Biology Reference
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between breeds and between individuals within a breed ( Dantzer and
Mormede, 1983; Grandin, 1993a; Murphey et al., 1980b, 1981; Tulloh, 1961 ).
Fearful, flighty animals become more agitated and struggle more violently
when restrained for vaccinations and other procedures ( Fordyce et al., 1988;
Grandin, 1993a ). Fear is likely to be the main cause of agitation during
restraint in cattle, horses, pigs, and chickens. Recent studies on the genetic
effects on behavior during transport handling, and restraint of these animals
are discussed in Chapter 4.
Species Differences in Emotional Reactions to Similar Tests
In an open-field test, a single animal is placed in an arena ( Hall, 1934 ).
Rodents often stay close to the arena walls whereas cattle may run around
wildly and attempt to escape. Possibly the reaction of the rodent is motivated
by fear and the reaction of a single bovine may be PANIC (separation dis-
tress). The rodent has an instinctual fear of open spaces and stays close to
the walls. The motivation is to avoid open spaces where it likely to be seen
by a predator. The bovine is motivated to rejoin its herdmates. The open-
field test may be measuring different emotional systems in these two species.
When fear is the motivator, a frightened animal may react in two different
ways. It may run around frantically and try to escape or in another situation,
it may freeze and stay still. Chickens often freeze when handled by humans.
Jones (1984) called this “tonic immobility.” The chickens become so fright-
ened they cannot move. In cattle, brahman Bos indicus cattle are more likely
to go into tonic immobility than Bos Taurus breeds such as Hereford. In
wildlife, forceful capture can cause enough fear to sometimes inflict fatal
heart damage. Wildlife biologists call this capture myopathy. In summary,
much is known about the complex phenomenon of fear, but many questions
still remain.
BIOLOGICAL BASIS OF FEAR
Genetic factors influence the intensity of fear reactions. Genetic factors can
also greatly reduce or increase fear reaction in domestic animals ( Flint et al.,
1995; Parsons, 1988; Price, 1984 ). Research in humans has clearly revealed
some of the genetic mechanisms governing the inheritance of anxiety ( Lesch
et al., 1996 ). LeDoux (1992) and Rogan and LeDoux (1996) state that all
vertebrates can be fear-conditioned. Davis (1992) recently reviewed studies
on the biological basis of fear. Overwhelming evidence points to the amyg-
dala as the fear center in the brain. A small bilateral structure located in
the limbic system, the amygdala is where the triggers for “flight or fight” are
located. Electrical stimulation of the amygdala is known to increase stress
hormones in rats and cats ( Matheson et al., 1971; Setckleiv et al., 1961 ).
Destroying the amygdala can make a wild rat
tame and reduce its
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