Life Cycle of Vertebrates
The biological group known as vertebrates is an extremely diverse category. They all share at least one common trait and that is having a vertebral column which houses the spinal cord. Included in this category are fish, sharks and rays, mammals, reptiles, birds and amphibians. These animals may live different lives, but their life cycles are all strikingly similar.
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Sexual Reproduction
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One common bond of all vertebrates is that they are a product of sexual reproduction. Every vertebrate received half of its DNA from a sperm cell of its father, and the other half of its DNA came from one of their mother's eggs. When the sperm and the egg combine in a process called fertilization, the resulting single cell is called a zygote. That one zygote will copy itself into two cells, then four cells and so on until the entire organism is finished. Sexual reproduction can involve intercourse--as with mammals--but many vertebrates do not come in contact in order to reproduce. Many fish, for example, lay their eggs in the bottom of the river or ocean and hope that a male will come by and spray sperm in the general vicinity and that sperm then finds the eggs.
Methods of Giving Birth
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There are two primary modes of giving birth in vertebrates: oviparous and viviparous. Oviparous animals, from the term ovum which means egg, birth their young from eggs. Most reptiles, birds, fish, sharks and rays, and amphibians follow this method of giving birth. A few mammals, most notably the duck-billed platypus, follow this method as well. Some of these may carry the eggs with them and some may lay them in the sand, the dirt, or a nest. When the organism is ready, they emerge from the shell.
Most mammals follow the viviparous method which means they give birth to their young alive, not in egg form. Some snakes, lizards and even fish give birth by this method, but it is not common. With viviparous animals, the growing baby is supplied with nutrients from the mother throughout the period of gestation. It is only birthed when it can breathe and survive outside of the mother's womb.
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Sexual Maturity
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Vertebrates are not born sexually mature, and it can take some time before they are ready to have children of their own. The amount of time is usually directly related to their expected life span. Organisms with very short life spans usually sexually mature faster than organisms with very long life spans. Some animals--including fish--can be sexually mature within weeks of birth. Higher mammals may take many years before they are able to reproduce on their own.
Metamorphosis
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Most vertebrates are born in roughly the same shape that they will have their entire lives. A puppy looks like a dog, and a baby human looks like a human. However, few organisms undergo metamorphosis at an early point in their lives and alter their shape dramatically. While more common in insects, some vertebrates undergo metamorphosis as well. The most commonly known example of this are the amphibians known as frogs. Frogs are born from eggs that are underwater into an aquatic form known as tadpoles. Tadpoles have very small bodies and no limbs except for their tail. After metamorphosis, the frog has four limbs and no tail, and it is not bound to the water like tadpoles are. Animals are not sexually mature until after they have undergone metamorphosis and taken their final shape.
Death
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All life comes to an end at some point, and this is, of course, true for vertebrates. Life spans vary greatly with the vertebrates. Obviously not all animals live the longest time possible as outside factors like predation, birth defects and malnutrition can play a large role. Regardless of the life span, once an animal has reproduced, its lives on through its offspring and further descendants in the DNA. Life spans can vary from a few short years to over one hundred years.
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