Back to Breeding
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.
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.
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.
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.
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.
Posted on February 14, 2004, 08:53,
Last updated on December 18, 2008, 13:29