A little eight-legged pickpocket that darts around acacia trees could be the first known vegetarian spider.
Bagheera kiplingi belongs among the big-eyed, athletic predators in the family of jumping spiders and gets its name from a panther in a Rudyard Kipling story. Yet a population of these spiders in Mexico mostly eats bits of the acacia trees, says Christopher Meehan of VillanovaUniversity in Pennsylvania.
A few other spider species do taste vegetable matter now and then, says Yael Lubin of Ben-GurionUniversity in Sede Boqer, Israel. Male crab spiders that spend their brief mating-oriented adult lives sitting on flowers will sip nectar for a little energy boost. And some baby spiders eat spores that have stuck to a web. But on hearing about spiders specializing in stealing vegetarian food, “I was absolutely floored,” Lubin says.
These arachnid herbivores are no wimps. “The tree is full of biting, vicious ant guards,” Meehan said during the 12th International Behavioral Ecology Congress meeting August 9 through 15 at CornellUniversity. The little spider spends its life dodging patrols of ants and stealing their (vegetarian) lunches.
Acacia trees and their resident ants have become a textbook example of a mutually beneficial partnership. Tree thorns grow swollen bases the right size to shelter ants. Glands at the base of the leaves ooze nectar, far from flowers but just at the spot to offer refreshment for ants. Acacia leaflet tips sprout nubbins of protein and fat suitable for ant snacks.
Certain ant species take full advantage of these comforts and defend their home trees against all comers. In the course of their vigilance, the ants get rid of caterpillars and other invaders that might chew on the tree.
Meehan says the spiders manage to dodge the ants, perching on leaf tips and nesting in mature leaves, which aren’t as heavily patrolled as other tree parts.
Ecologists have studied the partnership for years, but “people who look at ant acacias — they look at the ants,” Lubin says. “It took the eyes of a student naturalist to see this.”
That fresh observer was Meehan, who, along with his Villanova colleague Robert Curry, noticed the spiders dining on the leafy snacks of acacias in Mexico. In videos of 140 spider meals, the researchers counted 136 acacia protein-fat snacks with a few nectar sips. On four occasions the spiders did turn to meat as they tugged away ant larvae from a passing nursemaid and ate the youngsters.
In Costa Rica, the spiders also steal ant food, though to a lesser extent, according to observations from Eric Olson of BrandeisUniversity. He independently discovered the spiders eating tree snacks in Costa Rica in 2001 and is working with the Villanova team on a report on the species.
Those meat moments don’t happen often, according to studies done in collaboration with Matt Reudink of Queen’s University in Kingston, Canada. The team checked spider tissue for the heavier form of nitrogen, N15, which becomes more concentrated as animals eat animals that have eaten other animals.
That carnivore signal does not show up in the acacia-tree spiders, which carry a relatively light concentration of N15, one that is typical of plant-eaters, according to the team’s data. The researchers also found that the concentration of the heavier form of carbon, C13, also looks typical for a vegetarian.