According to the U.S. National Research Council Committee on Recognition and Alleviation of Pain in Laboratory Animals pain is experienced by many animal species, including mammals and possibly all vertebrates.

According to Swiss scientist Gennady Ermak, an overview of the anatomy of the nervous system across the animal kingdom indicates that not only vertebrates but also most invertebrates have the capacity to feel pain.

Indications that an animal is experiencing pain include behaviours such as eating less food, disruption of their normal behaviour, suppressed social behaviour, unusual behaviour patterns, distress calls, respiratory and cardiovascular changes, as well as inflammation and release of stress hormones.