I'm going to take this one medium-seriously. The mice can clearly figure out how to clamber about in microgravity, and they also are seen eating so I assume they could take care of their basic needs if sustenance were available. I have no information to support this but I feel like they would figure out the copulation aspect too. "Life, uh, finds a way." So let's look at some problems our intrepid rodent friends might face:
Direct Obstacles of Microgravity
- We know humans experience myriad physical symptoms of extended spaceflight including decreased muscle mass, decreased bone density, higher risk of cancer (possibly for ISS astronauts), changing in the shape of the eyeballs, etc. Since we use mice as proxies for human experimentation quite often, let's assume they can turn the tables and expect to experience some/all of these symptoms based on our experience. These issues may not critically endanger the mice (especially over a shorter lifespan), but they will not be happy upon possible return to Earth.
- Humans can reason through the obstacles of movement in microgravity. The station is also designed for humans to move around easily and astronauts are trained in doing so. Even though the mice clearly figure out how to move around in a small space, I wouldn't envy being the mouse trapped floating in a large open area. Without human size or intelligence, one might float for quite a while without being able to reach and grab a wall. Not fun.
Danger to the ISS
Since the ISS will have to function properly for the mice to survive, anything they do to impede the smooth functions of the station will hurt their chances.
- Waste is going to be an issue. Anyone who has owned rodents (or been to a pet store) has some idea of the amount of excrement produced by a mouse colony. The ISS is a closed system and this waste isn't going anywhere. So imagine floating mouse turds and urine entering the air circulation and lodging in nooks and crannies. This will make the station dirty and dangerous for humans (if we leave and turn the lights out behind us the mice are SOL). This will also be a major burden on filtration systems - possibly too much for the system to handle.
- Mice are going to go poking about looking for comfy spots (aforementioned copulation is one reason) and in the process will not bother treating the hardware of the ISS with care. Any number of situations could arise in which a mouse chews through the wrong cable/bulkhead/seal/etc. and compromises the integrity of the ISS or some subsystem. Even barring some rodent-caused emergency situation, it's almost a certainty that this gnawing problem will eventually be too large a burden and, as before, if humans have to leave it's bad news for mousestronauts.
The Outlook Is Not Great
It might be possible for a multigenerational mouse colony to develop on the ISS if sustenance were available. However, in terms of human generations it would be a very quickly deteriorating situation in which the mice would render their environment unlivable just by behaving naturally.
Cats could be trained to use a litterbox in space, right?