Chapter 3
Nonhuman primate models of early development
Rhesus macaque model: important features
As primates they are reasonably closely related to humans (but less so than
apes such as chimpanzees or bonobos)
Very social: life in groups of 20-200; organized around female matrilines.
Significant differences in individual temperament within social groups, 15-20%
unusually fearful, 5-10% unusually impulsive or aggressive.
Development
Rhesus developmental course:
• First month in almost
constant contact with
mother. Strong emotional
attachment.
• Second, third months
increasing interactions with
peers
• Fourth month (weaning),
peers and play with peers
becomes predominant social
activity. Remains so until
puberty.
• Marked individual
differences in temperament
in peer play.
Effects of Peer-only raising in Rhesus macaques.
• Procedure: separated from moms at birth; raised
by humans for first month; moved to peer groups
of 3 or 4 beginning 2nd month; moved from peer
group to larger social group at 6 or 7 months,
larger social group usually has both MP (normal)
reared and PO reared members.
PO effects:
• Development of hyperattachments to peers,
dysfunctional, excessive clinging
• Heightened fear response, little
exploratory behavior, sig
reduced play behavior
• Heighted stress response:
Hypothalamic-pituitary-adrenal
(HPA) axis.
• Increased aggression with age
• Increased alcohol intake, when
available
Brain effects of PO:
• Reduced cerebral blood flow, reduced serotonin activity
• Enlarged regions of brain associated with stress:
dorsomedial prefrontal cortex, dorsal anterior cingulate
cortex.
• Effects on immune function also found, inflammation
response increase, antimicrobial response decreased
HPA axis
• Importance of HPA axis in
stress response: ultimately
leads to release of stress
hormone cortisol. Cortisol
feeds back on brain reducing
HPA activity, thus creating a
natural self-limiting effect of
stress response.
• However: Chronic activation of
HPA axis (as might occur in
stressful developmental
situations) is implicated in
parenting-related disorders
such as post-traumatic stress
and anxiety disorders.
• Implication: abandonment of
evolved parenting practices
(EPP) – increased stress –
increased probability of stressrelated disorders. EPP include:
children embedded within
families, multi-aged relatives,
embedded within cultural
practices.
Stress and cognitive development
Using hair cortisol levels (found to be more reliable measure than
other way of testing cortisol levels and replicable in humans)
• Increased hcl associated with increased trials necessary to pass
object permanence tests in infant rhesus macaques.
• In humans: increased HCL associated with lower performance on
cognitive tests and depression and bipolar. Decreased HCL
associated with generalized anxiety, PTSD, and panic associated
with lower HCL.
• In any non-typical rearing rhesus population, stress sharply
increases HCL, only MP-reared showed little change in HCL despite
stressors.
• HCL was also predictive of later life anxiety in PO rhesus.
Mother’s milk and stress
• Along with important nutrients, fatty acids
for brain development and energy; MM
also provides hormones for the
regulation of the HPA axis.
• In rats, ingestion of MM has been shown
to affect both cognitive (better spatial
memory) and social (more play behavior)
development. As adults, better regulated
stress response.
• In rhesus macaques: sons who received
higher concentrations of cortisol in MM
had higher “confidence” (bold, active,
curious, playful) scores. Not so for
daughters.
• However, only study in humans found
opposite result. Daughters had greater
negative affectivity. Not so for sons.