochoric/gonochoristic/dioecious (sexes separate); fertilization ; viviparous
Average birth mass: 4.55 g.
Average number of offspring: 7.3.
Because male members of R. rattus copulate with one female and then move on to the next, they don't contribute much to the care of the young. The young remain relatively helpless for about 2 weeks, until they begin to grow a pelage, their eyes open, and they are able to move around more. Weaning is accompanied by increased independence from the mother. Until these rats reach their full adult size, they stay in the nest built by their mother. Young rats are capable of reproducing by about 3 to 5 months of age.
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)