The expression of mutually-exclusive shared and sex-specific behaviors is decided by interactions of monoamine, peptide, and lipid hormone pathways. A. Food leaving is a behavior exhibited by C. elegans males in the absence of a mating partner. The probability of a male leaving food is regulated by serotonin, insulin, and signals from the gonad (adapted with permission from . Copyright 2004, Society for Neuroscience). B. Hunger may regulate the probability of expressing mating and courtship behaviors in mammals. Ghrelin, a peptide hormone signaling nutritional status, regulates the frequency of ultrasonic courtship calling behavior in mice (adapted with permission from . Copyright 2010, Elsevier Ltd.).