Male spiders can be tens of times smaller and weigh one hundredth of what their female counterparts weigh, and new research shows that these size differences may be in part due to a spider behavior called bridging.

Bridging is a means of transportation for spiders living in the trees and other vegetation of forests and meadows. In bridging, a spider casts a strand of its silk into the wind, and the silk is carried aloft to a neighboring plant. The spider then pulls the strand taut and crawls upside-down along the strand to its new turf, where it may find a willing mate or tasty prey. [Image: A spider engaged in bridging.]

In a new study, researchers led by Guadalupe Corcobado, a doctoral student at the Spanish National Research Council, found that bridging is much easier for the tiniest of male spiders than for their slightly larger counterparts. The researchers concluded that the advantages conferred on a small male that is efficient at bridging — such as mating with more females — could have driven the evolution of male spiders toward smaller sizes.

Spider setup

In the work, the scientists placed spiders on a stand that was about one foot (30 centimeters) away from a plant. To recreate a breezy day in the forest, they placed a fan about 10 feet (3 meters) away from the stand, on the side opposite the plant. They tested male and female spiders belonging to 13 species collected from all over Spain, including a less-poisonous relative of the black widow spiders found in America.

"We used spider species in which both females and males are tiny, and species in which females are giant, but males are small," said Jordi Moya-Larano, a tenured scientist at the Spanish National Research Council who worked on the study.

They found that smaller spiders were more likely than larger spiders to send their silk bridges to the plant and make the journey across to a new home.

"Size matters," Moya-Larano said. "Smaller individuals had a higher propensity to bridge, whether male or female."

Almost all spiders weighing more than 0.005 ounces (150 milligrams) found the feat impossible. The researchers tested spiders that weighed up to 0.03 ounces (1 gram).

But for spiders, the males are the mate-seekers, so bridging is particularly advantageous for them. For females, the competing demand of laying eggs, which is made easier by being larger, may limit how much their size can be affected by bridging.

Species sizes

Prior to this research, many hypotheses have attempted to explain the size differences between male and female spiders. Charles Darwin thought that males were smaller so they could escape from female attacks, Moya-Larano said. One hypothesis that is well-supported is that females that are larger are more easily able to produce more offspring.

However, this idea does not explain why some species of spiders show extreme size differences between the sexes while others do not, Moya-Larano said.

The new idea — that bridging behavior has played a key role in driving males of some species toward smaller sizes — can explain why, in some species, males and females are vastly different sizes while in others they are nearly the same.

Spiders that live on the ground don't need to make bridges. And among those that live high up in tree tops, some are prevented from making bridges by other aspects of their anatomy, such as their body shape or the weakness of their silk. For those species, males and females tend be similarly-sized.

"Our prediction is that no same-sized species use bridging," Moya-Larano said, and future work may show that this prediction holds up.

The research will be published online tomorrow in the journal BMC Evolutionary Biology.

This article was provided by Life's Little Mysteries, a sister site to LiveScience.