The term ‘aposematic colouration’ describes the often vivid markings of animals which may as a deterrent or warning signal to any potential predator. These signals are a secondary defence mechanism, advertising that the animal is toxic, noxious or otherwise able to defend itself in a manner which may result in injury to the predator. Such colours may also be the result of Batesian mimicry, whereby an otherwise harmless species closely resembles an unpalatable species with such a degree of accuracy that it too is avoided by experienced predators. Mutual mimicry between species sharing similar anti-predator colouration may occur (Mullerian mimicry). Colour, along with startling behaviours and/or sounds may form an effective predator deterrent.
The very definition of aposematic colouration demands that an animal’s colour be bold or vivid enough to be visible against an often homogenous background of neutral shades such as greens and browns – the more conspicuous an animal is, the more likely it will be seen by a predator. The most visible colours are most often those at the red end of the spectrum and indeed reds, oranges and yellows are extremely common. While some individuals will perish as a result of their enhanced visibility, the predator will learn to associate such marking with unpalatability or danger and thus ignore similar in the future. To quote Spock1: “The needs of the many outweigh the needs of the few. Or the one.”.
Aposematism is most frequently associated with invertebrates and is indeed much more common in such taxa than in vertebrates.Ladybirds provide a classic example; their brightly coloured elytra warning of their toxicity. Many moth and butterfly caterpillars are similarly brightly coloured and may combine colouration with other defences such as eye-like markings. Warning colouration need not be immediately apparent or even visible. Some, like the caterpillar of the swallowtail butterfly, are cryptic from afar but quite alarmingly conspicuous close-up, a dual defence which proves to be quite effective. Similarly Poecilotheria regalis, a tarantula from India (above, left), is very well camouflaged from above but when disturbed it rears on its hind legs, displaying a black ventral surface surrounded by startlingly white or yellow limbs.
Of course, vertebrates may also – and do – express aposematic colouration. Poison dart frogs are often extremely colourful, vividly advertising their toxicity (though the degree of toxicity varies between species). Larger species such as skunks and porcupines are more boldly patterned, their greater size ensuring that they are readily visible. Though the colouration of these two species is somewhat similar, they both advertise very different characters. The skunk (and some mustelids, though to a much lesser degree) famously excretes a foul-smelling liquid which can be sprayed a considerable distance. The porcupine on the other hand advertises its impressive armoury of sharp spines; modified hairs coated in keratin which can inflict a painful or even fatal wound on an attacker.
The evolution of aposematic traits may appear somewhat paradoxical if taken from the commonly assumed starting point of successful crypsis (camouflage). If such vivid secondary defences evolved in an already successfully cryptic prey animal, it would seem logical to assume that the new rare and more conspicuous morphs would experience a greater degree of predation. Such frequency-dependent selection would surely result in the early removal of these individuals from the population. It would therefore seem more reasonable to explain the evolution of aposematic colouration in terms of species which are already conspicuous by virtue of their behaviour. In such instances the gradual evolution of brighter colouration imposes fewer costs in comparison to those imposed on already cryptic species. Enhanced colouration may also confer an array of benefits, from deterring predators to increased mating success. Evolution in this regard may thus be focussed by any one, or a combination of, sexual selection, facultative aposematism or the enhancement of pre-existing traits.
 Alright, Spock and Kirk. Start Trek II: The Wrath of Khan.