The life cycle of organisms is vast and varies drastically based on the organism. We will look at many different life cycle examples, including: mammals, insects, fish, frogs, birds, and flowering plants. While all of these life cycles are different, they all follow the same theme in that they start with the fertilization of an egg or seed, and end with reproduction and then the death of the organism.
Mammalian Life Cycle
The mammalian life cycle, including the life cycle in humans, can be generalized into four main areas, which include:
- The fertilized egg or pre-birth and embryonic state.
- The infant stage, in which the young depend on one or two of their parents.
- The adolescent stage where the mammal learns traits from their parent(s).
- The adult stage, which is usually signified by the ability to mate with the opposite sex.
The word mammal comes from the word Mammalia, which is a class within the phylogenic tree. Mammals characteristically share a few traits that help us identify them. These include possession of body hair, presence of special glands, separate sexes, internal fertilization, feeding of young from milk endowed mammary glands, and various other traits.
There are many different types of mammals, and marsupials are a great example of a mammal with different life stages than the ones that humans experience. A marsupial is a mammal with a very different life cycle from other placental mammals such as humans. Marsupials are often characterized by their pouches, which are very important to the survival of their offspring.
An example of a marsupial is a kangaroo.
- The lifecycle of a kangaroo starts with a male and female kangaroo mating, which then produces an embryo within the uterus of the female kangaroo.
- Once the female kangaroo is pregnant, the gestational period is very short, and so they essentially give birth to embryos called joey.
- The joey makes its way out from the vagin* and into the females pouch, where it will stay and feed on the mother's milk until it is around eight months old.
- Once the joey is old enough, it will go back and forth out of the mother's pouch to feed on the milk until it is too large, at which point it is considered an adult.
Human Life Cycle
There are five main stages of the human life cycle, which are:
- The fertilized egg (0-9 months in utero).
- Infancy (0-3 years old).
- Childhood (3-10 years old).
- Adolescence (10-19 years old).
- Adult (20+ years old).
Of course, these stages are very generalized and vague, so we can identify twelve more specific stages of human life. The more specific stages can be classified as:
- 1) Pre-birth or fertilized egg stage, which includes the transition of the embryo to a fetus.
- Infancy, which is broken up into two stages: 2) infant and 3) baby.
- Childhood, which can be broken into three categories: 4) early childhood or toddler, 5) middle childhood, and 6) late childhood.
- 7) adolescence, which is marked by puberty.
- Adulthood, which can be broken into five stages: 8) early adulthood, 9) middle-aged adults, 10) mature adults, 11) older adults, and 12) the elderly.
In human reproduction, there must be a male and a female to produce offspring. Cells needed for reproduction start in the reproductive organs of the male testes, which produce sperm, and female ovaries, which produce eggs. Half of the cells come from the male and half come from the female, which together forms a zygote. After the sperm has fertilized the egg, the egg becomes an embryo, which means that the female is pregnant.
The development of the fetus begins at week eight of the female's pregnancy. This development will continue until the thirty-eight to forty week mark, wherein the fetus will be considered full-term and will be born. The female human will give birth after undergoing labor through several stages of childbirth in which the body will change to allow for the infant to exit the body through the vagin*l canal.
Once born, the fetus enters the infancy stage of life. Because humans are also mammals, the female will produce milk to feed her infant through the childhood stage of life. Children tend to be more resilient and can learn independence, social norms, and survival techniques.
After childhood, around the age of ten, children transition into the adolescence stage, usually characterized by puberty. Once puberty and significant mental growth have occurred, the adolescent will transition into the final phase of adulthood. This is the final stage, where the human has reached sexual and mental maturity.
Insect Life Cycles
Insects, or insectus, is an extremely diverse group categorized under arthropods. Because of this large diversity of species, there are two main life cycle patterns of insects, the holometabolous metamorphosis, and the hemimetabolous metamorphosis.
A very small percentage of insects reproduce asexually, meaning that they encompass both male and female parts, or that they do not need a partner to reproduce. However, most insects reproduce sexually, meaning a male and female are needed to reproduce. Fertilization can either be internal or through the use of spermatophores, which is basically a small capsule that holds sperm outside of the body.
There are two main types of life cycle patterns for insects, the holometabolous metamorphosis and the hemimetabolous metamorphosis. There is a third life cycle pattern, ametabolous development, which has three stages (egg, juvenile, and adult) wherein the juveniles are similar to the adult.
Holometabolous metamorphosis, which is also known as complete metamorphosis, is a life cycle pattern of insects that has four stages, which are:
- Egg
- Larvae
- Pupae
- Adult
Butterflies undergo holometabolous metamorphosis — these insects have visible changes in each of their different life stages.
