The Life Cycles of Mammals
Mammals are a unique order of animals in the animal kingdom. Mammals are characterized by being warm blooded, having sweat glands, having mammary glands and bearing live young (except for five species known as monotremes that lay eggs.) Mammals are unique from many other animals in that the young are born helpless and require parental care for an extended period of time before being able to fend for themselves.
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Reproduction
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The mammalian life cycle begins with mating. Mammals reproduce sexually. The egg that resides within the female and is fertilized by the sperm of the male, then attaches in the uterus where it begins to develop into a baby. Depending on the species, this can occur with the union of a single egg and sperm, resulting in one offspring, or with many.
Meosis
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The fertilized egg will begin to undergo rapid changes after fertilization. The zygote, as it is now called, will begin to divide into many difference cells without actually increasing in size of weight. This process is called meiosis. This is where the main germ layers are created, but well before any cell specialization takes place.
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Gestation
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From this point forward, the zygote will develop into an embryo and then a fetus, with cells branching off and specializing into organ systems and other structures. The time it takes for a zygote to become a live baby is commonly referred to as gestation. Gestation lasts for different lengths of time, depending on the species of mammal and can range anywhere from a week to a year or longer.
Birth
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After gestation has completed, the mother will give birth to live offspring. Depending on the species, the baby or babies will be born either head first, as in the case of humans, or tail first, as in the case of most mammals that live underwater. The mother will immediately begin to care for the young, cleaning it and feeding through the use of the mammary glands.
Development
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Young mammals do not undergo stages of development like larvae or any kind of metamorphosis, as they are born as miniature versions of the adult, although many of them do go through a maturation process in which they become sexually mature. After maturation, the life cycle begins again with mating and conception.
Considerations
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It is the parenting process that sets mammals apart from many other animal species. The defining characteristic of mammals is the ability to nurse their young. This milk provides constant nourishment and antibodies to the young, allowing them the best possible start to life. The mother also provides shelter and protection from the elements and would-be predators. It is these advantages that allow mammals to thrive and flourish.
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