Young jumping spiders hang on a thread in a box in the laboratory overnight. From time to time, their legs curl and their spinnerets contract – and the retinas of their eyes, visible through their transparent exoskeletons, move back and forth. “What these spiders are doing seems very similar to REM sleep,” says Daniela Rößler, a behavioral ecologist at the University of Konstanz in Germany. During the REM phase (Rapid Eye Movement), a sleeping animal’s eyes move unpredictably, among other things.
In humans, most dreams occur during REM sleep, especially the most vivid ones. This leads to an interesting question. If spiders have REM sleep, does that mean their poppy-sized brains could generate dreams? Rößler and his colleagues reported their findings about spiders that move their retinas in 2022. By observing 34 of these animals with cameras, they found that they had short REM-like episodes every 17 minutes. Rapid eye movements were typical of these moments. This did not happen at night, when the jumping spiders moved or when they stretched, realigned their silk threads, or cleaned themselves with their legs.
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Although spiders are immobile during the period before these REM-like episodes, researchers have not yet been able to prove that these animals sleep. But if it turns out that this is the case – and that what looks like REM is actually REM – dreaming is entirely possible, says Rößler. He can easily imagine that jumping spiders, as highly visual animals, could benefit from dreams to process the information they receive throughout the day.
Rößler is not the only scientist asking these questions about animals that are far away from us. Today, researchers are finding evidence of REM sleep in more and more animals: spiders, lizards, squid and zebrafish. The growing numbers have led some to question whether dreaming, a condition once thought to be limited to humans, is much more widespread than previously thought.
In addition to rapid eye movements, REM sleep is generally characterized by a number of other features: temporary paralysis of skeletal muscles, periodic contractions of the body, and increased brain activity, breathing, and heart rate. REM was first observed in sleeping babies in 1953 and was soon identified in other mammals such as cats, mice, horses, sheep, opossums and armadillos.
The processes in the brain during REM sleep are well understood, at least in humans. In non-REM phases, also known as resting sleep, brain activity is synchronized. Specifically, in the cerebral cortex, neurons are simultaneously activated and then inactivated, producing waves of activity known as slow waves. During REM sleep, however, the brain shows bursts of electrical activity that are reminiscent of wakefulness.
Even among mammals, REM sleep is not the same. Marsupial mammals called echidnas exhibit characteristics of both REM and non-REM sleep simultaneously. Studies of whales and dolphins suggest that they may not experience REM sleep at all. Birds have REM sleep, which is accompanied by spasms in the beak and wings and a loss of tone in the muscles that support the head. Nevertheless, researchers are beginning to find similar sleep states in many branches of the animal tree of life.
Scientists are discovering different stages of sleep in more and more creatures across the animal kingdom. ADAPTED BY NC RATTENBORD AND G. UNGUREAN / TRENDS IN ECOLOGY & EVOLUTION 2023/ KNOWABLE
In 2012, for example, scientists found a sleep-like state in octopuses, as well as strange REM-like behavior during this putative sleep state: The animals periodically moved their eyes quickly, contracted their limbs, and changed the color of their eyes. there bodies. During a fellowship at the Marine Biological Laboratory in Woods Hole, Massachusetts, behavioral biologist Teresa Iglesias further studied the phenomenon, collecting terabytes of videos from half a dozen squid.
The six samples showed episodes of REM-like activity that repeated approximately every 30 minutes: these were bursts of limb and eye movements, while their skin simultaneously displayed a spectacle of color changes and patterns that repeated repeatedly. The creatures emitted camouflage signals and other attention-grabbing signals common in waking behavior. Because this cephalopod’s brain directly controls skin patterns, “that suggests that brain activity is a little out of control,” says Iglesias, who now works at the Okinawa Institute of Science and Technology in Japan.
Researchers have since observed a similar condition in octopuses. When octopuses and octopuses dream, “it just breaks down the walls of our idea of humanity’s specialness,” Iglesias says.
