Male Black Widow Spiders Find Potential Mates by Following Other Suitors’ Trails

Finding love can be a dangerous game, but for black widow spiders, courtship carries more than just the risk of a broken heart. Per a paper recently published in Proceedings of the Royal Society B: Biological Sciences, a mere 12 percent of prospective male suitors survive the search for a mate—a shockingly low success rate stemming from the low number of available females.

As Knvul Sheikh reports for the New York Times, researchers from the University of Toronto have found that male black widows track down females by following silk trails left by rivals. Although this tactic may seem counterintuitive, lead author Catherine Scott tells Sheikh, “Males have to race to find females. It makes sense for them to try to use all the tricks they can to find females as soon as possible, even if there are other males that have already found her.”

In a press release, study co-author Maydianne Andrade explains that black widow courtship can last for hours, making it possible for late arrivals to enter the fray and still emerge victorious. Making it to the female’s web is only half the battle; once there, the male must fight off other suitors, then perform an elaborate mating ritual involving vibration-transmitted information, dancing and silk-making. But as the scientists explain in the paper, it can be the determining factor in successful courtship, increasing the speed and efficiency of finding females without significantly increasing chances of competition. (Given the fact that males outnumber receptive females by a ratio of more than 10 to 1, the team deems “intense competition … inevitable.”)

According to a post on Scott’s spider-centric blog Spiderbytes, female black widows attract males by producing a pheromone that conveys information on location and sexual receptivity. Scientists have long believed that males rely on this chemical cue to find females, but as Sheikh notes for the Times, shifting winds and other changing conditions can interrupt transmission, forcing the spiders to resort to alternative tracking methods.

The bizarre mating behaviours of male black widow spiders

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To gauge which factors guide males’ searches, Scott and her colleagues set up a nearly 200-foot-long racecourse on the sand dunes of British Columbia’s Vancouver Island. At sunset, the team released groups of males placed at roughly 30-foot intervals along the track. In the first trial, strong winds blew the females’ pheromones directly toward the males, enabling all of them to successfully locate the cage at the end of the course. But in the second experiment, males released from distances of more than 130 feet away had trouble tracking the females, likely due to the interference of weak, variable winds.

These results were fairly unsurprising, Scott explains on Spiderbytes. The unexpected aspect of the experiment stemmed from the fact that males starting out furthest from females achieved the fastest average speeds even when wind conditions were so poor it became difficult to smell a female directly.

“The silk threads act like a highway connecting the tops of plants,” Scott says to Sheikh. “So following this path is more efficient than trying to make their own way, climbing over obstacles and making sure they are still going in the right direction just based on pheromones carried by the wind.”

After conducting the outdoor races, the researchers decided to move their research into the lab. Here, the team placed males in a maze and presented them with two options: retrace a rival male’s silk threads or avoid the trail and rely exclusively on the female’s pheromene signaling. Ultimately, the Times reports, 95 percent of males opted to follow in the footsteps of their competitors rather than forge ahead on their own.

This newly documented strategy is just one of several in male widow spiders’ reproductive arsenals. In 2015, an Animal Behavior study found that males use their sense of smell to avoid mating with hungry females—contrary to popular belief, females only cannibalize their mates around 2 percent of the time. In 2016, a paper published in Biology Letters posited that male redbacks and brown widows use their fangs to create openings in immature females’ shells and access their genitals. The process, which leaves females unharmed, increases males’ chances of passing on their genes—and enables them to escape the encounter without ending up on the dinner menu.

Meilan Solly | | READ MORE

Meilan Solly is Smithsonian magazine's associate digital editor, history.