Mammal life cycles vary based on the species, but mammalian life cycles share the same fundamental infancy, adolescent and adult stages. Mammals begin as an egg cell fertilized by a sperm cell. Mammalian young are born after an incubation period in a womb.
The mammalian life cycle begins when a male's sperm cell fertilizes a female's egg cell by mating. The length of the incubation period in the womb varies based on the species. The fetus develops and grows inside the womb for the entirety of the incubation period. Most mammals give birth to live young, as opposed to laying eggs. Like the incubation period, the length of each stage of the life cycle varies based on the species.
Newborn mammals continue to grow and develop once they are out of the womb. The mother nurses the newborn as it learns how to perform necessary functions for survival during the infancy stage. The infant is usually nourished and protected by its elders until it is an adult.
The adolescent stage of the mammalian life cycle involves the development of reproductive organs. In most mammal species, the adolescent stage involves sexual, mental and physical maturation.
Following the adolescent stage is adulthood. Mammals in the adult stage of the life cycle seek to reproduce and care for their young. Adults are independent but may reside in a pack or herd depending on species.
Adulthood is followed by old age and the eventual death of the mammal as it declines in health and physical strength.