ONE autumn day in 2020, drove home with a car full of jumping spiders. Her lab was closed due to covid-19 regulations, so, after a day in a dry field spent corralling her specimens, RöĂler had no option other than to bring them back to her house. When, by chance, she checked on them that night, the spiders were by threads of silk.âI had never seen this before,â says RöĂler, a behavioural ecologist at the University of Konstanz in Germany, who soon went back to the field with her colleagues. âWe started filming them, just out of curiosity,â she says.
They observed the same behaviour, but only at night. Stranger still, some months later, close monitoring in the lab using a night vision camera revealed not only that the spiders were . That is similar to what happens when humans dream, which raises the irresistible prospect that spiders could be dreaming too.
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Jumping spiders arenât the only non-human animal in which we have recently found evidence of dream states. We are seeing hints of dreaming, and even nightmares, in species throughout the animal kingdom â from pigeons to octopuses. âIf we appreciate the functions that could be connected with dreaming, it totally makes sense for animals to dream,â says RöĂler. And yet the question remains: do other animals dream like we do and, if so, what they are dreaming about? Figuring this out isnât easy but it is worth doing, not least because it might even help us to fathom the purpose of human dreams.
To dream, first you have to sleep, and there is no known animal that doesnât slumber, says RöĂler. Recent studies show that sleep is essential for even the simplest organisms to reset their minds and bodies. Even some jellyfish, which have a distributed net of neurons rather than a true brain, . âMaybe sleep evolved before brains evolved, as a very fundamental thing that is required on a cellular level,â says RöĂler. It may enable animals to get rid of waste products and toxins that build up when they are active, she adds.
When humans sleep, we do so in two ways that alternate throughout the night. The first is rapid eye movement (REM) or active sleep. During this stage, our eyes move, even though our eyelids are closed. Our muscles also twitch slightly, though they are largely paralysed so we donât hurt ourselves. In contrast, we also engage in non-REM or quiescent sleep, during which we barely move at all. Most dream states, and certainly those with the most vivid dreams, happen during REM sleep.
Ancient dreamers
There is plenty of evidence that other mammals . In 2022, for example, researchers and found increased activity in neurons that respond to the direction the animal is facing. That suggests the animals were looking around, as humans do in REM sleep.
Finding evidence of dreaming itself in non-mammals has proven more difficult. Non-mammalsâ brains are very different to those of humans, and it can often be difficult to image them while they are sleeping. In June 2023, however, at the Max Planck Institute for Biological Intelligence in Seewiesen, Germany, and her colleagues succeeded in recording brain activity in awake and sleeping pigeons. They first habituated the birds inside a fake functional magnetic resonance imaging (fMRI) scanner before swapping it out for a real one to obtain the first brain scans of sleeping birds. As in mammals, the recordings .
Intriguingly, REM sleep activity was high in brain regions involved in processing visual information, especially images that slide across your field of view when you are in motion â a phenomenon called optic flow. âThese activities, put together, are indicative of birds flying,â says Ungurean, who tentatively suggests this is what the pigeons were dreaming about.
Ungurean also found that during non-REM sleep in pigeons, cerebrospinal fluid flushes through the brain, washing out toxins â but this happened much less in REM sleep. The implication, she says, is that REM sleep has an essential function too. âWe need this REM state,â she says, otherwise we wouldnât have traded cleansing, non-REM sleep for it.
That said, dreaming and REM sleep probably arenât universal in the animal kingdom, says at the University of the Witwatersrand in South Africa. For example, sponges donât have brains, or even neurons, so .
There are also some animals with unusual sleep patterns. These include whales and dolphins, which sleep with half of their brains awake and show no sign of REM sleep. One interpretation is that they only experience less vivid non-REM dreams. This feels counter-intuitive, says Manger, because we tend to think of whales and dolphins as having complex inner lives. He reasons that during REM sleep, the bodyâs thermoregulation system is impaired, leaving animals more vulnerable to extremes of temperature. This could be dangerous for marine mammals sleeping in cold water. âTo me, it looks like itâs an adaptive thing to prevent hypothermia,â he says.
Nevertheless, in many cases REM sleep does seem to have benefits. Growing evidence from suggests that REM sleep and dreaming are important for forming memories and learning. âEvents are replayed during sleep,â says Ungurean, which helps to integrate memories into longer-term storage. As soon as animals evolved moderately complex lifestyles, they would have needed to dream in order to âlearn and contextualiseâ, says at Alaska Pacific University in Anchorage. This happens by rerunning variations on experiences.
