Scientists Discover a Vegetarian Spider, Upending a Longstanding Assumption

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Scientists Discover a Vegetarian Spider, Upending a Longstanding Assumption

By Beth Daley on Boston.com

The discovery of Bagheera kiplingi (named after the panther in Rudyard Kipling’s The Jungle Book) is not simply a eureka discovery about a twist in the biological world. It is a lesson on the importance of keeping an open mind on long-held scientific assumptions about how that world works.

For years, scientists were confident the world’s 40,000 spider species shared a common trait: All were meat-eaters.

Now, Brandeis University senior lecturer Eric Olson and Villanova University researcher Christopher Meehan have found a vegetarian spider.

The discovery of Bagheera kiplingi (named after the panther in Rudyard Kipling’s The Jungle Book) is not simply a eureka discovery about a twist in the biological world. It is a lesson on the importance of keeping an open mind on long-held scientific assumptions about how that world works.

Scientists had known for decades that a stinging species of ants and a type of acacia tree had a symbiotic relationship: One could not survive without the other. The ants feed primarily on small nutritious pellets, called Beltian bodies, that develop at the tree’s leaf tips. The ants, in return, defend the tree from other plant eaters so successfully that researchers believed few other insects lived on - or even visited - the tree.

In fact, experiments have shown that ant acacia trees can't live without the ants: Herbivores take over and kill antless tree. If you’ve taken Ecology 101, you’ve probably learned about this special relationship.

Olson knew of it. So when he began a long-term spider inventory in Costa Rica in 1999 as part of a larger forest study, he specifically told assistants to avoid the acacia trees. Of course there would be no spiders, he said. And the ants could deliver a nasty sting.

Olson focused on jumping spiders, famous for their bright colors, courtship dances, and foraging behavior. Almost every day, assistants would bring back jumping spiders for Olson - a task that broke up otherwise tedious work.

One day, some volunteers brought him a jumping spider found on an acacia tree. In an e-mail, Olson described what happened next.

"So I asked these sweet volunteers of mine, 'What in the world were you looking at ant acacias for, during this spider survey?' I asked this because I just 'knew' that no spider could possibly exist on this tree. And the answer was, they were just watching the ants, for fun really, and by chance they noticed a jumping spider at the same time. . . . I figured, 'This is just a fluke, it must have fallen on the plant from above just before the people found it' - and I went on with my business.

"Some weeks later it happened again, and one of my field assistants called me outside and said, 'Cmon boss, let’s go see what that spider is doing,' and so I went, and watched, and within about 5 minutes, ZOUNDS! it suddenly grabbed a Beltian body and ran off with it. I remember going into the lab at that point and announcing 'Yo! See this animal, this animal shall from today and for all time be known as the Ant Acacia Jumping Spider!' And so it has come to be."

It turns out the spider’s eyesight, agility, and cognitive skills allow them to avoid the stinging ants and at times even steal ant larvae to eat. The spiders don’t appear to defend the plant, but rather have found a niche to take advantage of the tree and ants.

"They probably drive the ants crazy," said Olson.

Olson was not the only one to notice. In 2007, Meehan independently observed the same behaviors in a spider population in coastal Quintana Roo, south of Cancún, Mexico, during a field project for a tropical biology course taught by Villanova professor and study coauthor Robert Curry. They and Olson combined efforts to jointly publish the discovery in Current Biology last week.

"It’s humbling," said Olson. "What is right under our nose isn’t always apparent."