WHALES, AND SPIDERS HAVE SOMETHING IN COMMON
by Whit Gibbons
June 20, 2004
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.
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
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
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.
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.
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.
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.
you have an environmental question or comment, email