Читать книгу The Behavior of Animals - Группа авторов - Страница 33

Common toads, Bufo bufo, interpret small elongated objects—a worm or millipede—as prey. Experimentally, it can be shown that a 2.5 × 40 mm stripe oriented parallel to the direction of its movement releases eager prey-catching (see Further Reading, Movie A2). The same stripe suddenly oriented crosswise to the direction of movement—in a split-second—leads the toad to “freeze.” That configuration signals threat. Prey-like again, the stripe immediately elicits strong prey-catching activity, etc. The discrimination of these dynamic configurational “opposite stimuli” is invariant to changes in movement direction (Figures 2.3d, 2.4a–d) and velocity. This phenomenon was also observed in terrestrial urodeles (Finkenstädt & Ewert 1983).

Interestingly, the mudskipper Periophthalmus barbarus, an amphibious fish (Burghagen in Kutschera et al. 2008), and the preying insect Sphodromantis lineola (Kral & Prete 2004; cit., 2004) respond to such test-stripes in the same way as toads. Toward the threat-configuration, mudskippers may even raise their dorsal fins, thus threatening back.

Generalizing, a configurational feature perceived as threat here, is a contrast-border aligned crosswise to the direction of its movement. Unlike earthworms, snakes—the “archenemies” of toads—reveal such visual cues during locomotion: elevating head, undulating sidling body. Faced with a snake (Figure 2.5a) or a dummy snake (Figure 2.5b) a toad keeps eye contact, stops locomotion, discharges skin poison glands, swells up, assumes a stiff-legged defensive stance, and, eventually, presents its head and back like a shield difficult for the snake to handle (Ewert & Traud 1979, cit. Ewert 1984).

Figure 2.5 (a) Common toad displaying anti-predator behavior toward a ring snake or (b) to a snake dummy. (Courtesy of R. Traud.) .

Threatening postures, as components of agonistic behavior, shown for stickleback (Figure 2.1A), perch (Heiligenberg et al. 1972), or great blue heron (Figure 2.1B), are widespread in the animal kingdom. The caterpillar of Hemeroplanes triptolemus, if attacked, displays a scaring snake-like posture that is disregarded as prey. If human divers encounter a shark, experts recommend assuming an erect posture that will not fit shark’s prey schema.

We are tempted to speculate that during human evolution, the upright gait had—inter alia—a bearing on emitting a threatening signal. Wagging the erected forefinger against somebody as threat gesture, we interpret as a ritualized behavior. In fact, the significance of signs of that kind was known phylogenetically for a long time, such as with the amphibian genera Bombina and Bufo that emerged in the Jurassic and Paleocene, respectively (cf. Ewert & Burghagen 1979; cit. Ewert 1984).