Why are my pet mice being aggressive and eating their cage mates?

Thanks for reaching out about Sadie and including a photo. It appears as if the two new females where bonded and are bonded and ganging up on your first mouse. They can be aggressive but rarely do serious damage. But provide multiple hiding areas, food dishes, water dishes and area of rest. But if aggression continues, they might have to be separated. Good luck.

I am sorry to hear about your mice! If they are still fighting you can try putting them in cages directly next to eachother or place one in a smaller cage inside the larger cage to allow them to smell/see eachother and get used to eachother again without the possiblity of fighting. Also adding another feeding bowl and water bottle to provide several locations for resources so less likelihood of competition may help.

In addition to the big cage, provide adequate numbers of food/water bowls, toys and hides to prevent resource protection or food related aggression. If this doesn't help, you may need to separate the bully. You can try reintroductions after a week or two of separation.

Consider bumping him up to a juvenile rat (or small adult) once a week. An exam with an experienced herp veterinarian would also be recommended to diagnose illness or injury. Frequently, either may result in behavioral changes or sudden aggression.

Your friend’s mice were probably just startled for some reason when you approached them today. I would recommend that you continue to engage and interact with them regularly to help gain their trust. The mice are in a relatively new environment and are probably very prone to stress. I would also recommend separating the pregnant mouse from the other two mice. Her pregnancy and birth can be very stressful and so anything you can do to help relieve her anxieties/stress would be good for her. I hope this helps. Thanks for using Petco Pet Education Center, formerly Petcoach. Best of luck.

Mice are prolific breeders. Gestation is about 3 weeks and they can reproduce throughout the year having 5-10 litters each year. Separating males from females is necessary to stop breeding. Females can breed at 3 weeks of age, so it is important to separate them prior to this age.

I am so sorry that this change is happening - it is never easy when one pet starts to be aggressive to the other. I don't know when the last time that you had a wellness exam performed on your rabbits, but I think that would be a good place to start with these two. Sometimes a more dominant animal will "pick on" the other if it isn't feeling well - this is often picked up by the animals in the house long before it is noticed by the humans in the household. Your veterinarian would perform a physical exam, and then could also perform some blood work to evaluate systemic health. Since it looks like your rabbits spend some time outside, they may also evaluate the feces for any evidence of parasites as well that they may pick up. I would also recommend weighing both of the rabbits - it sounds like a lot of the negative interactions occur when food is present, and this may make it hard for your lop to get adequate amounts of food, and weight loss can occur. Additionally, there may be a process going on in the dominant rabbit that is making it need more nutrition and, therefore, is becoming more aggressive about food items. One thing that you may try while waiting to get into the vet is separating the rabbits with a physical barrier that they can still see through - then if they still want to lie next to one another against the barrier they can, but the lop can get away easier if getting picked on. You can feed them near the barrier as well, in case they are social eaters. This may make them feel more comfortable than being completely separated from one another.