Evolution favors successful and successive reproduction. Survival before reproduction is essential for passing genetic materials to the next generation for all organisms, while survival after is important for species whose post-reproductive actions such as nurturing, affect the next reproductive cycle. During reproduction however, physical and energetic resources from the parent(s) are used to bring the offspring to life, often reducing the post-reproductive fitness. To survive or to reproduce? How is the switch between the two modes affected by exogenous factors such as environmental stressors and endogenous ones such as hormones? We utilize the Japanese rice fish (Oryzias latipes), to explore this complex decision-making process. Understanding the rationale behind the body’s choice may create opportunities to influence it, and provide insights into the ongoing international human population structure changes.