In the 17th century, famed philosopher Rene Descartes proposed that animals experienced emotions at a level equaling machines [source: Leinhard]. Pet owners have long argued that cats and dogs feel many of the same emotions as humans, but scientists and the general public often have disagreed. It wasn't until long after Descartes -- in the 1980s -- that the concept of animal emotions began to catch on, leading to new questions about how animals should be treated and what rights they deserved.
Throughout the 20th century, much of the research on animal emotion was based on anecdotes with little scientific basis. In 2009, a study published in "Current Biology" described how domestic cats could alter their purring to manipulate their owners. By increasing how frequently they purred, the cats could inspire the same human reactions as crying babies, leading to faster feeding or other types of care [source: Viegas]. This suggests that cats not only experience emotions, but posses a level of emotional intelligence that allows them to understand and tap into their human owners' emotions.
What about animals in the wild, who have no coddling humans around to manipulate? Many experts believe that emotions help wild animals make critical life decisions. These emotions may be as simple as a sense of optimism or pessimism that is based on the animal's experiences and helps it to identify danger or spot hidden prey [source: ScienceDaily]. Emotions also may help animals form partnerships that often are critical to survival. In 1964, scientists performing experiments on rats and monkeys gave the animals food in exchange for activating a device that would give another member of their species a painful electric shock. Seeing the pain their actions caused, the monkeys and rats were more likely to turn down the food than to activate the shock [source: Sohn].
As these types of findings emerge, some people argue that human treatment of animals must improve as our understanding of our fellow creatures evolves. Activists suggest that part of the reluctance to accept animal emotions lies in the moral dilemmas and lifestyle changes we face along with accepting their emotions. After all, if you knew for a fact that animals could feel pain, fear or a sense of their own mortality, could you still eat the same food or wear the same clothes and accessories? It's a decision based largely on personal beliefs, but it's a topic worthy of further research and contemplation.
It was Charles Darwin who first pointed out that animals have emotions. In his book, "The Expression of the Emotions in Man and Animals," Darwin explained that "the lower animals, like man, manifestly feel pleasure and pain, happiness and misery" [source: International Wildlife].
Despite these early words, it wasn't until Jane Goodall's work with chimpanzees in the 1960s that perceptions started to change. Goodall was the first to suggest that chimpanzees have cultural and social behaviors linked to emotional responses. For example, chimps seem to be capable of empathy and mourning. It's not uncommon for a primate mother that loses her baby to carry the corpse around for days, holding it and rocking it in an obvious mournful state [source: New York Times].
Few scientists now dispute the possibility that animals have emotions, although most concede that it's hard to prove. There are also obvious differences between species -- Darwin believed that different animals feel different degrees of emotions, some more complex and some much more basic. This could explain why elephants, which have very advanced memory and learning capabilities, also seem to mourn their dead, but mosquitoes display no obvious signs of emotion.
In some species, emotions can be key to a group's survival. For example, Capuchin monkeys monitor that all members of the group get equal portions of food after a gathering session. This could show empathy and fairness, or could be a sign of the animals trying to ensure that all members of the group eat well and stay healthy, simply for survival's sake. Other examples lean toward emotion. Marc Bekoff, Ph.D., an animal behaviorist and former professor at the University of Colorado, cites an experiment where a rat refused to push a lever to receive food once she realized that another rat would get an electric shock every time she did. The first rat was choosing not to inflict pain, even if it meant going without food [source: Psychology Today].
Many scientists have also studied the depression-like symptoms dogs sometimes develop after losing animal or human companions. These symptoms, which include lack of interest in play and food, are similar to those experienced by humans. The problem is sometimes so serious that some dogs require treatment with antidepressant medications. Scientists are still trying to understand why this happens and the most basic explanation (that animals experience sadness and loss) is gaining ground.
Social necessity is one of the reasons it might be beneficial for animals to have emotions. When animals interact with one another -- say in a pack or a pod or a herd -- emotions could help an individual in a number of ways. For example, guilt could help smooth over a conflict; empathy could encourage the adoption of an orphaned infant; or fear could help develop a pecking order. When social animals are raised by themselves, they often have trouble interacting with others of their species if they're forced to socialize later in life.
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