Introducing Mice

Neutral, fermented feed, a clean target habitat – this simple recipe for introducing new individuals to an existing group is known to many mouse caregivers. When done according to this method, merging often ends successfully, especially with purebred mice from reputable breeders who ensure that their rodents have calm and balanced temperaments. However, there are situations where, after merging new mice this way, the caregiver observes chaotic scenes in the mouse habitat for several days or even weeks: squeaking, chasing, nibbling, and even more serious fights. Prolonged stress drastically lowers mice's immunity, making them more susceptible to various diseases. It also has a negative impact on their psyche and can lead to depression, for example. Of course, some mice may have a more aggressive, dominant character and will try to establish their dominance at all costs, but very often, such behaviors are a result of stress caused by incorrectly conducted merging. So, how should merging be done to minimize stress associated with this process?

How's Your Health?

When welcoming a new mouse into your home, you should first ensure that it is healthy. If the animal comes from a reputable breeder, this is straightforward because breeders always make sure that the mouse is in good health before handing it over to a new home. However, if the mouse comes from adoption (e.g., from a foundation or a private individual), it is essential to take care of its health independently. In such a situation, the best solution is to schedule a visit to a veterinarian specializing in the treatment of rodents, who will examine the mouse and perform deworming.

Welcome Home!

Merging should not begin immediately after bringing a new mouse home. It's essential to give the mouse time to acclimate to its new environment. Even the abundance of new scents and sounds can be uncomfortable, so the animal should be given a few days to get used to the new situation. During this time, the mouse should be kept in a temporary habitat that meets all its needs. This habitat should be placed away from the living quarters of other mice, preferably in separate rooms to prevent them from detecting each other's scents. During the acclimation period, all mice can be given herbs or calming supplements. This could include melissa (fresh or dried) or, after consulting with a veterinarian, products containing CBD or tryptophan. These measures will help keep the mice calm and relaxed during merging, making the process more comfortable for them. It's also essential not to stress the mice further with your behavior. While it's natural for caregivers to be excited about their new pets, it's best to ensure calm interaction with the new resident so they can feel safe in their new environment from the start. The acclimation phase with herbal supplementation should last about two weeks. Ideally, you should always introduce a minimum of two mice to an existing group because this provides them with companionship during the waiting period. Isolation is very stressful for mice, and it should always be minimized.

A Bit of Theory

Unlike humans, mice are macrosmatic organisms, meaning that their sense of smell is highly sensitive and plays a crucial role in communication. Therefore, introducing mice is primarily based on familiarizing them with an unfamiliar scent while simultaneously building positive associations (desensitization with counterconditioning). A mouse's natural reaction to an unfamiliar scent is defensive, initially attempting to flee, and if that doesn't work, they may attack to force the intruder to retreat. During the preparation for merging, we will gradually acquaint the mice with the unfamiliar scent while trying to show them that whatever emits that scent poses no threat and may even be a harbinger of pleasure – treats will be helpful for this. All the described actions should be performed in both directions: we introduce the new mice to the scent of the existing ones, and the "old" mice to the scent of the newcomers. In the case of merging, all mice are exposed to stress, so the preparation process aims to minimize this stress. A properly conducted merging process takes quite a long time, about 3-4 weeks. It's essential not to rush this process or shorten any of its phases. After each change or introduction of a new element, give the mice time to get used to it and solidify the desired behaviors that arise at that stage. Conduct every step calmly and without sudden movements or noises. Pay close attention to the mice's behavior. This patience and calmness will pay off when the entire procedure is completed.

Nose to Nose... Sort of!

