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Jumping spider close up. Macro photography. Portrait of spider

In What Ways Are Spiders Intelligent?

The ability to perform simple cognitive functions does not appear to depend on the vertebrate brain as such

This year saw a huge uproar in science media over claims that prominent researcher of spider behavior Jonathan Pruitt, a behavioral ecologist currently at McMaster University in Canada, had fabricated evidence of complex, seemingly intelligent behavior in spiders.

After many colleagues recently raised concerns in blogs and tweets that behavioral ecologist Jonathan Pruitt had fabricated the data behind a slew of provocative results regarding animal personalities and social spiders, he denied the charges, saying any problems were inadvertent mistakes.

Elizabeth Pennisi, “Embattled spider biologist seeks to delay additional retractions of problematic papers” at Science (March 12, 2020)

Some of Pruitt’s data management mistakes seem rather curious:

More than 20 scientists — co-authors, peers and other interested observers in the field — mobilized to pore through the data in almost 150 papers on which Pruitt is a co-author, looking for evidence of manipulated or fabricated numbers. They found similar signs of copy-and-paste duplications. In at least one instance, researchers identified formulae inserted into a published excel file, designed to add or subtract from a pasted value and create new data points.

Giuliana Viglione, “‘Avalanche’ of spider-paper retractions shakes behavioural-ecology community” at Nature (February 7, 2020)

At Nature the episode has been described as the unraveling of a complex web. For one thing, the 17 papers under question (some now retracted) have been cited over 900 times. Clearly, it was news many wanted to believe.

What’s really remarkable about this story is not actually the claims for spiders but the skepticism, the minute examination of data, that followed. Many in in animal behavior science today very much want to believe that human-like intelligence, even abstract thought, can be attributed to non-human life forms. Such a find is bound to be seen as a long overdue blow to “human exceptionalism,” the idea that humans are something more than just big-brained primates.

As a result of this mood, researchers who seem to have overinterpreted animal subjects’ behavior were generally indulged. That may well have happened with, for example, with Sue Savage-Rumbaugh and her bonobos and John C. Lilly and his dolphins. And also with a number of researchers who have attempted to teach apes to converse.

But then we are talking about… spiders here, not apes or dolphins. What had Pruitt claimed that “rocked the field” and attracted attention to irregularities?

He was testing individual differences in spider behavior. Spider behavior is much more complex than we used to think.

If Pruitt made merry with the facts in his own research, it appears that he’ll answer for it. But here are some better attested findings that help us understand the spider’s world:

First, there is a difference between what we humans understand as thinking or consciousness and the ability to process complex information efficiently. This distinction has not always been recognized clearly in the past, as a science writer discussing spiders that spin webs (orb weavers) acknowledges:

Early naturalists concluded that orb web construction was highly stereotyped. They noted that these spiders do not need any experience or learning to build perfect orbs and the overall order in which different portions of the orb are built are invariable.

This view of orb weavers as small automatons, unable to adjust their pre-programmed behavior to altered circumstances, prevailed for a long time. However, more recent research has revealed orb weavers to be flexible in many ways. They adjust their web-building behavior to different types of stimuli, including their supply of silk, the size of the open space available in which to build, the orientation of the web with respect to gravity, the wind, and the spider’s own size and weight.

Mary Bates, “Do Spiders Think?” at Psychology Today (March 7, 2019)

Generally, the spiders work in the dark, using their sense of touch for weaving. They follow a rule of not crossing the just-woven sticky loop. But, as William Eberhard of the Smithsonian Tropical Research Institute found, they don’t follow that same rule when making repairs to the web. He attributes spiders’ decision-making flexibility to “simple cognitive abilities.”

The senses of web-weaving spiders bear little relation to ours. They catch their prey by using the web as a sonic communication device, according to a recent paper:

They weave their webs to catch prey and attract mates. But while humans can generally spot an insect trapped in a web, spiders—which have extremely poor eyesight—can’t.

Instead, they appear to rely on vibrations from the silk strands to locate prey and find deformities in the web. As a spider plucks and pulls on the web, sending out ripples in every direction, it can sense the vibrations in each of its eight legs.

Carrie Arnold, “Spiders Listen to Their Webs” at National Geographic (June 5, 2014)

We are told that web weavers can sense movements of a hundred nanometers, which is 1/1000 of a human hair’s width.

Ronald R. Hoy, Cornell University professor of neurobiology and behavior, considers the spider “one of the smartest of all invertebrates.” But while its behavior is comparable to that of many vertebrates, its anatomy is not:

Dr. Hoy and his colleagues wanted to study jumping spiders because they are very different from most of their kind. They do not wait in a sticky web for lunch to fall into a trap.

They search out prey, stalk it and pounce. “They’ve essentially become cats,” Dr. Hoy said.

And they do all this with a brain the size of a poppy seed and a visual system that is completely different from that of a mammal: two big eyes dedicated to high-resolution vision and six smaller eyes that pick up motion.

James Gorman, “Unexpected Complexity in a Spider’s Tiny Brain” at New York Times (November 3, 2014)

One curious thing Hoy’s research team discovered is that a single neuron, not the expected complex network, integrates the information from the two independent sets of eyes.

Another new study provides evidence that jumping spiders can plan their attacks:

In the 1980s and 1990s, Robert Jackson of New Zealand’s University of Canterbury demonstrated that Portia fimbriata, a member of this spider-snacking subfamily, methodically plans winding detours to sneak up on prey spiders. Portia can even find hidden prey, suggesting that the predator can visualize its prey’s location and a path to get there.

Michael Greshko, “Jumping Spiders Can Think Ahead, Plan Detours” at National Geographic (January 21, 2016)

Like many life forms, spiders also have a basic sense of low, one-digit numbers:

Fiona Cross and Robert Jackson at the University of Canterbury in New Zealand discovered that jumping spiders of the species Portia africana display a sort of numerical competence.

Cross and Jackson’s paper in the journal Interface Focus, from the Royal Society, explains that these spiders of the family salticidae, which hunt other spiders for food, were “proficient at distinguishing between numbers of prey in the range of 1-3, but apparently not proficient at making the discriminations 3 versus 4 or 3 versus 6.”

Barbara J. King, “What Goes On In The Minds Of Spiders?” at NPR (June 8, 2017)

So, do spiders think? Yes, in the simple sense that many vertebrates do. That has come as a surprise because we simply assumed that arachnids and insects would not have the same mental capacities as vertebrates. But the ability to perform simple cognitive functions does not appear to depend on the vertebrate brain as such. It seems that we have a great deal to learn yet about the relationship between types of brain and the types of intelligence they can mediate.

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Denyse O'Leary

Denyse O'Leary is a freelance journalist based in Victoria, Canada. Specializing in faith and science issues, she is co-author, with neuroscientist Mario Beauregard, of The Spiritual Brain: A Neuroscientist's Case for the Existence of the Soul; and with neurosurgeon Michael Egnor of the forthcoming The Human Soul: What Neuroscience Shows Us about the Brain, the Mind, and the Difference Between the Two (Worthy, 2025). She received her degree in honors English language and literature.

In What Ways Are Spiders Intelligent?