Do Spiders Think?
Spider webs contain clues to a spider's decisions.
Posted Mar 07, 2019
Spider webs can be beautiful, intricate, and deadly—but what can they tell us about the spiders who build them?
There are over 3,100 species of orb-weaving spiders, which range in size from one to several centimeters. They are commonly found in gardens, fields, and forests, where they construct spiral wheel-shaped webs.
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.
“This kind of flexibility begs the question of whether the adjustments result from pre-programmed instructions and are executed automatically or whether they result from more complex processes such as ‘insight’ or ‘understanding,’” says William Eberhard, a scientist at the Smithsonian Tropical Research Institute.
Orb webs consist of a series of concentric circles starting from the smallest at the center, known as the hub, and radiating outward. The circles are divided into sectors by lines of silk, like slices of a pie. When building a web, the spider first builds up the framework with nonsticky silk before adding a final sticky line that spirals inward from the outer edges of the hub.
Interestingly, orb weavers complete this process blindly: They cannot see the lines in their web and typically construction takes place in the dark. Instead, they rely on their sense of touch.
“The spider taps with one leg, like a blind person with a cane, toward the site where the inner loop of sticky line is likely to be,” says Eberhard. “Once it touches this line, it immediately turns to attach the new line it is producing and then moves on to find the next radius and make the next attachment. The inner loop thus serves as a type of guard rail while the spider builds additional loops of sticky line.”
Working gradually inward from the outer edge of the orb and using this tactile cue, the spider avoids crossing the previous sticky loop. This can be thought of as the ‘do-not-cross rule.’
The positions of the lines in a web constitute concrete, precise records of hundreds of behavioral decisions by a spider. By tracing the lines a spider makes, it is possible to deduce when it fails to use the do-not-cross rule.
In a new paper, Eberhard takes advantage of these aspects of orb webs to evaluate behavioral flexibility in two species of orb-weaving spiders, Zosis geniculata and Uloborus diversus. He analyzed when spiders used the do-not-cross rule when building new webs from scratch and when making repairs to existing webs.
Eberhard shows that these spiders are flexible in whether they use this rule to guide sticky spiral construction, especially when making repairs.
“Normally, they very seldom broke the rule, only once every several thousand decisions,” says Eberhard. “But they routinely broke it when they found themselves ‘painted into a corner,’ with a large portion of the web lacking sticky lines and the only way to reach that portion was by breaking the rule.”
By occasionally ignoring the do-not-cross rule, spiders gained access to otherwise inaccessible portions of their webs. For instance, Z. geniculata spiders repairing webs with a single damaged sector never broke the do-not-cross rule, but those repairing two- or three-hole webs broke the rule about half the time.
Thinking Like a Spider
Traditionally, behavioral flexibility in a tiny animal like an orb weaver has been ascribed to either learning or pre-programmed, innate instructions. Learning is unlikely for several reasons, including the often substantial time elapsed between construction decisions and prey impact with the web, which makes linking the events difficult.
Neither can the pre-programmed instructions hypothesis explain the observations in this study. Temporarily violating the do-not-cross rule was only one of several different ways in which spiders entered otherwise inaccessible portions of their webs during sticky spiral construction. Additionally, the details of the situations when the rule was broken and the variation of behavioral adjustments made (for instance, the sharpness of turns), also suggest that an innate response is unlikely. There would have to be many different pre-programmed behaviors to deal with the many different contexts in which the rule was broken.
“The results lead me to suspect that these spiders have some kind of ‘understanding’ (though obviously not exactly what a human experiences) of the physical location of sites in the orb where there are open spaces not yet covered with sticky lines and of how to reach such spaces by breaking the do-not-cross rule that otherwise strongly influences where they lay sticky lines,” says Eberhard.
As Eberhard writes in the paper, “At some point, it becomes less convincing to invent more and more complex explanations based on pre-programmed responses than it is to suppose that the animal is using simple cognitive abilities to solve problems.”
The biggest challenge—and also the most exciting aspect of his work, according to Eberhard—is working with an animal whose sensory world is so different from ours.
He shared one particularly pleasing experience he had watching a female orb weaver apparently try to start an orb. He understood her behavior so well that he could perceive what she was trying to do and directly help her. On a windy morning, with few supports nearby, the spider made repeated descents and then ascents on a single line after she had built only a single horizontal line. Eberhard interpreted these behaviors as fruitless attempts to find additional support for her web by floating a new line.
“I got to be one of a very select group of human beings to participate directly and knowingly in the construction of an orb web by grabbing the loose tip of the line that she was producing and snagging it onto a nearby support,” he says.
“The spider immediately walked along the line to this support, strengthened the attachment, and then immediately began the next steps in building an orb.”
For Eberhard, it is rewarding to be able to deduce what a spider did and why it did it (i.e., the cues that it probably used) from photographs of orb webs. His work suggests that the traditional assumption that spider behavioral flexibility is pre-programmed is not correct.
“The big surprise in this paper is that such a small, ‘simple’ animal could have anything even approaching insight or understanding,” he says.
Eberhard, W. G. (2019). Adaptive flexibility in cues guiding spider web construction and its possible implications for spider cognition. Behaviour 156(3-4). DOI: https://doi.org/10.1163/1568539X-00003544.