The length and diameter of a honeybee's tongue hairs determine how efficiently it can collect nectar from various flowers, a discovery that could help communities plant the right flowers for this declining insect.
The speed with which a bee retracts its tongue determines its ability to capture honey. A team of researchers has discovered that the optimal retraction time is determined by the properties of the hairs that coat a honeybee's tongue.
The speed with which a bee retracts its tongue determines its ability to capture honey. A team of researchers has discovered that the optimal retraction time is determined by the properties of the hairs that coat a honeybee's tongue. ×
Honeybees are constantly on the move, visiting approximately 2 million flowers to produce one pound of honey. All that flying is exhausting, so honeybees must efficiently collect the nectar they require to produce their sweet, golden liquid. Honeybees collect nectar with their tongue, which is covered in tiny hairs, as scientists have known for a long time.
However, they were unaware of the strategies used by honeybees to maximize nectar intake. Jianing Wu of Sun Yat-sen University in China and colleagues have now studied how a bee's tongue-retraction time affects its energy intake rate mathematically and experimentally . The optimal retraction time is found to be related to the length and diameter of the hairs covering a honeybee's tongue.
Honeybees have long, thin, hairy tongue that is protected by a sheath. When a bee lands on a flower, the sheath opens, and the bee's tongue moves in and out of the sheath, collecting nectar in the vertical spaces between the tongue's hairs. The addition of nectar to the tongue increases the diameter of the tongue and thus its drag. As a result, tongue retraction occurs at a slower rate than tongue protraction (when the tongue is free of nectar and the hairs lay flat).
While the process by which honeybees are drinking is well understood, the impact of changes in the properties of the various elements involved is not. Wu and his colleagues investigated the problem by developing a fluid-transport model that included all of the elements. They calculated the optimal tongue-retraction time—the shortest time in which the bees collected the most nectar—for six honeybee species with masses ranging from 12 to 256 mg by varying nectar concentration, tongue-hair unfolding time, and tongue length.
The team discovers that the optimal retraction time decreases as a honeybee's body mass increases. For example, the model predicts an optimal retraction time of 135 ms for the Western honeybee, which has an average mass of around 100 mg and is one of the world's most common honeybee species. Meanwhile, the optimal retraction time for the Pearly-banded bee, which has a body mass of around 40 mg and lives in Southeast Asia, is around 200 ms.
The body mass of a honeybee correlates with both tongue length and the tongue-hair length-to-diameter ratio. As a result, the length-to-diameter ratio of tongue hairs is related to retraction time. The model also predicts slower retraction times for lower nectar concentrations.
Wu and colleagues conducted experiments to validate these predictions. They put honeybees in a transparent setup so they could watch them drink. The researchers injected sugar water between two glass slides and then used a high-speed camera attached to a microscope focused on the slides to capture the motion of the bees' tongues. Wu and colleagues discovered that, as predicted by the model, retraction time decreased with body mass.
Wu hopes that his research will aid in the preservation of honeybee populations, which have been in decline for several years. According to Wu, it is critical for the health of our planet to ensure the survival of honeybees. "We lose 40% of our crops if honeybees die." Fabian Brau, a physical chemist at the Free University of Brussels who has also studied how honeybees drink nectar, agrees. According to Brau, the model developed by Wu and colleagues could be used by communities to determine which flowers to plant, ensuring that local honeybee species have access to flowers with the nectar concentrations they require to collect it efficiently.
Brau also mentions that the predictions could change people's perceptions of how different honeybee species behave. Wu and colleagues demonstrate that the rate at which honeybees of a given species dip their tongues into and out of flowers is constant, but not for different species. "We assumed that this frequency was constant for all bees," says Brau. "And it was true for the bees [previously studied], but the bees [in those studies] turned out to be all the same size."
Soft Matter No. 38, B. Wang et al., "Optimal kinematics of the bee tongue for viscous fluid transport,"