Choreographed Web-Building Routines Showcase Spiders' Architect Tendencies
Courtesy of Abel Corver
(Inside Science) -- Like any good architect, an orb-weaving spider builds its residence in stages. Unlike human architects, however, they start without the final plan in mind. It's a strategy that always works for them, almost as if spiders are born knowing only the steps of a choreographed dance -- no pattern is revealed until it's finished. Scientists have observed the same structural progression in this construction process with many spider species, though elements such as silk thickness and location vary greatly.
After centuries of fascination with the 2D webs created by a group of spiders known as orb-weavers, scientists are working to piece together what the pages of that intuitive instruction manual look like, and to understand what they can teach us about the animal mind. To do this, a group of researchers catalogued and indexed the movements spiders use to build their webs and published the results in the journal Current Biology.
Web-building is just one example of a particularly complex feature of animal cognition, easy to study in spiders because of their relatively small brain size. In studies of the animal mind, "a lot of work has focused on single behaviors, like a rat tapping a lever, or an animal engaging in sexual activity -- single action sequences," said Andrew Gordus, a biologist at Johns Hopkins University in Baltimore who oversaw the study. "But we know that animals are capable of organizing [action sequences] in a higher order way."
Whatever the cognitive processes are behind these multistage behaviors, it's likely that they're mirrored across many categories of animals, Gordus said. However web-building is encoded in the spider brain, it probably looks a lot like how dam-building is encoded in a beaver's brain, or nest-building in a bird's brain.
For a spider, that action sequence manifests as distinct stages of web-building. "The geometrical differences between these stages have been observed and are very striking," said Abel Corver, a graduate student at Johns Hopkins and the lead author of the study. If spiders built their similar-looking webs by making the same series of small movements, Corver thought, it would suggest a pretty complex algorithm at work in the spiders' brains.
When it finds a place to crash for the night, a spider begins by pitching a proto-web, a rudimentary structure believed to be a sort of test run. It's a warmup to the dance, a space-exploring exercise guaranteeing that the rest of the movements can proceed as needed. Using the proto-web as a guide, it forms a frame with radii reaching the center, where it will then wind a first spiral back out to the edges. The final stage is the creation of the capture spiral, the sticky or stringy bit in the middle designed to catch prey. At any moment during the process, a spider could be using its front legs to probe or walk along threads or its back legs to dispense silk at various speeds.
In order to track the specific movements common to each stage, the research team trained a computer to analyze long videos of web-weaving while tracking the movement of spider limbs. Using a tool known as a convolutional neural network, the team collected an "alphabet" of individual actions that the spiders rely on or repeat as they move through web-building stages.
It's the first time the web-building choreography has been catalogued step by step, bringing precision and detail to a body of work studying web architectures that first began in the 1980s and '90s "What this study is doing is basically putting a 21st-century spin on that type of research," said Todd Blackledge, a biologist at the University of Akron in Ohio who was not involved with the study.
Matching each "dance step" to a physical part of the web in a one-to-one fashion allows researchers to begin asking an even bigger question that has implications for animals of all sizes: How are these steps stored and executed by the brain? Compared to how humans build, this deeply coded ground-up choreography is a "very different approach," said Blackledge. "But it's probably more or less ubiquitous in the evolution of all sorts of different animal structures, whether you're thinking about spider webs, bird's nests, or termite mounds." Understand how spiders learn and perform the web-building dance, and you're likely to discover a cognitive pattern that's echoed across the animal kingdom.
Studying this new "alphabet" in more detail, Gordus said, could include altering or knocking out genes to understand where this complex choreography originates from within the genetic code. Another approach is to study how external cues such as the web's stability or the temperature outside can change or interrupt the spider's motions. Previous research has identified plenty of factors that influence how webs are built. When spiders are hungrier, they make stickier webs. Older spiders tend to make webs with more irregularities. In zero gravity, they use light to orient their webs. But the basic choreography for orb-weaver web-building have remained the same.
This new "alphabet" of actions could also be used to compare orb-weavers' movements with those of spiders who spin different styles of webs, such as the false widow spider's 3D cobweb. The researchers hope to uncover a common evolutionary blueprint that all animals who engage in multistep building actions share.
Gordus hopes it might even help people appreciate spiders a bit more.