5.5Growth and Senescence
The fetus develops continually in the mother's body, and once it is able to sustain its own life, it is ready for birth. For example, it is ready for birth once its respiration, digestion, absorption, exercise capacity, etc. are functioning sufficiently. Of these functions, the development of the respiration function is particularly important. This is because efficient respiration is important in the survival of prematurely born infants.
For a certain period after the infant is born, its body parts grow rapidly, and its organ functions develop considerably. Such a physical development of animals, including mammals, generally continues until the breeding period. After the breeding period, each function of the animal's body gradually deteriorates, and the animal eventually dies. The changes that occur after the breeding period in this life cycle are called "senescence" (aging). This change occurs relatively slowly in humans. In contrast, more familiar examples, such as salmon and cicadas, change and die very rapidly after the breeding period. The length of the period of this aging is related to the length of the breeding period and to the amount of time necessary for growth of the next generation (nurturing period). However, regardless of the length of this aging period, once the breeding period has passed, the deterioration and death of the body that occurs with senescence is an inevitable process common to all animals.