A bird’s-eye view of the forest

What would the world look like if humans could see UV light? It’s an interesting question to ponder. Many animals, however, do perceive their environment through the UV. Because humans do not have that ability, it can be hard for scientists to predict which objects animals see as having high UV contrast and the advantages that their enhanced vision might confer.

Cynthia Tedore from the University of Hamburg in Germany and Dan-Eric Nilsson from Lund University in Sweden re-created the visual perspectives of animals by taking images of natural vegetated environments in both UV and visible light. The researchers modeled a type of cone cell found in the eye by having each color channel collect light from one part of the spectrum. The channels specifically mimicked a bird’s vision, which includes the entire visible spectrum from red to blue and goes into the near-UV.

Images of the leafy environments were surprisingly low contrast in the green channel. The UV channel had uncovered additional contrast between the upper and lower leaf surfaces because the natural illumination is much brighter from above than below. The light intensity difference is exaggerated in the UV because leaves can transmit a lot of visible light but almost no UV. As shown in the figure, that change in the illumination made the structure of the foliage easier to see in the UV channel. Such an enhancement could make the complex forest environment easier for animals to navigate.

The researchers analyzed images of vegetation under three forest-canopy densities and found that the images’ contrasts were higher in different parts of the UV spectrum. Denser foliage had higher contrast at longer wavelengths, whereas sparse foliage had higher contrast at shorter wavelengths that were further into the UV. An optical model showed that the difference was likely due to the illumination source: When the incident light came primarily from light transmitted through overlying leaves instead of the sky or direct sunlight, the foliage showed higher contrast at longer wavelengths. That could explain why animals have evolved cones to see UV light of different wavelengths. (C. Tedore, D.-E. Nilsson, Nat. Comm. 10, 238, 2019 .)