Читать книгу The Behavior of Animals - Группа авторов - Страница 31

To understand how an animal interprets a stimulus means investigating how the animal responds to it. In his famous textbook The Study of Instinct, Tinbergen (1951) introduced the topic of stimulus perception with an experiment (Figure 2.1A):

Figure 2.1 (A) The “reflection experiment.” Toward its mirror-image a male stickleback assumes a vertical threat posture. (Abstracted after a photo in Tinbergen 1951). (B) Great blue heron, Ardea herodias, in threat-display: bill/neck vertically straightened. (Abstracted after a photo in: http://redandthepeanut.blogspot.de/2010/06/two-great-blue-herons-face-off-in.html [accessed: 08/11/20]).

A male three-spined stickleback, Gasterosteus aculeatus, seeing its reflection in a mirror, assumes a vertically oriented body posture with the head pointing downward.

Tinbergen listed a set of issues that must be addressed in order to answer why, in a causal sense (Chapter 1), the animal does this. These questions concern the processes hidden in a “black-box” that, so to speak, translates the stimulus into the behavioral reaction (Figure 2.2).

Figure 2.2 Looking into a “black-box”: principles of brain function mediating between sensory input and motor output. (Modified after Ewert 1976).

First, we must classify the reaction in a behavioral context (e.g., reproduction, male–male aggression). The fact that a male stickleback in reproductive state shows this behavior suggests the involvement of hormones, which requires neurochemical investigations. Then we should examine the releasing features of the visual stimulus and analyze the neuronal instruments that extract the features, exploring the processes responsible for stimulus recognition and localization. Furthermore, the releasing value of a stimulus may depend on motivation and experience. The information regarding stimulus features, locus, memory, attention, and motivation yields a releasing mechanism that activates motor pattern generation responsible for eliciting the adequate behavior. What Tinbergen is suggesting is that “neural orchestration” in the whole brain participates in what appears, initially at least, as a relatively simple stimulus–response.

Back to the “reflection experiment” (Figure 2.1A): what is the ethological background? A male stickleback encountering a conspecific male in its territory changes its longitudinal body axis into a vertical head-down position, thus signaling threat. In fact, when placing a male into a narrow glass tube (Figure 2.3a), the responses of territorial males were stronger when the tube was oriented vertically compared with when the tube was oriented horizontally (Ter Pelkwijk & Tinbergen 1937; cited in Tinbergen 1951). Furthermore, the red coloration of the male’s belly contributed to the release of aggressive attacks. Strongest responses were obtained when both features—vertical posture and red color—occurred together. Later, Heiligenberg et al. (1972) demonstrated quantitatively in the male perch, Haplochromis burtoni, that its black eye-bar delivered a threat-signal to conspecific males that, too, was enhanced if the fish assumed a vertical head-down posture.

Figure 2.3 Configurational features in sign-stimuli. (a) Three-spined stickleback, in a glass tube, in threat posture and prevented from assuming a threat posture. (Ter Pelkwijk & Tinbergen 1937.) (b) Parent thrushes simulated by a head(h)/rump(r) model. Nestling’s gaping (arrow) is directed toward parent’s head. (Tinbergen & Kuenen 1939.) (c) A bird model moving overhead turkeys resembles a goose-like bird or a hawk, depending on the movement direction. (Tinbergen 1948; cit. 1951) (d) A moving small stripe signals either prey or threat to a toad (cf. Figure 2.4), depending on its orientation relative to the direction of movement (arrow). (Ewert 1968; cit 1984.).

Figure 2.4 The prey (a, c) vs. threat (b, d) configuration of a stripe traversing a common toad’s visual field in different directions (arrows). (Ewert et al. 1979; cit. Ewert 1984; cf. Suggested Reading, Movie A2.).

Tinbergen referred to such a stimulus, composed of different features, as a sign-stimulus. More generally, he pointed out that a feature A combined with a feature B may provide a certain sign-stimulus, but that feature A in combination with a feature C may provide a different sign-stimulus. For example, in male sticklebacks:

 red belly and head-down posture addresses a threat signal to conspecific males, but not to females;

 red belly and zigzag dance (Chapter 3) addresses a courtship signal to conspecific females, but not to males.

It is the combination—configuration or “Gestalt” (e.g., see Koffka 1922)—of behaviorally relevant features that determines the releasing value of a sign-stimulus in the sense of a “stimulus-pattern.” Its perception requires pattern recognition—a process, in which genetic and/or learning factors can be involved.

A configuration is perceived as the whole. This means that the sum of the responses to the features, when each feature is presented alone, is significantly less than the response to the complete array. Furthermore, recognition is independent of certain changes in other stimulus parameters as long as these do not affect the configuration. This phenomenon is called invariance.

Sign-stimuli provide parsimonious ways of encoding information to release adequately motivated behaviors. They also have survival value, since they are recognized quickly and responded to rapidly and unambigiously. With these attributes in mind, we continue to use the term sign-stimulus. Its efficacy can be analyzed in experiments using dummies by changing, omitting, adding, or exaggerating certain features.

Sign-stimuli allow humans to communicate with other animals. The wildlife biologist Kent Clegg used his ultralight aircraft as a sign-stimulus for captive-bred endangered whooping cranes, Grus americanus. Simulating their parents, he painted the wings of the plane white with black tips and thus instructed the young cranes to fly and follow the small aircraft. After leaving Idaho and making three overnight stops, he succeeded in having the young follow the plane on their first migratory trip at 35 mph—matching crane’s flight-speed—for 800 miles to their winter residence in New Mexico (see also https://friendsofthewildwhoopers.org/whooping-cranes-facts-management [accessed: 08/11/20]).