Sex differences in behavioral decision-making and the modulation of shared neural circuits

Animal behavior is flexible. From moment to moment, a given sensory stimulus can elicit qualitatively different behavioral responses. Novel objects may be approached or avoided, food items may be pursued or ignored, conspecifics may be courted or attacked. Moreover, animals presented with multiple stimuli generally select a single behavioral strategy from a repertoire of possible responses. This behavioral flexibility is born, at least in part, out of necessity. Most complex behaviors engage the body of the animal as a whole, and thus are expressed in a mutually exclusive manner. This enforces a decision-making process, through which behaviors are prioritized according to the current physiological needs and reproductive goals of the animal [1,2]. Thus, animals do not behave as automatons. Rather, the mapping of sensory stimuli to motor output is flexible, and responsive to changes in their "internal state" (a rubric representing the synthesis of physiological needs and motivational drives). If an animal is malnourished, it will vigorously pursue food-related stimuli; if well-fed, it may ignore them and save valuable energy or avoid the risk of predation. In this way, behavioral flexibility makes a critical contribution to an animal's survival and reproductive success.As animals reach sexual maturity, new dimensions are added to this calculus of internal state. In addition to growth and survival, the organism's behavioral decision-making process must now incorporate drives to locate and select mates, foster progeny, and compete for territory. This transition to reproductive maturity can differentially affect the sexes' behavioral decision-making in several important ways. The most prominent of these changes is the emergence of new behaviors that are sex-biased or sex-limited in their expression. These include many behaviors closely connected to reproduction, such as mating, courtship, offspring care, and aggressive behaviors. Notably, both sexes often retain the capa