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CHAPTER
10
Behavioral Development
BEHAVIORAL ONTOGENY
of the circuit from eye to brain and brain to motor
output is ready and waiting for this input. Early
behaviors tend to be imperfect at first and then fine-
tuned in a manner that matches the fine-tuning of the
development of synaptic connections, which goes on
in humans through childhood and beyond. Some early
behaviors are anticipatory in the sense that they reflect
ancestral nervous systems and modes of behavior
evolving toward later forms. There is evidence that
mammalian embryonic spinal cords have rhythmic
activity similar to swimming patterns of fish, and that
at early stages even human embryos display swim-
ming movements. The grasp reflex is an interesting
example of this phenomenon (Figure 10.1). This reflex,
present at birth in a human infant, disappears at about
3 months of age. When the palm of a baby is touched,
it causes inward curling of the fingers and a forceful
hold on any object. This grasp reflex is thought to be a
relic of a more primitive primate nervous system in
which clinging to branches or mother was important
for survival. Perhaps these embryonic reflexes that are
vestiges of ancestral behavioral traits, reflect important
steps in the development of the neural control of our
more derived behavioral repertoire.
Some embryonic and juvenile behaviors are useful
and adaptive , and serve specific functions at particular
stages of development. Hatching behaviors are a good
example (Oppenheim, 1982). Quail embryos make
clicking noises in the shell, which helps to synchronize
hatching (Vince and Salter, 1967, Vince 1979). More fre-
quent clicks accelerate hatching, and less frequent
clicks retard it. In reptiles, birds, and insects, hatching
is composed of repeated stereotypical movements that
are transient, specific to that stage of life and clearly
adaptive. In human babies, the rooting reflex, turning
the face toward a touch on the cheek, is a transient
reflex important for breast-feeding. It is important to
We often think of behavior in terms of the lives of
postnatal animals, but behavior begins well before
birth. As a fetus, you were making coordinated,
though perhaps not goal-directed, movements. You
were kicking, swallowing, putting your thumb in your
mouth, and moving to songs. With ultrasound, we can
now see the emergence of behaviors in humans, and
even diagnose behavioral deficits prenatally. A bird
embryo in its shell also moves and peeps. Even fly
embryos wiggle and squirm before they hatch. The
first movements and motor responses that an animal
makes are far simpler than the sophisticated move-
ments of an adult. Do these embryonic behaviors serve
any strategic purpose, or are they merely the conse-
quences of a nervous system that is wiring up and
becoming electrically active? Which behaviors arise
first in the embryo, how does the repertoire of behav-
ior grow, is there any logic in the sequence, and how
does behavior feed back onto the building of the
nervous system? These are very intriguing questions
for a developmental neurobiologist.
Early behaviors have often been classified as antic-
ipatory, adaptive, or substrative (Oppenheim, 1981).
Clearly, some behaviors develop in an anticipatory
manner, with forward reference to actions that will be
of value in later life (Carmichael, 1954). For instance,
prematurely born humans can respond to light
although they would normally not be exposed to light
in the womb. Since the nervous system is constructed
over a period of time, behaviors cannot be slapped
together the moment they are needed, but arise with
the developing circuitry. In the case of sensing light,
one of the last things that happens in the nervous
system is the addition of active photoreceptors; the rest
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