Can Apes Ape?Clues to Understanding Human Evolution

David F. BjorklundDepartment of PsychologyFlorida Atlantic University

Boca Raton, FL USA

Deferred (Delayed) Imitation

• Reproduction of observed behavior after significant time interval

• Involves the representation of actions held in long-term memory stores, which requires symbolic representation (Piaget; