Results of keeping social species alone

Keeping mice alone will show unwelcome results. But it is very different in what way the rodents show their suffering. Sometimes you will notice it clearly, sometime you have to know what you are looking for to notice anything.
Some results which are often missed but might be a sign of suffering from loneliness are:

They would suffer psychically without their friends

• moving less then they would do with conspecifics
• reduced or excessive eating
• extreme, unnatural tameness
• extreme, unnatural shyness
• snappishness
• immunsystem compromised
• wound healing might be worse
• dysfunction of the autonomic nervous system
• neglected grooming
• lowered intelligence
• contact behaviour disorder
• depressions
• early death

These symptoms often occur, but they are usually simply missed, or at least misinterpreted. It requires some experience in the behaviour of the considered species or genus to recognise them. Some, such as the dysfunction of the autonomic nervous system or depression are difficult to show outside of a laboratory - but this does not mean that they do not exist.
Some symptoms don’t become apparent until the lonely mouse gets friends again. Comparing health and behaviour like reduced moving, reduced or excessive eating and unnatural tameness / timidity of the individual before and after getting friends, the difference will be obvious. Also contact behaviour disorders become apparent when introducing the lonely one.
In particular not provable - but can be conjectured by comparison with litter siblings - is the early death of an individual. For humans early death of solitary confinement is scientificly approved.

Symptoms that are not always interpreted correctly, but usually at least noticed, are the following:

• Apathy
• contact behaviour disorders shown in an introduction to conspecifics
• refusing to eat anything at all
• cannibalism → chewing parts of the own body

Especially the contact disorders become apparent soon when they make an introduction more difficult than expected. Beginners are soon overstrained with such individuals and call them simply mad or aggressive without any reason.
Apathy and food refusal are also rare and they are quickly misinterpreted as a symptom of a physical illness rather than a psychical one.
Cannibalism/mutilation of the own body is the rarest, although most massive disorder. The victim itself eats its limbs or other parts of the body. Usually the keeper is pretty helpless concerning such a problem. In most cases they don’t think of solitary confinement as a cause.

Source
www.wikipedia.org

Translation
Angelus Noctis
Proofreading
Jedediah