Whether animals feel emotions in the same way people do is a mystery. However, their behaviors are commonly interpreted as reliable expressions of mood—for example, relaxed, fearful, or aggressive. Based on observed changes in behavior, it is thought that some dogs and cats grieve after losing a close human or animal companion. In 1996, the ASPCA conducted a study of mourning in companion animals and found that more than half of dogs and cats had at least four behavioral changes after losing an animal companion. Many of these changes, such as eating less and changes in sleep patterns, were similar to behaviors exhibited by grieving people.