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SCIENCE OBSERVER

The Scent of a Spider

Noah Eisenkraft

Jane was a solitary sort of girl. She didn't like boys. Didn't want them bothering her. They'd approach sometimes, but she knew the right signals. A little shake here, a certain way of standing—most boys could tell she wasn't interested.

Of course, one day she'd want a family. "Won't be too hard to come by," she thought. "I'll just put on some perfume and wait. The boys'll show up—guaranteed."

A few days back, some hot-blooded stud thought he could just barge into her life. Fat chance. Without a second thought, Jane killed and ate her would-be suitor.

Jane is not a psychopath or a character from a Stephen King novel. In fact, she's just like all the other desert spiders her age. This stereotypical spider personality presents a puzzle for animal behaviorists. How does a male spider know when a female spider is, to put it bluntly, in the mood? A mistake in the male's judgment doesn't mean a night on the couch—it may mean his last night on earth.

Male and female spiders mate . . .Click to Enlarge Image

The long-standing consensus among investigators was that would-be mates communicate their intentions through vibratory and visual signals. Spiders use this form of communication everywhere in their lives. If they want to talk through their web or scare rivals from their territory, spiders use vibrations and visual displays. Surely, the researchers assumed, mating is no exception.

Observations seemed to support this hypothesis. Before mating, both male and female desert spiders engage in complex courtship rituals. The male's ritual involves 21 unique leg-waving, abdomen-swinging, web-plucking steps evolutionarily proven to win him the heart of his favorite arthropod.

Susan Riechert of the University of Tennessee, and Mirjam Papke and Stefan Schulz, both with the Technische Universität Braunschweig, disagreed with this speculation. Research they presented in the May issue of Animal Behaviour claims that, when it comes to mating, the type of desert spider they studied primarily communicates with chemicals. When a female desert spider is ready to mate she secretes a pheromone, 8-methyl-2-nonanone, that drives male spiders wild.

In the investigators' experiments, whenever male desert spiders were presented with a pheromone-soaked paper disk, male desert spiders waggled, vibrated and preened. The males quickly initiated and performed most of the steps in their complex mating ritual. But when males were placed before a paper disk lacking any pheromone, they did not waggle or vibrate—they weren't the least bit aroused.

But how extensive is the pheromone's role? Male spiders near a pheromone-soaked disk also spent a significant amount of time trying to locate the female spider—never engaging in a subset of the courtship ritual steps (lunge and retreat). Only when a pheromone-producing, virgin female spider was placed on a dry paper disk did the male perform his courtship ritual with all its nuances—a performance that was reciprocated and well received. The research team concluded that the pheromone helps male spiders locate the female and stimulates most of the courtship sequence.

The complex mating system of the desert spider is not easily reduced to a simple stimuli/response mechanism. True, when standoffish Jane wants a man's attention she need only put out a scent. But if she wants more than just his attention, she too will have to play her part. And would-be suitors beware—if the male is too thin or obviously between webs ? well, let's just say that female spiders have standards. And when those standards aren't met, female spiders have lunch.—Noah Eisenkraft


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