Do Spiders Dream Like Humans Do? This Researcher Wants to Find Out

Source: Do Spiders Dream Like Humans Do? This Researcher Wants to Find Out., Scientific American, 8:30, uploaded 2024-10-30, category Science, playlist index 497.

Jumping spiders have eight eyes, including two large forward-facing eyes. They stalk prey and pounce on it, then turn towards anything that moves to investigate. That response gives the impression that the spider is looking back. Daniela Rößler studies what happens when the same animal stops moving and goes to sleep.

The sleeping spiders on the windowsill

Rößler is a behavioural ecologist whose work begins with a general question about sleep. Sleep supports learning and memory formation, and it carries health effects across animals. She points out that the range already includes jellyfish, which have no centralised brain in the usual sense. The history of sleep itself remains unclear: researchers have little account of where it began, how its parts evolved, or how sleep relates to sensory processing.

Most sleep research uses different methods for different species. Rößler wants to compare spiders with the same methods so that the differences become meaningful. The project began during the pandemic, when she could not leave home to collect a new study animal. She found Evarcha, caught several, and kept them in boxes on a windowsill.

One night she looked into the boxes and found every spider hanging from the lid on a silk thread. The position appeared only at night, although these spiders are active during the day. That pattern suggested a sleep behaviour. The spiders also twitched at regular intervals of roughly twenty to thirty minutes, which recalled the movements people see in sleeping dogs and cats.

Baby spiders made the next observation possible. Their bodies are transparent enough for researchers to see the retinal tubes. The twitches occurred with movements in the retinas, giving Rößler’s team a reason to consider REM-like sleep.

Behaviour before brain recordings

Rößler describes REM sleep as a state in which the muscles become inactive whilst the brain remains active. Animals with movable eyes show eye movements, and they can show muscle twitches that do not follow a controlled movement. The same pattern has been documented in mammals, birds, reptiles, and cephalopods. The spider’s behaviour resembles that pattern, although resemblance alone does not establish the state.

Direct recordings are difficult. A spider’s hydraulic pressure makes the usual approach to brain activity hard to use, so the team turns to behaviour. Animals in REM sleep tend to need a stronger stimulus before they wake and react. Reduced responsiveness during the spider’s leg-curling period would therefore support the claim that the period is REM-like.

The team first looks at web spiders in the field. They use a sound that matches the wing-beat frequency of wasps and bees. The common orb-weaver Araneus diadematus responds to that sound with an anti-predator signal. During the night, when the spider is immobile, the stimulus has to be much stronger before it wakes and reacts. That change gives the researchers a behavioural measure for the state they cannot yet inspect inside the brain.

The lab work turns to Portia, a jumping spider that hunts other spiders. It changes its hunting strategy according to the kind of spider it is approaching, which makes it useful for later tests of cognition. The researchers let the spiders fall asleep in small vials, place the vials on top of a speaker, and wait for the REM-like behaviour before playing the stimulus. The video shows a spider sensing the sound and beginning to move. Rößler presents this as an early stage of the work, with the separate observations still being fitted together.

REM-like behaviour and the question of dreams

The footage invites a quick conclusion. A twitching spider looks as though it is dreaming, especially because people associate REM sleep with dreams. Rößler draws a firm line between those ideas. Researchers can study human dreams through dream reports. A spider cannot report what it experienced, so the evidence can support a REM-like state without proving that a dream occurred.

She also treats the limit as part of a larger evolutionary question. Complex behaviour has deep histories, and human versions of it did not appear without predecessors in other animals. A spider’s sensory world differs so much from ours that researchers have no way to know what a spider’s dream would feel or look like. Rößler imagines prey capture or escape from a predator as plausible content because those events organise the spider’s waking life. A form of dreaming could also serve an ecological purpose, although the video presents that as a possibility to investigate.

The study has already widened beyond the first animals. Since the paper was published, Rößler says the team has examined about fifteen other spider families and found the same REM-like behaviours in all of them. She sees a pattern that may prove widespread across spiders, alongside a large gap in knowledge created by the simple fact that researchers had not looked closely before.

Limits of the evidence

The video follows a research programme in progress. The hanging posture, retinal movements, leg curling, and reduced responsiveness support the description of REM-like sleep. They do not establish brain activity in the strict physiological sense, and they supply no route for proving a spider’s subjective dream. The source gives no paper title, sample sizes, measurements, or statistical results. Its account of fifteen spider families remains Rößler’s report within the film. The durable claim is narrower and more interesting: spiders offer a way to test how sleep-related behaviour evolved across animals whose bodies and sensory systems differ sharply from ours.