HUMANS, WHALES, AND SPIDERS HAVE SOMETHING IN COMMON

by Whit Gibbons


June 20, 2004


The world is full of predators. Consequently the world must be full of even more prey, as the total amount of biomass, the living weight, of prey must be greater than that of the predators that eat them. But although predators require prey, their victims do not willingly offer themselves up as sustenance. Consequently, successful predators must have effective strategies for capturing their food. The diverse array of such tactics provides an excellent platform to show just how complex and fascinating the natural world is.

Predatory animals use an impressive panoply of techniques to catch other animals for food. Many simply attack unaware prey, such as a red-tailed hawk swooping down on a cotton rat in a field or a praying mantis pouncing on a small lizard. Among the least imaginative and most common approaches to capturing prey are ones in which the predator outruns, outflies, or outswims the prey. A cheetah's attack on a gazelle, a peregrine falcon's capture of a dove in flight, or a barracuda's raid on a school of mullet are brute force power plays in which the predators win often enough to keep the species extant.

Some mammals demonstrate more sophisticated prey acquisition tactics by using herding techniques. For example, several dolphins will operate in unison to encircle a school of fish, drive them toward shore, and actually force them onto a sandbar, on which the dolphins themselves emerge. Wolf packs will function as a social unit to cut off a single caribou from the remainder of the herd.

Some predators invest in devious strategies such as using a tail or tongue as a lure to attract an unwitting prey animal. Baby copperheads wiggle their yellow tails to attract small frogs, and alligator snappers open their mouths underwater to display a large pink tongue that to a passing fish looks like a worm. Some tropical lightning bugs flash the mating signal of other species to attract males that think they are headed toward a romantic encounter only to become prey of the deceiver.

Archer fish are in a special class for prey capture techniques. These little fish, found in coastal areas from India to Australia, eat flying insects. They capture insects by waiting until one lands on a limb above the water. The fish positions itself below the bug and squirts a powerful stream of water that knocks it into the water, whereupon the fish snaps it up. Had I not seen this myself at the Ft. Worth Zoo, I would be skeptical of such talent in a fish.

One capture technique used by only a few animals is to trap the prey so that it is contained until the predator subdues it. Aside from humans, spiders are the paragon of trappers among the animals. The number of flying insects that perish each day in spider webs is staggering. Another trapper is the ant lion, which we called doodlebugs when I was a kid. Ant lion adults look somewhat like damselflies, but the voracious larvae are the ones smaller insects need to beware of. Ant lion larvae dig conical pits in dusty or sandy soil and wait at the bottom for ants or other small insects to walk over the rim and slide to the bottom. The larval ant lion grabs the helpless prey with its pincers.

At the other end of the size spectrum of trappers are humpback whales. Two dozen or more will swim together in a circle deep below the ocean's surface while blowing bubbles. The rising bubbles form a cylinder called a "bubble net" through which fish will not pass. The whales then swim up through the center of the bubble net, eating the congregated fish. Why don't the fish simply swim through the bubbles? Some scientists believe that the whales make loud sounds as the bubbles rise. Because the air bubbles are more likely to trap the sound than is the water within the cylinder, the fish stay in the quiet area of water inside the cylinder.

Every species of animal has to get its food in some manner. Discovering--and understanding--the techniques that various animals use is an exciting part of ecology.



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