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346 Biomimetics: Biologically Inspired Technologies

Irish elk antler after more than 10,000 years in a peat bog. Instead, neutron diffraction, which
measures the degree of preferred orientation of the hydroxyapatite crystals that comprise bone,
showed that the orientation of the hydroxapatite is predictable from the presumed forces generated
during fighting. Thus on the tensile faces of the antler, the orientation was along the length of the
antler, whereas on the compressive faces, the orientation was more orthogonal to the long axis —
exactly what the theory of fibrous composite materials predicts (Kitchener et al., 1994).

13.3.2 Passive Camouflage

Many American hunters recommend that more effort should be put into the research on camou-
flage, and that body armour should be a second priority to finding effective concealment. The logic
is that what you can’t see, you can’t hit. Body armour is required only when you can be seen and
identified.
Many animals and plants, especially insects, can look like inert objects such as bits of wood
or stones (e.g., the succulent South American plant Lithops). Because of their colored wings,
many moths can conceal themselves when placed against a suitable background such as the bark
of a tree. The peppered moth (Biston betularia) in industrial areas of England has been held
as a classic example of natural selection, with birds eating those moths that they could see
only when they were sitting on an unsuitably colored bark. In this instance the moth was
originally light with small black speckling, but pollution produced in the early industrial revolution
blackened the trees, so an initially rare dark form of the moth was selected by being less easily seen
and eaten (Kettlewell, 1955). Later, with reduced pollution and clearing of the woods, the bark was
lighter and better lit and the lighter-colored form again predominated. Similarly many nesting birds
are difficult to see; ground-nesting birds have camouflaged eggs and chicks. Many insects,
especially grasshoppers, have bright hind wings which disappear when the insect stops flying,
settles, and folds its wings thus becoming camouflaged. This sudden change makes it difficult to
spot the insect.
Another basic component of passive camouflage, well known to technology, is countershading,
in which, those parts of the body that are normally well illuminated are darkly colored, and those
that are normally shaded lightly colored . This is seen in both terrestrial and aquatic animals; the
corollary is the larva of the privet hawk moth (Sphinx ligustri) which is dark on the underside and
light on the upperside, and habitually hangs inverted beneath its twig. The effect is to flatten the
aspect of the animal, making it difficult to judge its size and how far away it is.
The literature of camouflage in biology is very large (Wickler, 1968).

13.3.3 Warning Coloration

The announcement that you are strong or dangerous is useful since it can deter an enemy from
attacking, and gains its best effect by the strong making themselves easily seen. But one can also
pretend strength. This is not novel, and has been used for hundreds of years with armies making
themselves appear larger than they are with hats on sticks, unattended guns protruding through the
battlements, and soldiers circulating past a small gap for the enemy to see . . .
Many animals and plants (especially fruits) advertize that they are poisonous or that they have
a very nasty sting or bite. Typical warning colors are bright, for instance red and yellow associated
with black, mutually arranged to maximize contrast and visibility (aposematic coloration). There is
a vast amount of literature on this aspect of coloration, which includes mimicking of an unpalatable
animal by a palatable one (Batesian mimicry) and mimicry of palatable mimics of unpalatable
animals (Mu ¨llerian mimicry). Such mimicry is probably commonest amongst butterflies, where the
main selection agent is predatory birds and the habitat is thick forest or woodland (Wickler, 1968).
Thus, the predatory bird probably only ever gets a fleeting glimpse, poorly illuminated of its
prospective prey, and with this minimal information it has to decide whether or not to attack. It