Happy International Women’s Day!
KNOXVILLE, Tenn. — Jackie Walker knew the routine of the dress rehearsal, as well as the routine of the football practice. He knew blousy shirts and an actor’s pose just as well as he knew how to hunch over in a linebacker’s menacing stance. He knew how to sing with grace in a tenor’s voice, and he knew how to make a less graceful noise — the “thwack!” of his pads against a ball carrier. Walker sailed comfortably in the two contrasting realms of entertainment — the theater and the gridiron — and into each endeavor he took those wide eyes, which were always ablaze with passion.
Walker was sleek and imperturbable onstage, and he was sleek like a missile on the football field and kept his poise over the taunts of opposing fans. Playing for the University of Tennessee from 1969 to 1971, he was among the early wave of black players when the Southeastern Conference was integrating. Jackie Walker was not quite Jackie Robinson, but like Robinson, Walker knew when to bow and when to bite.
We humans have three types of colour receptors in our retinas, sensitive to blue, green, and red light. Birds have four types of receptors, and see more complexity in colours than we do. Some birds, including the honeyeaters, have violet-sensitive receptors along with blue, green and red receptors. Other birds use receptors sensitive to ultraviolet rather violet light.
In our study, we examined the reflectance profiles (patterns of how an object reflects light of different wavelengths) of the flowers of 234 native Australian species. We measured these profiles across the ultraviolet and visual wavelengths of light, and these profiles were then translated into single points in a “colour space” based on the sensitivities of birds’ four classes of colour receptors. (A colour space is a way of positioning reflectance profiles within a four-dimensional grid so that colours lying near each other would appear similar to a bird.)
Of the 234 plant species we looked at, 154 were pollinated by insects and 80 by birds. The insect-pollinated flowers had a colour range that we see as violet and blue to yellow. Although birds don’t pollinate these flowers, birds would also see them as having a broad range of colours.
The flowers that were bird-pollinated formed a more interesting pattern. About half the species overlapped with the colour space arc of insect-pollinated flowers. As bird pollination usually evolves from insect-pollinated ancestors, this similarity in colours is not surprising.
The other half of bird-pollinated species, however, were crowded in a narrow and separate stretch that occupied just 1% of the total volume of the colour space. These flowers appear red to humans, and we named this cluster the “red arm”. Their reflectance profiles would also strongly stimulate the red receptors in bird eyes.
It appeared that many flowering species had evolved to “talk” to birds using a very particular set of colour “words”. In particular, the distinctiveness of the “red arm” appeared only when the colour space was based on the visual system of honeyeaters.
Models of the other visual system in birds with ultraviolet receptors, common in parrots and songbirds, did not reveal a distinctive red arm of floral colours.