Herbivorous animals have been selected evolutionarily, in general, to not openly show signs of suffering. Because of this, there are animals who may appear by outward signs not to be suffering but who are actually suffering, and may even be suffering terribly, in silence. It follows that if we want to be able to detect whether or not an animal is suffering we have to consider other indicators, such as those relating to their health.
If we accept that certain behaviors in humans are indicators of
suffering, then evolutionary logic tells us that these same
behaviors in nonhuman animals show us that they are suffering. For
example, we can often tell an animal is suffering from the way they
cry out, whimper, writhe, or start favoring an injured body part.
Over longer time periods, injury and chronic pain are suggested by
certain abnormal postures an animal adopts or when their activities
are different from their habitual ones.
There are cases, however, in which we don’t know how to interpret
the behavior of nonhuman animals. There are also situations in which
animals hide their emotions. For example, for most of their
evolutionary history herbivorous animals have been threatened by
predators whose success in hunting depends on them finding sick and
weak individuals.
For this reason, those who show signs of suffering are more likely
to be captured than those who don’t. This means that herbivorous
animals have been selected evolutionarily, in general, to not openly
show signs of suffering. Because of this, there are animals who may
appear by outward signs not to be suffering but who are actually
suffering, and may even be suffering terribly, in silence. It
follows that if we want to be able to detect whether or not an
animal is suffering we have to consider other indicators, such as
those relating to their health.
Additionally, we can infer a lot about the state of an animal’s
suffering or wellbeing from the type of situation they are in. Even
if we don’t have any other information, if we see an animal who is
being burned or who is wounded, we know that animal is suffering
(under normal circumstances) because we know that injuries are
painful. This obvious example illustrates the way we can glean
information based on the situation, but there are much less obvious
cases as well. There may be situations that negatively affect the
wellbeing of certain animals that are not obvious, but which we know
as a result of past observations. Once this has been sufficiently
validated, we don’t have to keep examining them on a case by case
basis in order to infer that animals are harmed in such
circumstances.
There are also physiological indicators that can indicate suffering,
such as trembling, sweating, dilated pupils, accelerated heartbeat,
rapid breathing, respiratory problems, etc. And, of course, a more
comprehensive physical exam can find other indications of suffering,
but this won’t usually be an option.