Initially, allow the mice to smell the unfamiliar individuals' scent for a short time, which you will gradually extend. Associate this stimulus with something very pleasant – treats are the easiest way to do this. It's best if the treat is something small that the mouse will consume quickly and eagerly. Larger treats slow down the process and pose a risk of obesity, so be mindful of that. Good choices include water with honey given in tiny droplets using a pipette or Sinlac porridge administered in minimal amounts on the end of a wooden skewer. Sunflower or pumpkin seeds may not be suitable treats for this purpose as mice love them but consume them slowly, and they are very caloric. How to begin familiarizing them with the scent? You can, for example, rub your hands with bedding from one set of mice or take a blanket they slept on and place it in the other set of mice's cage. Avoid chasing the mice or presenting an unfamiliar scent directly to their noses, but rather put your hand/blanket at a distance and observe. Mice are curious by nature, so you won't have to wait long for their reaction – they will approach and sniff. Treats should be prepared so that you can react quickly to the mouse's behavior. Try to reward any interaction with the unfamiliar scent, even the smallest ones. Initially, this could be as simple as taking a small step toward the source of the scent. The treat delivery should be swift so that the mouse associates the unfamiliar scent with pleasure. Over time, you can raise the bar – after a few sessions, reward the mice less frequently, and they will have to "earn" the reward, perhaps by placing a paw on the blanket with the unfamiliar scent. These "training sessions" should be conducted several times a day, gradually extending the exposure time to the new scent. Initially, these sessions will be very brief, but over time, you should aim for sessions lasting several minutes. Continue observing the mice's behavior. This patience and calmness will pay off when the entire procedure is completed. When you notice that the animals are relaxed, willingly approach the unfamiliar scent, and clearly expect a treat, you can move on to the next step, which involves exchanging bedding between the cages. Take a handful of bedding from one cage and place it in the other, preferably far from where the mice sleep. Leave something delicious (at this point, you can use seeds or small cookies) near the unfamiliar bedding a few times a day. Give the mice a few days to get used to this situation. Again, when you see that the mice are behaving calmly near the unfamiliar scent, you can proceed further.

On Neutral Ground

Just before introducing them in a neutral environment, you can place both cages next to each other so that the mice constantly smell each other for a day or two. If the mice are kept in plastic containers like Samla boxes, exchanging scents this way will be ineffective, so you can skip this step without harming the merging process. The next crucial step is introducing the mice in a neutral environment. It should be a safe and as extensive as possible area – a bathtub or a shower cabin is ideal. The mice should have the option to decide whether they want to approach the unfamiliar individuals or keep their distance. In such a neutral area, you should provide mice with access to food and water, as well as several hiding places. These should be easily accessible for the caregiver to intervene quickly if necessary. You can also prepare something to quickly separate the mice without risking being bitten (e.g., the lid of a plastic container) and something to make noise (e.g., a bunch of keys) to disperse the mice in case of a fight. Release the mice into the neutral ground and observe them. Allow them to approach each other or move away freely. Try to reward the mice as often as possible with treats (similar to the initial familiarization with the scent) for approaching unfamiliar individuals and for calm, friendly interaction to continue reinforcing positive associations. If the mice enjoy contact with humans, you can also include petting near the unfamiliar mice as a form of reward. These meetings on neutral ground should be held for several days, gradually extending their duration until you see that the mice are behaving entirely calmly and peacefully in each other's company, then you can move on.

...But Home Is Best!

The final stage is placing all the mice in the target habitat. It should have appropriate dimensions for the enlarged group and as many enrichments as possible so that the mice don't have to compete for them. Ensure that there are several different hiding spots and toys to choose from, and for larger groups of mice, several water sources and places to take food. A rough estimate is to have 3 enrichments for each mouse. Before introducing the rodents, thoroughly clean the habitat to remove the scents of previous occupants. For the first few days, spend time observing the mice several times a day, especially when they are active, to ensure that nothing harmful is happening. Continue rewarding the mice for friendly interactions. The entire merging procedure using this method takes quite a long time, but it is undoubtedly worth investing the time to minimize the stress experienced by the mice, take care of their health, and later enjoy observing a stable, peaceful group without behavioral problems resulting from merging through aversive methods.

Article written by Marta Borkowska.