Taking care of orphan babies when a mouse foster mother is not an option. This is extremely hard to do but if you are up for the challenge, it can be very rewarding. Often times the bond that is formed between a human and a foster mouse is unbreakable. This can be extremely heartwarming to the human as well as the mouse. However, it can be very heartbreaking if the baby doesn't make it. In my opinion, the risk is worth taking :). Before you get started, know that this is very time consuming and sometimes no matter how well you take care of the orphan(s) they still may not make it. If that happens and you did your best, rest assured that you did everything possible… it just wasn't meant to be. Also know that you gave that orphan a wonderful life, even if it was a short one.
When taking care of an orphan mouse you will need some supplies. It is absolutely necessary that these are acquired as fast as possible. The orphan needs nourishment quickly. You will need kitten formula (KMR is recommended). This comes in powder as well as pre-mixed. I prefer powder because it lasts longer. *Unless your formula goes by weight, make sure you dilute it twice the recommended amount.* KMR is designed for kittens (obviously). Pinkies are so much smaller that they can't swallow the KMR unless its diluted more than recommended. You will also need a syringe (no needle) for hand feeding. If the baby is young enough (he/she has eyes still closed) a shoe box with holes in the lid for ventilation will work well. If the baby has its eyes open, a small fish tank, kritter keeper or something else that they can not chew out of is best. Bedding material can consist of clean rags (make sure there are no lose threads that they can get their legs or toes tangled in as his can cut off circulation), paper towels, unscented TP, non-lotion Kleenex, etc. You will also need to keep the baby warm but not to warm! It is just as easy to over heat a baby as it is to not make it warm enough. You can use a heating pad. Turn it on the lowest setting and place it under the tank. It is best to place a towel in between the heating pad and the bottom of the tank. Check it often to make sure it isn't to warm! A mouse nest is around or slightly above 80 degrees F. As the baby gets older (over 2-1/2 weeks) it will be able to maintain its own body temperature easier as long as the room temperature is not to cool. Lastly, you will need Q-tips to stimulate the baby to go potty.
In order to know when/how often to feed the baby, you must first establish its age if it is not known. Try to judge the age using the day by day pictures of mouse babies on my site. Orphan babies are often smaller than babies that have a good mother. Don't let this discourage you when taking care of the baby. The baby might actually be older than what it appears to be. However, take care of it by the age it looks, and not necessarily how old it actually is. For instance, a baby might be 2 weeks old but looks one week old. Take care of it as if it was 1 week old. If your orphan is a new born you will need to feed it every 1-2 hours as a minimum. You must do this around the clock. This will make for many sleepless nights and one of the reasons that taking care of orphan mice is so challenging. If the baby has its eyes open then it is around or over 2 weeks of age. You can decrease feedings some at this point. Every 3-4 hours (weaning gradually) should be sufficient. If the baby at any stage appears to lose weight, try to feed more if the baby is willing. Be sure not to over feed (don't force it). A baby should eat .05cc per gram of their body weight per feeding. This means that a baby of 10 grams should eat .5cc of milk per feeding. Once the baby has its eyes open you can introduce solid mouse food as well as a water bottle. Make sure the bottle is low enough for the baby to reach. It may not drink from the bottle right away. Give it time. I do not recommend water dishes. This can be a huge hazard as the baby can drown in even a small amount of water. Foods you can introduce to a 2 + week old baby are store bought mixes (hamster food is often best), cooked rice (mice generally accept white rice over brown but both are good. White rice often helps them get started as it's more appetizing) mixed with KMR, human baby food (select things that are easy on the digestive track. Stay away from things that might give it the runs, such as spinach, etc), soft kitten food, etc. When the baby is around 3 weeks old it should start to wean itself. Don't worry if it doesn't happen at 3 weeks. Your orphan will come around in his or her own time. It might even be as late as 4+ weeks when it starts to wean. No mouse should be fully weaned before 3-1/2 to 4 weeks of age. Also, orphans usually need to be weaned late. You may feel frustrated or concerned but know that it will happen when the time is right. It's actually quite amazing how it magically happens. Soon enough your baby will be solely on solid foods.
When hand feeding an orphan make sure not to get any formula in its nose!! This is extremely important! If the baby has a nose full of formula it can suffocate easily. Dispense formula in the side of the mouth (as best as you can) using the syringe. Only give it as much as it can eat. Take it slow and don't force it. When first starting to feed an orphan it can prove trying, frustrating, etc. Give it time and keep your patience. After feeding make sure you rub the orphans belly gently with a Q-tip. Try to massage until the baby goes potty. A baby can NOT go potty by itself, you need to assist it. If you do not assist it, it will die from being backed up. Do not rub too hard or irritate the skin. If you have been massaging for a long time and it will not potty, stop for a while and try again later.
Survival rate of hand raised orphan mice depends on age and health. If the mouse shows signs of illness, at any age, the survival rate is low. A new born orphan has a smaller chance at surviving, but don't let that discourage you! I know of many success stories! A baby that has its eyes open has an excellent chance at surviving.
Things will often come up that I did not address here. It is impossible to cover everything. Please refer to the forum for one on one help with your orphan mouse.