Hemimetabolous metamorphosis, which is also known as incomplete metamorphosis, is a life cycle pattern of insects that has three stages, which are:
- Egg
- Nymph
- Adult
Grasshoppers undergo hemimetabolous metamorphosis — these insects do not have a larvae or pupae stage. Instead, once past the egg stage, they have no significant visible changes to the adult stage other than growth and maturity.
Fish Life Cycles
Fish have many different life cycles, and, depending on the type of fish, these terms can vary. The average life cycle of a fish, however, can be summarized into five stages. The general life cycle of a fish is
- Egg
- Larva
- Fry
- Juvenile
- Adult
- Eggs within the fish lifecycle can vary based on many factors, including: weight, color, type of fertilization, size, and whether they are guarded or not. Generally speaking, most fish fertilize their eggs externally, or they are dioecious.
- Larvae emerge from their eggs for different reasons, and at different times — this can be in response to temperature changes or simply because they are ready. Most larvae carry the yolk sack externally on their bodies as they continue to grow, and eventually they absorb it.
- A Fry is a recently hatched fish who has fully absorbed its yolk sack. Their swim bladder has formed, and, generally speaking, they can live independently by starting to eat foods.
- The juvenile stage is important, as the fish generally continues to change color. They closely resemble the adult version, but they continue developing.
- The adult phase is the final phase in the fish life cycle. Typically, this is when the fish has reached sexual maturity and can have offspring.
Frog Life Cycles
There are many unique life cycles for frogs. However, the average life cycle of a frog can be summarized into three stages:
- Egg
- Tadpole
- Adult
To see the full picture of a frog's life cycle, though, we can split the stages into five sections, which are:
- Egg
- Tadpole
- Tadpole with legs
- Froglet
- Adult frog
- Male and female frogs externally fertilize eggs in a process called amplexus. As the female lays the eggs, the male exerts his sperm directly onto them, after which a protective jelly layer is formed over the eggs for protection.
- Once the eggs are fertilized, they begin to undergo changes, and once the embryo has formed, and a tail emerges from the protective barrier, the organism becomes a tadpole. During this time, the tadpole will grow legs, form a distinct head, develop gills for breathing, and continue to grow.
- Once this growth is complete and the tadpole has reached sexual maturity, the tadpole becomes an adult frog.
Bird Life Cycles
There are many different life cycles of birds, mainly because there are so many different species, but the average life cycle of birds can be summarized into four major stages. The four general life stages for birds are:
- Egg
- Hatchling
- Fledgling
- Adult
Birds are unique to the animal kingdom in that most are monogamous, meaning that they have only one sexual mate during any given mating period. Some will only have one mate through their entire life cycle.
- In most species of birds, the male's sperm comes into contact with the female's left ovary, where the fertilization of the egg will occur. While still in the female, the egg receives a shell or protective coating before being laid by the female bird. Most bird species incubate the eggs by laying on them — this can be done by one or both of the parents in a nest.
- After adequate time, depending on the species of bird, the eggs will break or hatch. As the baby birds emerge from the eggs, they will enter the hatchling stage. A hatchling is usually vulnerable; it cannot move or protect itself, and it relies on the parent(s) for food and warmth.
- Once the hatchlings have grown their feathers, specifically the feathers used for flight, they will start to venture out on their own and learn the behaviors associated with their particular species. This indicates that they have entered the fledgling stage.
- Once they have fully matured, including reaching their sexual maturity, and mastered the particular behaviors unique to their species, the birds will be considered adults.
Flowering Plant Life Cycles
A flowering plant, or angiosperm, lies under the umbrella of seed plants. In the life cycle of a plant, one major characteristic of angiosperms is that the ovule is completely surrounded by sporophyte, or carpel tissue, which originates in the plant's leaves and stays with it throughout its life cycle. In the early stages, the carpel protects the ovules of the plant and later becomes part of the fruit.
Another unique characteristic of the angiosperm is double fertilization, in which half is produced into pollen and half is produced into the nuclei of the central cell.
There are three main stages of a flowering plant's life cycle, which are:
- Seed
- Seedling
- Mature plant
- Seeds are produced from the flower, which is a part of the larger, mature plant.
- Once seeds have met a number of environmental conditions, they will start to germinate and enter into the second phase, which is the seedling stage.
- After the seedling has taken root and had adequate time to grow, it will enter into the third stage, being a mature plant. During this stage flowers will form, which are the main identifying factor for angiosperms. After this, the flower will produce fruit, which then holds the seeds to start the whole process over again.
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