Researchers have also discovered a REM-like state in bearded dragons by recording signals via electrodes placed in these animals’ brains. And they have identified at least two sleep states in zebrafish based on brain markers in the fish. In one of these states, neuronal activity was synchronized, as is the case in the non-REM state of mammals. In another condition, the fish showed neuronal activity reminiscent of an awake state such as occurs in the REM phase. (Fish do not have rapid eye movements.)
By looking at multiple sleep stages in a relative so evolutionarily distant from us, the authors suggest that different sleep types arose hundreds of millions of years ago. It is now known that flies can also flit back and forth between two or more sleep states. Nematodes appear to have only one condition.
Researchers are considering the possibility that nonhuman animals dream during REM sleep because they exhibit similar behaviors in this state as they do when awake, such as the changing patterns in the skin of cephalopods or the contractions of spider glands. —. In pigeons, sleep scientist Gianina Ungurean from the Max Planck Institute for Biological Intelligence in Munich and the Medical University of Götingen, together with her colleagues, observed that the pupils of these birds contract during the REM phase of sleep. in the same way they do this in courtship behavior. This raises the question of whether the pigeons are dreaming or in some way reliving what happened during their courtship sessions while awake, the expert says.
In some animals, REM sleep has also been linked to repeating experiences. For example, when researchers analyzed the electrical activity of the brains of sleeping mice that had previously navigated a maze, they saw activation of neurons involved in navigation related to head direction, even though the mice’s heads were not moving. They also saw activity in neurons associated with eye movements. The combination suggests that the mice may have had a dreamlike experience exploring the environment, Ungurean says.
Given all these signs, it is reasonable to suspect that the animals could be dreaming, says Ungurean. “However, if we look at these reasons individually, none of them are sufficient.” Brain activity that involves repetition, such as in mice running through mazes, does not only occur during REM or sleep, says Ungurean. It can also occur when planning or daydreaming. And the connection between REM sleep and dreams is not absolute: People also dream in non-REM states, and when drugs are used to suppress REM sleep, participants in a study can still have long, strange dreams.
Ultimately, people know they’re dreaming because they can tell it, says Ungurean. “But animals can’t do that, and that’s the biggest problem we have when it comes to proving it in a purely scientific and well-founded way.”
There are still debates about what REM is for. “Nobody really knows what the function of REM or non-REM sleep is,” says Paul Shaw, a neuroscientist at Washington University in St. Louis. One of the most widely held ideas is that REM helps the brain form and reorganize memories; Other theories suggest that REM supports brain development, helps develop the body’s motor systems, maintains the circuits needed for waking activities so they do not deteriorate during sleep, or increases brain temperature.
However, if REM turns out to be present in distant species across the animal kingdom, it suggests that its role, whatever it may be, could be very important, says Iglesias.
Not all scientists believe that researchers identify REM sleep. They may simply be confirming the preconceived notion that all animals have two sleep states and interpreting one of them as REM sleep, says Jerome Siegel, a neuroscientist who studies sleep at UCLA. Some of these animals — like spiders — may not even sleep, he says. “Animals can do things that look the same, but the physiology is not necessarily the same,” he says.
Scientists are still looking for clues. Rößler’s group is trying to develop dyes that will allow them to image the brains of spiders – this could reveal activation in areas functionally similar to those we use when dreaming. Iglesias and other colleagues implanted electrodes into the brains of cephalopods and recorded their electrical activity during two sleep states – one that shows wake-like activity and another that represents a state of rest, with neural markers similar to those found in mammals were observed. And Ungurean trained pigeons to sleep in an MRI machine and found that many of the areas of the brain that light up during human REM sleep are also activated in birds.
When octopuses, spiders and a variety of other animals also dream, it raises interesting questions about what they experience, says David M. PeñaGuzmán, a philosopher at San Francisco State University and author of “When Animals Dream: The Hidden World of” . Animal awareness. Because dreams unfold from the viewer’s perspective, dreaming animals should have the ability to see the world from their perspective, he says.
Dreaming would also indicate that they have imagination, he adds. “We believe that only people can make this break with the world,” he says. “Maybe we need to think a little more about what other animals are capable of.”
Article translated by Daniela Hirschfeld.
This article originally appeared on Knowable en español, a nonprofit publication dedicated to making scientific knowledge accessible to everyone.
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