"How do we even define what an emotion is in an animal?"...

It was back in 1991 that Anderson and Jennifer Mather, a psychologist from the University of Lethbridge in Alberta, Canada, first decided to undertake a joint personality study of 44 smaller red octopuses at the aquarium as a way to begin to codify and systematize what they thought they had been observing. Using three categorizations from a standard human-personality-assessment test - shy, aggressive and passive - their data would ultimately show that the animals did consistently clump together under these different categories in response to various stimuli, like touching them with a bristly test-tube brush or dropping a crab into the tank.

"The aggressive ones would pounce on the crab," Anderson told me. "The passive ones would wait for the crab to come past and then grab it. The shy animal would wait till overnight when no one was looking, and we'd find this little pile of crab shell in the morning."

Anderson and Mather's resulting 1993 paper in the Journal of Comparative Psychology, entitled "Personalities of Octopuses," was not only the first-ever documentation of personality in invertebrates. It was the first time in anyone's memory that the term "personality" had been applied to a nonhuman in a major psychology journal."...

<i>Long but fascinating article on individuality and self in the animal kingdom, at the <a href="http://www.nytimes.com/2006/01/22/magazine/22animal.html?_r=1&pagewanted=all" target="new" >New York Times</a> (free registration required).</i>