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Defense and Attack Strategies and Mechanisms in Biology 347

is imaginable that under these conditions even a slight resemblance to an unpleasant species is
enough to convince a bird not to attack.
Most insects, in particular beetles, butterflies, and moths, get their noxious chemicals from the
plants they feed on. The first bird to be discovered with warning coloration and toxic feathers is the
Pitohui of New Guinea (Dumbacher et al., 2004). The source of the alkaloids, also found in poison-
dart frogs, is Melyrid beetles.

13.3.4 Active Camouflage

Created by dynamically matching the object to be camouflaged to its background colors and light
levels thus rendering it virtually invisible to the eye. This is conceptually the same camouflage
process as that used by a chameleon. This is accomplished through a sophisticated color and light
sensor array that detects an object’s background color and brightness. This data is then computer
matched and reproduced on a pixel array covering the viewing service of the object to be
camouflaged.
Pattern control is achieved by flatfish such as the plaice (Pleuronectes platessa) that can change
its shading and patterns to suit a variety of backgrounds — including a chequer board! However, it
can manage only black and white, and then only slowly, over a matter of minutes, since its color-
change cells (melanophores) are hormonally controlled. They change color by moving pigment
around inside the cell going from ‘‘concentrated’’ (the pigment is centered making the cell white or
translucent) to ‘‘dispersed’’ (the pigment is spread around the cell which now appears dark) (Fuji,
2000; Ramachandran et al., 1996).
Color control in octopus and squid (cephalopod — literally ‘‘head-footed’’ — molluscs) is
managed by colored cells — chromatophores — that are found in the outer layers of the skin. Each
comprises an elastic sac containing pigment to which is attached radial muscles. When the muscles
contract, the chromatophore is expanded and the color is displayed; when they relax, the elastic sac
retracts. The chromatophore muscles are controlled by the nervous system. Differently colored
(red, orange, and yellow) chromatophores are arranged precisely with respect to each other, and to
reflecting cells (iridophores producing structural greens, cyans and blues, and leucophores, reflect
incident light of whatever wavelength over the entire spectrum) beneath them. Neural control of the
chromatophores enables a cephalopod to change its appearance almost instantaneously (Hanlon
et al., 1999), a key feature in some escape behaviors and during fighting signalling. Amazingly the
entire system apparently operates without feedback from sight or touch (Messenger, 2001).
The primary function of the chromatophores is to match the brightness of the background and
to help the animal resemble the substrate or break up the outline of the body. Because the chroma-
tophores are neurally controlled, the animal can, at any moment, select and exhibit one particular
body pattern out of many, which presumably makes it difficult for the predator to decide or
recognize what it is looking at. When this is associated with changes in shape or behavior, the
prey can become totally confusing. Consider this performance by an octopus found in Indo-
Malaysian waters. It is seen on the seabed as a flatfish and swims away with characteristic
‘‘vertical’’ (remember the flatfish swims on its side) undulations. As it does so it changes into a
poisonous zebra fish. It then dives into a hole and sends out two arms in opposite directions to
mimic the front and back ends of a poisonous banded sea snake (videos of these behavior patterns
are available to download with the paper by Norman et al.). It also sits on the sea bed with its arms
raised, possibly in imitation of a large poisonous sea anemone. Or it can sink slowly through the
water column apparently imitating a jellyfish (Norman et al., 2001). Each of these types of animal
requires a different response on the part of the predator, which presumably is totally confused. Such
dynamic mimicry is seen only in cephalopods and the films of the Marx Brothers.
Countershading in animals is widespread and cephalopods are no exception. On the ventral
surface, the chromatophores are generally sparse, sometimes with iridophores to enhance reflec-
tion; dorsally the chromatophores are much more numerous and tend to be maintained tonically