The world of spider sex is full of interesting phenomena. There are examples of cannibalism, genital mutilation and even chastity belts. And widow spiders are no exception. During courtship, males become home-wreckers and destroy part of the prospective female’s web, bundling up chunks of it with their own silk. While this is well-documented, why they engage in this curious behavior was somewhat of a mystery. But a new study has offered some insight, demonstrating that it dissuades other males from coming along and thus reduces competition.

Perhaps even more interestingly, the females didn’t actually seem too bothered by the males’ destructive act, and the researchers have offered a possible explanation. By reducing the web’s attractiveness to other potential suitors, the female experiences less harassment and thus spends less time fighting off males, allowing her to get on with other activities.

This particular investigation focused on western black widows, Latrodectus hesperus, a venomous species found in western North America. Like all widows, these spiders are decorated with a distinctive, brightly colored hourglass pattern on their underbellies. As with many web-building species, male western black widows are attracted to pheromones that the females’ silk is laced with. Alongside alluring members of the opposite sex, the chemicals actually convey a lot of information about the female.

“The silk pheromones that female black widows produce are like scent-based personal ads,” lead author Catherine Scott said in a statement. “One whiff of the pheromone can tell a male about the age, mating history and even hunger level of the female. These complex chemical messages are just one part of the spiders’ communication system, and web reduction is a fascinating behavior that allows a male to interfere with a female’s message.”

To find out how effective this destructive technique is at deterring competition, scientists from Simon Fraser University collected female western black widows and allowed them to build webs in enclosures before taking them to a spot they naturally inhabit on Vancouver Island. They then removed the females and examined how many males arrived at the boxes under different conditions. The aim was to see whether the males’ behavior was important for the apparent decreased web attractiveness, so in some of the boxes the researchers partially destroyed the webs themselves, whereas in others they allowed a male to do the job himself.  

As described in Animal Behavior, intact webs proved very popular, attracting as many as 10 males within just six hours. But while the webs tampered with by the researchers were equally as attractive as the intact ones, those that had been partly bundled up by the male spiders lured in only a third as many suitors. This demonstrates that it is not merely the web destruction that is important in reducing attractiveness, but something that the male is specifically doing. The researchers think that the males may be removing spots that are heavily laden with pheromones, or possibly dowsing the silk with their own chemicals to deter rivals. 

So why does the female not seem to mind that her web is wrecked by a pesky male? It may actually be beneficial to her given the fact that courtship ensues over several hours. The female may therefore have numerous visitors before she has even done the deed with one suitor, so deterring further males from coming along and disrupting the act would allow her to get on with it.