Like teenagers walking into a cafeteria, bumblebees can make quick social calculations about the safest place to alight for a meal. If they don't sense danger, they'll land wherever's convenient. But if the setting is treacherous, a bumblebee will check out all its options and look for a seat near a friend. No matter how nerve-wracking the lunch room is, of course, kids are at little risk of spider ambush. But for bees, the stakes are life-and-death. In their new paper, Erika Dawson and Lars Chittka of Queen Mary University of London explain that bees must watch out for predators that sit on flowers and wait for them to land. Some crab spiders can even change color to match the flowers they're perched on.
To learn about how bumblebees handle this threat, the scientists used an artificial crab spider—that is, a pinching device made of foam. They also built a miniature artificial meadow for the artificial spider to reside in. The enclosed space was a little smaller than a cubic meter and held a grid of eight "flowers," which were really just squares of yellow or white plastic baited with sugar water. Each flower had a landing platform, and some of those platforms had foam arms that the scientists could quickly close on a bee—not harming it, but simulating a failed attack by a spider. With its plain gray backdrop and minimalist blooms, the artificial meadow didn't look much like the real thing. But the bees weren't bothered; they flew around as normal, visiting and drinking from the flowers. Then the researchers began teaching the bees that some flowers were dangerous. Half the bees learned that yellow flowers were safe, but white flowers would try to grab them; the other half learned the opposite. After 100 or more training flights, the bees had learned which color of flower to avoid and were ready for the next test. Dawson and Chittka introduced the bees, one at a time, to the same meadow—but now the flowers were all one color, either white or yellow. Three of the bee's hive-mates were already sitting and feeding on one of the blooms. (The researchers had previously trained those confederate bees to feed on only that flower.) If all the flowers were the color the bee had learned was safe, the bee simply landed and fed at the first flower it saw. Most ended up alone, though two out of the 14 bees alit on the flower that was already occupied, apparently by chance. But if the flowers were the dangerous color, the bees paused. They "displayed hesitation," the authors write, "hovering in front of each unoccupied flower for a few seconds before rejecting it and moving onto the next." Only when they arrived at the flower their nest-mates were already sitting on did they land and feed. Almost all bees chose the flower that was already occupied
. Bumblebees seemingly ignored each other when they were safe. Faced with a risky situation, though, they looked to other bees for evidence of a safe place to land. If some bees were already sitting on a flower and weren't half-devoured by a spider, that spot was probably safe. It's the first time, the authors write, that bumblebees have been seen banding together to avoid predators. The strategy won't work on small flowers that only hold one source of nectar. But there are many flowers in the aster family, for example, that have multiple nectar organs. In that case, like at a friendly lunch table, anyone can pull up a chair and survive another day.
Images: bumblebees by Vicki; crab spider by Lynette Schimming (both via Flickr)
Dawson EH, & Chittka L (2014). Bumblebees (Bombus terrestris) use social information as an indicator of safety in dangerous environments. Proceedings. Biological sciences / The Royal Society, 281 (1785) PMID: 24789891
