The effect of a neuron is typically short lived and local: A neuron may, for example, connect to another neuron or cause a muscle to contract (Chapter 35). One way in which a stimulus may provoke a more widespread and prolonged effect is through the production of hormones. Hormones can affect multiple cells in target organs simultaneously (Chapter 38).

Anolis lizards demonstrate the important role of hormones in sexual behavior, showing both how social stimuli can affect the release of hormones and how hormones can affect behavior (Fig. 45.5). If females of Anolis carolinensis are isolated from males during the breeding season, about 80% of individuals have active egg follicles in their ovaries (Fig. 45.5a). If one male is added to a group of females, that figure increases to 100% (Fig. 45.5b). The courting behavior of the male lizard stimulates the females to produce hormones that cause the full development of the ovaries, making the females reproductively active. If a group of males instead of a single male is added to an all-female population, on the other hand, only about 40% of the females undergo ovarian development (Fig. 45.5c). This unexpected result seems to occur because the males fight among themselves rather than court the females, and therefore the courtship stimulus is lacking. Castrated males (which do not produce testosterone) that are added to a group of females have no significant effect on rates of ovarian development, which remains about 80% (Fig. 45.5d). They fail to court because the behavior is caused in part by testosterone. However, a castrated male that is injected with testosterone does display courting behavior and has the same effect as the presence of a single male, inducing all the females to undergo ovarian development (Fig. 45.5e).

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These simple laboratory experiments demonstrate the complex interplay between hormones and behavior. Testosterone mediates both male–male interactions and the male–female interactions that cause females to produce hormones that promote reproductive development.