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In this chapter, we will examine the way behavior
develops, especially as it reflects on the maturation of
neural connections. We will look in detail at the devel-
opment of various specific motor, sensory, and social
behaviors, and try to understand from a neural, molec-
ular, and genetic perspective how and why they
develop in the order that they do. We will also examine
the mechanisms by which behaviors become more
precise and skillful, and the role of sensory and motor
experience in the neural circuitry that controls them.
The grasp reflex demonstrated in a newborn baby.
Donald Wilson, who worked on the neural basis of
insect flight, was amazed that a locust, spreading its
wings for the first time, could fly without practice and
make appropriate adjustments to wind speed and
visual signals. “How perfect is the motor score that is
built into the thoracic ganglion?” he wondered. It
seemed to him that the CNS is developmentally pro-
grammed to contain nearly everything that is neces-
sary for flight before actual flight occurs (Wilson,
1968). Michael Bate, who works on the development
of the Drosophila nervous system, expresses a similar
surprise, but for a different reason (Bate, 1998). “How
do we explain,” Bate asks, “the remarkable fact that
behavioral 'sense' of this kind is inherited and built
into the nervous system as it develops?” Genes that
affect behaviors function to control the differentiation
and physiology of neurons. We have not found genes
whose job it is to organize an entire neural circuit. It is
difficult to comprehend the genetic basis of the neural
circuits that underlie complicated behaviors, yet all of
the mechanisms that we have discussed in Chapters
1-9 lead toward building a nervous system with func-
tional circuits that orchestrate adaptive behavior.
Clearly, genetic factors are at the root of behavior;
flies do not behave as humans, no matter how similar
their rearing environment. Yet the nervous systems of
both insects and mammals are built of the same type
of neurons, synapses, and neurotransmitters. They
also obey the same developmental rules often using
homologous molecules. In the 1960s, Seymour Benzer
began to address the question of behavior genetics by
searching for single genes which, when mutated,
would lead to aberrant behaviors in fruit flies (Benzer,
1971). Surprisingly, many of the genes he and his
colleagues discovered by this process are conserved
among different species. For instance, the genes found
to govern the 24-hour circadian rhythm in flies have
homologs that are involved in the same process in
recognize that embryonic and adult animals often live
in very different environments. Each animal tends to
exhibit stage-specific morphological, molecular, and
behavioral adaptations to its specific environment.
This is particularly obvious in the case of animals that
go through metamorphosis, such as moths and frogs.
Here, the larval and adult forms have a radically dis-
tinct appearance and behavior. The nervous systems,
the substrates of these behaviors, undergo substantial
modifications in response to metamorphic hormones
(ecdysone for insects and thyroxine for amphibian),
including death of larval neurons and genesis of new
adult neurons.
Finally, we can consider embryonic behaviors as
substrates for building more complex behaviors and
thus for the continued maturation of the nervous
system itself (Carpenter, 1874). Through function and
feedback, neural circuits become finely tuned. The
learning that we perform as adults may be little more
than a continuation of the mechanisms used to adjust
the embryonic nervous system. The behavioral pat-
terns that we see in embryonic and juvenile animals
are the integrated beginnings of more complex pat-
terns that continue to develop. We must necessarily
crawl before we can walk. If this view is correct, dis-
ruption of these early behaviors should have a signif-
icant impact on the development of later behaviors. A
good example of this is the fine motor skills of the
forearm. Experience is thought to be important for the
development of these motor skills. Preventing normal
forelimb use during early development not only
causes defects in fine forelimb movements that last
throughout life, but also produces defective develop-
ment of the axonal terminals of the corticospinal
neurons that control these movements (Martin et al.,
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