During the evolution of animals, survival and reproduction depended upon mechanisms that maintained internal homeostasis in the face of environmental change. These environmental changes included fluctuations in ambient temperature, food availability, humidity, day length, and population density. Most, if not all, of these variables have effects on the availability of energy, and most vertebrate species have mechanisms that sense energy availability and adjust behavioral priorities accordingly. For example, when the availability of food and potential mating partners is stable and abundant, brain mechanisms often inhibit ingestive behavior, increase energy expenditure, and give priority to courtship and mating. In response to severe energy shortages, brain mechanisms are likely to stimulate foraging, food hoarding, and overeating. These same deficits often delay reproductive development or inhibit adult reproductive behavior. Such adaptations involve the integration of sensory signals with peripheral hormone signals and central effectors, and they are key to understanding health and disease, particularly obesity, eating disorders, and diabetes.
The link between energy balance and reproduction recurs repeatedly, whether in the context of the sensory-somatic system, the autonomic nervous system, or the neuroendocrine cascades. Peripheral signals that are detected by receptors on vagal and splanchnic nerves are relayed to the caudal hindbrain. This brain area contains the effectors for peripheral hormone secretion and for chewing and swallowing, and this same brain area contains receptors for humoral and metabolic signals from peripheral circulation. The caudal hindbrain is therefore a strong candidate for integration of multiple signals that control the initiation of meals, meal size, energy storage, and energy expenditure, including the energy expended on reproduction. There are some differences between the reproductive and ingestive mechanisms, but there are also many striking similarities. There are still gaps in our knowledge about the nature and location of metabolic receptors and the pathways to their effectors. Some of the most promising research is designed to shed light on how hormonal signals might be enhanced or modulated by the peripheral energetic condition (e.g., the level of oxidizable metabolic fuels).