Alligator

Alligator common name for two carnivorous reptiles in the crocodilian order.Alligator is the Spanish el name,"the lizard”. Alligators and their close relatives the caimans have broad, flat, and rounded snouts, as opposed to the longer, sharper snouts of other crocodilians; also unlike other crocodilians, their lower teeth cannot be seen when their mouths are closed. Alligators feed on fish, frogs, snakes, turtles, birds, mammals, and carrion. In North America they are also known to attack humans occasionally. Alligators can survive a wider range of temperatures than other crocodilians, and they are found in more temperate regions. Their breeding season is generally restricted to spring.When alligators search for a mate they bellow often, perhaps to announce their presence to females and to warn other males to stay away. Unwanted intruders are confronted with ritual gaping, lunging, and hissing, but courtship behavior is sedate. After mating, the male seeks his own territory while the female builds a nest of mud and plants nearby, above flood level. The eggs, from 30 to 60 in a clutch, are covered with mud and vegetational debris. The female stands guard as the eggs incubate in the heat from this decaying vegetation and from the sun. When the eggs are ready to hatch, in about 60 days, the young begin to croak softly within the egg. The female may then assist the young in escaping from the nest and may even carry them in her mouth to the water's edge. The female may remain near her young for a year or more. A young alligator in distress will give a series of sharp croaks that may quickly bring the female to investigate.
Only two species of alligator exist: the Chinese alligator and the American alligator. The Chinese alligator makes its home in the Yangtze River Basin of China. It is more timid and much smaller than the American alligator, seldom exceeding 2.5 m (8 ft) in length, and is considered little threat to humans. The American alligator lives mainly in freshwater swamps, lakes, and bayous in the southeastern United States, but it ranges as far west as the Río Grande in Texas. It is larger, reaching up to about 6 m (about 20 ft) in length, and is potentially dangerous to humans. Attacks occur infrequently, usually in areas where humans have recently encroached on alligator habitat or where alligators have become accustomed to the presence of humans. Hunted for generations both for sport and for its hide, populations of the American alligator dwindled until, in 1967, it was declared an endangered species. Under this protection it made a strong comeback and, little more than a decade later, hunting of the American alligator was again allowed in some states.

Raccoon

Raccoon is carnivorous mammal, found throughout the United States, southern Canada, and Central and South America, whose head is broad, tapering to a pointed muzzle, and whose ears are short and erect. The body is generally short and plump, with a long-haired coat of fur, and the tail is bushy. The legs are short; each foot has five toes, and the soles are naked. The animal walks on the soles of its feet with the heels touching the ground, similar to the practice of bears and humans. Raccoons are grayish-brown above and light gray beneath, with black cheek patches that narrow into a vertical stripe extending from the space between the eyes to the top of the head. The rest of the face is pale gray, with dark eyes and white whiskers. The tail is marked with six or seven brownish-black rings. A raccoon ranges in length from 0.62 to 1 m (2 to 3.3 ft), including its tail, which is 20 to 40 cm (8 to 16 in) long.The common species found throughout the United States usually lives in trees or near ponds and streams of forests close to civilization. During the night it hunts for poultry, mice, birds' eggs, various insects, fish, and frogs. Occasionally it varies its diet with nuts and wild fruit. It is a skillful swimmer. Northern raccoons spend the winter in a den, usually high in a hollow tree, sleeping but not hibernating, and emerging during relatively warm periods. From four to six young are born in a single litter each spring, and members of the same family live and travel together for about one year.
Another species, distributed from Costa Rica to northern South America, is the agouara, or crab-eating raccoon. This animal is longer than the other species, although the tail is slightly shorter; the teeth are thicker and stronger and the fur shorter. The crab-eating raccoon is dark gray with patches of yellow.The fur of the raccoon, especially that of the northern species, has been highly valued in North America since the 17th century. Coonskin caps, and coats and robes made of the coarse but attractive fur, are still worn today. Coon hunting is practiced extensively in the southern United States. The animals are hunted at night with dogs and are usually found near swamps or streams.

Angelfish

Angelfish is name used for several different fishes, including the true angelfishes and the butterfly fishes. Angelfishes differ from butterfly fishes in having a spine near the lower edge of the gill cover. These laterally compressed fishes are among the most beautiful of the tropical reef fishes. Most species are only a few centimeters long, but some grow to a length of 61 cm (24 in). In many species the juveniles and adults are colored differently; the young French angelfish of tropical Atlantic waters is black with bright yellow bands; the adult is predominantly black. The well-known queen angelfish is also differently colored as a juvenile. Most angelfishes feed on small invertebrates. Certain species are often called freshwater angelfish and are popular as aquarium fishes (see Cichlid). In the United States a spadefish is sometimes incorrectly called an angelfish.
Angelfishes belong to the order Perciformes. True angelfishes make up the family Pomacanthidae. Butterfly fishes constitute the family Chaetodontidae. Freshwater angelfish belong to the family Cichlidae. French angelfishes are classified as Pomacanthus paru. Queen angelfishes are classified as Holacanthus ciliaris. Spadefishes are classified as Chaetodipterus faber.

