Rat
Rescue :: Short
Notes on Reproduction in Rattus norvegicus
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.
References
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.
