All living organisms have a life cycle beginning with the initiation of life and continuing until death. Scientists generally consider a life cycle to encompass the changes that an organism experiences from the beginning of a specific developmental stage until inception of the initial developmental stage in the next generation. The stages an organism will experience and the length of the life cycle varies widely across species. Species survive only when sufficient offspring complete the life cycle to continue the species. Such cycles vary from minutes to multiple years.
The time to complete a life cycle is generally correlated to the size of the organism. Bacteria complete their life cycle in 30 minutes by dividing to form another generation. Higher organisms take much longer. The giant sequoia, for example, produces its first fertile seeds after 60 years. Some plants and animals die after reproducing, with reproduction symbolizing the end of the life cycle. Other animals, notably humans, live for many years after their reproductive cycle is complete.
Most simple organisms complete their life cycle in one generation with an initial splitting in two of an adult, the growth of the new organisms, and upon reaching maturity, the division of each one into two new individuals. Higher animals can also have a single generation with the fusion of male and female sex cells to create the embryo and the growth of the offspring to reproductive maturity when they produce sex cells that create new offspring. Plant life cycles, however, are usually mul- tigenerational. A spore will germinate and grow into a gametophyte. At maturity, the gametophyte forms gametes. With fertilization, the gametes develop into sporophytes. Once reproductive maturity is reached, the sporophytes produce spores and a new life cycle begins. This process is frequently referred to as “alternation of generations” and occurs in some fungi and protists as well.
The time the young animals spend developing prior to being born (the gestation period) or hatched varies across species. Small marine animals frequently produce large numbers of tiny eggs that hatch at a very early developmental stage, while others produce fewer, larger eggs with the young exiting at a more advanced developmental stage. The smaller young will need longer to develop before becoming adults, while the more fully developed young move quickly into adulthood. This effect is common in higher organisms as well. Generally the more developed the young, the fewer produced in one reproductive cycle, as there is more chance of the young surviving. Larger animals take longer to bear their young, as the young must be ready for the hardships of life. Elephants have the longest pregnancy period of any mammal at 22 months. Smaller animals have shorter gestation periods. For example, the domestic cat has a pregnancy term of approximately 9 weeks.
Hibernation is a state of dormancy that some animals enter into during cold months of the year. Bears are well known for this; however many other animals hibernate, including rodents. Hibernation allows the animal to exist during times of food shortage and unfavorable conditions. In the spring, the animal rises and is generally ready to reproduce. Plants and seeds also experience dormant periods until conditions improve to allow them to continue their life cycle.
A number of animal species include metamorphism in their life cycle. Metamorphosis is a process of great change between stages in the cycle. The stages provide a way for the organisms to live separately from other stages in order not to compete for food or habitat. For example, frogs lay eggs in the water. The embryo quickly grows and hatches into a tadpole. The tadpole lives in the water and breathes through gills. As the tadpole grows, its tail shrivels, and legs and lungs develop. When the gills disappear, the tadpole comes to the surface and is now considered to be an adult frog. Most species of frogs grow into adults within one season and reach reproductive maturity within a year. Thus, mature frogs and tadpoles live in very different habitats, lessening the competition for food and enhancing other aspects of survival.
Butterflies also use metamorphosis in their life cycle. Each one moves quickly from a caterpillar (larval) stage to a pupal or chrysalis stage, when the larva is dormant, and then emerges from the pupa as a full adult ready to reproduce, with one cycle normally taking a year to complete. Fruit flies also undergo metamorphosis, changing from one form to another completely different form, moving from an egg to larva to pupa to adult. Each phase is a natural phase of the life cycle. The whole process normally takes about a week. Other animals that use metamorphosis are amphibians (toads, newts, etc.) and a wide variety of insects.
