If you look around a flower meadow, you will discover many bright colors – but rarely shiny petals. A study has now discovered the reasons for this. Accordingly, dull flowers have a clear advantage for pollinators: They present a clear, reliable color signal that bees and co. can use as a guide. With shiny flowers, on the other hand, the color can be difficult to see depending on the viewing angle and light. But why do some plants still rely on a shiny appearance? The experiments also provide an answer to this.
Whether bright yellow dandelions or deep blue lavender, flowers use their colors to attract pollinators. Most specimens rely on matt surfaces that consist of tiny, cone-shaped cells that scatter light in all directions and whose coloring is therefore easy to see from all angles. But in all parts of the world you can also find plants whose flowers have flat surface cells and which therefore shine. In our latitudes, this includes the buttercup, for example. Depending on how the light falls on them, such flowers produce a reflection that can outshine the actual color signal. But why did predominantly matt flowers, but also some shiny flowers, develop in the course of evolution? What advantages and disadvantages do each offer for pollinators?
Matte surfaces are more reliable
A team led by Alexander Dietz from the University of Würzburg investigated this question. “Visual signals are usually most effective when they are consistent in time and space,” explain the researchers. “Shiny visual effects contradict this principle because the bright, directional light pulse that dominates their appearance is highly variable in space and time.” But how do insects react to these variable signals? To answer this question, the team built artificial matte and shiny flowers in various shades of blue and yellow and presented them to bumblebees in an experimental arena. Some of the artificial flowers offered delicious sugar water as a reward, while others remained empty.
In various experiments, the researchers varied which types of flowers provided a reward. This showed that the bumblebees were better able to distinguish between different colors on the matt surfaces and quickly learned which dummy flowers were worth approaching. Shiny surfaces, on the other hand, made it difficult for them to differentiate, so they more often headed for empty artificial flowers. “The reflective light reflections interfered with the reliable interpretation of the color signal,” explain Dietz and his team.
Tradeoff between visibility and distinctiveness
But further experiments revealed another advantage of the shiny surfaces: If the researchers placed the artificial flowers so far away that they were in the border area of the bumblebees’ field of vision, the shiny specimens were apparently more noticeable. “Glare appears to be an evolutionary strategy for certain ecological conditions in which improved visibility from a distance outweighs the disadvantage of more difficult color recognition up close,” says Dietz’s colleague Johannes Spaethe.
Shiny surfaces on flowers therefore represent a visual compromise. “Up close, their shine makes it difficult to see details,” explains Dietz’s colleague Casper van der Kooi. This is comparable to reading a glossy magazine in the sun: the letters are difficult to see because of the shiny surface.” For bees, foraging on shiny flowers is therefore less efficient because they have to spend more time and energy finding the right flowers. For the plants, the risk also increases that pollinators will transfer the pollen to plants of other species, which can reduce reproductive success. Only if the improved visibility from a distance outweighs these disadvantages can it become established in an evolutionary manner.
From the researchers’ perspective, the results could also be important for other biological disciplines. “Similar principles also play a role in the interaction between predators and prey,” says Dietz. For example, the shine of insect shells hinders predators from accurately pursuing their prey, and the flashes of light from fish scales can reduce the likelihood of birds being hit in an attack.
Source: Alexander Dietz (University of Würzburg) et al., Science Advances, doi: 10.1126/sciadv.adz9010