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the first moving object that they see, forming a very
stable attachment. Ordinarily, the mother goose fills
this role, but hatchlings can also learn to follow inani-
mate objects, and even the experimenter himself. This
learned behavior is termed filial imprinting . Filial
imprinting has the immediate advantage of keeping
offspring with the provider, and it can also have impli-
cations much later in life. When mature, the male birds
will court a member of the species on which they
imprinted, whether it is a bird, dog, or human.
Filial imprinting is not unique to birds. Tree shrew
pups will imprint on the nursing mother during the
second postnatal week. If removed from the nest
during this period, a pup will not learn to follow its
real mother, and it can be induced to follow a cloth per-
meated with the odor of a foster mother (Zippelius,
1972). Similarly, rat pups come to prefer their nest
based on the mother's odor during the first few post-
natal weeks, and this preference can be modified by
providing a novel odorant during this period (Brunjes
and Alberts, 1979). Subantarctic fur seal pups must
learn their mother's vocalization within five days after
birth. The mother seals set out on two to three week
foraging trips, and the pups locate their mother within
minutes of her return using the sound of her vocaliza-
tion (Charrier et al., 2001). Newborn humans also
display a preference for their mother. When infants are
able to elicit either their mother's voice or the voice
of another female by the rate at which they suck on
a nipple, they preferentially activate their mother's
voice (DeCasper and Fifer, 1980). What is the evidence
that this preference is learned? When mothers read a
story aloud during the last six weeks of pregnancy,
their babies will subsequently prefer to hear that story
over one that was not read aloud. Unexposed new-
borns display no preference between the two stories.
Thus, even though an infant's hearing is quite limited
in utero, he may already be forming certain auditory
preferences (DeCasper and Spence, 1986).
What exactly is the nervous system learning during
filial imprinting? Do infant animals simply learn their
mother's smell or image? These questions have been
explored in newly hatched ducklings, and the results
suggest that several factors are necessary for filial
imprinting to occur: visual cues, auditory cues, and
social environment. When one-day-old mallard duck-
lings are allowed to follow a stuffed mallard hen for
30 minutes, they develop a preference for this replica,
presumably based on its visual appearance (Johnston
and Gottlieb, 1981). However, mother ducks also
produce an “assembly” vocalization, and this auditory
cue maintains filial imprinting as the ducklings begin
to grow. The assembly call is such a powerful signal
that ducklings will preferentially follow an unfamiliar
red-and-white striped box that is producing this call
rather than a familiar mallard hen model.
Why is the mother's assembly call such a powerful
cue? One possibility is that the mother's call is necessary
to maintain her duckling's attachment when the entire
family leaves the nest and begins to move about the
environment. Older ducklings become very attached to
their siblings as they grow, and this “peer imprinting”
can actually interfere with filial imprinting (Dyer et al.,
1989). For example, socially reared ducklings will not
preferentially follow a silent, familiar mallard model,
although individually reared ducklings will do so.
However, the mallard maternal call will induce socially
reared ducklings to follow a familiar mallard or an unfa-
miliar pintail model (Dyer and Gottlieb, 1990).
This raises an important question: Do ducklings
respond innately to their mother's call, or does it
depend on sensory experience? Interestingly, duck-
lings have an inborn preference for the mother's call
rate, 4 notes per second. However, to maintain prefer-
ence through hatching, the duckling must either hear
its own “contentment” call or that of its siblings
(Figure 10.23). When ducklings are devocalized and
reared in isolation with a “contentment” call that is
slowed down to about 2 notes per second, they subse-
quently show no preference for the mother's “assem-
bly” call (Gottlieb, 1980). Thus, imprinting is a far more
elegant form of learning than was originally suspected.
Although ducks, geese, and chicks can visually imprint
on an object after walking behind it, many other factors
regulate this learning. By studying the animal in its
natural setting, it becomes clear that developing
animals are “prepared” to learn certain cues they will
likely encounter, such as the vocalization of its siblings.
Newly hatched domestic chicks also display filial
imprinting, and the neural substrates have been
studied in some depth. When chicks are presented
with tones pulsed at about 3 Hz, they will develop a
strong preference for this acoustic stimulus and selec-
tively approach it (Wallhausser and Scheich, 1987).
There is a dramatic reduction of spines in two differ-
ent higher forebrain regions during the period of
imprinting, and this is not observed in naive chicks.
Furthermore, both imprinting and spine elimination
depend on functional NMDARs within these forebrain
regions (Bock et al., 1996; Bock and Braun, 1999a,
1999b). Chicks can also learn to imprint on the visual
characteristics of an object and follow it around, just as
duckings do. Destruction of a specific forebrain area
impairs imprinting, and this same region displays an
increase in NMDARs following imprinting (Horn,
2004). Therefore, some of the cellular mechanisms that
have been implicated in synaptic plasticity (Chapter 9)
have an important role in this early form of learning.
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