Pigeons, though, are much more closely related to humans than spiders are. Our last common ancestor with arthropods and other invertebrates lived over 500 million years ago. Sleep and dreams in invertebrates have been almost ignored, says RöĂler. That is partly due to prejudices about invertebrates â that they are primitive and lack complex inner lives â and partly due to the practical challenges of studying them. But in recent years, the field has begun âslowly, finally, cracking openâ, she says.
Unable to keep spiders still inside a brain scanner, RöĂler had to find new methods for measuring sleep activity. Jumping spiders have retinal tubes inside their eyes that they can angle to shift their field of vision. Using tracking software that precisely measured how these tubes moved, she observed behaviour akin to REM sleep.
RöĂler is careful to specify what her findings show. âWe havenât properly demonstrated that theyâre even asleep,â she says, as sleep is conventionally defined by the behaviour of mammals. Still, RöĂler points out that when the spidersâ eyes were moving, their legs were curled inwards â a behaviour only otherwise seen in dead spiders, due to a loss of internal pressure. âWe feel like this is a very similar thing to the muscle atonia and paralysis that we see in sleeping mammals and birds,â she says. RöĂler is now studying sleep in 20 different spider species and so far has found âthe exact same patternsâ in every single one.
Patterns like these are suggestive, but we still donât understand how this outward behaviour relates to internal experience. It seems impossible to know what it is like to be a spider or a pigeon, let alone imagine their dreamscapes. We are quick to interpret the twitching limbs and muted barks of sleeping dogs, but the truth is that we donât know if there is an internal experience of chasing rabbits that comes along with that.
Another invertebrate dreamer offers insight here. Cephalopods, such as octopuses and cuttlefish, were over a decade ago by inferring from changes in colouration and arm twitching while the animals slept. But in the past few years, close observation of cephalopod behaviour, alongside brain imaging studies, hint at an even richer inner world.
In 2019, while making a documentary, Scheel housed an octopus named Heidi in a tank in his living room. At one point, in the middle of the night, Heidi seemed to dream: her limbs and head moved, and her skin rapidly changed colour, as though she was pursuing a crab.
Similarly, in May 2023, a report emerged of a sleeping octopus apparently having a nightmare. Costello, as the octopus was called, thrashed around, extended his mantle as if trying to make himself look bigger, and squirted ink as though he were being attacked by a predator. The nightmare study is intriguing, says Scheel, but is only based on one animal. He argues that as well as outward behaviour, to show that the octopuses are replaying sequences of activities from their waking lives in dreams.
This kind of parallel brain activity has already been observed for octopuses hiding from danger. In June 2023, at the University of Washington in Seattle and her colleagues reported implanting electrodes in nine octopuses to observe their brains while awake and asleep. Daytime patterns of activity associated with octopuses camouflaging themselves were also replicated during sleep.
For Scheel, the evidence is at least consistent with the idea of octopuses having a nightmare while they sleep. If humans dream in order to process emotionally intense experiences, then perhaps octopuses do too. An octopus that shows evidence of experiencing fear in a dream is likely to be dreaming of a familiar predator, says Scheel. But in the case of Costelloâs apparent nightmare, he says we donât have enough data on âhow octopuses respond in the moment to traumasâ to prove it.
The trouble is, says Scheel, that we will never be able to experience any animalâs dreams other than our own. That goes for other humansâ dreams too. But we can try to imagine what these dreamscapes are like by meeting animals on their own terms, says RöĂler. For example, vision is the dominant sense for many humans, and so our dreams are heavily visual too. Dogs primarily navigate the world using smell, and bats echolocate. Spiders, meanwhile, rely much more on vibrations. So, if jumping spiders do dream of helpless insects, their inner world is likely to be composed of thuds and shakes.
These experiences emerged hand in hand with the environments animals have evolved in. Given these vast possibilities, it is likely that dreaming has served multiple purposes since the first complex animals evolved, says Scheel. And this is how animal dreams might shed light on the true purpose of our own.
If we take a more systematic approach, sampling across the animal family tree to discover which groups dream, and how the events of their lives affect their dreams, we could figure out whether dreaming evolved just once, in the earliest animals, or multiple times independently. That, in turn, could offer fresh insights into the evolutionary pressures that cause species to evolve the capacity â and thus shed light on why animals, and humans, unconsciously conjure life-like experiences.
Michael Marshall is dreaming of a white crustacean being devoured by an octopus
