Behavioral Genetics and Animal Science - Genetics and the Behavior of Domestic Animals - page 25

NERVOUS SYSTEM REACTIVITY CHANGED BY
THE ENVIRONMENT
Raising young animals in barren environments devoid of variety and
sensory stimulation will have an effect on development of the nervous
system. It can cause animals to be more reactive and excitable as adults.
This is a long-lasting, environment-induced change in how the nervous
system reacts to various stimuli. Effects of deprivation during early devel-
opment are also relatively permanent. Melzack and Burns (1965) found
that puppies raised in barren kennels developed into hyperexcitable adults.
In one experiment, deprived dogs reacted with “diffuse excitement” and
ran around a room more than control dogs raised in homes by people.
Presenting novel objects to the deprived dogs also result in “diffuse
excitement.” Furthermore, the EEGs of the kennel-raised dogs remained
abnormal even after they were removed from the kennel ( Melzack and
Burns, 1965 ). Research by Simons and Land (1987) show the somatosen-
sory cortex in the brains of baby rats do not develop normally if sensory
input is eliminated by trimming their whiskers. A lack of sensory input
made the brain hypersensitive to stimulation. The effects persisted even
after the whiskers had grown back.
Emotional reactivity develops in the nervous system during early gesta-
tion. Denenberg and Whimbey (1968) showed that handling a pregnant rat
can cause her offspring to be more emotional and explore less in an open
field compared to control animals. This experiment is significant because
it shows that handling the pregnant mother had the opposite effect on the
behavior of the infant pups. Handling and stressing pregnant mothers chan-
ged the gestational environment of the fetus resulting in nervous offspring.
However, handling newborn rats by briefly picking them up and setting
them in a container reduced emotional reactivity when the rats became
adults ( Denenberg and Whimbey, 1968 ). The handled rats developed a
calmer temperament.
The adrenal glands are known to have an effect on behavior ( Fuller and
Thompson, 1978 ). The inner portions of the adrenals secrete the hormones,
adrenaline and noradrenaline, while the outer cortex secretes the sex
hormones androgens and estrogens (reproductive hormones); and various
corticosteroids (stress hormones). Yeakel and Rhoades (1941) found that
Hall's (1938) emotional rats had larger adrenals and thyroids compared to
the non-emotional rats. Richter (1952, 1954) found a decrease in the size of
the adrenal glands in Norway rats accompanied by domestication. Several
line and strain differences have been found since these early reports.
Furthermore, Levine (1968) and Levine et al.(1967) showed that brief
holding of baby rats reduces the response of the adrenal gland to stress.
Denenberg et al. (1967) concluded that early handling may lead to major
changes in the neuroendocrine system.