Reproductive time frames will vary between different rats and/or lines of rats.
Environment, health, lineage, and nutrition are a few of the factors that can affect reproductive life.

Female Reproductive Life

Although it is not advisable, some female rats can be capable of breeding at 6 weeks. In rare circumstances breeding has occurred as early as 5 weeks. This is why it is best to separate the sexes when a litter reaches 5 weeks of age.

When bred at a very young age, 5 or 6 weeks, it has been noted that the litters are usually smaller but that the birth weight of the individual babies tends to be normal. It has also been noted that as the young mothers reached maturity their size was comparable to their female siblings.

Breeding Age Standards

Most fancy breeders tend to wait until the female is at least 4 months old (usually 4-5 months) or until they have reached a certain weight (usually 9-12 ounces). At times breeding is delayed, 10-12 months, to evaluate the health and temperament of the potential mother. There is no evidence of increased problems during pregnancy or birth in females bred up to a year.

Pelvic fusing is a myth that has been perpetuated within the fancy. This has led to recommendations of early breeding to avoid endangering the mother rat. There is no scientific evidence that rats experience pelvic fusing at any age.

In the laboratory the traditional breeding age begins at 3-4 months and ends at 10-12 months. Maximum fertility is reached at between 3-10 months.

Middle Age

Middle age in a female rat begins roughly around 9 months. At this age, the reproduction system is still usually fully functioning and many breeders have reported that they successfully breed their female rats for the first time between 9 months and a year.

“Menopause”

Although female rats do not experience actual menopause, their reproductive ability can begin to subside anywhere between 9-12 months of age according to some studies², but it’s typically more common for it to occur between 15 and 18 months of age. At this time there is a reduction or cessation of estrus cycles (termed estropause) due to disruption of hormones. As they age their reproductive organs will become less suited for pregnancy and birth and the eggs will become less viable.

The age at which a female reaches the post-productive period of her life will vary between individual rats and lines. Often, by 18 months of age, the estrus (heat) cycle has become irregular or even ceased. It is still possible for a female to get pregnant even though it appears she is no longer going into heat.

As the female ages fertility may decrease and the litters may tend to be smaller. An aging reproductive system in mammals can also increase the percentage of birth defects, problematic pregnancies, and labor difficulties.

Male Reproductive Life

Male rats can impregnate a female as young as 5 weeks, although that is not common. They are able to breed for most of their lives.

Breeding older males is common. There are times when it is even preferable to wait to better determine health, temperament, and longevity. This may be particularly important when dealing with a male from an unproven line or unknown lineage. With an older male you don’t have the danger of birth defects or health issues one might see from breeding an old female.

As male rats get old their sperm are generally still viable, although the number of sperm produced may decrease resulting in smaller litters.

Beyond the Statistics

Now that the statistical facts about rats and breeding age have been discussed, keep in mind that in nature there are always exceptions. There have been cases of females over the age of 2 years giving birth as well as elderly males fathering litters.

One old male was housed with a pair of older females after he lost his cage mate. The owners considered it “safe” due to his age and the fact that he had lost the use of his back legs. They also assumed that the females had passed the age of reproduction. The old male impregnated both old girls within a few days after they were put together and the shocked owners ended up with over 30 babies.

Never assume that a rat is too old, or too young, to reproduce. The only time you can safely mix your colony is when one or both sexes have been neutered or spayed.

References

1. Beek, van der, E. (1998, March 26). Neuro biological mechanisms of reproductive aging in the female rat. Symposium Platform Reproductive Research in Production Animals, NRLO-research report, Leerstoelgroep Fysiologie van mens en dier (Human and Animal Physiology Group), Wageningen Institute of Animal Sciences (1994 – 1999), Lelystad.
2. Koebele, S. V., & Bimonte-Nelson, H. A. (2016, May). Modeling menopause: The utility of rodents in translational behavioral endocrinology research. [Abstarct]. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4829404/#S3title
3. Suckow, M., Weisbroth, S., & Franklin, C. (2005). The Laboratory Rat, Second Edition (American College of Laboratory Animal Medicine). Toronto: Academic Press.