Neural implications of ethological results

The behaviour of an animal is to a large extent the product of activity in its nervous system. The patterns of behaviour that are recognised in ethological studies must therefore reflect the underlying organisation of the nervous system. In the case of the elementary components of behaviour studied by the early ethologists, this correspondence may be fairly close. Consequently, a careful study of behaviour patterns at the level of the intact organism will often produce results that provide valuable clues about the underlying neural organisation.

Consider the classic case of the egg-retrieval behaviour found in many ground-nesting birds, which was first studied in the greylag goose (genus Anser) by Lorenz and Tinbergen in the 1930s. A nesting goose employs a stereotyped sequence of movements to retrieve an egg that has become displaced from the nest. The bird leans out of the nest, places its beak beyond the egg, and then draws the beak back towards its chest so that the egg is rolled back into the nest. Superimposed on this movement towards the chest are little side-to-side movements of the beak, which serve to keep the egg in place. This sequence of movements is used by all members of the species for egg retrieval;none uses an alternative method. Indeed, a very similar pattern of movement is found in other birds, such as the herring gull (Larus), on which many tests have been carried out (Fig. 1.1). Stereotyped movements of this kind were originally called fixed action patterns; nowadays, more general terms like motor pattern are used instead by most ethologists.

Figure 1.1 Egg retrieval in the herring gull (Larus): an incubating gull will retrieve an egg that has become displaced from the nest, using a stereotyped pattern of movement. Here, the retrieval response is being used to test what the gull perceives to be an egg. Two different models, both of which differ considerably from the real egg in the nest, are placed on the rim of the nest to compare their effectiveness in eliciting the retrieval response. (Redrawn after Baerends & Drent, 1982.)

Figure 1.1 Egg retrieval in the herring gull (Larus): an incubating gull will retrieve an egg that has become displaced from the nest, using a stereotyped pattern of movement. Here, the retrieval response is being used to test what the gull perceives to be an egg. Two different models, both of which differ considerably from the real egg in the nest, are placed on the rim of the nest to compare their effectiveness in eliciting the retrieval response. (Redrawn after Baerends & Drent, 1982.)

It was noticed that many such motor patterns seem to occur in response to specific stimulus situations in the natural environment. During the 1930s, ethologists developed the technique of using models, in which one feature at a time could easily be varied, to find out what features of a situation are important in triggering an animal's response. Lorenz and Tinbergen found that the greylag geese would retrieve wooden models painted to resemble natural eggs. The goose would still retrieve the models when they were made the wrong shape, such as cubes or cylinders, or when they were made the right shape but the wrong size, including models that were much larger than a normal egg. It was evident from these results, and many others, that only certain features of the natural stimulus are needed to produce a response. These essential features were called sign stimuli or, where they were found in the context of social behaviour, social releasers.

Ethologists rightly sought to account for the fact that animals often respond to only a small selection of the available stimuli by postulating neural mechanisms in the responding animal. Response selectivity might

Figure 1.2 A flow diagram showing early ethological concepts of the mechanisms involved in a simple behaviour pattern such as egg retrieval. (Redrawn after Shepherd, 1983.)

be due partly to the capacities of the sense organs, but it was already known that an animal may respond to a specific sensory cue in one behavioural context and not in another. Hence, the occurrence of sign stimuli must also be due to stimulus selection by more centrally located mechanisms processing the information received from the sense organs. The term releasing mechanism was coined for this central processing and, because it was assumed to develop independently of experience with the sign stimuli, the adjective innate was attached to it, giving innate releasing mechanism (IRM). The adjective innate is not much used by modern ethologists, but the term releasing mechanism continues to call attention to an important phenomenon of behaviour.

