Sign stimuli in amphibians

The way in which frogs and toads recognise their prey provides another example of a releasing mechanism. In this case, the ethological results are even more compelling because they have been combined with a neuro-physiological study of the same system. This combined approach clearly shows how the selective properties of nerve cells (neurons) are involved in releasing particular patterns of behaviour (Ewert, 1985, 1987).

In the visual world of a frog or toad, just a few, simple criteria serve to categorise moving objects as prey, enemy or lover. Once the visual system has placed a given object in one of these categories, the animal reacts accordingly. These reactions can be used to analyse the criteria involved in prey recognition because the animals are readily deceived by small cardboard models moving in front of them. A special study of prey detection has been made in the common European toad (genus Bufo), using such models to analyse the behavioural responses of the intact animal and the responses of specific classes of neuron in the visual system. The natural prey of Bufo consists of small animals such as beetles, earthworms and millipedes. If one of these animals appears in its peripheral visual field, the toad responds by turning its head and/or body so as to bring the animal into the frontal visual field. The toad then walks towards the prey in order to capture it.

The sign stimuli, by which the prey is recognised, can be analysed quantitatively in the laboratory. A hungry toad is confined in a glass vessel, from which it can see a cardboard model circling around (Fig. 1.4 a). If the toad interprets the model as a prey animal, it tries to bring it into the frontal visual field, and in doing so turns around jerkily after the moving model. The number of orientating turns per minute elicited by a given model, compared to the number elicited by others, can therefore be taken as a measure of the resemblance between that model and prey, from the toad's point of view.

In this experimental situation, the toad is not much impressed by a small 2.5 X 2.5 mm model, which elicits only a few orientating movements. However, the stepwise elongation of this shape in the horizontal dimension (Fig. 1.4b, shape x) greatly increases its releasing value. That is to say, elongation of the model in the direction of movement increases its resemblance to prey, up to a certain limit, and this long, small stripe has been called the

Figure 1.4 Analysis of prey recognition in the toad (Bufo). (a) The experimental set-up, with the toad confined in a glass vessel and a prey model (P) circling around it. The toad turns to follow the model when it has moved through a sufficient angle, the effective displacement (D). (b) The response of the toad to moving models of three shapes (x, y, z) as these are enlarged in one dimension (shapes x, y) or two dimensions (shape z). The toad's response is measured by the number of times it turns to follow the model in 1 min. (Redrawn after Ewert, 1980, 1983.)

Figure 1.4 Analysis of prey recognition in the toad (Bufo). (a) The experimental set-up, with the toad confined in a glass vessel and a prey model (P) circling around it. The toad turns to follow the model when it has moved through a sufficient angle, the effective displacement (D). (b) The response of the toad to moving models of three shapes (x, y, z) as these are enlarged in one dimension (shapes x, y) or two dimensions (shape z). The toad's response is measured by the number of times it turns to follow the model in 1 min. (Redrawn after Ewert, 1980, 1983.)

worm configuration. If the small, square shape is elongated in the vertical dimension (Fig. 1.4b, shape y), its releasing value decreases to zero. In fact, the toad often interprets it as a threat and freezes in a defensive posture. This shape has been called the antiworm configuration. If both dimensions of the model are lengthened equally, so that the toad is presented with squares of increasing size (Fig. 1.4b, shape z), the prey-catching activity initially increases but then declines rapidly to zero. This is probably the result of non-linear summation of the horizontal (worm) and vertical (antiworm) edges.

The toad's ability to distinguish between worm and antiworm does not vary with other stimulus parameters, such as the colour of the model or its velocity of movement. It is also independent of the direction in which the stimulus traverses the toad's visual field. If the models are moved past the toad in a vertical direction, then the vertical stripe elicits prey catching and the horizontal stripe elicits no response or a defensive posture. Thus, the worm/antiworm distinction is based on the combination of just two stimulus parameters: the elongation of the object in relation to its direction of movement. These parameters, then, are the sign stimuli that release prey-catching behaviour in a hungry toad, and it is obvious that they correspond only very approximately to a real worm. Nevertheless, they will normally enable a toad to distinguish correctly between potential prey and inedible objects in its natural environment.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Post a comment