Species differ in their responses to painful stimuli.
For example, dogs and humans make much noise but
sheep do not, because loud vocalisations may elicit
help from social group members in dogs and humans,
but just attract more predator attention to an injured
sheep. Hence, different responses are adaptive in
different species. The feeling of pain may be the same
even if the responses are very different. However,
even if immediate responses vary, avoidance of the
painful stimulus and the effects of learning to avoid
such stimuli on subsequent exposure to the stimulus
would be observable in fish or invertebrates as well as
in mammals and birds. Other feelings such as fear,
anxiety, and various forms of pleasure have also
been deduced to exist by careful observation and
experiment.