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.
Rhesus developmental course:
• First month in almost
constant contact with
mother. Strong emotional
• Second, third months
increasing interactions with
• Fourth month (weaning),
peers and play with peers
becomes predominant social
activity. Remains so until
• 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:
(HPA) axis.
• Increased aggression with age
• Increased alcohol intake, when
Brain effects of PO:
• Reduced cerebral blood flow, reduced serotonin activity
• Enlarged regions of brain associated with stress:
dorsomedial prefrontal cortex, dorsal anterior cingulate
• 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
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
• 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
• However, only study in humans found
opposite result. Daughters had greater
negative affectivity. Not so for sons.

Chapter 3 Nonhuman primate models of early development