Filial imprinting refers to the process where the social behavior of the young animal becomes limited to a particular object or class of objects, as a result of exposure to an object. In most cases studied, the stimulus object is the social mother, hence the young learn to recognize and attach to a parent through filial imprinting. The typical example is the young of ducks and geese, which instantly follow their mother. This is often called the following response. The chicks of these birds imprint on any individual present at hatching time, including a human, and will follow this individual as a mother figure. In fact, ducklings and goslings have also been shown to imprint on various inanimate objects in the laboratory. Generally, the attachment to the mother figure is very strong, and hence when a gosling is imprinted on a human, it will not change the attachment although it is given access to its true mother after only a day post hatch. Hence, a characteristic of this imprinting is that it is more or less irreversible. Cases of filial imprinting similar to those seen in birds are also found in mammals. For instance, a juvenile, single lamb will follow a human who provides milk from a bottle, also when the lamb is not hungry. The attachment may last well into adulthood.
At least two perceptual mechanisms are involved in the development of filial behavior, namely filial imprinting and predispositions. The latter refer to perceptual preferences that develop in young animals without any prior experience with the particular stimuli involved. The two mechanisms are neurally and behaviorally dissociable but are supposed to interact during the development of filial preferences, where the predisposition may bias the animal's responses. This bias may differ strongly among species, as observed in precocial birds. For instance, the bias may be relatively weak in a goose but strong in a wader.
Numerous experiments have shown that the young may imprint on a wide range of objects, and individuals that form an attachment to the wrong kind of object will certainly be disadvantaged under natural circumstances. However, in nature the adult present at this early stage of life will usually be the caring parent and so this imprinting method may work perfectly. In species with parental care, the benefits of filial imprinting are obvious because it helps an offspring to attach to its parent. If such a bonding does not take place, the parent may not start investing in that particular young and it will soon die. The following response of goslings and ducklings is adaptive because in these birds the nest is often placed at some distance from water and hence the mother has to lead the young to the water soon after hatching, often through various obstacles, like dense vegetation, where it is important that the young would follow closely. The strong attachment to the mother would also have other advantages, like avoiding dangerous sites and learning to recognize enemies, where the mother may give a specific call and the chicks would freeze and remain motionless until danger has passed. Attachment to the mother will, in many cases, also help the young to learn foraging behavior, to locate shelter, and to socialize with other members of their own species.
Typically, juveniles do not avoid any object initially but tend to approach and explore them. Imprinting on the caring parent would help them to avoid novel objects, and this acquired ability to discriminate may effectively bring the sensitive period for the filial imprinting to end. However, experiments have also shown that after having become familiar with an imprinting stimulus, the juvenile may begin to prefer stimuli slightly different from the initial one. This may have the effect of familiarizing the offspring with different aspects of the mother. It may be important to recognize a parent from many angles, which seems only possible if the juvenile builds up a composite picture of its parent's characteristics.
Apparently, the juvenile identifies as the caring parent the first object it meets that possesses some simple characteristics. Various tests have been made to study which imprinting stimuli are more important. In case of the geese studied by Konrad Lorenz, most important was the movement of an object away from the chick. The effect was even stronger if the object produced some sound, although the sound did not need to be from a goose or bird; even the sound of a ticking clock could work. Furthermore, the model body did not need to be of an animal or covered by feathers but could be a simple box or a block of woods. However, more detailed studies have shown that young birds tend to show some innate preferences for certain features, such as color, shape, and size that may steer them toward the real mother figure rather than toward some arbitrary objects. Similarly, juvenile rhesus monkeys prefer a cloth surrogate mother to a wire surrogate mother. Such innate preferences are often referred to as predispositions. The variation among species in sensitivity to stimuli seems related to which stimuli are important in the wild. For instance, sound is very important in wood ducks. These birds nest in tree holes and the mother calls to induce the ducklings to leave the cavity. There also seems to be a change in relative importance of stimuli with time. For instance, in ducks, the following response is largely influenced by auditory cues from the mother soon after hatching whereas visual cues become important later on. Likewise, offspring of many species may show a preference for the more conspicuous signals, but if the signal is too startling, it may elicit fleeing rather than approach. The following response can also be enhanced with food rewards, which makes sense in species where the parent provides food or leads the young to a food resource.
Studies have also been made to identify the sensitive period for the filial imprinting, for instance, by quantifying the following response in ducks and geese to a proper mother model presented at various points in time. Some authors use the term 'critical period' for the part of the sensitive period when the learning response is greatest. In precocial birds, like geese, where the chicks can run around and find food soon after hatching, the sensitive period for the filial imprinting is only the first one or two days post hatch. In fact, the young seem to learn the call of their mother already before hatching. However, the duration recoded for the sensitive period may be dependent on the method used, for example, whether the response is measured as the percentage of following responses after a single exposure, or by the percentage of birds following during the first exposure; the former measure may show a much more sharply defined period of sensitivity than the latter. One also has to take into account whether single individuals are tested, or a brood of young, because there may also be an effect of siblings (social effects, or imprinting on siblings). For instance, juveniles kept singly may remain responsive to moving objects much longer than juveniles kept in groups. Imprinting on sibs may help the juveniles to stay together.
The sensitive period for filial imprinting is much later in altricial birds than in precocial birds. Altricial young are blind and very helpless when hatching, which may limit their opportunity to imprint during their first days of life. Moreover, the necessity for very early imprinting on parents clearly differs between the two groups of birds. Precocial young move around and feed at least partly by themselves shortly after hatching, and may thus need immediate tuition in, say, feeding behavior. Moreover, this behavior may lead to many encounters with various adults apart from their mother. Altricial young, on the other hand, do not need to distinguish their parents from other birds until quite late in the nestling period since they receive food in their nest where there usually will not be any adults around except their parents.
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