Short Notes on Reproduction in Rattus norvegicus

Rats are known for their prolific breeding. Young rats are normally weaned by their mothers when they are 21 days old. However, they usually remain in the family group for a short while later until they are fully independent and are able to cope socially in the environment on their own. Rats are considered to be pups or juveniles until around 60 days of age (two months or eight weeks old). It appears that the complete dispersion of the young does not occur before 60 days of age (Meaney and Stewart, 1981). This is under normal circumstances in the wild, and such circumstances may be applicable to domesticated rats if they were given the opportunity to behave in the social manner that their wild counterparts do.

Rats start to become sexually mature at between 40 to 65 days old (Rowett, 1957; Weihe, 1989; Harkness and Wagner, 1995; Wolfensohn and Lloyd, 1989; Daly, 2002). This means that they are going through puberty. It is during this pubertal stage that the first oestrus occurs. However sexual attraction occurs long before puberty, at around 36 to 45 days old (Meaney and Stewart, 1981). The vagina of the female opens at 35 to 90 days (n = 72), and the testes descend at 20 to 50 days (n = 40) (Donaldson, 1924; Harkness and Wagner, 1995). There is thus great variability on the reproductive time-line of rats of the species that we are dealing with. However one thing is quite clear cut and that is oestrus (the time that a rat can conceive) occurs every four to five days. Once fertilisation has taken place, gestation is between 21 and 23 days in length.

This species of rat, although capable of reproducing at a very young age, very rarely does this in the wild. Domesticated rats are also capable of reproducing at a young age. However it is inadvisable to allow them to do so. First mating should be delayed until the female rat is at least 100 to 120 days old when the females weigh approximately 250 g and the males 300 g (Rowett, 1957, Weihe, 1989; Harkness and Wagner, 1995; Warren, 1995). Young females bred at too early an age give rise to small litters and the babies will be small and weak. Breeding at an early age will also shorten the reproductive life of the female. At 120 days, the mother is strong enough and mature enough to raise a litter successfully (Warren, 1995). Leaving the first mating to this later age of around 110 days also gives rise to strong, healthy, vigorous young (Harkness and Wagner, 1995). No female older than 12 months of age should be mated. After 12 months of age, litter size decreases, the litter interval increases and the young are not strong (Harkness and Wagner, 1995). Menopause occurs at between 450 and 500 days (Weihe, 1989; Harkness and Wagner, 1995).

Like most rodents, these rats display a post-partum oestrus. This means that very shortly (within 48 hours) after having given birth, she is able to be impregnated again. However, the use of the post-partum oestrus to breed rats is not advised. Although one maximises the amount of young born, they are not strong, and mortality is increased. In addition to which, if the post-partum oestrus is not used, the mother provides more milk, and larger young and litters (Harkness and Wagner, 1995; Wolfensohn and Lloyd, 1998). The use of the post-partum oestrus for breeding puts an enormous strain on the mother’s body, since she now must not only support her own tissues and organs, but she must also produce sufficient milk for her pups who have been born while she is gestating another litter at the same time (Daly, 2002). If a female rat has been mated in the post-partum oestrus, and is lactating at the same time, there will be a delayed implantation of the new foetuses leading to an increase in the length of gestation by three to seven days (Wolfensohn and Lloyd, 1998). In addition to this, it was found that female rats who conceived in the post-partum oestrus, and then lost their suckling litter, bias the sex ratio of the new litter towards females. This means that more females will be born. It appears that male embryos are less successful in implanting in a uterus immediately after a previous birth and are lost (Bacon and McClintock, 1999).

This post-partum oestrus occurs only when conditions are favourable (Meehan, 1984). This would mean that there would have to be sufficient food available, that the environmental conditions are at best, and that the mother rat is in peak health. In the wild, females rarely produce more than five litters per year (Corbet and Harris, 1991). As far back as 1950, Farris stated that at the Rowett Research Institute, no stock female was permitted to have more than seven litters during her reproductive life, and that the female was rested for seven days after her litter had been weaned. Rowett (1957) recommended that the mother should be isolated until she has weaned her young and then rested for a further two weeks until the next mating. Many 21st Century animal laboratory breeding systems favour this rest period as well.

