Why is Blue So Rare in Nature?

Why is Blue So Rare in Nature?

by Jasmine Chan 

Source: “Why Blue Animals Are So Rare.” BrightSide, 16 May 2020, brightside.me/wonder-animals/why-blue-animals-are-so-rare-797189/amp. 

Introduction

Blue, a primary colour, a colour which is often found around us in a bustling city such as Hong Kong - where Science on Sundaes is located. Before I delve into the science, I want you to think about the associations of blue and what makes it special. According to a survey conducted across ten countries in four different continents, blue is the most popular colour amongst all. [1] Humans love the colour blue, as it is hard to obtain naturally. I want you to recall the last time you have visited nature, a place away from the city, whether it may be a small hike, visiting a botanical garden or even the wild. What is the first colour that comes to your mind? Most likely, your answer would be green, but blue is never near the first thought. Why is blue, especially blue pigments so uncommon in nature?

Fig.1 Chart which shows the ratios of favourite colours in each country.

Source: “Why Is Blue the World's Favorite Color?” YouGov, 13 May 2015, hk.yougov.com/en-hk/news/2015/05/13/why-blue-worlds-favorite-color/. 

Blue is an infrequently occurring colour in the biological world as established from the beginning. You may be thinking, ‘What about the blue butterflies, like the ones that can be found on our emoji keyboard?’ or ‘I’ve seen blue flowers, what about those?’ Statistically, there is less than 10% of all species which have the colour blue. [2]

Blue Flowers

Fig.2 Examples of Blue Flowers.

Source: “Top 55 Types of Blue Flowers with Names and Pictures.” Florgeous, 10 Dec. 2019, florgeous.com/blue-flowers/.

True blue in plants does not technically exist. The blue colours that we see in flowers are blue-toned purple or a cool-toned red. Most blue flowers such as blue roses and blue orchids are artificially made through white flowers and dye. The blue colour that is seen in the plant is usually a combination of other coloured pigments and mineral ions from the soil. 

The most common pigment that exhibits this ‘blue’ colour in plants is known cyanidin-3-glucoside, a major anthocyanin pigment. Cyanidin-3-glucoside can be found in ‘blue’ flowers such as cornflowers and morning glories. [3] By varying the concentration of any anthocyanin pigment, different hues of blue can be made. [2] Some anthocyanins can also be altered by changing the surrounding pH. For example, blue poppies have a defect in their proton pumps, which causes the cells to become more basic/alkaline and hence appearing blue.

How do most animals exhibit colour?

Most animals exhibit the colour they are seen to the human eye is due to pigments in their fur/skin from the food they consume. [4] One major example is salmons. Salmons are naturally grey. The pink colour that we see in salmon is due to the primary consumption being shellfishes, which are pink. This diet causes a pigment known as astaxanthin to be incorporated into their bodies, turning them pink. [5] Another example is goldfinches. Their yellow colour on their fur is obtained from the yellow flowers they eat, causing carotenoids to be in their feathers and exhibit their yellow colour. [4]

Blue in Animals

Fig.3 Image of a Blue Jay

Source: “Blue Jay.” Audubon, 17 Mar. 2020, www.audubon.org/field-guide/bird/blue-jay. 

Fig.4 Image of a Blue Morpho Butterfly

Source: “Blue Morpho Butterfly.” Zoo around The World, 17 July 2014, zooaroundtheworld.wordpress.com/south-america/blue-morpho-butterfly/. 

There are some animals which do have true blue pigments which I will address later on. However, if you think about the feathers of a bluejay, or the wings of a morpho butterfly, blue is seen by the human eyes due to refraction of light instead of incorporated pigments. [5]

Bluejays (Cyanocitta cristata), have pigments in their feathers known as melanin which is brown. Through the human eyes, we see their feathers as blue due to Tyndall (light) scattering. [6] The feathers in their wings are made of small pockets made with air and a protein known as keratin and when light hits these pockets, all of the other wavelengths aside from blue are absorbed. Hence, blue is refracted. Light scattering only occurs in normal lighting conditions. If a person attempts to backlight the feathers of the bluejay or any blue birds, they would appear brown as the colour of melanin is brown. [7] 

 

Fig.5 Close-up Picture of Blue Jay's feathers

Source: “r/MacroPorn - Feathers in a Blue Jay Feather [2569x1440] [OC].” Reddit, www.reddit.com/r/MacroPorn/comments/bexfwg/feathers_in_a_blue_jay_feather_2569x1440_oc/. 

