1. Critical sensitive period
Imprinting occurs at a particular time termed the sensitive period during early postnatal life. For example, in anserine birds such as ducks and geese, the time for imprinting is 24-48 hours after hatching when the ‘following response’ is learnt. At this time a gosling learns to follow his mother who is normally the first large moving creature in his world. In fact, of course, the visual stimulus that he imprints on does not necessarily have to be Mother Goose. In these species imprinting can occur on any object within a certain size range regardless of its colour or shape. Movement helps to attract attention but is by no means essential.Although the dominant sense involved in imprinting is sight, sound and olfaction are also involved. In a variety of experiments, young chicks and ducklings were imprinted on humans, wooden blocks and classically even old gum boots. They bonded with a single item and would follow it wherever it went. Rather like Mary and her little lamb, Konrad and his little gosling were to go on to form a life long association. Although Lorenz was the first to record his observations in a scientific manner, the essence of imprinting had long been recognised. Indeed, Chinese peasants have for centuries capitalised on the tendency to imprint in making ducks more effective in the control of snails that otherwise damage rice crops. By imprinting ducklings onto a special stick, the peasants can not only take their brood out to the paddy fields as required but, by planting the stick sequentially in different parts of the plantation, they can ensure that molluscs in all areas can be subjected to predation.
Imprinting seems more important in precocial species, in which the offspring are less dependent on their mothers for food and warmth, than in altricial species which often confine their more vulnerable, and often hairless, young to nests. This is why many horse breeders are recognising the life-long benefits of thorough handling of their foals during the first 24 hours of life. Altricial neonates, on the other hand, are unlikely or unable to stray from their home base in the first few days of life and therefore do not need the same response. They learn similar lessons rather later in life during what are called “socialisation periods”. These apply when the animal’s sensory, motor and thermoregulatory systems are fully functional and they learn to move away from their mother and to interact with others of the same and other species. The window of opportunity for learning varies according on the species. In dogs it is from 3-10 weeks and in cats 2-7 weeks, while in primates it is usually 6-12 months. Stimuli that the youngsters of each species are exposed to during these window periods will be accepted as being “normal”. We do well to exploit this limited learning opportunity in our companion animals.
2. Imprinting is irreversible:
The imprinted knowledge is retained for life. Of all forms of learning, imprinting is the least likely to be forgotten or unlearned.
3. Imprinting establishes an individual animal’s preference for a certain species
Contrary to what one might predict to be their genetic tendency, once they have imprinted, animals will always prefer to follow the learned stimulus rather than a member of their own species. The following response in ducks that have imprinted on humans means that the ducks will preferentially follow any human rather any duck.
4. Some behaviours are affected by imprinting more than others
Not all behaviours are affected by imprinting. Lorenz noted with some amusement that jackdaws that had imprinted on him would court his favour by presenting him with juicy fresh earthworms and would even attempt to introduce these into his ear-holes. However, when not sexually aroused, these birds would happily join other jackdaws in flight. In sexually dimorphic species (in which the external appearance of males and females differ), sexual imprinting varies depending on whether the youngster is male or female. So, while a male mallard duckling will identify his future mate by relating it to the appearance of his mother (or attachment figure), the same does not apply for a female. While falcons imprinted on humans require a combination of human and avian stimuli to elicit sexual responses.