At the American Board of Veterinary Practitioners Symposium in 2002, one of the topics of discussion was distress in animals [source: Colorado State University]. Animal experts from different disciplines came together to argue whether animal stress, fear or pain is worse in animals and how veterinarians can deal with it. There was little discussion among experts on whether animals can feel fear or pain -- most of the discussion was based on how the levels of pain and suffering vary from one animal to the next.
The most accepted theory is that pain and suffering are associated with the size and complexity of the animal's brain [source: Colorado State University]. Because mammals and birds are more advanced and have more complex brains, it's likely they all possess capacities to experience pain. The "gray area," as scientists call it, comes with animals such as fish and lower invertebrates.
Peter Singer, professor of Bioethics at Princeton University and author of the renowned animal rights book "Animal Liberation," argues that it's illogical to think animals don't experience pain, because they display many of the same behavioral signs and physical responses that humans do when exposed to pain. Singer also argues that humans and nonhuman mammals evolved side by side and didn't diverge until after the nervous system (which controls pain responses) had already developed. As a result, both humans and other animals should have similar feelings of pain.
The argument for fear is more complex because fear is a psychological response. Accepting that animals feel fear would be accepting that they're capable of having some kind of understanding about danger. A 1997 study led by the Sub-Department of Animal Behaviour at the University of Cambridge showed that when deer were hunted by a pack of hounds, they had high levels of stress hormones in their bodies at the time of death and their muscles presented signs of stress damage. These signs weren't present in animals that were killed with single clean shots [source: University of Cambridge]. This type of fear stress response can be found across the animal world.
Animal rights activists such as Singer argue that just knowing that animals feel pain and fear is enough of a reason to change the way we treat them and think about them. Although not all scientists agree on the subject, it's at least enough to warrant pause for thought.
If faced with sufficient stress or discomfort, this chimpanzee from the Taronga Zoo in Sydney, Australia, could begin to bite itself or exhibit other self-injurious behaviors. (Ian Waldie/Getty Images)
It may seem obvious, but animals do feel fear and pain. Fear is an emotion that is part of an animal's instinct to survive in the wild. For instance, a field mouse might flee in response to the shadow of a predator. Much like fear, pain in animals also produces observable behavior. Psychologist B.F. Skinner proved that animals feel pain in a clinical test in which animals were shocked with electrical current as they approached their food. Greater understanding of animals' fear and pain led to stricter meat packing regulations beginning in the mid-1900s.
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