Think your airline's bag fees are burdensome? Try flying after swallowing part of your luggage and strapping the rest to your legs. That's how bees do it. And depending on how a bumblebee loads herself up with nectar and pollen, her flight back to the hive might be less of a beeline than usual. Like honeybees, bumblebees gather both nectar and pollen, bringing them back to the hive for food. They collect nectar simply by drinking it. After being slurped up a bee's long tongue, nectar is stored in a kind of pre-stomach. Pollen, meanwhile, goes into "pollen baskets." These are scoop-shaped surfaces on the bee's back legs, surrounded by stiff hairs. After a flower sprinkles a worker bee with pollen, she can groom the grains into her baskets for storage. In the photo above, the big yellow blob on the bee's haunch is a full pollen basket. To learn how these loads affect bee flight, Harvard University researchers put bumblebees into wind tunnels and filmed them with high-speed cameras. The researchers simulated pollen and nectar loads using teensy steel ball bearings. They temporarily glued one ball to each of a bee's pollen baskets to represent pollen, or glued both to her back to represent a nectar load. (The site on the back was just above the actual nectar stomach.) The ball bearings added about 15 percent to the bee's total mass. In nature, this would be a light load. One study found bumblebees carrying about a quarter of their body weight in pollen and nectar, on average. Another study found bumblebees hauling as much as 91 percent of their weight in nectar alone. The researchers had trained the bumblebees to fly toward an artificial flower while in a wind tunnel. Now they tested the load-bearing bees under three scenarios: unsteady airflow, a flower waving back and forth, and both challenges at the same time. All of these scenarios forced the bees to keep making small adjustments to their flight paths. In this video, a bee carrying weights on her back to represent nectar is trying to land on a target that's moving around. The triangle on her back will let the researchers track the bee's flight and digitize it afterward.
Fake nectar loads added mass near a bee's center of gravity, while fake pollen loads hung at the back the body. The researchers could see a resulting difference in how bees flew. Bees with weighted legs were more stable in flight. In other words, they didn't rock around as much in the air. But this also meant it was harder for them to maneuver; they changed direction more sluggishly. So bees loaded with pollen can fly a little faster when a breeze is knocking them around, but bees weighed down with only nectar can zigzag through the air more easily. (This might explain why in a family of bees called the Megachilidae, females carry pollen on their bellies instead of their legs. They need to perform quick maneuvers in midair to evade large, harassing males.) The differences are small—in unsteady wind, a bee carrying weights on her legs flew just 2.3 percent faster than a bee with a weighted back. But for a bumblebee trying to carry pollen back to the hive on a windy day, that difference might matter. The hive spends most of its collective energy on flying, the authors write. If every bee returning with pollen could save a tiny bit of energy by spending a little less time in the air, those savings would add up. The study didn't look at bees carrying pollen and nectar at the same time. But it suggests that bees could benefit from adjusting their food search to the weather: pollen when windy, nectar when still. On our own travel days, maybe we'd benefit from threatening to swallow half our luggage if we see another fee.
Images: top by Hornet Arts (via Flickr), bottom Mountcastle et al.
Mountcastle AM, Ravi S, & Combes SA (2015). Nectar vs. pollen loading affects the tradeoff between flight stability and maneuverability in bumblebees. Proceedings of the National Academy of Sciences of the United States of America PMID: 26240364
