Ants on Acacia plant

Photo Credit: By Ryan Somma [CC-BY-SA-2.0], via Wikimedia Commons Ants on Acacia plantPhoto Credit: By Ryan Somma [CC-BY-SA-2.0], via Wikimedia Commons

A population of spiders in Mexico subsists almost entirely on plants.

Transcript

Leaf-hunting spiders. I'm Bob Hirshon and this is Science Update.

A Central American jumping spider, known as Bagheera kiplingi, is the first-known spider to eat mostly plants. Their veggie of choice: acacia leaf tips, guarded by venomous ants that live on their nectar. Biologist Christopher Meehan, formerly of Villanova University, says the spiders case out the shrub first.

Meehan:

And at some point, they just decide to go in for the kill. And they use these incredibly entertaining acrobatics to literally just steal a vegetable from under the noses of the ants.

Meehan says the leaf tips keep the spiders well fed—even in the dry season, when insect prey would be scarce. Plus, the ants fight off all the competition. However, he says not all Bagheera spiders are equally herbivorous: while a Mexican population subsists on 90 percent acacia leaves, their Costa Rican cousins split meat and veggies 50-50. I'm Bob Hirshon for AAAS, the Science Society.

Making Sense of the Research

We think of spiders as quintessential predators. Although the web-spinning kind dominate books, movies, and Halloween decorations, many species simply pounce on their prey, like cats. The species Bagheera kiplingi belongs to this second group of spiders, known as jumping spiders. Jumping spiders are usually solitary and nomadic, meaning they hunt and live alone, and move around a lot.

This population of Bagheera shows how a population can adapt its behavior to suit the local environment. It's important to note that we're talking about a population, not an entire species; as you heard, a population of Bagheera in Costa Rica relies equally on live prey and vegetation, while other populations may not eat the plant material at all.

The Mexican Bagheera aren't pure vegetarians: about ten percent of their diet comes from meat, mainly the larvae of the ants that guard the precious acacia leaf tips. But over time, these spiders have adapted to live mostly on the leaf tips. All things being equal, insect prey would deliver more nutrition per ounce than the plant material. However, as you heard, all things aren't equal. In the dry season of the Mexican forest, the population of insects drops considerably. Specializing in leaf-eating has allowed the spiders to enjoy a consistent and reliable food source.

Interestingly, the spiders "hunt" the leaf tips much as they would hunt live prey: they stalk it, wait for the right moment, and then jump on it. Meehan says they're incredibly nimble. In countless filmed observations, he's never seen a guardian ant actually catch the spider. That doesn't mean that the ants are useless, though. They successfully fight off other creatures that would eat the acacia. That benefits the plant, which the ants rely on for their own food. But it also benefits the spiders, because it keeps away all the competition.

Meehan says that the Mexican spiders have changed their behavior in other ways to suit their leaf-eating lifestyle. They live in much closer quarters than most other spiders. They spend more time caring for their nests and young. Males have more responsibility to the nest than other male spiders. They're also loyal to their homeland, usually staying put for multiple generations.

All this doesn't mean that the spiders have transformed into social creatures. At their cores, they're still aggressive hunters. But the circumstances in which they live have created an incentive for them to put their basic instincts aside and live in relative peace with each other. Meehan says this may actually mirror the early days of human evolution, in which nomadic hunters began to form more closely-knit, interdependent societies in order to reap the benefits of agriculture.

Now try and answer these questions:

What do the Mexican Bagheera spiders eat? What are the benefits of this food source? How has this food source affected the spiders' behavior? What do you think would happen if you took a hundred spiders from the Mexican population and transferred them to the Costa Rican population? How would the passage of time factor in?

You may want to check out the October 16, 2009 Science Update Podcast to hear further information about this Science Update and the other programs for that week. This podcast's topics include: debunking 2012 doomsday hype, developing a better rocket fuel, vegetarian spiders, and the genetics of aggression in Africanized bees.

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