Watching orchid bees (Euglossia imperialis) busily sucking up nectar from flowers in their native Panama and Costa Rica made Brendan Borrell from the University of California, Berkeley, wonder what factors affect how bees feed. Many bees feed with their tongues, but orchid bees sup nectar through a proboscis, a long tube which they fold up and flatten when not in use. To suck up nectar, a bee contracts muscles which expand a compartment inside its head. This reduces the air pressure inside the compartment and the proboscis relative to atmospheric pressure, forcing nectar up the tube. As well as the drop in pressure, the sweetness and viscosity of nectar also affects the rate at which a bee can slurp a snack. By enticing bees to feed from artificial flowers, Borrell investigated how nectar sweetness, viscosity and external air pressure affects a bee's feeding rate().
Capturing bees by luring them to baits, Borrell measured bees' feeding rates by weighing them before and after feeding to see how much of the sweet solution they consumed and by timing each meal. First he fed bees solutions containing different concentrations of sucrose. The feeding rate went down as the concentration went up, but sweeter solutions are more viscous, so Borrell wondered if bees were feeding more slowly in response to a solution's sweetness or viscosity.
Knowing that hawkmoths slurp faster as nectar sweetness increases, Borrell measured the bees' sucking rate while they fed on solutions with a constant viscosity, but different sweetnesses. Unlike hawkmoths, the bees' feeding rate didn't change, no matter how sweet the nectar was. Next Borrell tested the effects of increasing a solution's viscosity on feeding rate. While keeping sucrose concentration constant, he added an inert sugar called tylose, which makes solutions more viscous, but doesn't affect the sweetness. The bees'feeding rate went down as the viscosity increased, showing that a meal's viscosity influences how fast bees feed. Borrell explains that their ability to suck up nectar probably depends on how quickly the muscles inside the head compartment contract and how much force they produce to reduce the air pressure in the proboscis: thicker solutions are much harder to suck up.
Having shown that viscosity and not sweetness affects feeding rate, Borrell wanted to test how nectar intake rate would be affected by reducing atmospheric pressure. He reasoned that if he dropped external atmospheric pressure, the feeding rate for viscous nectars would decrease more than feeding rate for more liquid nectars because liquid nectars are much easier to drink. Placing bees inside a hypobaric chamber as they fed, he reduced atmospheric pressure from 100 kPa to 40 kPa, an equivalent move from sea level to the top of Mount Everest. But, the results didn't confirm his prediction. As atmospheric pressure decreased, the feeding rate declined by the same amount for viscous and runny nectars.
Borrell admits that this is a puzzling result, and suspects that it is because of the physical properties of the proboscis: it is similar to a collapsible tube, bulging or collapsing depending on the nectar flow rate. A bee has to keep the balance of forces in the proboscis just right to keep the nectar flowing; if not, it could go hungry.
