One species, two opposite answers
Mice catch people out because the correct housing depends on the sex of the animal in a way few small pets match. Females live naturally in groups, share nests, groom each other and raise litters communally; a female kept alone loses all of that and it shows. Males, once adult, treat another male in their territory as a rival to be driven off or fought, and mouse fights escalate to serious wounds with little warning. So 'can mice live alone?' resolves to: a female should not, a male almost always must. Any shop or seller giving one answer for both has not kept many mice.
Life for a lone female — and the fix
A single female mouse typically becomes restless and repetitive: bar chewing at the cage top, frantic climbing circuits, excessive grooming of her own coat, and a startling, nervous manner when handled. Female groups also pile together to sleep, sharing warmth and scent, so a lone female loses her sleeping companions as well as her groomers. These are the standard stress signatures of a social animal short of company, and they resolve far better with companionship than with toys. The fix is straightforward — two or three females together, ideally littermates bought together, in an enclosure with multiple hides, wheels and food stations so the group's mild hierarchy never turns into queueing for resources. Female groups are usually stable and are the easiest introduction in the whole small-pet world.
The solo male, set up properly
An adult male mouse alone is not a welfare problem to feel guilty about; it is the standard recommendation, and males often bond noticeably to their owners, being bolder and more handleable than females. What a solo male does need is a cage worth living in: deep bedding for burrow systems, a correctly sized wheel, cardboard architecture to shred and rearrange, scatter feeding, and rotation of chew toys so the territory keeps offering small projects. Brief gentle handling most days completes the package. Watch for the genuinely relevant risks instead — barbering his own whiskers is cosmetic, but obesity creeps up on inactive males, and any male that stops eating or sits hunched and puffed needs a vet, not a cage mate.
The one combination never to attempt
The costly mistake is solving a solo mouse by adding the wrong companion: an unneutered male put with females produces litters on a roughly three-week cycle and overwhelms owners within two months, and a second adult male added to 'keep him company' usually produces a fight, not a friendship — male introductions succeed only rarely, on neutral ground, with expert patience. Neutering a male to live with females is possible and done by some keepers, but it is a vet conversation about anaesthetic risk in a very small animal, not a casual fix. Buy the grouping right at the start: a female group, or a single male, planned as such.
Because mouse households run on who-is-with-whom, a written record prevents the classic helper error. Noting each mouse's group and solo status in KinsLe Care Teams means the neighbour feeding them while you travel will never helpfully reunite animals that must stay apart.