Figure 2. Hypothalamus as a central coordinator of organismal life history programs.
The growth, reproduction, and maintenance arms of life history theory correspond on the organismal level to the growth hormone - insulin-like growth factor (GH-IGF), hypothalamic - pituitary - gonadal (HPG), and hypothalamic - pituitary - adrenal (HPA) axes, respectively. These axes all initiate at the level of the hypothalamus, and can be engaged or disengaged depending on inputs reporting on the quality of the environment. The GH-IGF axis regulates hepatic secretion of IGF-1, which is known to be essential to growth. The HPG axis controls the gonadal secretion of sex hormones, which are necessary for reproductive maturation and function. The HPA axis governs adrenal secretion of glucocorticoids, which are a common component of responses to environmental stress.