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.
The Animal Welfare Research Group of the University of Edinburgh has a website many materials and resources for learning to recognize when an animal might be suffering.
Guidelines on the Recognition and Assessment of Pain in Animals
The Animal Welfare Information Center of the United States Department of Agriculture has a website with numerous references.
A Reference Source for the Recognition & Alleviation of Pain & Distress in Animals
Other resources on the topic:
USDA Farm Animals – Pain and Distress
International Veterinary Academy of Pain Management
Recognizing Pain in Animals, Institute for Laboratory Animal Research
These materials can be good sources of information, but it should be kept in mind that they are written with a focus on the interests of those who use nonhuman animals as resources.
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