Indirect evidence for emotional / affective states can come from neuro-anatomy, neuro physiology, and behaviour, with animal behaviour providing one of the most unique insights into an animal's subjective state. However, behavioural changes can either arise with an animal's mental awareness of a situation or simply because of changes in its neurophysiology (without any conscious awareness). For an animal to be aware of its own circumstances, this likely requires some degree of cognitive sophistication.

Cognitive abilities can be identified by testing whether information is retained by an animal as procedural behaviour (i.e., a simple, unconscious reflexive response to stimulus) or a declarative behaviour (i.e., selective attention to stimuli that includes anticipation, expectation, and goal-directed activity, allowing for flexible and adaptive responses). It is often argued that declarative representations are evidence for higher levels of consciousness (including sentience).

Examples of declarative representations include: 

- A significantly elevated fear response to natural predators for which an animal has not previously been exposed to before (e.g., Zebrafish avoidance of Indian Leaf Fish, their natural predator, but no fear response evoked to Compressed Cichlids, an allopatric predator).

- Individual recognition and ability to assess fighting abilities of potential opponents/conspecifics simply through observation (e.g., Siamese fighting fish, or Atlantic salmon during conspecific aggression).

Behavioural changes that may have a protective function in response to potentially damaging/painful events are also important indicators of a negative affective component (e.g, pain or fear) that could be associated with the sensory experience.

The following video provides a few examples from research on fish congition.

NB: This third-party video is linked for illustration only and copyright and control belong entirely to the originating organisation