The way in which the various components might interact to produce a behaviour pattern is illustrated in Fig. 1.2, which represents the results of the early ethological period. In egg retrieval, the visual stimuli from around the nest are passed from the sense organs along a neural pathway to the central nervous system, where the releasing mechanism responds to the sign stimuli that indicate 'egg'. This central mechanism then releases or triggers activity in the motor regions of the nervous system that generate the fixed action pattern for retrieval. This sequence is not invariable in its operation but is enhanced or prevented by other factors. Thus, the releasing mechanism is inhibited in the short term (arrows from above in Fig. 1.2) when the bird is away from the nest foraging or escaping from a predator, and in the long term (arrows from below) retrieval cannot be elicited outside the breeding season, which is controlled by reproductive hormones.

Further insight into this phenomenon has been made possible by the detailed studies of egg retrieval in the herring gull carried out by Baerends and his colleagues (Baerends & Drent, 1982; Baerends, 1985), who placed two egg models side by side on the rim of the nest and then watched from a hide to see which of the models the gull retrieved first. Thousands of these tests were made, carefully varying only one feature at a time, in order to determine what the gulls' preferences were. It was found that the gulls preferred larger eggs to smaller ones, green eggs to any other colour, speckled eggs to uniformly coloured ones, strongly contrasting speckles to weakly contrasting ones, and natural egg shapes to abnormal ones. This last preference was not nearly as strong as might have been expected, and a cylindrical model was almost as effective as an egg-shaped model of the same size and colour.

These results show that the gulls do, indeed, respond selectively to a limited number of stimuli, which match a real gull's egg only in a rough way. It is not even necessary for all the stimuli to be present for a response to occur. The stimuli that are present add together independently to determine the overall effectiveness of an egg model in producing a response. For instance, a smaller green egg will be as effective as a larger brown egg; if speckling is then added to the green egg, it will become more effective than the larger brown egg. One consequence of this property is that models can be made more effective than the real object they represent. A gull will retrieve a model 50 per cent larger than normal, green and with black speckling in preference to one of its own eggs; such a model is what ethol-ogists call a supernormal stimulus.

The experiments with models show that this releasing mechanism involves perception of a number of simple visual cues, which add together quantitatively to determine the degree of 'egginess' as far as the gull is concerned. Clearly, these properties reflect the way in which visual perception occurs in the gull's nervous system, and the flow diagram shown in Fig. 1.3 tries to incorporate this. The response to a limited number of simple cues may well reflect the occurrence in the early stages of the visual system of units that respond selectively to visual cues such as colour, contrast, edges and shapes (represented as selectors, S1to S9 , in Fig. 1.3). The way in which the separate cues add together suggests the presence of a more central unit

Figure 1.3 Releasing mechanism for egg retrieval in the herring gull: a flow diagram based on experiments with egg models. The boxes represent major systems or operations and the circles indicate sites where summation of inputs occurs. Visual perception (top) is represented as a series of selectors (S1 to S9) that respond to particular features of the stimulus. Some of these feed on to a specific detector for egg recognition, which in turn feeds on to the motor control for egg retrieval. This response is maintained during the period of incubation but may be overridden by other factors such as the need to escape (left) or the bird's memory based on experience with real eggs (right). (Redrawn after Baerends, 1985.)

Figure 1.3 Releasing mechanism for egg retrieval in the herring gull: a flow diagram based on experiments with egg models. The boxes represent major systems or operations and the circles indicate sites where summation of inputs occurs. Visual perception (top) is represented as a series of selectors (S1 to S9) that respond to particular features of the stimulus. Some of these feed on to a specific detector for egg recognition, which in turn feeds on to the motor control for egg retrieval. This response is maintained during the period of incubation but may be overridden by other factors such as the need to escape (left) or the bird's memory based on experience with real eggs (right). (Redrawn after Baerends, 1985.)

that combines information from a specific set of selectors so as to act as a detector for specific objects in the environment, in this case an 'egg detector'. Units that correspond closely with this description are found widely in the visual systems of both vertebrates and invertebrates, as shown in the following example of prey detection in toads (see also Chapter 5). It is easy to see how these units could be excited more strongly by a supernormal combination of stimuli than by the natural combination.

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