Home & Garden Columns

Wild Neighbors: Orbweaver Brains: Is Bigger Always Better?

By Joe Eaton
Tuesday July 31, 2007

About the time of year the robins wind down and the naked ladies begin to bloom, we start seeing the garden spiders. They’re orbweavers, probably Araneus diadematus, and at this stage they’re just little orange-and-black specks with legs. Between now and Halloween they’ll get a lot bigger, and plumper. 

The garden spiders take over the garden, of course, but they don’t confine themselves to it. Most years we have a clutch of them on the front porch, anchoring their webs to the railings. They often get into the car, and have to be delicately removed. Once we woke up to find a sizable web across the back door. Some might find that ominous. I live with an arachnophile, though, and I’ve come to terms with them. 

The large conspicuous spiders are females. The males would be smaller and more furtive. It’s not clear what arrangements the garden spiders make, but males of a related species, the black-and-yellow agriope, spin their own webs at a safe distance from the females’. The males’ webs are shoddily constructed and often littered with beer cans and pizza boxes. 

But a female garden spider’s web is an architectural marvel—all those precisely arranged spokes and spiral struts. A typical web has 25 to 30 radial threads forming regular 12-to-15-degree angles. The younger the spider, the more threads. The center of the web is stickier and has closer-spaced spirals. From that hub she monitors the web for tremors that announce an arriving insect, holding on to a signal thread.  

Webs are delicate things, subject to damage from struggling prey. Rather than patching up the old web, a spider begins her day by eating whatever’s left of it—thus conserving the silk proteins—and spinning a new one. 

Webmaking is a complex act, and it’s hard to see how a spider’s minuscule brain can hold all the necessary programming. A spider’s central nervous system consists of a pair of ganglia—clusters of neurons—that are wired to its muscles and sensory systems. You wouldn’t expect a lot of bandwidth there. 

What about a really small spider? William Eberhard of the Smithsonian Tropical Research Institute and the University of Costa Rica was interested in the tradeoffs that a miniaturized spider brain would require. Brains are metabolically expensive to run, and Eberhard expected that small-brained organisms would have to jettison some of their behavioral repertoire. Webmaking precision seemed a likely candidate. 

So Eberhard chose five Costa Rican orbweaver species, ranging in weight from the 50-milligram Leucage mariana to the tiny Anapisona simoni that tips the scales at .005 milligrams. That’s five orders of magnitude’s difference. The smaller species, he says, had descended from larger ancestors, downsizing their brains along the way. 

He measured the webs of each species, expecting the smaller spiders to be sloppier webcrafters. To the contrary, there was no loss in precision with decreasing size. Eberhard speculated to a New York Times reporter that the smallest spiders “have done something subtle or special with the neurons in their brain to be able to do the same behavior that larger ones can.” 

That’s an almost heretical thought. We all know big brains are best, right? Primates, cetaceans, corvids all have higher brain-to-body mass ratios than other mammals and birds. Even octopi and squids, arguably the most intelligent invertebrates, are large-brained for mollusks.  

I can think of only a few other instances of brain shrinkage in the evolutionary process. Slender salamanders, wormlike creatures that are probably hiding under the leaf litter in your back yard, have smaller and less complex brains than their ancestors. But the lifestyle of these sedentary amphibians doesn’t require complex behavior. A slender salamander that travels more than ten feet in its entire lifetime would be exceptional. 

Somehow, though, arachnids have found a way of shrinking the brain (along with the body) without losing key behavior capabilities: some kind of compromise between size and connectivity. Whatever they’ve done, it’s one of the neater evolutionary tricks. 



Photograph by Ron Sullivan: A female garden spider waits for visitors. 



Joe Eaton’s “Wild Neighbors” column appears every other Tuesday in the Berkeley Daily Planet, alternating with Ron Sullivan’s “Green Neighbors” column on East Bay trees.