Spiders Getting Bigger -- Global Warming to Blame?

May 5, 2009

As if global warming isn't giving us enough to worry about, now scientists say it could lead to bigger—and possibly more—spiders of at least one species.

A group of Danish scientists wondered whether global warming would make the hairy, meat-eating wolf spiders of northeastern Greenland bigger, since longer summers mean more hunting time. The little-known species, Pardosa glacialis, grows as long as 1.6 inches (four centimeters), study co-author Toke Høye of Aarhus University estimates.

(More wolf spider news: "Spider 'Resurrections' Take Scientists by Surprise.")

The spiders can live for at least two years, and the researchers found that, in years when spring came early, the animals grew larger, on average.

For example, when spring came 30 days earlier than usual, some spiders grew exoskeletons that were 10 percent thicker than average, resulting in bigger bodies overall.

Likewise, in colder years average exoskeleton thickness shrank.

At the end of the ten-year study, average exoskeleton thickness averaged 0.104 inch (2.65 millimeters), a 2 percent increase over the 0.102 inch (2.6 millimeters) commonly found in the early years of the study—a big difference to see in just a ten year period, according to the researchers.

Big Mamas

Warmer temperatures also seem to be giving the females of the species a size advantage over males, the new study suggests.

Pardosa glacialis females tend to be only slightly bigger than males—as was the case in 1997, when the spring thaw came 160 days into the year. But in 2005, for example, when the thaw occurred 143 days into the year, females' exoskeletons were 2 percent bigger than males', on average.

But Why?

Why warming seems to be making these spiders bigger is a mystery.

It could be that the spiders are growing larger because their prime hunting season is longer. Or perhaps longer summers allow the spiders to molt—shedding old exoskeletons—more often and thereby grow bigger during their lifetimes.

It's not known whether the wolf spiders are being born any bigger, as the study focused on adult spiders.

Nobody has any idea what effect these larger spiders will have on the local environment, Høye said.

But he's pretty sure the spiders will be not only bigger but more plentiful.

Larger adult females will probably increase spider populations, because larger females produce larger and/or more offspring Høye said.

On the bright side—at least for arachnophobes—cannibalism is common in many spider species. The practice could help keep populations in check, Høye said, especially as adults grow even bigger, making spiderlings even easier prey.

Findings published in the current issue of the journal Biology Letters.

SOURCES AND RELATED WEB SITES

ADVERTISEMENT

NATIONAL GEOGRAPHIC'S PHOTO OF THE DAY

NEWS FEEDS     After installing a news reader, click on this icon to download National Geographic News's XML/RSS feed.   After installing a news reader, click on this icon to download National Geographic News's XML/RSS feed.

Get our news delivered directly to your desktop—free.
How to Use XML or RSS

National Geographic Daily News To-Go

Listen to your favorite National Geographic news daily, anytime, anywhere from your mobile phone. No wires or syncing. Download Stitcher free today.
Click here to get 12 months of National Geographic Magazine for $15.