Catfish

Catfish is common name for a group of more than 2,500 species of fish, classified in about 30 families and found worldwide. Catfish are mostly nocturnal scavengers, living near the bottom in shallow waters. Two families are primarily marine, and all other families inhabit fresh water.Catfish are named for the feelers, or barbels, located around the mouth suggesting the whiskers of a cat. These feelers are used for finding food. The body is scaleless, either naked or with bony plates. The dorsal and pectoral fins are often edged with sharp spines that are used for defense. They can inflict severe wounds and are poisonous in some species.
Catfish range in size from about 32 mm (1.25 in) and 7 g (0.25 oz) to 4.5 m (15 ft) and 320 kg (700 lb). The huge sheatfish native to Europe can weigh up to 180 kg (up to 400 lb). Smaller species include stone cats and mad toms, which build nests in the mud or under stones and guard the eggs and young. In many of the marine catfish of the Ariidae family, the pea-sized eggs are carried and cared for in the mouth of the male until hatched.The blind catfish, found in caverns in eastern Pennsylvania, has atrophied eyes. The electric catfish of the Nile and tropical central Africa is capable of giving an electric shock with enough voltage to stun an animal (350 to 450 volts) (see Electric Fish). Some catfish of the family Mochokidae normally swim upside down.
The walking catfish can travel across land to areas of deeper water during dry spells. On land, it moves using a slithering motion combined with a thrashing of its tail. A stout spine in each pectoral fin digs into the ground to help balance and propel the fish. It is able to breathe air by means of a modified gill that forms an air chamber. This catfish originally occurred in eastern India and Southeast Asia but was discovered near Boca Raton, Florida in 1968, following its importation by tropical-fish dealers. Its maximum length is 56 cm (22 in).Of the numerous catfish species native to North America, the bullhead is commonly fished for eating. Of greatest commercial importance are fish in the Mississippi River valley and the Gulf states, some of which weigh as much as 70 kg (150 lb). The blue catfish, or chucklehead, and the channel catfish, the flesh of which is esteemed as equal to that of black bass, form the major part of the harvest.Many species of catfish have been listed as endangered on the International Union for the Conservation of Nature and Natural Resources (IUCN) Red List of Threatened Species. These include the giant catfish, the cave catfish, and the Andean catfish.

Angler

Angler is one of the common name for any of about 265 related saltwater fishes having appendages resembling fishing rods or lures with which they entice their prey. The common anglerfish is found along the coasts of Europe and North America from the British Isles and Nova Scotia to Barbados. Up to 1.5 m (about 5 ft) long, they live on the ocean floor, creeping along on modified pectoral fins in search of food. With a huge mouth and distensible stomach, an angler can swallow other fish as large as itself. In the United States certain anglers are also known as goosefishes and are often marketed under the name monkfish. Other groups of anglers include batfishes, frogfishes, and sea toads.
One of the most unusual aspects of anglers is their reproductive behavior. In many species of deep-sea anglers, the male is less than one-tenth the size of the female and lacks her characteristic lure. The parasitic male attaches himself to the body of his mate by biting through the skin of the female host. The circulatory systems of the two fish then join, with nutrients from the blood of the female thereafter providing the male angler with his only source of nourishment. Anglers are the only fish that exhibit this type of extreme sexual dimorphism.Anglers make up the order Lophiiformes. Those known as goosefishes make up the family Lophidae in the suborder Lophioidei, including the common anglerfish, or European goosefish, classified as Lophius piscatorius. Sexual dimorphism is characteristic of anglers belonging to the suborder Ceratioidei, often referred to as deep-sea anglers. Other groups of anglers include the batfish family, Ogcocephalidae, the frogfish family, Antennariidae, and the sea toad family, Chaunacidae.