Similarly, in morpho butterflies (Morpho peleides), they have microscopic scales on their backs to create an iridescent blue effect which is bright and eye-catching in nature. These scales also absorb all wavelengths except blue and hence, reflecting a vivid and intense blue colour. [8] This phenomenon is known as coherent scattering. [6] Each scale contains microscopic ridges which creates a repeating pattern. These ridges form a multilayer system made of a series of overlapping lamellae. [6] As the lamellae overlap, it causes constructive interference to occur within the wings of the butterfly which increase the rate of light waves to reflect. This means that the chance of two waves having the same phase (where the position of peaks and troughs in both waves are identical) increases and therefore intensifies the blue colour in morpho butterflies. [9] Compared with Bluejays, in butterflies, reflection occurs whereas refraction occurs with bluejays. In each of these scales, they are shaped specifically to reflect and bend light. If these structures are shaped differently, blue would not be reflected. [10] 

Fig.6 Diagram showing how Constructive Interference works

Source: “Wave Interference.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., kids.britannica.com/students/assembly/view/53869. 

Fig.7 Close-up of microscopic scales on butterfly wings and microscopic ridges

Source: Oh, Jenny. “What Gives the Morpho Butterfly Its Magnificent Blue?” KQED, 16 Dec. 2014, www.kqed.org/science/24552/what-gives-the-morpho-butterfly-its-magnificent-blue. 

Fig.8 Diagram displaying how microscopic ridges in scales reflect blue wavelengths of light via constructive interference.

Source: “Blue Things In Nature: Why Is Blue So Rare In Nature?” Science ABC, 20 Oct. 2019, www.scienceabc.com/nature/why-is-blue-difficult-to-find-in-nature.html.

Fig.9 Example of iridescence in butterfly wings.

Source: “Something Blue – Butterfly Wings.” Ray Cannon's Nature Notes, 9 Jan. 2019, rcannon992.com/2019/01/08/something-blue-butterfly-wings/. 

Blue Pigments in Nature

The only exception in nature that has true blue pigments incorporated into them is the obrina olivewing (Nessaea obrinus). Even predators of the obrina olivewing are not able to incorporate this pigment into themselves after consumption. The blue pigment in the obrina olivewing is called pterobilin. There is no current evidence to why the obrina olivewing has pterobilin in their wings. However, pterobilin is believed to be used as a signalling method or some cryptic functions towards other obrina olivewings nearby. [11]

Fig.9 A Picture of a Obrina Olivewing.

“Nessaea Obrinus.” Flickr, Yahoo!, 17 June 2018, www.flickr.com/photos/lpjconflickr/42134572204.

Conclusion

Blue in nature is incredibly unique and special. Although I have not personally seen any blue animals in nature, I have definitely seen blue flowers and was very attracted to them! Maybe in the future, we are able to see more organisms having blue pigmentation in their bodies!

Bibliography

[1] “Why Is Blue the World's Favorite Color?” YouGov, 13 May 2015, hk.yougov.com/en-hk/news/2015/05/13/why-blue-worlds-favorite-color/.

[2] “Blue.” In Defense of Plants, 30 Sept. 2015, www.indefenseofplants.com/blog/2015/9/30/blue. 

[3] Falkenthal, Gayle Lynn. “Our Love For Blue Flowers: It's Complicated.” Good Earth Plants, 23 May 2019, www.goodearthplants.com/our-love-for-blue-flowers-its-complicated/. 

[4] “8 Animals That Get Their Color From Food.” Mental Floss, 25 Aug. 2016, www.mentalfloss.com/article/85268/8-animals-get-their-color-food. 

[5] Pomranz, Mike. “Your Salmon Might Be Lying To You: Farm-Raised Salmon Isn't Naturally Pink.” Food & Wine, www.foodandwine.com/news/your-salmon-might-be-lying-you-farm-raised-salmon-isn-t-naturally-pink. 

[6] “Something Blue – Butterfly Wings.” Ray Cannon's Nature Notes, 9 Jan. 2019, rcannon992.com/2019/01/08/something-blue-butterfly-wings/.

[7] “A Trick Of Nature: Blue Jays Aren't Really Blue.” Forest Preserve District of Will County, www.reconnectwithnature.org/news-events/the-buzz/nature-curiosity-why-are-blue-jays-blue. 

[8] “Species Profile: Blue Morpho Butterfly.” Rainforest Alliance, www.rainforest-alliance.org/species/blue-butterfly. 

[9] Horton, Jennifer. “Where Do Butterflies Get Their Striking Colors?” HowStuffWorks, 3 Apr. 2008, animals.howstuffworks.com/insects/butterfly-colors.htm.

[10] Oh, Jenny. “What Gives the Morpho Butterfly Its Magnificent Blue?” KQED, 16 Dec. 2014, www.kqed.org/science/24552/what-gives-the-morpho-butterfly-its-magnificent-blue. 

[11] “The Rarest Pigment On Earth Is Blue Pigment, But Why Blue Is Everywhere?” Earth Buddies, 31 July 2018, earthbuddies.net/the-rarest-pigment-on-earth-is-blue-pigment-but-why-blue-is-everywhere/.