Male spiders drum out mesmerizing syncopated beats to woo mates

Male wolf spiders (Schizocosa stridulans) that improvise intricate dance moves are big winners in the mating game, wooing females with showstopping tap routines. Now, new research finds that the more complex the dance, the more likely the spiders are to find love. 

The study researchers found that improvised steps benefited the spiders, which live in humid, mostly forested areas worldwide. The ability to bust a complicated move wasn't associated with size or strength in males, but it may hint to females that the male possesses a certain athleticism and grace.

"Females aren't necessarily looking for the biggest male or the loudest male or the strongest male," study co-author Eileen Hebets, a biologist at the University of Nebraska–Lincoln (UNL), said in a statement. "But maybe they're looking for a male that is really athletic and can coordinate all of these different signals into one display."

Feel the noise

S. stridulans are brownish-gray spiders that can grow to be 1.4 inches (35 millimeters) in body length. But behind that drab coloration lies a flashy and flamboyant performer, with mating dances that involve males tapping their forelegs and vibrating their abdomens. Females feel these vibrations and decide whether or not to let the suitor get close enough to mate.

Recent UNL doctoral graduate Noori Choi, a student of Hebets', wondered what exactly the females found so intriguing about the mates that they eventually chose. He analyzed one of Hebets' experiments in which ready-to-mate female spiders were put in a soundproof chamber with one amorous male at a time. The researchers placed the spiders on top of thin filter paper, which easily transmits vibrations, and monitored them with cameras and a laser to detect every last shiver and twitch created by the male's dance.

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Out of 44 hopeful males, nine spiders were deemed acceptable by the female test subjects. The spiders that successfully mated also had the most complex dances, Choi found.

Getting into a groove

Choi analyzed the complexity of the spiders' dances with computer-science analyses that have been used to quantify the complexity of patterns in data signals, part of the process of data compression. These methods have never before been applied to arachnid vibrations. Previously, Hebets said, scientists looked at features of a spider's dance individually, focusing just on factors like vibration alone, or looked at very basic interactions, such as those between visual signals and vibrations. 

"Now we're at the point, with some really talented people who have quantitative skills, of coming up with computational ways to look at how all of these things might interact, and how the entire package might be important in ways that we would never understand if we were just looking at components A, B or C," Hebets said. 

Males danced with more complexity for heavier females, which are desirable mates because they're likely to be able to bear and take care of large broods of spiderlings, the researchers found. Successful males also amped up their dance complexity as the courtship went on — dances can last up to 45 minutes —which may have indicated that the females were communicating their interest in some way. 

"When you're talking about spiders," Hebets said, "I think that's something people don't tend to appreciate; that signalers are paying attention to the receivers, they're paying attention to their environment, and they're adjusting accordingly."

The complexity of these spiders' moves is the equivalent of a person dancing on a syncopated beat, changing up the tempo, or otherwise making unpredictable artistic choices. These moves didn't correlate with spider size or a male's ability to produce loud vibrations, the researchers reported May 18 in the journal Biology Letters. Instead, the important qualities seemed to be related to vigor and skill, the researchers said. 

Or maybe these males just stood out from the crowd by abandoning preplanned choreography and thinking on their feet.

"There are a lot of studies that show that animals prefer novelty, in some capacity," Hebets said. In the case of the lovelorn wolf spiders, "the males constantly changing things up" might be the best way to catch — and keep — females' attention, she added. 

Originally published on Live Science.