Scientists at the University of Bonn have found evidence that the Australian jumping spider Portia fimbriata experiences distinct stages of sleep, including a REM-like state marked by eye movements and muscle twitches. The findings are described as the first comprehensive experimental confirmation that spiders undergo regulated sleep rather than simply becoming inactive, raising new questions about how sleep evolved across animals with very different nervous systems.
A small spider with sophisticated hunting skills
Researchers at the University of Bonn’s Institute of Organic Biology made the discovery while studying Portia fimbriata, a small but highly capable predator. Fully grown, the Australian jumping spider is about the size of a 1-euro coin.
Unlike many spiders, which primarily prey on insects, species in the genus Portia specialize in hunting other spiders. They use a range of complex strategies, including mimicking the vibrations made by insects trapped in webs.
These deceptive signals can lure a web-building spider toward what appears to be an easy meal, only for it to encounter a hunting Portia instead. Such behavior suggests that these spiders have unusual learning abilities, making them valuable subjects for studying the links between sleep, nervous-system function and cognition.
Two distinct stages of sleep
Although sleep has been extensively studied in humans and other vertebrates, much less is known about sleep in animals with very different nervous systems. To investigate whether Portia fimbriata was truly sleeping and whether its sleep included separate stages, the researchers monitored the spiders over several days and nights.
They identified two distinct sleep stages. One resembled REM sleep, the stage associated in humans with rapid eye movements, muscle activity and vivid dreams.
During this REM-like stage, the spiders displayed characteristic eye movements and muscle twitches. The observations suggest that some features of sleep previously studied mainly in vertebrates may also occur in spiders.
However, the researchers stress that identifying REM-like movements does not show whether spiders dream. The finding does indicate that sleep in these animals is more complex than previously understood.
How the researchers tested whether the spiders were sleeping
The researchers placed the spiders in plastic dishes measuring 12 centimeters in diameter and monitored them under different lighting conditions. They recorded the animals’ daily activity and rest cycles, sleeping positions, periods of quiet rest and episodes of REM-like movement.
An infrared camera allowed the team to observe the spiders throughout the night without using visible light. The researchers also used sound stimuli to measure how quickly the spiders responded to disturbances, helping them distinguish sleep from ordinary inactivity.
Another part of the study involved sleep deprivation. The team investigated whether the spiders showed “sleep rebound”—a compensatory increase in sleep after their rest had been interrupted.
Together, the behavioral observations, response tests and sleep-disturbance experiments provided evidence that Portia fimbriata experiences regulated sleep in distinct stages.
The challenges of studying spider sleep
Determining whether a spider is asleep is more difficult than observing a sleeping human. Spiders sense their surroundings differently, so the researchers first had to identify sound stimuli that produced consistent responses during the experiments.
The team also had to avoid assuming that the observed eye movements or muscle twitches represented exactly the same processes seen during human REM sleep.
A major limitation was that the researchers could not measure the spiders’ brain activity. Instead, they relied on carefully documented behavior to identify different sleep states. This required high-resolution video recordings collected around the clock, generating large amounts of data for analysis.
Despite these challenges, the combination of behavioral monitoring, reaction tests and sleep-disruption experiments gave the researchers a detailed picture of how these spiders sleep.
What spider sleep could reveal about evolution
The discovery could have implications beyond spiders. Most scientific knowledge about sleep comes from vertebrates, particularly mammals and birds. Finding comparable sleep characteristics in animals with fundamentally different nervous systems could help researchers determine when and why different forms of sleep evolved.
The findings may also open new opportunities in behavioral science, neuroscience and chronobiology, the study of biological rhythms. By examining sleep in a wider range of animals, scientists hope to identify traits shared across species and investigate the fundamental biological functions that sleep may have served early in evolutionary history.
The researchers argue that future studies should include species with unusual adaptations or simpler nervous systems, rather than focusing mainly on traditionally studied animals. Such comparisons could show whether particular sleep mechanisms are widespread across the animal kingdom or evolved independently in different lineages.
Despite decades of intensive research, many fundamental questions about sleep—including important aspects of human sleep—remain unanswered.
Can spiders dream?
The University of Bonn team is expanding its work to examine how sleep, especially REM-like sleep, varies among spider species. The researchers also plan to compare spiders sleeping in their natural environments with those observed in laboratories to determine how environmental conditions affect their behavior.
Another goal is to investigate the relationship between sleep and cognitive performance. The researchers want to learn what happens in the brains of spiders and insects during sleep, and whether different sleep stages contribute to learning and other mental processes.
Environmental changes are also a focus. The team plans to study how climate change and human activity—particularly artificial light at night, or ALAN—affect the sleep patterns of spiders and other arthropods.
The findings show that even tiny jumping spiders can experience surprisingly complex forms of sleep. Whether spiders actually dream remains unclear, but their nocturnal behavior is giving scientists new clues about one of biology’s enduring mysteries.
Source: www.sciencedaily.com
