Abnormal behaviour in animals can be defined in several ways. Statistically, abnormal is when the occurrence, frequency or intensity of a behaviour varies statistically significantly, either more or less, from the normal value. This means that theoretically, almost any behaviour could become abnormal in an individual. Less formally, 'abnormal' includes any activity judged to be outside the normal behaviour pattern for animals of that particular class or age. For example, infanticide may be a normal behaviour and regularly observed in one species, however, in another species it might be normal but becomes 'abnormal' if it reaches a high frequency, or in another species it is rarely observed and any incidence is considered 'abnormal'. This list does not include one-time behaviours performed by individual animals that might be considered abnormal for that individual, unless these are performed repeatedly by other individuals in the species and are recognised as part of the ethogram of that species. Most abnormal behaviours can be categorised collectively, however, many abnormal behaviours fall debatedly into several of these categories and categorisation is therefore not attempted in this list. Some abnormal behaviours may be related to environmental conditions whereas others may be due to medical conditions. The list does not include behaviours in animals that are genetically modified to express abnormal behaviour.
Activity anorexia; a condition where animals exercise excessively while simultaneously reducing their food intake.
Adjunctive behaviour; an activity reliably accompanying another response that has been produced by a stimulus, especially when the stimulus is presented according to a temporally defined schedule.
Barbering, or fur and whisker trimming; removing the whiskers or fur of another animal.
Broodiness; sitting on a clutch of eggs to incubate them.
Cannibalism; eating the flesh or internal organs of another animal of the same species.
Feather pecking; one bird repeatedly pecking or pulling at the feathers of another.
Feather-plucking ; birds chewing, biting or plucking their own feathers with their beak, resulting in damage to the feathers and occasionally the skin.
Forced moulting; commercial egg-laying hens losing their feathers due to the deliberate removal of food and water for several days.
Herbivory in carnivorous animals; eating plant material by an animal that is considered to usually be meat-eating.
Infanticide; killing of young offspring by a mature animal of its own species.
Learned helplessness; failing to respond even though there are opportunities for the animal to help itself by avoiding unpleasant circumstances or by gaining positive rewards.
Lignophagia; eating wood.
Obsessive-compulsive disorder; a specific, unnecessary action or series of actions that is repeated more often than would normally be expected.
