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The Case of the Missing Perfume

For a female Sierra Dome Spider, a web serves as a safe haven from predators and a way to catch a meal—until her last molt. When she sheds her skin for the last time, the female becomes sexually mature and her web becomes a trysting bed.

If the female lives in a dense population—a spider city, so to speak—she’ll have no need to actively attract a mate. Many males visit her web, testing her mood by touching her body. Most of the time she shoves them away. Just before she molts, however, the female signals her immanent maturity by ceasing her rebuffs. One masterful male then begins a vigil: days of fending off rivals. The male who holds the fort when the female finally molts will mate with her and fertilize most of her eggs. All this jousting gives the male a mighty appetite, and he eats most of the prey caught on the female’s web. Fortunately the female fasts before molting.

If males are scarce, the female Sierra Dome spider turns her web into a scented hankie to wave in the air. Applying an attractive scent to her web, the virgin female sends a powerful sexual signal far and wide. That’s good for the female, who wants to attract many males. But what of the male who responds? His interest lies in being her only mate and in avoiding conflict. A flock of suitors suits him not at all.

Furthermore, copulation in Sierra Dome spiders is a complicated affair lasting up to seven hours. Unlocking their complex sex organs once they’ve begun can take a full minute. An interrupting rival male, also attracted by the female’s scented web, could spoil the mating male’s whole day. So the male silences the signal by wadding up the web. This behavior, called “web reduction” results in a tightly packed ball of scented silk that the male may cut from the web and drop to the ground. If he has acted quickly enough, he mates immediately. If a rival male arrives, however, the two males may fight, with the winner completing the web reduction before mating.

To nail down whether the male’s web web reduction behavior stems from sensing a female pheromone emanating from the web or from the female herself, arachnologist Paul J. Watson, then of Cornell University, made an extract of 84 webs of mature virgin females. He applied the love potion to an unoccupied web and introduced a male. Within a minute the male wadded up just that portion of the web Watson had sprayed. The Sierra Dome spiders not only send and receive complex signals tuned to the circumstances, but the male also interrupts the female signal.

Original text 1996, updated 2004

More photos of Sierra Dome Spiders and webs on Olympic Natural History