Even some very complex forms of behavior could be thought of as chain reactions of simple stimulus-response mechanisms, The zigzag courtship dance between a male and a receptive female illustrated this more complex mechanical pattern (Figure 14.3). When a female enters a breeding territory, the male swims vigorously toward her. Such an aggressive approach causes most nonreceptive females to flee, but an egg-bearing female responds by facing the approaching male and exposing her swollen abdomen. This simple response acts as a new stimulus that switches the male from aggression to courtship, He responds by leading the female in a zigzag pattern toward the nest.