Colour matters

Colour change is a surprisingly widespread feature in the animal kingdom. Rapid colour change occurs in both invertebrates and vertebrates. The feature has been observed in crustaceans, insects, cephalopods, amphibians, reptiles and fish.

There are two main methods for changing colour: morphological and physiological colour change. Morphological colour change is based on changes in the number and quality of pigmentophores, whereas physiological colour change is based on changes in the number of organella within the pigmentophores. Melanophores are the most common pigmentophores to have melanosomes. Physiological colour change is much faster than morphological colour change. It can happen in microseconds. Physiological colour change is regulated by the neuromuscular system in cephalopods and by the neuroendocrine system in other classes. Environmental factors, such as background, lighting conditions, temperature and moisture, along with behaviour and stress can trigger physiological colour change.

Animals capable of changing colour usually have more than one colour-change strategy. Environment, the number of predators, predator species and the presence of individuals of the same species influence the colour-change strategy. For example, the daisy parrotfish (or bullethead parrotfish, Chlorurus sordidus) has three different colourations: individuals have stripes, are all black or have an eye-dot on the tail. The purpose of the eye-dot is to frighten predators, whereas the all-black daisy parrotfish tries to blend in with its background and the striped daisy parrotfish tries to bluff or dazzle its predators. The occurrence of these colourations is influenced by environmental background, the body size of the daisy parrotfish and its social relationships. On the other hand, the common cuttlefish (Sepia officinalis) chooses its strategy by whether a predator hunts using vision or chemical signals (watch how the common cuttlefish changes its colour). Chameleons (Chamaeleonidae), however, change their colour according to the environmental background rather than to mimic or to frighten.

The common octopus (Octopus vulgaris) can change its colour. © Sari Holopainen

Temperature affects the melanocyte-stimulating hormone (MSH) in many colour-changing animals such as fish, amphibians, reptiles and crustaceans. MSH is in charge of dispersing melanin. Changing to a dark or light colour helps an animal to either reflect or absorb heat. On the other hand, changing colour can concurrently predispose the animal to predation, because the animal is unable to blend in with its environment. The colour change of over 25 desert reptile species has been proven to depend on both environmental temperature and body temperature regulation. When it gets very warm (over +40°C) reptiles change to a lighter colour despite their background being somewhat dark. The reptiles usually still escape from predation because predators are inactive at such high temperatures. In proportion, when it gets cooler reptiles become darker than their environment, especially if they are near to cover.

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