14 October 2009

herbivorous spider

photo by Robert L. Curry: An adult female Bagheera kiplingi harvested this tasty yellow treat from the tip of an ant-acacia leaf.

Scientists have discovered a species of spider that does not prey on other insects, according to a study published in Current Biology, Volume 19, Issue 19, R892-R893, 13 October 2009. Abstract
Spiders are thought to be strict predators [1]. We describe a novel exception: Bagheera kiplingi, a Neotropical jumping spider (Salticidae) that exploits a well-studied ant–plant mutualism, is predominantly herbivorous. From behavioral field observations and stable-isotope analyses, we show that the main diet of this host-specific spider comprises specialized leaf tips (Beltian food bodies; Figure 1A) from Vachellia spp. ant-acacias (formerly Acacia spp.), structures traded for protection in the plant's coevolved mutualism with Pseudomyrmex spp. ants that inhabit its hollow thorns [2]. This is the first report of a spider that feeds primarily and deliberately on plants.
See also reporting in (UK) Daily Telegraph and NPR. Excerpt from BBC News (includes video).
Running the gauntlet

The jumping arachnid, which is 5-6mm long, has developed a taste for the tips of the acacia plants - known as Beltian bodies - which are packed full of protein.

But to reach this leafy fare, the spider has to evade the attention of ants, which live in the hollow spines of the tree.

The ants and acacia trees have co-evolved to form a mutually beneficial relationship: the aggressive ants protect the trees from predators, swarming to attack any invaders; and in return for acting as bodyguards, the ants get to gorge on the acacias' Beltian bodies themselves.

But the crafty Bagheera kiplingi has found a way to exploit this symbiotic relationship.

One of the study's authors, Professor Robert Curry, from Villanova University, Pennsylvania, told BBC News: "The spiders basically dodge the ants.

"The spiders live on the plants - but way out on the tips of the old leaves, where the ants don't spend a lot of time, because there isn't any food on those leaves."

But when they get hungry, the spiders head to the newer leaves, and get ready to run the ant gauntlet.

Professor Curry said: "And they wait for an opening - they watch the ants move around, and they watch to see that there are not any ants in the local area that they are going after.

"And then they zip in and grab one of these Beltian bodies and then clip it off, hold it in their mouths and run away.

"And then they retreat to one of the undefended parts of the plant to eat it."

Like other species of jumping spider, Bagheera kiplingi has keen eyesight, is especially fast and agile and is thought to have good cognitive skills, which allows it to "hunt" down this plant food.

Fierce competition

The spider's herbivorous diet was first discovered in Costa Rica in 2001 by Eric Olsen from Brandeis University, and was then independently observed again in 2007 by Christopher Meehan, at that time an undergraduate student at Villanova University.

The team then collaborated to describe the spider for the first time in this Current Biology paper.

Professor Curry said he was extremely surprised when he found out about its unusual behaviour.

He said: "This is the only spider we know that deliberately only goes after plants."

While some spiders will occasionally supplement their diet with a little nectar or pollen, Bagheera kiplingi's diet is almost completely vegetarian - although occasionally topped up with a little ant larvae at times.

Professor Curry said there were numerous reasons why this spider might have turned away from meaty meals.

He said: "Competition in the tropics is pretty fierce so there are always advantages to doing what someone else isn't already doing.

"They are jumping spiders, so they don't build a web to catch food, so they have to catch their prey through pursuit. And the Beltian bodies are not moving - they are stuck - so it is a very predictable food supply."

Acacias also produce leaves throughout the year - even through the dry season - which would make them attractive.

And Professor Curry added: "Because the plants are protected by ants, they have none of their own chemical defences that other plants do."
Bagheera kiplingi looks like a spider, but maybe it gets bullied by the other insect eating spiders. It would be worth finding out which species are above it in the food chain.

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