Dreams are strange, misunderstood things. This week, Science of Us is exploring the latest research that helps explain what they are, what they might mean, and how they affect our waking lives.

Sleep helps humans learn: If you give people a virtual maze to navigate, and then let them take a nap, and then tackle it again, they’ll do better than people who stayed awake. Indeed, one of the most sensible explanations for why dream imagery is so weird is that the kaleidoscopic visions you get are the product of new and old experiences being tied together in memory. But, according to a range of research, humans aren’t the only animals who simulate and stimulate their lives during sleep.

The observation isn’t exactly new: Aristotle mused on how “all viviparous quadrupeds” dream (that’s in reference to animals that give birth to live young, like horses, dogs, oxen, sheep, and goats). “Dogs show their dreaming by barking in their sleep,” the philosopher wrote in his The History of Animals. But this century, scientists have started peeking into animal brains. In 2001, MIT researchers found that if you direct rats through mazes, their brains fire in the same way while sleeping. In 2006 they found that rats likely dream in images, and in 2012 they successfullymanipulated the dreams of the rodents. The researchers trained the rats to run through mazes by playing different auditory tones for different directions, then, in sleep, played random tones, apparently shifting the rats slumbering experience. “When the sound associated with the left side of the maze was played, the dream content switched to memories of running down the left side of the maze,” lead researcher Matthew Wilsonsaid in an interview. “When the sound associated with the right side was played, the dream content switched to the right side of the maze.”

Cats and dogs, though beloved, are understudied — perhaps because people get uncomfortable with invasive necessities of experimental neuroscience. In the 1960s, French scientists removed the part of the brai nstem responsible for sleep paralysis, the pons, in cats, and the animals proceeded to walk around whilst sleeping, as Jason G. Goldman notes at BBC Future. Dogs have been shown to have REM cycles quite similar to humans, so theyought to dream, though more research needs to be done; the pons is weaker in very young and very old canines which explains why puppies appear to have less sleep paralysis — which keeps you from acting out your dreams. Thus why they’re such cute, active dreamers.

dog animated GIF

Birds and their songs also rely on sleep, and what appears to be dreaming. Using fine wires to record individual brain cells, University of Chicago neuroscientist Daniel Margoliash and his team tracked the waking and slumbering activity of male zebra finches. For a paper published in Sciencein 2000, the researchers found that the bird’s neuronal activity during sleep mirrored what their brains were up to while awake and singing. Not only that, but the birds were tuning their crooning while snoozing. “We saw that the patterns of activity overnight changed, changed modestly, but definitely changed,” Margoliash tells Science of Us. “Not all birds and not all neurons, a large percentage of them. What that means is that the representation, how the motor system represents the song, changed overnight, and without the bird singing.” In a later review, he and a co-author described this auditory replay as “absolutely remarkable”: When the birds replay a song during sleep, the neuronal firing is within a millisecond of the version they sing while awake.

You can’t formally conclude that they’re “dreaming,” Margoliash says, in the way that humans do, with the sequences of images and sensations. Also, given a bird’s mode of expression, you can’t exactly ask it what it dreamt about last night, similar to how you can’t formally know that an infant was dreaming. But even if they don’t have the words, the research indicates that birds train their voices as they sleep.