Animal

B

Other Feeding Strategies

Most predators hunt the largest animals that they can catch without putting themselves unduly at risk. However, some animals concentrate on food items that are much too small to be worth collecting one by one. Instead of catching food individually, they have special feeding adaptations for sweeping it up in bulk.

On land, these animals include insect-eating mammals, such as anteaters and pangolins. Using their long and sticky tongues, they lick up ants and termites and can consume over 20,000 insects a day. In water, this kind of feeding strategy is mirrored by animals called filter feeders, which sieve small animals or food particles from their surroundings. Many of these filter feeders are sessile animals that sieve food from the water immediately around them. Others, such as some whales, scoop up their food while on the move and filter it out in their mouths, using specialized gills or plates of a fibrous material called baleen. This feeding technique is extremely efficient, allowing whales to grow to an immense size.

In another feeding technique, predators seek out sources of food that are much larger than themselves but only eat part of their prey—usually its blood. This way of life is has been pursued with great success by several groups of flying insects, such as mosquitoes and horseflies. But in the animal world as a whole, fluid diets are much more common in animals that feed on plants. Aphids, cicadas, and other true bugs use piercing mouthparts to suck sap from plant stems. Many different animals, including moths, butterflies, hummingbirds, and bats, use probing beaks and tongues to reach nectar in flowers.

To avoid the need to track down food, some animals use a highly specialized feeding strategy, called parasitism (see Parasite). A parasite lives on or inside other animals and simply siphons off some of its host’s food or, more commonly, feeds on the host itself. External parasites, such as fleas, have well-developed senses and adaptations that enable them to cling to their hosts. Internal parasites, such as tapeworms and liver flukes, are highly modified for a life inside their hosts. The sense organs of internal parasites are rudimentary or absent because they do not need to find food or avoid enemies. Instead, they devote their time entirely to the twin tasks of feeding and reproduction.

---

---

V

Breathing

Wherever they live, animals need oxygen in order to survive. By breathing, or respiring, they extract oxygen from their surroundings and dispose of carbon dioxide waste (see Respiration).

Very small animals do not need any special adaptations for obtaining oxygen. Oxygen simply diffuses in through their body surface, with carbon dioxide traveling out the same way. Larger animals cannot rely on this system because they have a much bigger volume relative to their surface area. To obtain sufficient oxygen, large animals have to boost their oxygen intake by using special respiratory organs. In water, many animals breathe by using gills. A typical gill consists of a stack of thin flaps connected to the animal’s blood supply. Water moves past the flaps in a one-way flow, either when the animal moves, or when it pumps water through its body. The flaps extract oxygen from the water and pass it into the blood, which transfers it to needed tissues. The blood releases carbon dioxide in exchange.

Gills do not work on land because their flaps collapse and stick together. Instead, land animals have evolved two different kinds of respiratory organs: tracheal systems and lungs. Tracheal systems are found in insects and many other arthropods. They consist of slender hollow tubes, called tracheae, that reach deep into the body, delivering oxygen from outside. Lungs are hollow cavities that have a large surface area. They are found in vertebrates and also in some invertebrates, such as terrestrial mollusks.

In tracheae and most lungs, gases move in a two-way flow. Most vertebrates actively pump air in and out of their lungs to step up the rate of gas exchange. By stretching and squeezing their bodies, some arthropods behave in a similar way.

VI

Movement

All animals can move parts of their bodies. The majority are also capable of locomotion—movement of the whole body from place to place. Many simple animals, such as rotifers and flatworms, move with the help of microscopic hairlike structures called cilia. These beat in a coordinated way, propelling the animal through water or making it glide over solid surfaces at the rate of a few inches an hour. Another form of creeping movement, seen in earthworms, involves changes in body shape. The worm’s segments extend and contract in a set sequence, allowing it to force its way through the surrounding soil.

Some of the earthworm’s relatives have flaps called parapodia that help them to move, but even with these, their speed is fairly modest. With a few notable exceptions—such as squid and octopuses, which can move by a form of jet propulsion—the fastest animals by far are ones that have skeletons and jointed limbs.

A

Jointed Limbs

Jointed limbs are found in only two groups of animals: the arthropods and vertebrates. An arthropod’s limbs are made of a number of hard tubular segments, which form part of its external skeleton, or exoskeleton. The muscles that operate them are hidden away inside this strong outer framework. In vertebrates, the plan is reversed. The bony skeleton forms an internal framework, with muscles attached around it.

During the course of evolution, both these kinds of limbs have become modified in many different ways. Aquatic animals often have paddlelike limbs that push against the water, enabling them to speed away from predators or after food, or to maneuver their way around confined spaces. On land, the fastest animals, such as the horse and cheetah, have long legs and a flexible backbone, which helps to increase the length of their stride. Land animals that move by jumping often have highly developed hind legs, with extra-large muscles. In fleas, the muscles squeeze an elastic material called resilin, which flicks the legs back when released. This extremely rapid flick is faster than a jump triggered by muscles alone, and it throws a flea up to 30 cm (12 in) into the air.

Many animals can glide, but only insects, birds, and bats are capable of powered flight. The fastest flying insects are dragonflies, which can reach speeds of about 29 km/h (about 18 mph) in short bursts. However, in terms of speed and endurance, birds are by far the most successful animal aviators. Swans and geese can cruise at 64 km/h (40 mph) for many hours at a time, while peregrine falcons can briefly reach 145 km/h (90 mph) when they swoop down on their prey.

B

Patterns of Movement

Being able to move gives animals many advantages, but it also generates its own demands. For any animal, random movement can be unhelpful or even dangerous. To be useful, movement has to be carefully guided.

Animals are guided by their senses, which provide feedback about their changing surroundings. In animals that have radial symmetry (symmetry around a central point), such as jellyfishes, sensory nerves are arranged more or less evenly around the body. This arrangement makes the animal equally sensitive to stimuli from any direction. In bilaterally symmetrical animals (animals made of equal halves), sensory nerves are concentrated in the head. They convey signals to the brain from organs such as ears and eyes, telling an animal about the surroundings that it is about to encounter.

These sensory systems help animals to move toward food and away from possible danger. On a longer time span, they also guide them through much more complex patterns of movement that are essential for their survival. These movements include special kinds of behavior needed to locate a partner, and also seasonal movements or migrations.

Some of the shortest migrations are carried out by microscopic flatworms that live on sandy shores. These worms migrate up to the surface of the sand at low tide and back into it at high tide—a total distance of about 20 cm (about 8 in) roughly twice a day. In the open ocean, many planktonic animals carry out larger daily migrations, rising to the surface at dusk and then sinking at sunrise. By doing this, they reduce the chances of being eaten.

The longest migrations are annual ones, undertaken by animals in response to the changing seasons. By carrying out these journeys, animals can breed in places where food is abundant for just a few months each year. Long-distance annual migration is seen in some plant-eating mammals, such as wildebeest and caribou, and also in whales, but it is most common in animals that fly. Some birds, such as terns and shearwaters, migrate over 32,000 km (20,000 mi) each year. Research has shown that during these epic journeys, they use a variety of cues to help them navigate. These include familiar landmarks, the position of the sun and stars, and the also the orientation of Earth’s magnetic field (see Animal Migration).

	More from Encarta
	•  Presidential Myths Quiz •  Coffee break: Recharge your brain