nest, and sometimes the jump toward the female ends in attack and biting. Thus, the zigzag dance can be considered a case of successive ambivalence. Ambivalent behavior is behavior that includes motor components belonging to two different behavior systems; in successive ambivalence , these components occur in rapid succession.
A somewhat similar case is provided by the “upright” posture of the herring gull (Larus argentatus; Figure 3.6). This display often occurs during boundary disputes when two neighboring gulls meet at their mutual territory boundary. The bird’s neck is stretched and its bill points down; the carpal joints (wrists) of the wings are raised out of the supporting feathers; the plumage is sleeked. The position of the bill and wings are characteristic of a bird that is about to attack (fighting in this species includes pecking and wing beating the opponent), and the stretched neck and sleeked plumage are characteristic of a frightened bird that is about to flee. Further, actual fighting or fleeing often follows the upright posture. Thus, the upright posture is a behavior pattern that includes motor components belonging to two different behavior systems. Unlike the zigzag dance of the stickleback, however, these components occur simultaneously. The upright posture can be considered a case of simultaneous ambivalence . Figure 3.6 also shows that the upright posture can occur in varying forms. In the “aggressive upright,” components of attack predominate, whereas in the “anxiety upright,” components of fleeing predominate.
Figure 3.6 Upright postures of the herring gull: (a) “aggressive” upright; (b) “intimidated” upright; (c) “anxiety” or “escape” upright. (From Tinbergen 1959).
The simultaneous occurrence of components belonging to different behavior systems greatly increases the number and variety of behavior patterns in a species’ repertoire. A technique called motivation analysis can be used to explore such ambivalent behavior patterns, which include many of the bizarre displays exhibited by many species. In a motivation analysis, one looks at the form of the behavior, the situation in which it occurs, and other behaviors that occur in association with it (Tinbergen 1959). An example is provided by Kruijt’s (1964, p. 61) analysis of “waltzing” by the male junglefowl (Gallus gallus spadiceus), the wild ancestor of the domestic chicken (Figure 3.7).
Figure 3.7 “Waltzing” in a male junglefowl. (From Kruijt 1964).
It is a lateral display: the waltzing bird walks sideways around or toward the opponent. Back and shoulders are held oblique, the inner side (the side nearest the opponent) lower than the outer side. Both wings are lifted out of the supporting feathers; the upper and lower arms are slightly lowered so that the rump becomes visible. Otherwise, the inner wing and upper and lower arm of the outer wing remain folded. The hand of the outer wing is lowered perpendicularly to the ground and pulled forward, its plane near the body. The primaries touch the ground and the outer foot makes scratching or stepping movements through the primaries. Head and neck are held at the level of the back and either in the medial plane or slightly turned toward the opponent. The tail spreads and is turned toward the opponent; breast and belly feathers are often ruffled, especially those of the other side.
Kruijt noted that the side of the bird’s body near the hen expressed many components of escape behavior, whereas the side further from the hen expressed many components of attack behavior. It was “as if the part of the animal which is nearest to the opponent tries to withdraw, whereas the other half, which is further away, tries to approach” (Kruijt 1964, p. 65). He also noted that waltzing was always directed toward a conspecific. Somewhat surprisingly, young males directed waltzing equally to males and females, even though adult males almost always direct it toward females. In about two-thirds of the cases it was performed immediately before, during, or immediately after fighting, and in some of these cases behavior associated with escape was also seen. Thus, on the basis of form, situation, and associated behavior, Kruijt could conclude that waltzing is indeed an ambivalent behavior pattern expressing both attack and escape, with attack predominating. Sexual motivation appears to be unnecessary.
Motivation analysis of many complex courtship displays in both birds and mammals has revealed that they are ambivalent activities very frequently involving primarily the attack and escape systems. Such activities are usually essential for successful courtship and reproduction. This means, as mentioned above, that the sex system by itself is often insufficient for achieving these ends, and illustrates clearly why causal and functional questions need to be kept separate.
Redirection
When two herring gulls meet at their mutual boundary, causal factors for both attack and escape behavior are present. As we have just seen, the birds usually adopt the ambivalent upright posture in this situation. A common occurrence during this mutual display is that one of the birds viciously pecks a nearby clump of grass and then vigorously pulls at it. In form, “grass pulling” resembles the feather pulling seen during a heated fight between two gulls. This behavior can be considered a case of redirected behavior because the motor components all belong to one of the behavior systems for which causal factors are present (i.e., aggression), but it is directed toward an inappropriate object. The causal factors for the other behavior (in this case, escape or fear) must be responsible for the shift in object. Redirection of aggressive behavior seems to be especially common in many species including humans (Lorenz 1966).
Displacement
Ambivalent behavior and redirected behavior are appropriate responses to causal factors that are obviously present in the situation in which the animal finds itself. Sometimes, however, an animal shows behavior that is not expected, in that appropriate causal factors are not apparent. A male stickleback meets its neighbor at the territory boundary and shows intention movements of attack and escape; then it suddenly swims to the bottom and takes a mouthful of sand (which is a component of nest-building behavior). A young chick encounters a wriggling mealworm and shows intention movements of approach to peck and eat the mealworm and of retreating from the novel object; then, while watching the mealworm the chick falls asleep. A pigeon, actively engaged in courtship, suddenly stops and preens itself. A student studying hard for an exam, puts down her book, walks to the kitchen, and makes herself a sandwich. These behaviors are all examples of displacement activities that are controlled by a behavior system different from the behavior systems one might expect to be activated in a particular situation.
In the case of the stickleback, it is reasonable to show components of attack and escape behavior at the boundary of its territory because the neighboring fish is an intruder when it crosses into our subject’s territory, and our subject loses the security of home when it ventures into its neighbor’s territory. But why should it engage in nest-building behavior? The stickleback has probably already built its nest elsewhere and, in any case, would not normally build it at the edge of its territory. What are the causal factors for nest building in this situation? Similar considerations apply to the other examples as well. In all cases, causal factors for the displacement activity appear to be missing. It is this apparent inexplicableness of displacement activities that has caused so much attention to be focused on them. Why does this unexpected behavior occur?
There have been two main theories put forward to account for displacement activities: the overflow theory and the disinhibition theory. The original theory was proposed independently by Kortlandt (1940) and by Tinbergen (1940) and is usually called the overflow theory. They proposed that when causal factors for a particular behavior system (e.g., aggression) were strong, but appropriate behavior was prevented from occurring, the energy from the activated
