Dreams are not the sole purview of humans. Scientists have found that other animals can dream, too, and that it may help with learning and memory.
How can you tell if an animal is dreaming? Pet owners routinely assert that cats and dogs dream all the time, citing the way their fuzzy friends paw at the air and meow or bark while sleeping.
A more scientific marker of dreaming is rapid-eye-movement, or REM sleep, when the eyelids twitch as the eyes beneath flicker and move as if watching a tennis ball game. During this stage of sleep, the brain behaves almost the same as it does when it’s awake, and that’s when most people report having particularly vivid dreams that they can remember once they wake up.
REM sleep has been observed in all mammals, although in varying amounts. Surprisingly, many animals get even more REM sleep than humans, and platypuses top the list. In fact, they get more REM sleep than any other animal, spending up to 8 hours a day in REM phase. This makes the strange duck-billed mammals the most prolific dreamers in the animal world. On the other side of the spectrum, dolphins get the least REM sleep, if any.
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So if we assume that all animals dream during REM sleep like humans do, what do they dream about? Surely not getting on stage and finding themselves completely naked.
By studying the physical behaviors of animals during REM sleep, scientists can guess what they might be dreaming about. Normally, our bodies are paralyzed while we dream so that we don’t physically react to whatever our minds are seeing; this state is called REM atonia. In 1965, researchers were able to interfere with REM atonia in cats by removing a part of their brainstem. Instead of lying paralyzed during REM sleep, the cats actually walked around and stalked the mice they were dreaming of.
Modern technology allows us to get a less invasive look at what animals dream about. A study performed on rats at MIT monitored their brains during two activities: learning to run through a maze for a food reward, and sleeping. They found that the rats’ brains repeated the same patterns of neuron firing during REM sleep that were first established while they ran the course. The researchers were even able to pin down how far the rat had progressed through the course, and whether it was running or standing still, during its dream.
Reptiles sleep in a different way entirely, that does not include a separate REM phase. Researchers have yet to study reptilian sleep closely enough to see if they might exhibit dream-like brain activities while sleeping. However, scientists have observed that birds experience REM sleep, which suggests that these sleep patterns evolved alongside warm-bloodedness. Like mammals, birds may also use dreams to learn tasks and form memories.
Zebra finches practice their songs in their dreams. Photo credit: Jim Bendon/Flickr (CC BY-SA 2.0)
Zebra finches, for example, must learn the melodies of their songs, and a portion of their brain called the robutus archistriatalis has neurons specially dedicated to singing certain notes. Biologists at the University of Chicago matched the patterns of neuron firings to the finches’ songs, and were able to construct a sort of neurological sheet music. They found that the zebra finches exhibited the same patterns of neuron activity while they slept, as if they were practicing the songs in their heads.
Because we’re unable to speak with these animals, it’s hard to determine whether any of them know that they’re dreaming, something we become aware of once we wake up. Likewise, since we can only compare their unconscious neural patterns with conscious activities, we don’t know if they also dream about more fantastical experiences that they wouldn’t encounter during the day—maybe cats have nightmares about being chased by giant mice, or giraffes dream of flying.
But these studies do show that dreams play an important role in development. Scientists still debate the physiological and evolutionary purpose of dreaming. The evidence that other animals dream about waking activities supports the theory that REM sleep and the dreams it induces are the brain’s way of consolidating and synthesizing experiences, forming memories, and learning lessons along the way. Studying how animals sleep and dream can give us a better understanding of our own dreams as well as the deep psychological framework that we share.