|
Imprinting
First described by Konrad Lorenz, imprinting is said to
occur when innate behaviours are released in response to a learnt
stimulus. Most imprinting promotes survival of newborn animals and
shapes their future breeding activities. Imprinting has a number of
characteristics.
Characteristics of imprinting
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.
5. Stressful stimuli fortify imprinting
If there is an increased level of stress at the time of the original
imprinting, the learning is more robust that normal. So if, in the
laboratory set up illustrated in
the
figure
below, obstacles are placed in
the runway between the duckling and the followed object then the
following response the duckling subsequently exhibits is more
determined and energetic. It may be that this enhances an individual
duck family's level of imprinting at times of greatest need, for
instance when the threat of predators or the distraction of other
broods is a particular problem.
After imprinting there are
two
major categories of learning,
associative and non-associative. In
non-associative learning the animal is exposed to a single stimulus
to which it can become habituated or sensitised, while in
associative learning a relationship between at least two stimuli
becomes established. There are two sub-divisions under the umbrella
of associative learning. These are called
classical
conditioning and operant conditioning.
The latter, as we will see, is important for animals to be able to
solve novel problems in their environment.
|
