Darwin asked why caterpillars are so gaudy, and Wallace answered the next day
In February 1867 Charles Darwin was stumped: why are some caterpillars so beautifully coloured when they are too young to attract mates? Alfred Russel Wallace wrote back the next day. Bright colours, he suggested, told birds that these caterpillars tasted foul, so they would never be mistaken for a tasty green or brown one.
That idea is now called aposematism, from the Greek for away and sign, a name coined by the zoologist Edward Bagnall Poulton, who popularised it in his 1890 book The Colours of Animals. An animal advertises to predators that it is not worth attacking, whether because it is toxic, venomous, foul-tasting, spiny or simply aggressive. The deal suits both sides: the predator avoids a nasty meal and the prey avoids a fatal bite. For a growing caterpillar, as Wallace noted, even a single peck can be deadly.
Red, yellow, black and white work best. They stand out sharply against green leaves, hold up under changing light and shadow, and some patterns even blend in from a distance while looking alarming close up. The signals are usually honest, because showiness tends to evolve alongside nastiness, so the brighter the animal, the more toxic it often is. Poison dart frogs, perhaps the most numerous warning-coloured vertebrates, carry some of the most toxic alkaloids known, while dull relatives in the same family lack them.
Warnings are not only visual. A rattlesnake's rattle is an acoustic version, tiger moths emit ultrasonic clicks that warn bats away, and the black-and-white stripes of skunks advertise their smell while the same pattern on honey badgers advertises claws and temper. Because the system depends on predators remembering a bad experience, warning-coloured animals are often slow, tough and sociable, which lets predators learn the lesson quickly and repeatedly.
Other species cash in. Harmless Batesian mimics copy a dangerous model to borrow its protection, while in Müllerian mimicry genuinely defended species converge on similar patterns so predators learn faster. In the sea the picture is murkier. Many reef creatures are brilliantly coloured with no link to defences, red fades quickly with depth, and even the famous flashing blue rings of the venomous blue-ringed octopus have rarely if ever been tested as a warning.
Source: Aposematism
