Hyperspectral imaging technology illuminates a bird’s colorful feathers

From the shimmering appearance of a peacock’s tail to the distinctive rosettes of a jaguar’s fur, animals display an incredible variety of colors and patterns. Quantifying color in animals has been a long-standing goal of evolutionary biologists who aim to understand how color has evolved over time and the physical and genetic mechanisms involved.

After all, studying animal coloration is important because it can reveal how evolutionary forces such as natural and sexual selection favor certain traits over others. However, it is difficult to fully capture the colors of animals. This is because researchers need both high spatial resolution (like traditional photography, which captures information in a limited number of color channels) and high spectral resolution (like spectrophotometry, which captures the reflectance spectrum in a single color channel). ) because you have to choose between them. point).

Evolutionary biologists at Princeton University recently investigated the color of bird feathers using hyperspectral imaging, a cutting-edge tool that measures detailed spectral information in each pixel in an image. Hyperspectral imaging works by dividing the spectrum of light into a series of narrow bands, each corresponding to a narrow range of wavelengths.

Essentially, images are taken in each of these narrow bands, producing a stack (or “data cube”) of images containing both spatial and spectral information. Each pixel in the data cube contains detailed information about the wavelength of the reflected light.

“Hyperspectral imaging offers the best of both worlds,” explained Mary Caswell Stoddard, Ph.D., a professor in the Department of Ecology and Evolutionary Biology and senior author of the study. “Researchers can obtain comprehensive reflectance data for the entire specimen in minutes, opening up new possibilities for studying animal coloration.”

Hyperspectral imaging, often used in agricultural and medical applications, has been used in a small number of studies on animal coloration, but widespread adoption has been slow. Hyperspectral data can be unwieldy, and commercially available cameras are expensive and rarely capture all wavelengths relevant to an animal.

“In our study, we developed a new computational pipeline, a series of step-by-step analyses, to demonstrate how researchers can acquire and study hyperspectral data from museum specimens. The hyperspectral data and the code we developed were used to help others replicate and build on our methods,” said Ben, associate researcher and first author of the study. Dr. Hogan says:

Hogan and Stoddard used a commercially available camera that is sensitive to wavelengths in the range of 325 to 700 nanometers, roughly corresponding to the spectrum visible to birds (usually 300 to 700 nanometers), including the ultraviolet range (300 to 400 nanometers). did.

Many birds have feathers that reflect ultraviolet light. “Hyperspectral imaging allows us to easily capture detailed ultraviolet images, sometimes revealing entire specks of ultraviolet color that are invisible to humans,” Stoddard said.

To demonstrate the power of hyperspectral imaging in studying animal coloration, Hogan and Stoddard focused on birds of paradise. These charismatic birds are native to New Guinea and neighboring regions and are known for their bright plumage and elaborate courtship displays.

The rare hybrid Dutch King’s Bird of Paradise has been a subject of significant interest. Only about 25 male specimens are known to exist in museums around the world, 12 of which are housed at the American Museum of Natural History (AMNH) in New York City. A cross between the King and Magnificent Birds of Paradise, this hybrid has plumage that combines characteristics of the two parent species.

By collecting and analyzing hyperspectral data from specimens borrowed from AMNH, Hogan and Stoddard were able to quantify the degree to which a hybrid’s appearance was truly intermediate, or the exact degree of blending of the parental species’ colors. It’s done.

“We found that in some feather spots (even those colored by very specific micro- and nanostructures) the color of the hybrid is actually a mixture of the phenotypic colors of the parents. I was surprised to discover the similarities,” Hogan said.

Hogan and Stoddard also integrated hyperspectral imaging with photogrammetry, a technique that stitches together hundreds of traditional images taken from different angles, to create a virtual 3D model of the bird specimen. These 3D models are valuable because they reveal how an animal’s body shape and morphology interact with its color. It also provides a detailed digital record of the specimen, which is easily accessible to researchers and the public and can be used for various morphometric analyses.

Hyperspectral imaging can be a powerful tool for studying avian camouflage, alert coloration, mimicry, courtship expressions, and more. This technique is ideal for studying other colorful taxa, such as butterflies and beetles. In the future, it may be possible to animate 3D models integrated with hyperspectral data to explore how motion affects signal design.

“We believe that the combination of hyperspectral imaging and 3D modeling has the potential to become a new ‘gold standard’ for many studies of animal coloration, especially studies based on museum collections,” Stoddard said. says Mr. “Hyperspectral imaging of moving animals in the field remains a challenge, as does capturing rainbow colors, but this approach has great potential.”

The study is published in PLOS Biology.