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Mating Systems Animals that reproduce sexually have evolved a wide variety of different systems for maximizing the number of young that can be raised. In the simplest system, each female is partnered by a male, and the partnership lasts for life. In more complex systems, the fittest adults have many partners while others have none at all. In polygynous breeding systems, successful males mate with more than one female. Polygyny is common in birds, particularly in species where the males establish breeding territories that provide access to food. A male with a good territory may attract several mates, while one with an inferior territory may attract few or none. Polygyny can also be seen in some mammals and is taken to extremes in species such as elephant seals. The largest and most powerful male elephant seals, weighing up to four times as much as the females, clash viciously for dominance on a breeding beach. A successful male can assemble a harem of over twenty females, but weaker males are excluded from breeding altogether. In polyandrous breeding systems, one female mates with several males. This kind of breeding system is rare and usually occurs in species where the males take on the work of raising the young. An example of a polyandrous bird is the North American spotted sandpiper. In this species, females compete for males. A single female can lay up to five sets, or clutches, of eggs, and each clutch is incubated by a different partner. The most specialized mating systems of all occur in animals that form permanent family groups. In social insects, which include many bees and wasps and all ants and termites, each group or colony is founded by a single female or queen. The queen is the only individual in the colony to reproduce. Her offspring, which can number more than a million, forage for food, maintain the nest, and care for the young. Parental Care With the exception of birds, the majority of egg-laying animals play no part in helping their young to survive. A large proportion of their young die, and to offset this, they often produce a huge number of eggs. A housefly, for example, can lay over a thousand eggs in the course of its life, while a female cod can lay 3 million. Most amphibians and reptiles lay smaller clutches of eggs, and some of them remain with their eggs and guard them until they hatch. Birds lay smaller clutches still, and the parents incubate the eggs, or keep them warm until they hatch, and continue to care for their young once they have hatched. Most ground-nesting species protect their young and lead them to food, but typical tree-nesting birds provide their young with both food and shelter until they are able to fend for themselves. Without this parental care, the young birds would have no hope of survival. Parental care is equally important in mammals, which provide food for their young in the form of milk. Raising a family in this way creates a close link between the mother and her young. This method also allows the young to learn important patterns of behavior by watching their mother at work. In small rodents, this learning period lasts for just a few days, but in larger mammals, it can last for more than a year. In the living world, resources such as food and space are limited. As a result, survival is a constant struggle. Through evolution, animals have developed a range of adaptations that give them the best chances of success. The most obvious of these adaptations are physical ones that affect the shape or structure of an animal's body. Equally important, although often less conspicuous, are adaptations that affect behavior and body processes. Together, these different adaptations allow each species to pursue a distinctive way of life. APhysical Adaptations The need to eat exposes animals to the danger of being attacked and eaten themselves. To avoid this fate, all animals have physical adaptations that enable them to escape being attacked or to survive an attack once it is underway. The simplest form of defense is a rapid escape, which calls for keen senses and well-developed systems for movement. Many plant-eating mammals depend on this strategy for survival and must maintain a constant lookout for danger. A less-demanding survival strategy, found in many small animals such as insects, involves deception. These animals use camouflage to blend in with their backgrounds, or they mimic inedible objects such as twigs or bird droppings. If a predator does come too close, they still have the option of making a dash for safety. A more sophisticated form of mimicry occurs in animals that resemble species that are poisonous. This is common in insects, and it also occurs in some snakes. Poisonous insects, such as bees and wasps, are often brightly colored to warn other animals that they are best left alone. By adopting these colors and developing similar body shapes, non-poisonous insects benefit from the same protection. The physical adaptations involved can be elaborate. The hornet clearwing moth, for example, is yellow and brown like a stinging hornet. On its first flight, it loses most of its wing scales, resulting in transparent wings that make the resemblance even more convincing. An alternative defense, seen in a wide range of animals, uses armor or spines to fend off an attack. Animal armor includes hard shells, overlapping scales, and in the case of armadillos, bands of hardened plates connected by areas of softer skin. If they are threatened, many of these animals can shut their bodies away inside their armor, making them difficult to attack. The disadvantage of this defense is that the animal cannot escape. If its armor is broken open, death is almost certain. BBehavioral Adaptations In simple animals, behavior is governed almost entirely by instinct, meaning that it is pre-programmed by an animal's genes. In more complex animals, instinctive behavior is often modified by learning, producing more-flexible responses to the outside world. Many forms of behavior help animals to survive severe environmental conditions. Two examples are hibernation, which enables animals to survive cold and food shortages in winter; and estivation, which allows animals to survive drought and heat in summer. True hibernators, such as brown bears and some rodents, become completely inactive during winter, and their body temperature falls close to freezing. While in this state, they survive entirely on food reserves stored in their bodies. Estivating animals, which include land snails and some amphibians, seal themselves up when conditions become dry and only become active again when it rains. Between these two extremes, many other animals show less drastic patterns of behavior that are triggered by cold or heat. Winter wrens, for example, often crowd together for sleep when temperatures fall below freezing. On warmer nights, they sleep on their own. Special forms of behavior also help animals to find food, to avoid being eaten, and to protect their young. One of the most advanced forms of this behavior is the use of tools. Several kinds of animals, particularly primates and birds, pick up implements such as twigs and stones and use them to get at food. More rarely, some tool-using animals seek out a particular object and then shape it so that it can be used. Woodpecker finches probe for insect grubs by making tools from cactus spines, and chimpanzees sometimes dig for termites using specially prepared twigs. Defensive behavior is exhibited by individual animals and also by animal groups. Group defense is common in herding mammals, particularly in species such as the musk-ox, which form a protective ring around their calves when threatened by wolves. It can also be seen in swallows, starlings, and other songbirds, which instinctively mob hawks and other birds of prey. By grouping together to harass their enemies, they reduce the chances that they or their young will be singled out and attacked. Individual defensive behavior is often based on threatening gestures that make an animal look larger or more dangerous than it actually is. Sometimes it involves some highly specialized forms of deception. One of the most remarkable is playing dead. Seen in animals such as the Virginia opossum and some snakes, this last-ditch defense is effective against predators that habitually hunt moving prey but leave dead animals alone.

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