Young male rat pups are easy to sex, even before maturity, by the relatively long ano-genital distance. The testes should have descended into the scrotum by about three weeks of age. Hold the buck (male) in a vertical position, with its head uppermost, to be sure of spotting the testes if you are in any doubt. A rat can withdraw them back into the body but nipples are visible in does (females) about one week after birth. It is common practice to separate rats into single sex groups by the time they are seven weeks old; after this time they will be approaching sexual maturity. Does are not usually used for breeding purposes until they are about thirteen weeks old and over 200gm (7oz) in weight. By comparison, males of a similar age under 275gm (9.7oz) are not usually recommended for stud purposes.
Various mating systems can be used. One particular method is to place one or two males with up to six females. This will serve to stimulate oestrus, or pheromones in the does as they respond to the male odors. Female rats cycle about every five days. Because of this, successful matings are likely to take place soon after the introduction of a buck. The level of lighting appears important for successful breeding, since their natural proliferation declines during the darker days of a temperate winter. Ideally, provide sufficient artificial light to give a total daily exposure of about fifteen hours. The normal gestation period is three weeks, or so, but will be longer if mating has taken place almost immediately after the doe has given birth. She will be ready to mate again in this so-called 'postpartum' breeding period within two days of producing pups, but the developing embryos will not implant into her uterine wall as rapidly as normal. It is often best to remove the doe to separate quarters to give birth, especially if she normally forms part of a colony. Two does may be housed together quite successfully in this way.
A litter may consist of up to fourteen pups. These will be independent about three weeks after birth with each young rat weighing about 50gm (1.8oz) at this stage. The doe will be ready to mate again within a week of weaning being completed if she did not conceive at the 'postpartum' heat. Rats can breed almost until the end of their lives but their reproductive capability declines after a year or so. Rats generally live between two and four years under normal captive circumstances but older individuals are not uncommon.
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