Science Focus (issue 26)

Introduction Amidst the myriad of phenomena that animals exhibit – from mimicry, hibernation, to bioluminescence – one of the most fascinating is the ability of some animals to change their colors in the blink of an eye. As a result, they can hide from predators, communicate with peers, intimidate when threatened, and regulate body temperature. How Do Animals Change Color? The process by which animals change color involves specialized cells called chromatophores. Located in the skin, they are colored cells that contain pigments or photonic structures [1]. Pigments absorb specific wavelengths of visible light and reflect those that are not absorbed, resulting in the colors we see. There are various types of pigment-containing chromatophores, named according to their color: melanophores for black or dark brown, cyanophores for blue, xanthophores for yellow chromatophores, and so on. In addition to pigmentary colors, there are structural colors created by a different mechanism. Iridophores in fish, amphibians and reptiles are made up of transparent guanine nanocrystals [2] that function to interfere with the light reflected off the skin at certain frequencies [3]. Iridophores can also produce iridescence, in which the color appears to change depending on the viewing angle [1]. Chameleons Chameleons are commonly known for being the representatives of color-changing animals. While most animals can change their color by controlling the dispersion and aggregation of chromatophores, chameleons have an additional level of control with two overlaying sets of iridophores [2]. The upper layer contains regularly arranged guanine nanocrystals. For panther chameleons, they can change their skin color by increasing the distance between these tiny crystals to manipulate the interference of light, leading to the reflection of longer wavelengths from blue to red. In the background of yellow xanthophores, the skin color of a male panther chameleon can thus change from green to yellow or orange when it is excited in male contests or courtship. The lower layer contains disordered guanine crystals of high reflectivity in the near-infrared region (700–1,400nm). It provides passive thermal protection to chameleons by reflecting direct and indirect “heat radiations” from the sun back into the environment, thus lowering their body temperature in the dry and sunny habitat. Mimic Octopus While chameleons have been the fascination of scientists for decades, the mimic octopus takes the art of adaptation to complex new heights. Among the diverse species of octopus, Thaumoctopus mimicus stands out for its remarkable mimicry skills, through a combination of sophisticated body movements and dynamic skin patterning [4]. The color-changing properties of the mimic octopus is enabled by the fine control over its chromatophores and iridophores [1]. Surrounded by radial muscles, their chromatophores Masters of Transformation: 偽裝大師:變色動物 Color-Changing