Once the breeding pair has been chosen preparation for mating is fairly simple. You will want to provide a suitable breeding area for the pair, decide whether to do an overnight or a prolonged pairing, and then wait for your female to go into heat.

The Breeding Environment

Some rats will mate anytime, anywhere, without any concern for what is going on around them. Others need a suitable area/environment to breed successfully. Often a one story cage or an aquarium is used. This gives the pair a larger flat area to mate in. And if your male is older, or your female is very active this will make her more accessible to mounting (and keep him from having to chase her all over the cage). Use appropriate cage bedding for the bottom making sure not to use too much, during mating it is not uncommon for the rats to kick out a lot of bedding.

If you are just leaving them in overnight you will probably not need anything more than a food dish and a water bottle for them. A soft towel added to the cage will give them a resting spot. If they are staying together longer a larger cage may be appropriate and a house and/or a hammock will make their environment more comfortable. Keep in mind to leave an open flat area for him to mount her.

Duration

Some people prefer to only put the pair together during heat while others leave the pair together until they are sure the female is pregnant. There are pros and cons with both methods

Overnight
The overnight stay assures that the pair is together only at optimal breeding times when the female is in full heat. The breeding rats, since they are only away from their cage mates for a matter of hours, can generally make an easy transition back into their respective cages.

You then will need to watch your female to see if she goes back into heat. If she does then you repeat the process until she becomes impregnated.

Extended Stay
The extended stay is particularly useful if your doe is having a difficult time getting pregnant. It is also useful if you have a doe that has heat cycles that are not obvious (some females hide their heat cycles well). You may have to go through a complete re-introduction process with your male before he can be a part of his colony once the female is pregnant.

The Receptive Female

While in heat the female will be receptive to the advances of the male rat. Female rats go into estrus (heat) every four to five days. Unlike many other mammals red blood cells are not prominent, so there is no vaginal bleeding. When in heat the female rat will usually display by arching her back when touched, vibrating her ears, and running in short fast bursts.

Heat lasts for various times depending on the individual rat and the environmental factors involved. Absence of darkness at night and light during the day can make the estrus cycle erratic. Heat can be suppressed by the introduction of another strange male or females.

It can also be brought on by the presence of an intact male. There is a higher risk of fighting when this is done as the female will kick and fight with the male if cornered. In this scenario it is a good idea to put a house in the cage for the female to be able to get away from the male. Usually a wooden box with a small hole (too small for a male to enter) will suffice.

Introducing the Breeding Pair

For the most part a female in heat can be put right in with an intact male with no problem. Unfortunately there are, as usual, exceptions. Breeding injuries can be caused by either the male or the female for various reasons.
It is wise to monitor the pair until you are relatively sure they are compatible.

Some causes of mating Injuries:

• The female is not yet receptive
• Rough copulation
• An unreceptive male may be attacked by the female in heat
• An inexperienced rat may attack out of fear
• An unresponsive female may be attacked by the male
• The rats may simply be incompatible

Courtship

When it comes to initiating sex the female in heat is usually the aggressor. It is not unusual for a female to sniff the males’ genitals, arch and quiver for him, and hop around the cage in what can only be viewed as a courting activity.

Sometimes a young inexperienced female will become so excited that the male will have a hard time catching her. If he does catch her she may dart off the moment he touches her. Usually after a period of time the female will slow down and allow the male to mount her.

More experienced females tend to more direct and sometimes even pushy with their mates and will go as far as backing up to the male and presenting themselves for mating.

Mating

Copulation may occur for 2- 24 hours depending on how long the estrus cycle lasts. The male will mount the female while she is standing causing her to arch her back and present herself for insertion (lordosis). Copulation is short and ends in the female darting off as the male bends down and cleans his penis. Copulation is generally repeated many times. Reports of up to 60-100 individual acts have been reported by a pair of rats in a single night.

There are rats that breed with much less abandon, though. There are cases of pairs being put together where no copulation was ever seen, yet the female became pregnant. If you are not sure the rats have mated look for a waxy plug (sometimes referred to as a mucous plug). It can often be found either in the cage or still in the vaginal opening of the female rat and will verify that mating has occurred.

Fertilization

The female rat ovulates approximately 8-12 hours after the heat cycle has begun. She can become pregnant anytime within 24 hours of mating. It has been suggested that female rats can “store” sperm for hours or days although this has yet to be laboratory verified.

Figures For the Mating Process

• Fig. 1: Mating Photos