Do animals display emotions?

That's a difficult question to answer. During Monday's class discussion some individuals argued that they have seen their pet animals display certain emotions after specified events. Many people generalize the overt behaviors of certain emotions that are displayed by humans to animals. These individuals believe that if a dog is not playful, sits around the house, refuses to eat, and moans after one member of the family has left for a vacation, that the dog is sad/unhappy/depressed. All of the behaviors the dog exhibited are typical of a depressed human. But how do we really know that the dog is sad? Can we attribute human-defined emotive states to nonhumans?