Many pet owners say that they know when their animal is happy or sad, and evidence has been found to show that animals do exhibit emotion. Researchers have found that emotions are accompanied by biochemical changes in the brain that can be measured. When scientists examine the physiological changes found in humans that correlate with certain emotional states (for example, anger, fear, or lust), they find that these changes can also be observed in certain animal species. For example, a study of stress among African baboons showed that social behavior, personality, and rank within the troop can influence the levels of stress hormones. In addition, field observations have recorded expressions that correlate with pleasure, play, grief, and depression. In one good example, primate ethologist Jane Goodall (1934–), watching the reaction of a young chimp after the death of his mother, maintained that the animal “died of grief.” Even with this evidence, it is impossible to truly know how another organism “feels.”