" Stress begs alleviation, which rhesus monkeys achieve through grooming. The relaxing effect of this activity wasn’t easy to prove, however, because for every time our monkey was being groomed we needed a perfect match, such as an almost identical situation in which she was not being groomed. The difference in heart rate could then be attributed to the grooming. We found indeed that grooming slows down the heart, which was the first such demonstration for any animal in a naturalistic setting. It confirmed the widely held assumption that grooming is an enjoyable, calming activity that serves not only to remove lice and ticks, but also to eliminate stress and foster social ties. Drops in heart rate have also been found in horses being petted by humans, and conversely, in humans petting their pets. In fact, animal companions are so effective against stress that they are increasingly recommended for heart patients. "

― Frans de Waal , The Age of Empathy: Nature's Lessons for a Kinder Society