Should a mother rat have her first litter with her when the litter from the post-partum oestrus conception is born, both litters will suckle from her (Gilbert et al, 1983). This suggests that even more of a strain is exerted on the body of the mother. It also suggests that each one of the pups will not obtain the necessary volume of milk required for optimal nourishment, growth and development, thus leaving them deprived.

In conclusion, this author is of the opinion that breeding rats at the extremes of their reproductive life is highly deleterious to both mother and the young. Similarly, to use the post-partum oestrus for mating is equally injurious to both mother and young. Not only are such practises cruel through the unnatural imposition of continuous breeding on the mother rat, they are also physiologically demanding to both parties. Such practises serve only to maximise the numbers of young rats born. The mother rats suffer as a continuous strain is put on their bodies. The young are not healthy and strong, and mortality is increased. In addition to which, such rats usually have a shortened life-span, and have a greater susceptibility to developing disease.

It is hoped that constant monitoring is undertaken of any facility where rodents are bred, especially where they are bred for the pet trade. Apropos which, it is hoped that ultimately the breeding and sale of live animals as pets is prohibited. It is further hoped that laws will immediately be put in place to make the sale of live animals illegal, and a prosecutable offence. We have to protect all living creatures, who are utterly dependent upon us as humans for their well-being and welfare.


Bacon, S.J. and McClintock, M.K.; 1999; Sex ratio bias in postpartum-conceived Norway rat litters is produced by embryonic loss in midpregnancy; Journal of Reproduction and Fertility; 117: (2), 403 – 411

Corbet, G.B. and Harris, S. (Eds.); 1991; The Handbook of British Mammals, Third Edition; Blackwell Scientific Publications; Oxford; p253

Daly, C.H. (DVM); 2002; Rats A Complete Pet Owner’s Manual; Barrons; New York

Donaldson, H.H.; 1924; The Rat Date and Reference Tables, 2nd Edition; The Weston Institute; Philadelphia

Farris, E.J.; 1950; The rat as an experimental animal; In: The Care and Breeding of Laboratory Animals; Wiley; New York.

Gilbert, A.N., Burgdoon, D.A., Sullivan, K.A. and Adler, N.T.; 1983; Mother-weanling interactions in Norway rats in the presence of a successive litter produced by postpartum mating; Physiology and Behaviour; 30: 267 – 271

Harkness, J.F. and Wagner, J.; 1995; The Biology and Medicine of Rabbits and Rodents, fourth edition; Lea and Febiger; Philadelphia.

Meaney, M.J. and Stewart, J.; 1981; A descriptive study of social development in the rat (Rattus norvegicus); Animal Behaviour; 29 34 – 45

Meehan, A.P.; 1984; Rats and Mice, Their Biology and Control; Rentokil; Felcourt, East Grinstead

Rowett, H.G.Q.; 1960; The Rat as a Small Mammal, Second Edition; Jarrold and Sons; Norwich.

Warren, D.M.; 1995; Small Animal Care and Management; Delmar Publishers; Albany.

Weihe, W.H.; The laboratory rat; In: Poole, T.B. and Robinson, R. (Eds); 1989; The UFAW Handbook on the Care and Management of Laboratory Animals, Sixth Edition; Longman Scientific and Technical; Burnt Mill, Harlow.

Wolfensohn, S. and Lloyd, M.; 1998; Handbook of Laboratory Animal Management and Welfare, Second Edition; Blackwell Science; Oxford

Author : Colleen McDuling, B.Sc(Med.Hons), MSc(Med.Sc.), Animal Behaviourist

Scientific Representative of the South African Rat Fan Club.