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Bar-Cohen : Biomimetics: Biologically Inspired Technologies DK3163_c013 Final Proof page 345 21.9.2005 11:51pm

Defense and Attack Strategies and Mechanisms in Biology 345

Unfortunately the data on ‘‘soft’’ body armor (e.g., Kevlar) does not quote performance in these
units, preferring to equate energy of an incoming threat to depth of penetration through the armor.
Presumably one has to go to reports from the old big game hunters to get similar information about
the rhinoceros. However, leather is still tougher than Kevlar, although nobody really understands
why, since the collagen fibres are not dissimilar from Kevlar in general morphology.
A concept that is entirely alien to the current design of man-made armor is the porcupine quill,
although the pikestaff of the medieval infantryman might be considered analogous, and parts of
mediaeval armor and their weapons were equipped with spikes to keep the enemy at bay. The
porcupine has several different types of quill; those with a length-to-diameter ratio greater than
about 25 are mostly rattles to warn enemies that there are quills here. Those with a lower length-
to-diameter ratio (15 or less) act as columns when they meet an end load, and with the sharp tip, can
easily penetrate flesh. They are sometimes brittle and the tip can break off, but they also have weak
roots in the porcupine’s skin and so can easily be pulled out when the impaled attacker moves away.
The quills are filled with a variety of reinforcing foams, struts, and stringers, so that they rarely
break when buckled (Vincent and Owers, 1986). Quills are modified hairs and are made of keratin.
In general, plants have totally passive defense mechanisms, which is energetically probably
much cheaper. They are thus built to survive a certain amount of damage due to grazing, and may
even grow more vigorously in response. Many plants, especially those living under dry conditions,
such as the acacia, have spines, thorns, or hooks that cause pain to the animals attacking them.
Presumably the giraffe, which feeds on such plants, has a reinforced surface to its tongue so that it
can cope with the abuse. Many of the grain-bearing plants (Graminae) have silica particles —
sometimes as much as 15% of the dry weight — which wears down the teeth of the animals feeding
on them. Indeed the performance of the teeth is frequently dependent on such wear, exposing a
complex of self-sharpening cutting and grinding surfaces (Alexander, 1983). The literature on
plant–animal interactions is large, mostly concerned with how plants control the ease with which
they can be grazed, commonly by limiting crack propagation with inhomogeneities such as
embedded fibres; and their chemical defenses which range from repulsive taste or smell, through
manipulation of the digestion or behavior of the grazer (by psychoactive drugs) to lethal chemicals,
mostly in those plants which cannot afford to be eaten since they grow so slowly.
In both plants and animals, spines and thorns are passive and are of use only at close quarters.
The closest equivalent is barbed wire which many claim to be biomimetic.
Horns and antlers can be used for both attack and defense, an unusual concept for technology —
the closest analogy is the sword, which can be used both to deliver a blow and to parry one. The
utility of antlers (dead, made of bone, replaced each season, grown from the tip) and horns (living,
made of a thick keratin sheath over a bone core, incremented each season, grown from the base) has
been questioned by animal behaviorists who find difficulty coping with the wide range in sizes of
horns and antlers, and the range in forces imposed on them during fighting. These problems were
largely resolved by Kitchener, who showed that there is a linear relationship between the second
moment of area at the base of the horn or antler and the body weight of the animal, and that this
relationship is constant for any single style of fighting. Most styles are ritualistic and akin to
wrestling; sheep and goats are far more agonistic, throwing themselves at each other resulting in
more random forces being exerted on their horns (Kitchener, 1991).
Ever since their discovery in the 16th century, the enormous antlers of the extinct Irish elk or
giant deer (Megaloceros giganteus) have attracted scientific attention. Mechanical analysis of the
antlers of the Irish elk shows that they are massively over-designed for display (for which, as John
Currey pointed out, they really only need to be made of waterproof cardboard) because the force
exerted by gravity acting on the antlers is less than 1% of their strength. In contrast, the antlers seem
to be optimally designed for taking the maximum estimated forces of fighting, that are more than
50% of the strength of the antler, as would be expected for a biological structure of this kind.
However, this analysis assumes that the mechanical properties of the bone of the Irish elk antlers
and living deer are similar. It would be unwise to measure directly the mechanical properties of