Organisms face constant changes in their environment. These changes often present physiological challenges or stresses that the organism must respond to by altering the functional state of its body. Changes in light or temperature, the threat of a predator, or the presence of a potential mate stimulate responses of an animal’s nervous system and endocrine system. Endocrine responses are commonly triggered by sensory signals received by the nervous system that are relayed to the endocrine system. The endocrine system, with its slower and more prolonged signaling, reinforces physiological changes in the animal’s body that better suit the environmental cues received by its nervous system.

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When a gazelle sees or smells a predator, its endocrine system helps to ready its body for rapid escape. When a female cardinal sees a bright red, singing male early in spring, endocrine signals released from within the female cardinal’s brain initiate changes in its reproductive organs and increase its behavioral responsiveness, encouraging it to select the male as its mate. And when particular smells stimulate a sense of hunger and digestive function in animals such as dogs and humans, the animals are attracted to food. In each of these cases, the endocrine system helps an organism respond appropriately to environmental cues.