Edited By: Joy Jiang
Dreams are interesting, fickle things. One night, we could be dreaming of flying across a city, and the next fighting zombies in a post-apocalyptic universe (interestingly enough, I’ve actually dreamed this on multiple occasions and I still have yet to figure out what my subconscious is trying to tell me). Every human being dreams, and interestingly enough, so do other animals. One night, I heard my puppy softly barking and fidgeting around in her sleep. A couple of minutes later, she looked up as if nothing happened and went about the rest of her dog to-do’s (which for her age was chewing up anything that I left on my bedroom floor unattended). It got me wondering, what in the world is she dreaming about, and do other animals dream too? Is there anything we can learn about our own dreams by looking at other animals?
Let’s look at the science behind dreams first. In the basics of it, dreams are images, thoughts, sounds, and sensations that we experience when we sleep. Dreams are still a complex thing scientists study about, and the reason we do it is still unknown. There are many theories as to why we dream, ranging from being prophetic to simply our brains storing memory from the day. Dreams are best remembered during a stage of sleep in which you experience rapid or random eye movement, or REM sleep. REM sleep occurs in cycles of about 90-120 minutes throughout the night and accounts for 20-25% of total sleep time in adult humans. The proportion of REM sleep to total sleep decreases with age, which may be why you had more vivid dreams as a child (a newborn baby can spend around 80% of their total sleep time in REM sleep!). REM sleep dominates the latter half of a sleepy cycle, especially around the hours before waking, which can also be a reason why you can remember a lot about your dreams as soon as you wake up, or why you wake up in the morning right after a dream.
REM sleep can be monitored by electrooculography, which is measuring the electrical activity of the retina of the eye over time. It is believed that the eye movement is related to the visual images of dreams during this sleep as well as associated with brain wave spikes found in regions of the brain involved with vision.
So how do we go about figuring out whether animals dream or not? We can apply what we know about our own dreaming habits into experiments that test whether animals dream in their sleep. Thus, one way to theorize whether animals dream or not is to look at their physical behavior during their phases of their sleep cycle. This is precisely what happened in the 1960’s, when reports started to appear describing people acting out movements in their dreams despite muscles being paralyzed during REM sleep. Since then, several studies have been made to determine whether animals dream or not, what specifically they dream about, and what it tells us about the evolution of dreaming.
Rats
As with most psychological studies, rats were one of the few test animals to test out theories on animal dreaming. In 2007, MIT scientists Kenway Louise and Matthew Wilson did research on neuron activity in the hippocampus, a structure involved with forming and encoding memories. They first recorded the neuron activity while the rats were in a maze, then while the rats slept. Louise and Wilson found that there were identical patterns of firing neurons during waking hours in the maze and during their REM sleep. Essentially, the rats were running the maze in their sleep! “The results were so clear that the researchers could infer the rats’ precise location within their mental dream mazes and map them to actual spots within the actual maze.”
Another study in 2015 by University College of London scientist, Hugo Spiers, further showed results that rats dream about where they want to go. When shown a food treat at the end of a path they cannot access and then falling asleep, the neurons in the hippocampus represented a route to access the treat in their sleep. In another experiment by Spiers’ team, “they placed four rats at the bottom of a T-shaped pathway, with entry to the top bar of the T blocked by a grille. Food was placed by the end of one arm, in a position visible to the animals.” After the rats went to sleep, the scientists put them back into the maze with the grille and treat removed. The rats ran across the arm where they had seen the food, with the same “place cells” firing in a pattern corresponding to the route to the food. Like humans, rats can dream about where they want to go and how to get there. This evidence suggests that animals in REM sleep do a lot of memory making.
Birds
A study in University of Chicago in 1998 looked into the brains of of Zebra Finches and found that they may have been practicing in their sleep the songs they sang during the day. Biologist Daniel Margoliash and colleagues noticed that when birds were asleep, their brains showed intense activity in the robustus archistratalis (RA), known to be involved in singing. In the experiment, the scientists played recordings of singing birds while the test birds were awake, asleep, and knocked out with anesthesia. At the same time, they looked at the electrical activity in the birds’ RA. The sleeping and unconscious birds showed intense activity from the RA, but signals went back to normal, oscillating patterns when awake. “This is surprising because the same neurons that show no response during the day have these strong responses to the bird’s own song when they are asleep,” Margoliash said. “It’s possible that songs learned during the day affect the bursting patterns of the RA at night, serving to solidify the newly learned songs in the animal’s mind.” Similar to rats, birds are committing to memory things they learned in waking hours through dreaming. If you study music, perhaps you’ll find yourself practicing in your sleep too!