Cockatiel

Cockatiel is the common name for a small, crested parrot native to Australia that is a popular pet. It is also known as a quarrion, weero, cockatoo parrot, or crested parrot. Cockatiels are gentle and affectionate birds that make good companions, especially when kept as solitary pets. Their soft call consists of a long, rolling kweel-kweel, but they can be taught to whistle basic melodies or speak simple words.Cockatiels in the wild live throughout Australia, but they are found mainly in the interior regions. They prefer open fields with groups of trees or bushes and a nearby water source. Australia bans the commercial export of this bird species. Cockatiels purchased as pets in other countries are the offspring of captive birds.From beak to tip of tail an adult cockatiel is a small bird, ranging in length from 29 to 34 cm (11 to 13 in) and weighing 75 to 125 g (3 to 4 oz). Male cockatiels (cocks) are larger than females (hens). In the wild, cockatiels display mostly slate-gray coloring. Cocks have a bright, lemon-yellow face and throat with prominent orange cheek patches and dark brown to black eyes. The pointed crest of cocks is predominantly yellow, with a grayish-yellow tint at the tip. Their white shoulder and wing patches contrast with a gray body; the beak and feet are also gray. The underside of the tail is black. Hens display similar but more subdued coloring, but the back and rump are a light gray with narrow white stripes. The underside of the tail is also striped with yellow and white markings.
Genetic mutations and selective breeding have produced eight color varieties in captive cockatiels, described as pied, pearl, cinnamon, fallow, silver, lutino, whiteface, and albino. The pied cockatiel may be predominantly white or gray, with an irregular pattern of yellow, white, or gray feathers. The pearl cockatiel is primarily yellow with a checkered or scalloped pattern on the back and wings created by feathers with a yellow or white center and darker edges. Cinnamon and fallow cockatiels are both brown in color, with fallow a lighter brown than cinnamon. The silver cockatiel has metallic gray coloring and red eyes. The lutino cockatiel is mostly white or yellow, with dark red eyes, while the whiteface cockatiel is mostly gray and white with no yellow or orange. Albinos result from the combination of whiteface and lutino coloring. Combinations of these eight color variations are also common.In the wild, cockatiels band together in flocks of 12 to 100, but they may also occasionally travel in pairs. They are nomadic or migratory birds that follow the availability of food and water. Their diet consists of grains, fruits and berries, seedling grasses, and seeds. Cockatiels forage on the ground but are quick to fly up into trees to avoid birds of prey, their most common predators. Because large cockatiel flocks may devour fields of crops, some farmers consider cockatiels as pests.
In northern Australia, the cockatiel mating season occurs from April to June; in southern Australia, cockatiels breed from August to December. During courtship, the cock displays his white shoulder patches to the hen, and he may drum his feet on the ground or a tree branch. If the hen flies away, the cock follows and repeats his display. Nests are built in hollow trees, usually eucalyptus trees, near fresh water. Both cock and hen take turns incubating the four to seven eggs, which hatch in about three weeks. Cockatiels are one of the few species of parrots in which both sexes share brooding responsibilities.Cockatiels do not display adult coloring until after their first molting (seasonal shedding of feathers) at six to nine months. They reach sexual maturity between 6 and 12 months. Cockatiels have an average life span of 12 to 15 years.

Gene Flow and Migration

Another source of genetic change in human populations is gene flow, the exchange of genes between populations. Gene flow occurs directly when individuals from one population mate with members of another population, thereby introducing their genes into the population. Increased gene flow between populations generally makes them more alike than they had been previously. Gene flow also occurs indirectly. For example, if population A interbreeds with population B, and population B interbreeds with population C, some genes from population A will pass to population C. In this way, gene flow occurs across vast geographic regions and connects distant populations. In fact, global gene flow maintains the unity of the human species, ensuring that people from any two populations in the world can successfully mate. If a human population became isolated and no longer shared gene flow with other populations, it might, over hundreds of thousands of years, lose the ability to breed successfully with other human populations. At that point the isolated population would be considered a new species.
In humans, gene flow often occurs as a result of migration. Migrations most frequently occur on a small scale, as when individuals or families move to a neighboring village, town, or city. Small-scale migration usually takes place at short distances and is reciprocal—that is, members of neighboring populations each migrate to the region of the other population. Large-scale or mass migrations occur when a large group of people moves to a new region, often because of the effects of war or natural disaster.
Mass migration and major population resettlements dramatically increase gene flow. For example, Africans who were brought to the United States as slaves, as well as their descendants, intermixed with white populations. Today the gene pool of those who identify themselves as African American is intermediate between that of American whites and African blacks. On average, African Americans in the United States have 30 percent European ancestry. Those African Americans in the northern United States may have up to 50 percent European ancestry while those in the Southern states—where laws and cultural values long prohibited racial mixing—may have as little as 10 percent European ancestry. This difference illustrates the power that psychological and cultural barriers can have in decreasing gene flow. People who feel deeply rooted in a particular racial or ethnic group may have some animosity toward the mating of people with different physical appearances or from different cultural backgrounds. Religious and socioeconomic differences can also act as barriers to gene flow. However, people are highly social by nature. Even with the effects of racism and ethnocentrism (a belief in the superiority of one’s own social or cultural group), people have always intermarried and interbred with members of neighboring groups.
Historically, natural barriers such as large rivers, seas, deserts, and mountain ranges have prevented migration and reduced gene flow between certain regions. Geographic distance also impeded migrations; people preferred to migrate only short distances. Over the course of the past several centuries, technological improvements in transportation have reduced the influence of geography and distance. For instance, people now can travel easily from one side of the world to the other within a day by airplane. In general, however, populations tend to be more similar to their neighbors and more different from populations that live far away.