Biological Notes: Nature's tooth, claw and weapon systems

MOST ANIMALS are either eaten or eat other animals. Plants, too, are often consumed. Consequently the chances of being devoured, or of eating some other organism to survive, are exceedingly high.

The evolutionary struggle that pits predator against prey, or plants against herbivore, may seem like an arms race, but the analogy is oversimplified. If the antelope evolved higher running speeds to escape the lion, which in turn became more fleet-footed to catch the antelope, which then became even faster to escape the lion - ad infinitum - they would both become the fastest animals on Earth. This competition, which characterised the arms race between the superpowers, is not nature's hallmark. One of the reasons is because killing, or avoiding being killed, is not the only item on an organism's agenda, and an organism cannot be optimally evolved for all things. The cheetah, for example, is the fastest land animal, reaching speeds of about 60mph. Its superior speed gives it a much higher hunting success over the lion, whose top speed is below 40mph. But cheetahs pay a high price for their speed: their smaller size and more slender build make them vulnerable to attack from lions, which often rob them of their quarry. Although the arms-race analogy is imperfect, it is useful to consider an organism from the perspective of its weapon systems.

We busy warm-blooded mammals are often dismissive of the "inferior" cold- blooded reptiles, like snakes, which spend so much time inactive. But mammals often become their prey, and some snakes have evolved a heat-seeking guidance system to locate them. All snakes capture their prey with the strike, which is when the poisonous ones inoculate their venom. The larger the prey, the bigger the dose. Venom is a corrosive cocktail of toxins and enzymes which begins digesting the animal from the inside. This is important because snakes are unable to chew their food. It is therefore in the snake's interest for the prey to linger, so that its dying heart can pump the enzymes throughout its body. To avoid injury, snakes release mammalian prey immediately after biting them. But birds are retained, otherwise they would fly too far away during their death throes for the snake to relocate them.

Some prey species have co-evolved defensive systems to counteract their prey's offensive systems. Moths, for example, which are nocturnal, are heavily preyed upon by bats, which use sonar to locate them. Many moths have co-evolved sensitive hearing, enabling them to detect the sonar signals of approaching bats. Some moths respond to the bat's sonar by diving to the ground. Some others, remarkably, have evolved a sonar-jamming system that interferes with the bat's own sonar.

Plants, for all their floral innocence, possess some of the most subtle, and deadliest, of all defence systems. The chemical arsenal of plants is vast, accounting for over 90 per cent of the Earth's biological compounds. They range in toxicity from mild stimulants like caffeine, to deadly poisons like strychnine. Tobacco plants manufacture nicotine to discourage herbivores. It can be deadly, as a family in the United States discovered when they boiled up tobacco leaves for a vegetable. Some plants produce chemicals that mimic insect growth hormones, interrupting the life-cycle of those that feed on them.

Nature is sometimes red in tooth and claw, but we should avoid judging what we see in human terms. It is just as much a part of the normal scheme of things for a raptor to snuff out a lamb as it is for the lamb to graze upon the grass.

Chris McGowan is the author of `The Raptor and the Lamb: predators and prey in the living world', published this week (Allen Lane, pounds 18.99)