Scientists at the University of Bonn have found evidence that the Australian jumping spider Portia fimbriata experiences multiple stages of sleep, including a REM-like state marked by eye movements and muscle twitches. The findings suggest that sleep in spiders is actively regulated and more complex than simple inactivity, while 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. A fully grown spider is about the size of a 1-euro coin, but its hunting behavior is unusually complex.
Unlike many spiders, which mainly prey on insects, species in the genus Portia specialize in hunting other spiders. They use a range of strategies to capture their prey, 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 sophisticated learning abilities, making them useful subjects for studying the links between sleep, nervous-system function and cognition.
Two distinct stages of sleep
Sleep has been studied extensively in humans and other vertebrates, but much less is known about sleep in animals with very different nervous systems. To investigate whether Portia fimbriata experiences genuine sleep and whether that sleep has distinct stages, the researchers monitored the spiders for several days and nights.
They identified two separate sleep stages. One resembled REM sleep, the phase in humans associated 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 traditionally studied mainly in vertebrates may also occur in spiders.
However, the researchers emphasize that REM-like movements do not show whether spiders dream. The findings instead indicate that sleep in these animals is more complex than previously understood.
How the researchers tested whether the spiders were asleep
The team observed Portia fimbriata in plastic dishes measuring 12 centimeters in diameter. The spiders were monitored under different lighting conditions so the researchers could examine their daily cycles of activity and rest.
An infrared camera allowed the scientists to observe the spiders throughout the night without using visible light. The recordings documented their sleeping positions, periods of quiet rest and episodes of REM-like movement.
The researchers also conducted experiments to distinguish sleep from simply sitting motionless. They used sound stimuli to measure how quickly the spiders responded to disturbances, then compared those responses with the animals’ normal periods of inactivity.
Sleep deprivation provided another test. The researchers looked for “sleep rebound,” in which an animal compensates for interrupted sleep by increasing its subsequent sleep.
Together, the behavioral observations, response tests and sleep-disturbance experiments provided evidence that Portia fimbriata experiences regulated sleep in distinct stages.
Why studying spider sleep is difficult
Determining whether a spider is asleep is more complicated 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.
They also had to avoid assuming that the eye movements and muscle twitches they observed represented exactly the same processes as 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 during the day and night, producing a large volume of data for analysis.
Despite these challenges, the combination of behavioral monitoring, reaction tests and sleep-deprivation experiments gave the researchers a detailed picture of how these spiders rest.
What spider sleep could reveal about evolution
The discovery could have implications beyond the study of 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 also create opportunities for research 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 look beyond traditionally studied animals and include species with unusual adaptations or simpler nervous systems. Such comparisons could show whether particular sleep mechanisms are widespread throughout the animal kingdom or evolved independently in different lineages.
Despite decades of intensive research, many basic questions about sleep remain unanswered, including important mysteries surrounding human sleep.
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 are also comparing spiders observed in their natural environments with those studied in the laboratory to determine how surroundings 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 or other mental processes.
Environmental change is also a focus. The team plans to study how climate change and human activity, particularly artificial light at night (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 providing new clues about one of biology’s enduring mysteries.
Source: www.sciencedaily.com