Why does growth in most animals slow down or stop after the juvenile period?

In most animals, growth slows and eventually ceases after the juvenile period because the organism reaches a state of maturity where energy resources are shifted toward reproduction rather than continued physical expansion. While the exact mechanisms are not fully understood, this process is governed by a combination of genetic control and internal regulatory systems.12

Key factors influencing this transition include:

• Genetic Limits: The total body and organ size of an animal are likely established by genetic mechanisms that dictate when growth should stop.2
• Hormonal Regulation: Growth is heavily influenced by hormones, particularly growth hormone produced by the pituitary gland. As an animal approaches adulthood, hormonal shifts—including the activation of sex hormones—coordinate the development of reproductive maturity, which often coincides with the slowing or cessation of somatic growth.231
• Feedback Mechanisms: Evidence suggests that organs may release substances, such as specific proteins, into the bloodstream that serve to limit their own growth, creating a feedback loop that stabilizes the size of the organism.2

While most species exhibit this determinate growth, some large-bodied animals may experience indeterminate growth, where they continue to grow very slowly throughout adulthood depending on food availability.1