Colors play a role in the intricate lives of animals. While many hues are common, purple stands out as a striking and less frequently observed color in the animal kingdom. This shade prompts curiosity about which creatures display it and the biological mechanisms that allow for its appearance.
Animals Adorned in Purple
Numerous animals across diverse phyla showcase purple coloration, making them visually distinct. In marine environments, the purple sea urchin (Strongylocentrotus purpuratus) has vibrant deep purple spines and body, inhabiting the eastern Pacific Ocean. The purple nudibranch, a sea slug, is known for its pinkish-purple body, often with contrasting white or orange features. Certain fish species also exhibit purple, including the purple tang, with its deep violet body and yellow fins, and the royal gramma basslet, which features a purple front half transitioning to an electric yellow tail.
On land, insects and birds also display purple. The male purple emperor butterfly (Apatura iris) has iridescent purple wings that shimmer in certain light conditions, making it a highly sought-after sight. Among birds, the violet-backed starling (Cinnyricinclus leucogaster) males exhibit brilliant metallic purple on their heads and backs, contrasting with a white belly, which helps attract mates. The purple sunbird (Cinnyris asiaticus) also features purple iridescence on its male plumage, though it can appear black in harsh sunlight. Even some amphibians, like the Indian purple frog (Nasikabatrachus sahyadrensis), can exhibit a purplish-gray hue.
The Science Behind Purple Hues
The manifestation of purple in animals primarily arises from two distinct biological mechanisms: pigmentary coloration and structural coloration. Pigmentary colors result from specialized chemical compounds, called pigments, that absorb certain wavelengths of light and reflect others. For instance, while true purple pigments are rare in animals, some carotenoproteins can produce purple-blue and green hues, particularly in marine invertebrates. Other pigments, such as porphyrins and indigoids, can also contribute to purple, red, green, or blue coloration in some animals.
More commonly, purple in animals, especially in birds and insects, results from structural coloration. This occurs not due to pigments, but from light interacting with microscopic structures on an animal’s surface, such as scales on butterfly wings or barbules on bird feathers. These nanostructures scatter or interfere with light, reflecting specific wavelengths perceived as purple. This phenomenon often leads to a shimmering, shifting color that changes with the viewing angle, as seen in iridescent birds like the purple emperor butterfly.
Why Purple Stands Out in Nature
Purple is a less common color in the animal kingdom compared to shades like brown, green, or black. This scarcity is due to biological challenges in producing this hue. Mammals, for example, are generally unable to synthesize pigments for purple, blue, or green colors, relying mostly on melanins. Animals that exhibit purple often require complex structural adaptations rather than simple pigment deposition.
Despite its rarity, purple coloration serves several ecological purposes. Its striking appearance can act as a warning signal to predators, a strategy known as aposematism, indicating an animal may be toxic or unpalatable. Some purple sea slugs use their bright colors for this purpose. Purple also aids in mate attraction, with vibrant displays signaling health and genetic fitness to potential partners, as seen in male violet-backed starlings or purple sunbirds. In certain marine environments, purple may aid in camouflage or visibility, as some wavelengths of light penetrate deeper into water, making purple visible at depths where other colors fade.