Rats need the company of other rats and usually coexist happily together as long as they have enough space, resources, and opportunities to engage in behaviours they find rewarding, especially chewing and hiding/burrowing.
Here are some guidelines to help you create a harmonious group of rats (also called a ‘swarm’ or ‘pack’ of rats):
- If possible, rats should be housed together in a group from a young age, and it is ideal to keep a group of siblings.
- Always try to introduce enclosure-mates to one another before or soon after weaning (at three weeks old) this decreases the likelihood that they will be aggressive towards one another. The optimal weaning age for rats is 21 days of age.
- If possible, avoid adding or removing rats from an established group, as this can upset the social organisation of the group and cause conflict and aggression which can result in negative welfare impacts and injury to the rats.
- 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.
- Provide enough resources so that all rats can access them at the same time (such as food bowls, water dishes, chewing and burrowing opportunities, and hiding places), avoiding competition. If there is enough room, ideally the resources should be physically separated from each other also.
Conflict between rats housed together can occur, although it is not common. You may observe more vocalisations and rats jumping at each other without causing injury and these behaviours usually resolve fairly quickly. Rats housed together do not often fight, especially once they are in a stable and familiar group. If there is fighting going on in the enclosure, you might notice that one rat will hide from the others (or specific individuals) and there may be injuries to one or more of the rats.
If you end up with a rat on their own because their group members die (which is not uncommon, especially with the short life expectancy of rats), it may be necessary to get a new rat or rats to keep the remaining rat company. You may also want to provide a home for an unwanted pet, or a rat who you meet and want to add to your family! Whatever the reason, it is likely that you will need, one day, to introduce new rats to your others. How can we make this as stress-free and safe as possible?
Rats are social animals, and usually adjust their lives to accommodate a newcomer quite happily. Introductions in neutral territory help facilitate the process. However, there will be a period of increased aggression when adults who don’t know each other are housed together.
It is important to observe your rats even more closely than usual after introducing a new rat, to watch for signs of trouble.
If the group of newly introduced rats continue to show aggression towards one another and/or injure each other, this may indicate that the new grouping will not be successful, and they should not continue to be housed together. Keeping rats together who do not get along is detrimental to their welfare and may be dangerous.
Things you can do to help reduce aggression (in addition to the guidelines above) include:
- Avoid anything that alters the smell of an individual as this may cause your rats to investigate their enclosure-mate more and could lead to unnecessary aggression. For example, if you need to take one of your rats to the vet, take the others too, or at least one of their enclosure-mates.
- Provide multiple shelters and visual barriers in their enclosure to help break up aggressive encounters by enabling rats move away from each other.
- Having multiple levels in their home enclosure can also help act as barriers to aggression. Make sure that shelters have multiple exits to avoid particular individuals stopping other enclosure-mates from leaving the shelter.
Other rodent species, such as mice, are not suitable companions for rats. They may carry diseases which are dangerous to each other and are usually incompatible when sharing resources such as space, food, and shelter. Rats and mice should not be kept in close vicinity to each other, even if in separate housing, as rats are predators to mice and so having rats in the vicinity can be very stressful to mice as the mice sense a predator nearby.
Evans E (2006) Small rodent behaviour: mice, rats, gerbils, and hamsters. In: Bays TB, Lightfoot T, Mayer J (eds) Exotic Pet Behavior. W.B. Saunders, pp 239–261
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