Rats mature and can breed at a young age (as early as four weeks of age), breed prolifically, and have large litter sizes. They will come into season every 4-5 days unless pregnant; their pregnancy duration is 21-23 days; their litter size is 6-13 pups; and they can be ready to mate again 24 hours after giving birth. The pups are weaned at about 21 days of age, by which time their mother may be ready to deliver another litter.
If possible, rats should be housed together in a group from a young age (before or soon after weaning), and it is ideal to keep a group of siblings.
Preferably, keep your rats in small same-sex groups and make sure young males and female rats are separated as soon as possible after they are weaned (to prevent unwanted breeding). If you are going to keep mixed sex groups, have your rats desexed. This will not only prevent unwanted pregnancies, but also competition that may result in aggression.
Unless you want to breed your rats, you only have two choices: preferably only keep rats of the same sex together or, if you must keep males and females together, have them desexed as soon as possible and keep males and females separate from before maturation (four weeks of age) until after they are desexed so no unwanted pregnancies result (speak to your veterinarian about the timing).
Make sure that undesexed rats are not left with other undesexed rats of the opposite sex if they are four weeks or older (including their littermates and/or mother/father), or unwanted pregnancies will likely result.
To do this, you will need to know how to tell the sexes apart. Even an experienced veterinarian, breeder, pet store owner, or animal shelter can make sexing mistakes, so take a look yourself to double check.
Sex identification
There are a few ways to determine if a rat is male or female, but no method is completely reliable all of the time.
- Males are typically twice the size of females.
- Adult males can be distinguished from females by the presence of a scrotal sac between the anus and the genital opening which contains the testes. Note though, that the testes may not be obvious until 3-4 weeks of age, and that rats can retract their testes into the abdomen, making it difficult sometimes to be sure.
- The distance between the anus and the genital opening (the anogenital distance) is longer in the male compared to the female (about twice as long). You may need to look at a few different rats to get an idea of the approximate appearance of the distances of both sexes.
- Male rats do not have nipples, but this needs to be looked at before too much hair grows, obscuring your view. These nipples in female rats are clearly visible from 8–10 days of age onward.
Desexing
Desexing prevents unwanted litters of rat pups and helps to prevent mammary tumours in female rats.
Desexing of both male and female rats is recommended. Speak to your veterinarian about the optimal timing for desexing your rats (and do it as early as possible!). Also ask if male and female rats should be kept separated after desexing and for how long so that there is no risk of unwanted pregnancy.
Breeding
If you do want to breed your rats, they should not be bred before nine weeks of age, and you should ensure that you can find good homes for all of the babies before breeding.
References
Banks RE, Sharp JM, Doss SD, Vanderford DA (2010) Rats. In: Exotic Small Mammal Care and Husbandry. Wiley, pp 81–92
Frohlich J (2020) Rats and mice. In: Quesenberry KE, Orcutt CJ, Mans C, Carpenter JW (eds) Ferrets, rabbits and rodents, 4th ed. W.B. Saunders, pp 345–367
Pilny AA (2015) Small Exotic Companion Mammal Wellness Management and Environmental Enrichment. Veterinary Clinics of North America – Exotic Animal Practice 18:245–254
