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But not all transitory embryonic motor behaviors
are useless or simply the result of partially completed
neural circuitry. One behavior that serves a clear
purpose is the hatching movements of bird embryos.
Chicks, like many other shell-bound embryos, go
through a very specific motor behavior pattern just at
hatching (Bekoff and Kauer, 1984). These behaviors
allow the embryo to get into the appropriate position
for breaking open the shell. If one places a post-hatch
chick into the hatching position within a glass egg, the
bird reinitiates its hatching behavior (Figure 10.10). If
sensory input from the neck is eliminated with a local
anesthetic, then hatching behavior is suppressed
(Bekoff and Sabichi, 1987). Therefore, it appears that
sensory receptors located in the neck provide a specific
input signal for initiating hatching behavior. Another
example of a transient embryonic behavior is the
migration that marsupial embryos make from their
womb to the mother's pouch. Born at an extremely
early phase of development with their hind limbs little
more than buds, these tiny embryos use their forelimbs
to crawl tens of body lengths into the pouch where
they attach onto a nipple and suckle (another transient
but adaptive behavior of mammals) for several
months. During this time they complete their embry-
onic development. Human infants that do not suckle
can be fed through a tube, and the absence of these
early suckling experiences does not impair the devel-
opment of adult eating. This demonstrates, once again,
that the juvenile behavior is not a prerequisite for the
adult motor program, even though it is adaptive to the
neonatal environment.
One of the most important transitory behaviors of a
metamorphic insect such as a fly is eating. Fly larvae
are eating machines, whereas adult flies are procreat-
ing machines. There are two styles of eating found in
natural population of Drosophila larvae, and this dif-
ference can be genetically mapped to differences in the
activity of a single gene, which encodes cGMP-
regulated protein kinase (PKG) (Osborne et al., 1997).
Larvae with the more active rover allele of PKG have
significantly longer foraging path lengths than do
those homozygous for the less active sitter allele. The
two Drosophila foraging variants do not differ in their
general activity in the absence of food, but when food
is available the two variants may fare differentially
based on the density of other animals feeding in the
vicinity. If it is crowded, larvae that forage further may
do better. Natural selection experiments done in the
laboratory showed that under high-density rearing
conditions for several generations, the rovers did better,
whereas the short path ( sitter ) phenotype was selected
for under low-density conditions (Sokolowski et al.,
1997). Interestingly, the age-related transition by honey-
bees from hive work to foraging is also associated with
an increase in the expression of the PKG gene and
experimentally controlled cGMP treatment or elevated
PKG activity cause premature foraging behavior in
bees (Ben-Shahar et al., 2002).
Metamorphosis signals a dramatic change in
lifestyle in insects, such as moths and flies, as well as
amphibia, such as frogs (Harris, 1990, Truman, 1992).
The larval behaviors of the swimming, filter-feeding
tadpole are completely inappropriate for the land-
dwelling, bug-eating frog. The transition from larval to
adult state is activated by specific hormones: ecdysone
for insects and thyroxine for frogs. Each of these hor-
mones has a widespread effect on gene expression and
A
Normal Hatching EMGs
ST
Hip
GL
Ankle
SA
Hip
BF
Knee
B
One day old post-hatch
chick folded into glass egg
C
Glass Egg
ST
GL
SA
BF
FIGURE 10.10 Chicken trying to hatch again in a glass egg. A.
Electromyographic (EMG) recordings of the normal hatching motor
program in the chick embryo. (ST and SA are hip muscles, GL is an
ankle muscle, and BF is a knee muscle.) Note the alternation between
the activity of hip and lower leg. B, A one-day-old chick is crammed
back inside a glass egg. C. EMG records from such a chick show that
it reinitiates the hatching motor program. (Adapted from Bekoff and
Kauer, 1984)
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