Bumblebees can sup on thick nectar just as easily as they slurp up thin nectar – and now we know why. It’s all down to the tiny hairs on their tongues.
Evolution has created some strange and surprising tools to help animals drink liquid. A close look at a bee’s tongue reveals a long rod-like stalk that is covered in thin hair-like protrusions. This makes it look a little like a tiny mop.
When bees are feeding, they quickly dip their tongue in and out of a flower to collect the sweet nectar. Pascal Damman at the University of Mons, Belgium, and his colleagues analysed videos of bees (Bombus terrestris audax) feeding from nectar with different viscosities, and made an unexpected discovery.
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They found that regardless of the thickness of the fluid, the bees lapped it up at the same rate, collecting the same volume of liquid each time. That’s a surprise because in theory, thicker liquids should be more likely than thin liquids to stick to an object dipped into the solution.
Tongue models
So Damman and his colleagues decided to try to mimic the action of bees’ tongues by 3D printing rods that were either smooth or covered in tiny structures to mimic the bees’ hair-like protrusions. They then dipped them into fluids of different viscosities.
It turned out that the distance between the microstructures on the rods explained the puzzle. If they are spaced close enough to one another then liquid is automatically pulled between them by what is called capillary action. This capillary action is fast enough to fill all of the gaps with nectar each time the bee dips its tongue in, and holds the liquid without dripping.
Like a mop and bucket, the bee squeezes the nectar out of its tongue hair when the tongue returns to the mouth.
Patrick Spicer of the University of New South Wales, Australia, who wasn’t involved in the study, says we often look at fluids in terms of their large-scale behaviour because humans are large animals. From this perspective, liquids that flow slowly appear thick.
Small scale
“But if you start going down to small enough length scales there is this quantity called the capillary length,” he says. At that scale what really matters are factors like molecular forces.
“That’s what the bee’s tongue does. It’s got all these hairs on the tongue and that space between the hair is able to pull fluid in,” says Spicer.
What this means is that a hairy tongue gives the bees two advantages. It can draw up fluids more efficiently than a hairless tongue, and it can also help regulate the fluid uptake regardless of its viscosity.
This second point is important for a bee that has to survive on nectar that can vary substantially in thickness depending on the weather. “You don’t want to starve just because your meal has gotten thinner,” says Spicer.
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