Spiders, like octopuses, have eight legs. But they share something else as well — like octopuses, once we got around to studying them, they turned out to be much smarter than expected. What makes spiders even more unusual is that they are smart with very small brains:
“There is this general idea that probably spiders are too small, that you need some kind of a critical mass of brain tissue to be able to perform complex behaviors,” says arachnologist and evolutionary biologist Dimitar Dimitrov of the University Museum of Bergen in Norway. “But I think spiders are one case where this general idea is challenged. Some small things are actually capable of doing very complex stuff.”
Behaviors that can be described as “cognitive,” as opposed to automatic responses, could be fairly common among spiders, says Dimitrov, coauthor of a study on spider diversity published in the 2021 Annual Review of Entomology. From orb weavers that adjust the way they build their webs based on the type of prey they are catching to ghost spiders that can learn to associate a reward with the smell of vanilla, there’s more going on in spider brains than they commonly get credit for.Betsy Mason, “Spiders are much smarter than you think” at Knowable Magazine (October 28, 2021)
There are some other life forms where the idea that brain size is a guide to intelligence is challenged. For example, lemurs, with brains 1/200 the size of chimpanzee brains, can pass the same IQ test.
But, as Mason goes on to report, jumping spiders, for example, can show very clever hunting behavior. One group of jumping spiders, Portia, lures female spiders of another species (Eurytattus) to their deaths by mimicking the way a courting male spider shakes her nest and then attacking. They also attack web-building spiders by mimicking the tug on the web of a trapped insect, adjusting its tug to the size of the spider it plans to devour. More remarkably,
If these strategies don’t work on a particular web spider, another of Portia’s tricks is to shake the whole web so it moves as if a gust of wind had hit it. This acts as a smokescreen for the vibration Portia makes as it crawls into the target spider’s web. In laboratory experiments, Jackson found that Portia will try different plucking methods, speeds and patterns until it finds just the right combination to fool each individual web spider it hunts — essentially learning on the job.Betsy Mason, “Spiders are much smarter than you think” at Knowable Magazine (October 28, 2021)
Portia spiders have large brains — relative to other spiders — but that probably isn’t the whole answer to how they develop so many clever strategies. Organization of the brain is likely a factor too.
Another recent discovery is that the bridge spider, Larinioides sclopetarius, turns its web into a giant ear, to hear across great distances:
The bridge spider uses its web as an engineered “external ear” up to 10,000 times the size of its body, according to a preprint study posted to bioRxiv on October 18. The discovery, which has not yet been peer reviewed, challenges many assumptions that scientists have held for years about how spiders and potentially other arthropods navigate and interact with the world around them.
“Evolutionarily speaking, spiders are just weird animals,” Jessica Petko, a Pennsylvania State University York biologist who didn’t work on the new study, writes in an email to The Scientist. “While it has been long known that spiders sense sound vibration with sensory hairs on their legs, this paper is the first to show that orb weaving spiders can amplify this sound by building specialized web structures.”Dan Robitzski, “Spider Uses Its Web Like a Giant Engineered Ear” at The Scientist, (October 29, 2021). The paper is open access.
Some spiders have also mastered the art of hunting in packs:
Spiders that hunt in packs use web vibrations to coordinate their attacks, allowing them to kill prey hundreds of times larger than they could on their own.
Of the 50,000 known spider species, just one or two hunt as a group, with thousands of individuals spread across webs that can span several cubic metres. When prey insects land on their web, the spiders synchronise their attack, moving as one to catch animals up to 700 times heavier than an individual arachnid.Christa Lesté-Lasserre, “Social spiders synchronise to catch prey hundreds of times their size” at New Scientist (March 7, 2022) The paper is open access.
When prey land, all the spiders stay still, to distinguish the vibration of the prey from the movement of other spiders, then they all charge. It’s still not clear exactly how they manage it.
Here is a spider using silk to hoist a shell up a tree:
Some analysts suggest that, with web-spinning spiders, it is best to see the web as part of the spider’s “mind,” which complicates the intelligence picture — and yet clarifies it in a way:
Spider intelligence isn’t about abstractions or moral choices; it’s about killing and eating other life forms. What’s spooky is how many different strategies they have developed for doing that.
Anyway, spiders are smarter than we used to think and we are only beginning to discover what they use their tiny brains for.
You may also wish to read: In what ways are spiders intelligent? The ability to perform simple cognitive functions does not appear to depend on the vertebrate brain as such. (Denyse O’Leary)
How do insects use their very small brains to think clearly? How do they engage in complex behavior with only 100,000 to a million neurons? Researchers are finding that insects have a number of strategies for making the most of comparatively few neurons to enable complex behavior. (Denyse O’Leary)