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Can Apes Ape?
Clues to Understanding Human Evolution
David F. Bjorklund
Department of Psychology
Florida Atlantic University
Boca Raton, FL USA
Deferred (Delayed) Imitation
• Reproduction of observed behavior after significant time
• Involves the representation of actions held in long-term
memory stores, which requires symbolic representation (Piaget;
• May reflect declarative/explicit, as opposed to
nondeclarative/implicit, memory (McDonough et al., 1995)
• Human infants show evidence of deferred imitation for simple
actions by 9 months and for more complex actions by 18
• “Apes raised by humans in something like a human
cultural environment (sometimes including
exposure to or training in symbolic skills); the
environment need not literally be a home but must
include something close to daily contact with
humans and their artifacts in meaningful
interaction (Call & Tomasello, 1996)
• Direct teaching
• Language
• Joint-shared attention
Tomasello, Savage-Rumbaugh, and Kruger (1993)
3 enculturated chimps (2 bonobos and 1 common)
3 mother-reared chimps (2 bonobos and 1 common)
18- and 30-month old children
4 deferred-imitation tasks of simple tool use.
• Baseline, objects from 4 tasks to interact with for 4 minutes
• Target behavior modeled for animal
• 24 hour delay, animal given objects for 4 minutes and look for evidence of
deferred imitation
Percentage of trials showing deferred imitation for
children and chimps (Tomasello et al., 1993)
Design of Deferred Imitation Experiment (Bering, Bjorklund, &
Ragan, 2000)
3 Enculturated Juvenile Chimpanzees (Pan troglodytes)
3 Enculturated Juvenile Orangutans (Pongo pygmaeus)
Sequence for Each of 7 Tasks
4-minute Baseline
5-minute Delay
Demonstration of Target Behavior (6 displays)
10-minute delay
4-minute Deferred Imitation Phase
• Target
• Approximation to the Target
• No Imitative Behavior
Percentage Deferred Imitation: Chimpanzees
Percentage of Deferred Imitation: Orangutans
Median Latencies to Imitate Target or Approximation to Target
Behaviors during the Deferred Trials: Chimpanzees
• Grub (5 behaviors):
• Kenya (5 behaviors):
• Noelle (2 behaviors):
Group median = 17.5 sec
% displayed with in 60 sec:
% displayed within 30 sec:
12.0 sec
35.0 sec
17.5 sec
Median Latencies to Imitate Target or Approximation to Target
Behaviors during the Deferred Trials: Orangutans
• Pongo (1 behavior):
235.0 sec
• Ruby (3 behaviors):
95.0 sec
• Christopher (2 behaviors):
25.0 sec
Group median = 65 sec
% displayed with in 60 sec:
% displayed within 30 sec:
Deferred Imitation in Nonenculturated Chimpanzees
• Five female lab-reared chimpanzees, all long-term members of a
stable social group at Yerkes Field Station
• Kristin Bonnie and Frans de Waal
Phases 1 and 2
– Georgia:
– Katie:
– Anja:
– Dona:
Phase 3
– Rita:
24 years, 2 month
15 years, 6 months
24 years, 10 months
14 years, 7 months
17 years, 1 month
No evidence of immediate or deferred imitation by any
animal in any phase
Generalization of Imitation
• Generalizing behaviors observed with one set of objects to
similar, but not identical, objects, to achieve a goal.
• Such generalization requires the actor to understand that a
similar goal, with a new set of objects, can be achieved by
executing similar actions.
Design of Generalization of Imitation Experiment: Chimpanzees
(Bjorklund, Yunger, Bering, & Ragan, 2002)
3 Enculturated Juveniles Chimpanzees (Pan troglodytes)
Sequence for Each of 8 Tasks
6-minute baseline with two sets of objects (e.g., cymbals and trowels)
5-minute Delay
Demonstration of Target Behavior (e.g, with cymbals)
10-minute delay
4-minute Generalization of Imitation Phase with different objects than
used in demonstration
• 4-minute Imitation Phase with same objects used in demonstration
Percentage Imitation and Generalization of Imitation:
Percentage of Imitation and Generalization of Imitation:
Orangutans (Yunger & Bjorklund,2004)
Species-atypical environments for human-reared chimpanzees
produced modified patterns of social cognition. What is the nature of
such changes?
• Produce only molar (i.e., behavioral) level changes in sociality
leading to enhanced learning abilities
– Socialization of attention (Tomasello)
– Apprenticeship hypothesis (Bering)
• Changes in the epigenetic system leading to the phenotypical
expression of cognitive abilities that are otherwise suppressed
under natural conditions
• Chimpanzees, and likely our common ancestor with
chimpanzees, possessed the necessary plasticity to modify
their social cognition in response to novel environments,
possibly leading to the exploration of new niches and to new
selection pressures, and eventually to evolutionary changes in
• Epigenetic theories of evolution view a developing organism’s
response to environmental changes as a mechanism for
evolutionary change. Natural selection still plays an important
role in evolution, but it is the developmental plasticity of an
organism that provides the creative force for evolution.