The colour of background vegetation reveals how bees see flowers

Wherever you go outside, chances are you’ll meet a pollinator: bats, flies, birds, butterflies and — most famously, bees. They play a key role in the sustenance of our ecosystems and agricultural fields by moving pollen. With such an important job, it is no surprise that plant-pollinator interactions have shaped the evolution of plants, including flower colour. 

The Fabaceae, the third largest family of flowering plants, has evolved different flower colours depending on the hue of green of the surrounding vegetation, a group of researchers reported in the American Journal of Botany last May. 

“Flower colour is adapted to pollinators’ preferences and visual sensitivity. If I’m talking about flower colours I think it’s important to consider how the pollinator is seeing the colours in nature because it’s different from what we see,” says ecologist Amanda Martins from São Paulo State University (UNESP) and first author of the publication. “There are a lot of flower traits or stimuli that attract bees, like the colour, scent, size, and shape, but the flower colour is one of the main traits that bees use to forage.” 

Martins and her colleagues in Brazil, Patricia Morellato and Maria Gabriela G. Camargo, as well as Montserrat Arista in Spain, aimed to better understand how flower signals have evolved through pollination pressures. 

Among the myriad of plants that she considered for her study, the legume family was ideal as members of the Fabaceae grow in a wide range of environments. And it’s not only their ubiquity that makes them great plants to study — they have a high diversity of flower types, colours, patterns and floral guides that bees love, allowing researchers to compare how bee perception has driven the evolution of flower colour in different ecosystems.

With this in mind, Martins and her colleagues wondered if plants growing in the savanna, the cerrado and the Mediterranean vegetation had flowers with different colours and signals. Using a spectrometer, the group of scientists collected reflectance and colour data from the petals of legumes pollinated by bees in the campo rupestre, cerrado and Mediterranean shrubland. The results surprised them. They found that the Mediterranean vegetation had a greater variety of flower colours while the tropical species had more diversity in colour patterns. They suspect this is due to the plants flowering in a concentrated small space in the Mediterranean compared to the tropical areas. 

“We first thought that we would find more diversity of flower colours in the tropical vegetation because we have more diversity of species,” Martins explains. “It was interesting. We found some particularities about each vegetation but overall, all of them are contributing to increase the diversity and flower detectability in each local environment.”   

They also collected information on the reflectance of the background vegetation from the 10 most abundant species in each vegetation. Using this information, the team classified flower colour and calculated colour signals such as chromatic and green contrasts, spectral purity and brightness. Instead of using the standard green background known as “pavo”, normally used to study colour and light reflectance, Martin and her team used the natural colour of the vegetation surrounding the plants they studied. Understanding contrast is especially important in flower evolution because bees will only navigate their way to flowers that strike out from the surrounding vegetation. This, of course, will depend mostly on flower colour. In other words, using the natural colour of the background would bring scientists closer to understanding how bees really perceive flowers. 

“[T]he green background of pavo was developed based on a green foliage, very similar [to] the colour of the Mediterranean vegetation background,” Martins says. 

But one size doesn’t fit all: their study used data from the Brazilian savanna and cerrado, which might create different visual effects for bees. They found that some differences in flower-colour signals were lost when they used the standard green background, while other signals differed depending on the background they used. This highlights the importance of considering the natural background in studies such as this.

Using this new approach, Martins and her team have gained a better understanding of how it is to see the world as a bee. She also says that this project has not only expanded her research, but also her scientific network. Martin began her work by collecting flowers and comparing two tropical communities from her native Brazil. However, during the course of her investigation she won a fellowship to travel to Seville, Spain. This broadened her research to also include species from the Mediterranean shrub scrubland. The positive outcome of the trip didn’t only reflect in the data; in Europe, Martins also found a group of women colleagues that broadened her scientific network with whom she hopes to continue collaborating.

RESEARCH PAPER:

Martins, A. E., Arista, M., Morellato, L. P. C., and Camargo, M. G. G. 2021. Color signals of bee-pollinated flowers: the significance of natural leaf background. American Journal of Botany 108(5): 788–797. 

Andrea Romero is a plant biology undergraduate student at the University of California Riverside with a passion for microscopic worms. When she’s not working you can find her reading sci-fi, climbing on rocks, or doing yoga with her dog. Follow her on Instagram @idig.worms and Twitter @idigworms.

Spanish translation by Lorena Villanueva Almanza