May 15, 2009

Flower Petals Help Bees Keep Their Grip

Scientists at the University of Cambridge in Britain have found that tiny conical structures on the surface of flowers provide bees something to hold on to, increasing their foraging efficiency.  Furthermore, the bees actually prefer these easy-to-grip petals, the study found.

Most insect-pollinated flowers have cone-shaped surface cells, in contrast to other flowers with flat surfaces.  It was long believed that these conical "bumps" existed to attract pollinators.

However, Beverley Glover and her team of researchers are the first to show precisely how these surfaces assist the bees.

"These cells also change the color of the flower by focusing light onto pigments, so researchers thought that might be their purpose," Dr. Glover told BBC News.

"But we've shown, in previous tests, that although they see this color change, they don't care about it. It's like choosing between different colored smarties - you can see the difference, but you still eat them all," she said.

Glover's team wanted to determine if the bees could sense these structures through their feet.  They began by testing the bees' sense of touch, to determine if the insects could differentiate petals with flat cells from those with conical ones.

The researchers used two types of snapdragon flowers, each with the same color of petals and the same scent.  However, one had a surface covered in these critical bumps, while the other had a flat surface.

Each flower was placed on top of a small container of liquid.

"The bees get a reward of sweet sucrose solution if they land on the right petal, and a 'punishment' - bitter quinine, which they don't like the taste of - if they chose the wrong flower," Dr. Glover said.

The bees soon learned to tell the difference between  the two.

"If they land on the wrong flower, they feel it through their feet and go 'uh-oh, this feels wrong, I'm not going to stick my tongue in there'," Dr. Glover explained.

To ensure the bees were able to select a flower based only on the feel of its petals, the team tested a separate group of bees with artificial epoxy resin flowers that virtually mimicked the two different surface structures of the real flowers.

These transparent discs were the same size and scent, but each identical fake flower contained a sweet reward of sucrose, regardless of its surface structure.

The scientists observed that if the flower-discs were set at a difficult upright angle, the bees much preferred to land on the flowers with conical surface cells.

"If the flowers are presented to them flat like dinner plates, they couldn't care less [which type they land on], but if they're upright like lollipops, then they choose the ones that are easier to grip," Dr. Glover told BBC News.

"We also saw that, on the conical cells, all six of the bee's legs are able to get a grip, and they can rest and turn off their wings."
The bees have tiny claws on their feet, which allow them to grip the cells.

"With the flat cells, their middle legs are scrabbling continuously, so they keep their wings moving to maintain their balance, which is a lot less energy-efficient," Dr. Glover said.

"It can't be easy to land on a flower - especially in the wind and rain, and the beauty of this is that evolution has come up with such a lovely, simple solution."


On The Net:

Current Biology

University of Cambridge