Webs of Perception
Understanding the tangled science of how spiders “outsource” processing power for their tiny brains
It makes intuitive sense to humans that the bigger the brain, the more intelligent the animal. Humans, chimps, gorillas, dolphins and whales all are good examples of intelligence powered by large, complex bundles of grey matter. But this bigger-is-better assumption might be a significant oversimplification. Some recent studies suggest it would be worthwhile focusing on intriguing examples at the other end of the scale.
Most people are familiar with the way bees perform near-miraculous feats of exploration and communication using fewer than a million neurons (we humans have 86 billion). Spiders present another fascinating example. For instance, jumping spiders, those that hunt down their prey, have impressive mental skills. One species in particular, Portia africana, attacks other spiders and insects and is known to have the ability to spot a target, then advance to a position from which it can attack, even if that approach takes the prey out of sight momentarily. Apparently, it holds that information in its tiny brain (which, at about 600,000 neurons and smaller than the head of a pin, merits the word tiny).
Some of the latest experiments with Portia have shown that it can, in a sense, count. If scientists place a single edible spider in its view, it will unhesitatingly choose an attack route that briefly obscures its view. However, if by the time the prey is back in sight, one or two extra spiders have joined the original, Portia will pause. The numbers don’t jibe. These experiments suggest Portia can keep track of one, two and many — a feat that demands mental skills that seem unlikely to emerge from a brain that size.
Comparing spiders whose brains vary widely in size also suggests smaller brains are no handicap. All spiders make mistakes when weaving webs, but small-brained species are no more error-prone than their large-brained cousins.
In a paper published in early 2017, biologists Hilton Japyassú and Kevin Laland suggested that spiders use “extended cognition,” or, in their words, are capable of “outsourcing information processing to the body or environment.”
What comes to mind immediately is storing your grocery list on your cellphone rather than in your frontal lobes, but there are more vivid examples. Octopuses, renowned for their smarts, have dispersed their central nervous system throughout their bodies: their brains contain only about two-thirds of their neurons, the rest residing in the eight arms. Those arms make decisions on their own, like deciding where to bend the arm so that the sucker holding a food item is brought directly to the animal’s mouth. But at least the octopus “brain,” scattered though it may be, still resides within the animal’s body. That may not be the case with spiders.
Japyassú and Laland contend that one of the issues confronting very small animals is how to maintain their smarts. It’s a complex problem. Even though as the body shrinks the brain volume does too, the brain occupies a progressively larger percentage of total body volume, according to a biological principle called Haller’s Rule. How to cram this organ into the head and how to fuel it as it becomes proportionally larger become serious challenges. In some spiders, like the tiny Anapisona simoni, the brain even overflows into the legs! And reducing the size of the brain’s neurons, while helpful, can only go so far.
The problem is particularly acute for spiders, which, as predators, must execute elaborate hunting manoeuvres. Even weaving a web that does the hunting for them is a demanding task. But there’s more to it: a spider can adjust the tension of the web threads to tune the web to detect larger or smaller prey. It’s a two-way street, because as the spider’s brain adjusts the web, the web feeds different information to the spider. That is extending cognition beyond the body.
It’s tempting to argue that the web, however beautifully designed and responsive it is, is a tool just like a chimpanzee’s termite-probing branch, or even a beaver’s dam. But Japyassú and Laland maintain that the key to an extended cognitive system is that it must work in both directions. The termite stick feeds information to the chimp but not vice versa. The dam itself isn’t communicating with the beaver; the animal is adjusting the height and composition of the dam depending on the materials available and the water level. In contrast, the web itself is informing the spider just as our fingertips inform us about textures they’re touching. We squint or cup our hand behind an ear to heighten our senses; the spider tweaks its web.
As strange as the idea is that we humans might one day enhance our own cognition by embedding a computer chip, stranger still is the notion that we are way behind the curve when compared with spiders.
AS THE SPIDER ADJUSTS ITS WEB, THE WEB FEEDS INFORMATION TO THE SPIDER. AMAZINGLY, IT IS EXTENDING COGNITION BEYOND ITS BODY