Male black widow spiders trash the webs of the ones they love

Male black widow spiders destroy large sections of the female's web during courtship and wrap it up in their own silk.

Sean McCann

Single guys will go to great lengths to get the girl. For black widow spiders, that sometimes means visiting a female's web and ripping it to shreds.

With 40 males for every female in a given population, a male spider faces fierce competition and will do about anything to hold onto his new companion. According to a study in Animal Behavior, the spiders destroy large sections of the female's web during courtship and wrap it up in their own silk. This home-wrecking behavior deters other males from approaching -- and doesn't seem to bother the female one bit.

Black widows build messy webs, which they use to communicate via vibrations and pheromones.

"The silk pheromones that female black widows produce are like scent-based personal ads," said Simon Fraser University's Catherine Scott, the lead author of the study. "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."

Scott focused on the western black widow spider, Latrodectus hesperus, which is native to western North America. Female black widows of this species are around 15 mm long and black, with a distinctive red hourglass-shaped mark on the abdomen. The male is much smaller, and a lighter tan color with a striped abdomen.

She and her team let female western black widow spiders build webs in cages in the lab over the course of a week, and then took those cages to a beach on Vancouver Island where the spiders are found naturally.

They removed the females from the webs, and compared the number of wild males arriving at cages containing three different kinds of web: intact webs, webs with about half of the silk cut out and bundled up by males, and webs from which half the silk was cut with scissors and removed.

The results showed that the female's webs were highly attractive: In the first six hours, some intact webs attracted more than 10 males. The webs that had been reduced by male spiders attracted only a third as many males as intact webs, showing that web reduction makes the web less attractive and reduces competition. The webs cut by the researchers, however, proved a similar draw to an intact nest.

"Whatever the males are doing during web reduction is much more effective than simply reducing the amount of silk by half," Scott said. "One possibility is that the female pheromone is concentrated in certain areas of the web, and males are bundling up those specific sections with silk, which stops the pheromone from being released. The males could also be adding their own pheromones, which rival males avoid."

Along with trashing nests, males have been known to guard their female mates. Scott and her colleagues, in an earlier study, also found the males engage in a mating dance of sorts, shaking their abdomens to produce carefully pitched vibrations that let females know they have "come a-courting" and are not potential prey.

Females don't seem to mind the male's home-wrecking behavior and researchers say it might be in their self-interest to tolerate it. With the number of suitors down, it saves the females from being harassed.

"Web reduction may be giving her the opportunity to rebuild her web without pheromones and get on with reproduction, rather than wasting time and energy chasing away a parade of redundant male suitors," Scott said.

  • Michael Casey

    Michael Casey covers the environment, science and technology for CBSNews.com