By Jenny Lam
One might say that blue is everywhere; it’s the color of the sky and oceans; look further than that, and you might realize that blue isn’t very common in living things. Many animals that appear blue, such as the blue jay or the blue morpho butterfly, aren’t considered truly blue, a category containing only less than 1% of the animal kingdom.
The color of an animal’s skin, fur, or feathers typically comes from pigments, chemicals that absorb specific wavelengths of light and others' reflections. Animals obtain these pigments from the food they consume, and a popular example of this is the color of a flamingo. Flamingos are born grey but gradually transition into a vibrant pink due to the pink dye beta-carotene found in their main diet of brine shrimp and algae. However, natural blue pigments are almost nonexistent. The phrase “truly blue” used in the first paragraph refers to animals with blue pigmentation, as most blue animals derive their blueness not from pigments, but from the structures of the molecules on their body and how they reflect light. In the case of the blue jay, its feathers have a specific bead design that scatters light in a way in which only blue light can escape, making the bird appear blue. The blue flowers of a plant are not due to blue pigments but are instead due to modifications of a common red plant pigment known as anthocyanin by shifting the pH and mixing it with other pigments and molecules.
As for the blue morpho butterfly, the ridges in its scales cause the butterfly to look blue due to constructive interference. When light hits the ridges of the scales, some light reflects off the top layer, and the rest goes deeper into branches of the ridges, some of which also gets reflected off the branch’s bottom layer, resulting in two rays of light. For most colors, the wavelengths of the two rays will be out of phase and cancel each other out (destructive interference), whereas for blue light, its wavelengths are in phase, becoming intensified and reflected (constructive interference). Like many blue animals, the structural blue color is created using optical effects, such as interference, rather than pigments.
Because most of the blue found in living things are due to their structure rather than blue pigments, this raises a question on why specifically blue pigments are so uncommon in the natural world. A theory states that as being blue became more beneficial for survival (to stand out to attract mates or warn predators), it was evolutionarily easier for animals to modify their structure to take advantage of the physics of light, rather than synthesize a new blue pigment. Additionally, most biological pigments absorb blue light since its wavelengths are shorter and higher in energy than most colors. According to researcher Radwanul Hasan Siddique, “blue light provides sufficient energy to raise an orbital electron to an excited state and hence force molecules to absorb it,” causing these pigments to appear more red or green.
The lack of blue has not stopped scientists from creating their own blue flowers. In 2017, researchers in Japan used genetic engineering to turn the commonly red, yellow, and pink chrysanthemum flower into a cool blue color. They inserted a gene from a blue Canterbury bells flower and one from a blue-flowering butterfly pea, which added a sugar molecule to the anthocyanin, shifting the plant’s pH to obtain its blue color.
Despite the rarity of blue pigments in nature, an extremely small number of organisms still produce a true blue pigment. The olivewing butterfly and mandarin fish are some of the only animals that have a pigment-based blue color. As of now, this list of blue-pigmented animals is quite small, but it is certainly possible that we will discover more truly blue species in the future!
What did you learn?
1. What are biological pigments, and how do they differ from structural colors?
Biological pigments are chemicals produced by living things that are a certain color due to selectively absorbing specific wavelengths of light. The pigment chlorophyll in plants absorbs violet-blue and red light and reflects green light, causing them to appear green. Structural colors are not made through absorption but are created through optical effects, such as microscopically structured surfaces that reflect light in a way in which waves of certain frequencies can constructively interfere. Unlike pigment colors, structural colors do not look the same from all angles because certain wavelengths of light can constructively interfere, or add up to make a stronger reflection at one angle, while destructively interfering, or cancelling out, at another viewing angle.
2. How do most blue flowers get their color?
These flowers usually get their blue coloring from modifying the red pigment anthocyanin through shifting the pH, mixing it with other pigments, or adding on molecules and ions to change the color of anthocyanin. In addition to giving some plants their color, anthocyanins also play roles in plant reproduction by attracting pollinators and can protect the plant from photoinhibitory (inhibition of photosynthesis due to high levels of high) effects of strong light.