Biomedical Engineering Reference
In-Depth Information
1996b ) and reduced immune function ( Schapiro et al.,
2000 ) compared to socially housed controls. These findings
are similar to those examining the response of primates in
social groups to actual removal and placement in individual
cage housing. In a study of chimpanzees housed socially
before being moved into individual cages, stereotyped
behaviors such as rocking, pacing, flipping, and spinning
increased during the 5 weeks of individual housing in
comparison to the week prior to relocation. However,
self-directed behaviors such as self-injurious behavior, self-
orality, and eye saluting did not change from pre- to post-
separation ( Brent et al., 1989 ). Increased abnormal
behavior during individual cage housing has also been
reported for rhesus monkeys ( Schapiro et al., 1996a ).
Although social housing is an important component of
a psychological well-being program, it should be
acknowledged that some monkeys do not fare well in social
groups because of their highly aggressive behavior. Such
heightened aggressiveness has been observed in some
individuals in free-ranging troops of monkeys ( Melhmann
et al., 1994 ). Furthermore, such monkeys show a behavioral
and physiological profile that is consistent with impulse
control disorders in humans ( Higley et al., 1996; Higley
and Linnoila, 1997 ).
stress in behavioral pathology remains poorly understood.
Stress might serve as a precipitating factor and/or it might
exacerbate an existing condition. Most of the evidence
supporting a role for stress comes from retrospective
analysis of colony records in which known stressors are
examined with reference to the presence of severe patho-
logical behavior in individual animals. Research has shown
that the number of relocations and number of minor
veterinary procedures (e.g. blood draws) are significantly
associated with SIB in monkeys ( Lutz et al., 2003; Rom-
meck et al., 2009b ). Although it might be assumed that
monkeys exhibiting SIB have a heightened physiological
response to stress, research has revealed that such animals
show a dysregulated HPA axis characterized by a blunted
response to the mild stress of restraint and venipuncture
( Tiefenbacher et al., 2000, 2004 ).
Whether environmental stressors can trigger episodes of
abnormal behavior (including SIB) cannot be determined by
associational studies. However, a more powerful approach
involving the major stress of relocation provided strong
evidence for a causal relationship between stress and SIB.
When individually housed monkeys with SIB and controls
were moved from pens to individual cages and placed in
rooms with higher animal density, all animals exhibited
a stress response as manifested by increased cortisol levels
in plasma, saliva, and hair. Importantly, monkeys with SIB
also showed elevated biting behavior that was still present
a year later ( Davenport et al., 2008 ).
Other Factors
Several nonsocial risk factors may also predispose some
nonhuman primates to develop severe forms of abnormal
behavior when housed in individual cages. These factors
include the temporal parameters of individual cage
housing, exposure to stressful events (e.g. relocations and
routine veterinary procedures), and the genetic character-
istics of each monkey.
Genetic Factors
Not all monkeys that are housed in individual cages
develop abnormal behavior. In fact, in one research facility,
only 11% of the animals developed SIB ( Lutz et al., 2003;
Novak, 2003 ). Recent evidence suggests genetic factors
may influence vulnerability to SIB. SIB has been linked to
polymorphisms in the gene for tryptophan hydroxylase-2,
the rate-limiting enzyme in the synthesis of neuronal
serotonin ( Chen et al., 2010 ), and to polymorphisms in the
mu-opioid receptor gene ( Kelly et al., 2009 ). Other forms
of abnormal behavior such as the heightened aggression
and impulsivity described above have been linked to
polymorphisms in the serotonin transporter gene ( Barr
et al., 2003 ).
Temporal Parameters of Individual Cage Housing
Two aspects of individual cage housing exert a powerful
effect on the development of severely abnormal behavior:
the age at which monkeys are first placed into individual
cages and the total length of time that monkeys spend
without normal contact with conspecifics. The earlier the
age of initial placement into an individual cage, the greater
the risk of developing self-injurious behavior (SIB). In one
facility, the average age of placement into individual cage
housing was 14 months for monkeys that developed SIB as
compared to 28 months for monkeys that did not develop the
disorder ( Lutz et al., 2003 ). Risk of developing SIB was also
related to the total length of exposure to individual cage
housing as noted at three different facilities ( Bellanca and
Crockett, 2002; Lutz et al., 2003; Rommeck et al., 2009b ).
THERAPEUTIC STRATEGIES FOR
REVERSING PSYCHOPATHOLOGY
Although it is not difficult to recognize abnormal behavior
in nonhuman primates, the alleviation or eradication of
such behavior has proven to be very problematic. The
failure to identify specific therapeutic strategies that are
effective in reducing or eliminating most forms of patho-
logical behavior can be attributed to several factors. First,
Stress Exposure
Stress is generally thought to contribute to the development
of abnormal behavior in at least some cases, yet the role of
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