Bigger, scarier spiders always get the girl

The bigger a male jumping spider’s tool seems, the more likely his rival will slink away quietly, leaving the larger guy a clear path to the waiting female.

Duke University graduate student Cynthia Tedore, working with her dissertation advisor, visual ecologist Sönke Johnsen, wanted to know what visual signals matter most to magnolia green jumping spiders.

Vision is clearly important to these quarter-inch animals, which can be “very predaceous (predatory) aggressive” when love is in the air, the journal Behavioral Processes reports.

Tedore’s lab is lined with wire shelves covered with row after row of Lucite boxes, each holding an individual chartreuse jumping spider, according to a Duke University statement.

In pairs, a group of males squared off for 10 minutes in “the arena”, a box festooned with female silk to put the males in a fighting frame of mind.  Over the course of 68 of these cage matches, the male with the bigger chelicerae (heavy, bristling fangs hanging in front of their mouth parts), usually scared the other guy off without a fight.

“The males wave their forelegs at each other for a period, and then the smaller male runs off,” Tedore said.

“That’s why we think they’re using vision to size each other up. Most of the time, the smaller one will run away before it comes to blows.”

Seven of the matches were scored as ties. Seventeen of the contests turned into shoving matches, with the spiders butting chelicerae against each other. Occasionally one would flip an opponent on his back, then chase and pounce on him. Tedore had to break up a couple of contests so that nobody got hurt.

Tedore said her work provides another glimpse into how these creatures, which have tiny brains and never met their parents, manage to make decisions and navigate their world.

“I don’t really think of them as conscious, but they’re following rules of some kind. I think of them more as robots,” Tedore said.