Because REM sleep is recognizable in mammals and birds, but not in snakes and other reptiles, scientists think that most warm- blooded animals dream. Studies have monitored the sleep of goats, sheep, cats, dogs, rats, mice, monkeys and apes, and all had dream periods and symptoms; all except the spiny anteater, which seems to be a dream-free mammal.
Watch a sleeping dog or cat sometime, and you can tell if it is dreaming of running after something. Its eyes twitch, sometimes it moves its paws—something could be happening in its dreams.