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Mating Behaviors
Males can ejaculate multiple times in a row, and this increases the likelihood of pregnancy as well as decreases the number of stillborns. Multiple ejaculation also means that males can mate with multiple females, and they exhibit more ejaculatory series when there are several oestrus females present. Males also copulate at shorter intervals than females. In group mating, females often switch partners.

Dominant males have higher mating success and also provide females with more ejaculate, and females are more likely to use the sperm of dominant males for fertilization.

In mating, female rats show a clear mating preference for unknown males versus males that they have already mated with (also known as the Coolidge effect), and will often resume copulatory behavior when introduced to a novel sexual partner.

Females also prefer to mate with males who have not experienced social stress during adolescence, and can determine which males were stressed even without any observed difference in sexual performance of males experiencing stress during adolescence and not.

Reproduction
Females are capable of becoming pregnant immediately after giving birth, and can nurse one litter while pregnant with another. Females are able to produce and raise two healthy litters of normal size and weight without significantly changing their own food intake. However, when food is restricted, females can extend pregnancy by over two weeks, and give birth to litters of normal number and weight.