Invertebrate animals have a variety of life cycles. Metamorphosis, a series of rapid physical changes, is common. Most insects lay eggs. Incomplete metamorphosis, or simple development, involves the eggs hatching into nymphs and moving to the adult stage. Complete metamorphosis, or complex development, includes a pupal stage between the larval and adult stages. As immature larvae and nymphs grow, they shed their exoskeleton, which is a tough exterior skin. Each time the exoskeleton is shed, the new stage is called an instar. The final instar develops into a pupa in complex life cycles or into an adult in simple life cycles. Most common insects generally have three to six instar stages; thus a complete insect life cycle includes the egg, larval, three to six instar, and adult stages. Ladybugs provide a good example of complex development. The adults lay eggs that hatch into wormlike larvae, which go through several instar stages, and the final instar covers itself in a hard pupa and emerges as a new adult. The life cycle takes 4 to 7 weeks. Most insects have a 1 year life cycle. However, wide differences exist. For example, the life cycle of a fruit fly is 1 week, while one species of cicada has a life cycle of 17 years.
Most reptiles are egg laying (oviparous). There is great variety in reptile mothers. Some eggs are laid and left on their own, although usually in well- protected and hidden nests. Sea turtles lay their eggs on sandy beaches and return to the water, leaving the young to fend for themselves. Other species stay with the eggs and aggressively defend the nests. Examples of these include crocodiles and pythons. Some reptiles are ovoviviparous, including many species of snakes and lizards, with very thin shelled eggs remaining in the mother’s body. These species appear to have live births, but in reality the eggs hatch just before the young are born, making it seem that the young are born rather than hatched.
Reptiles produce hatchlings that are exact replicas of the parents. The hatchlings must quickly grow and learn to defend themselves. Many reptiles grow through a process of molting, where the outer layer of their skin is shed to allow for physical growth. The molting times will vary. Most reptiles will molt shortly after hatching. During periods of rapid growth, molting can occur four or more times a year. Adults still molt but more infrequently, generally one or two times a year.
Fish are primarily egg laying. Many species lay the eggs and leave them alone to survive. Most eggs are soft shelled. Some species of sharks lay eggs with a tough outer coat to help protect the eggs. A few species protect the eggs through burying them in the sand or by being “mouth brooders,” where one parent will hold the eggs in his or her mouth until they hatch. Male seahorses have a special pouch where they hold the eggs. Baby fish are called “fry.” The young are generally able to reproduce within a year after birth.
Birds lay eggs on land even if, like ducks and loons, they spend more time on the water. Most birds stay with the nests (brood) to keep the eggs warm and safe from predators. Eggs vary in size, from the very small robin egg, approximately 1 inch around, to the large ostrich egg, which is 5 to 6 inches around. Hatching time is 12 to 14 days for robins and 35 to 45 days for ostriches. Ostriches will reproduce at 2 to 4 years of age, while robins reproduce within a year. Comparing the robin and the ostrich provides further evidence that the size of the organism affects the time taken for each stage of the life cycle.
Humans are generally considered to have four stages in their life cycle: infancy, childhood, adolescence, and adulthood. The gestation period is 9 months. Infancy is the stage between newborn and 2 years of age. Many changes occur in this stage, and the infant learns to walk and talk. Childhood occurs between 2 years of age and puberty (adolescence). Many physical changes occur in childhood, such as the rapid development of teeth and bones. The time of puberty varies but usually starts at 8 to 12 years of age. Adolescence is a period of many changes, including the sexual maturing of the human body. Males continue to grow physically until they are 21 years old, while females usually reach peak physical growth by 18 years of age. During adulthood, considered to be 18 years of age and over, the body begins to slow down. Changes occur, including thinning hair and body shape modifications. Sometimes old age is added to the life cycle stages. In developed countries the average life expectancy is 77 to 80 years. A few people live for more than 100 years. Life cycles are thus complex and show immense variation across species.
See also Birthrates, Human; Clocks, Biological; DNA; Dying and Death; Extinction and Evolution; Fertility Cycle; Gestation Period; Hibernation; Longevity; Malthus, Thomas; Maturation; Metamorphosis, Insect
Kalman, B., & Langille, J. (1998). What is a life cycle? New York: Crabtree.
McGraw-Hill encyclopedia of science & technology. (2002). New York: McGraw-Hill.