Cats
Contrary to popular assumption, when cats move around in their sleep, this does not mean that they are dreaming. Much like us, cats start to dream during REM, where they are “utterly slacked and…relaxed”. Sleepwalking only occurs in brain-damaged cats which have lesions around the locus coeruleus in the brain stem. This was tested in 1959 by Professor Michel Jouvet at Claude-Bernard University in Lyons, France. Jouvet surgically destroyed brain pons and found that, because the locus coeruleus is involved with the physiological responses to stress and panic, removing it also removed motor inhibition during REM sleep. Thus, when the experiment cats went into REM sleep, the cats began to act out the activities of their waking hours, crouching low and stalking dream prey, looking for dream food, etc. So, like the dreams of birds and rats, cats just seem to be committing to memory what they were experiencing in their waking hours.
Dogs
My puppy barking in her sleep is what made me curious about animal dreams in the first place. While dogs sleep more than people do, they do not spend a lot of their time in REM sleep. Dogs enter REM sleep around twenty minutes into sleeping and may stay there for only 2-3 minutes, which explains why you don’t see your dog running in their sleep for too long (assuming you haven’t woken them up yet). The brain pons responsible for keeping dogs from acting out in their dreams is underdeveloped in puppies and may not work as efficiently in old dogs. Much like cats, dogs dream of their daily lives, “so, pointers will point at dream birds, and dobermans will chase dream burglars.” According to Stanley Coren, a professor emeritus of psychology at the University of British Columbia. Interestingly enough, dogs also avoid sleep paralysis, a condition often a result of sleep deprivation and is a rare condition in dogs. Maybe we could learn a thing or two from dogs and their sleeping habits!
Reptiles
Most dream studies involve using mammals but until recently research has found that reptiles also go into REM sleep and possibly dream too. This study suggests that the sleep patterns seen in mammals may have evolved 100 million years earlier than previously believed, and may have first evolved in amniotes, a distant common ancestor of lizards, birds, and mammals, which lived between 300 million and 320 million years before our time.
The study involved placing electrodes on the surface of five bearded dragons’ brains and recorded the lizards going through the stages of sleep found in humans. The signs of sleep stages in the lizards were found in the dorsal ventricular ridge, a primitive part of the brain, unlike mammals where sleep occurs in the hippocampus. This suggests that the parts of the lizard’s brain associated with sleep are older than previously thought and could be traced back to the earlier ancestors. The study also found that the length of the cycles were not the same, the fastest sleep cycle being 80 seconds compared to a human’s 60-90 minutes. This can be a survival tactic, which we’ll talk about more with fish dreams. Nevertheless, theories for lizard dreams follow the same structure as mammal dreams, wherein they dream about the activities of their waking hours and “saving” them to memory: laying in the hot sun, catching prey, etc.
Fish
Much like reptiles, fish do not have eyelids, so there is little to no research done on fish sleep cycles and more specifically if they dream. Different species of fish have different ways of sleeping, developed as a survival tactic. For example, tuna rest motionless at night suspended in water while bass and perch sleep under or on top of logs. Most fish are never completely unconscious because doing so would mean being at risk to predators. Their brains sleep in shifts, resting different systems at different times.
While no official experiments have been done on fish dreams, we can make a hypothesis that they may do some sort of memory storing and learning at some point in their daily cycle. It has been proven that fish learn, as seen in an experiment at the University of Oviedo in Spain, where angelfish learned what corner of the fish tank food was delivered to over a few weeks, as well as knew when they would be fed. Additionally, experiments at the Manomet Centre of Conservation Science trained haddock to swim through nets via watching older, more experienced fish, a very advanced skill with hefty data processing.