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Tiny Spiders

by David Wheeler

Last year's blockbuster Spider-Man has much more entertainment value than it has scientific validity, but there's at least one scientific principle that holds true in the movie and in real life: if you're small, you can climb easier.

Admittedly, this principle may not be the reason behind Hollywood's decision to have a fairly short actor, Tobey Maguire, play the role of Spider-Man. However, it is the reason behind the success of a new theory published by UK spider researchers in the prestigious journal Evolution.

In an article titled "Climbing to Reach Females: Romeo Should Be Small," the scientists explain why male spiders of certain species are significantly smaller than their female counterparts. According to the researchers' "gravity hypothesis," smaller males have a physical advantage when they have to climb to find a mate. And the higher the females live, the smaller the males tend to be.

"It changes the whole picture of why males and females differ in size," says Jordi Moya-Laraño, a postdoctoral scholar in the entomology department. Moya-Laraño created the theory in collaboration with UK entomology professor David Wise and former postdoctoral scholar Juraj Halaj.

The prevailing explanation for size difference in male and female spiders—that females need to be a certain size to lay lots of eggs—gives only half the answer. The gravity hypothesis, explaining why males are small, is the flip side of the coin.

The researchers' gravity hypothesis not only caught the attention of Evolution, but also of The New York Times. "Publishing in a top journal is good, because your peers find out about your work," Moya-Laraño says. "But talking about science to the general public is important too. The Times article shows how catchy the idea is, and how understandable it is."

To test the theory, Moya-Laraño and entomology graduate student Cora Allard drove to a wooded area in Berea, Kentucky, collected several male and female spiders of the same species, and brought them back to the lab. They placed the spiders, one by one, on top of a pole, poked them, and watched them glide downward from a line of silk they produced instinctively in response to the stimulus. The spiders climbed back up the pole naturally.

As the spiders climbed, Moya-Laraño and Allard timed their speed on a stopwatch, measured their climbing distance, and weighed them on a lab scale that is accurate to the one-hundred-thousandth gram. "If you remove the effect of weight—and you can do that statistically—the correlation between sex and speed doesn't hold anymore," Moya-Laraño says. "So that means the males are faster than the females because they are significantly smaller."

Though the gravity hypothesis wasn't a part of Spider-Man, Moya-Laraño still points out a correlation between the actor's size and his ability to scale buildings. "He's kind of a small guy—the actor," Moya-Laraño says. "To be Spider-Man, you need to be small."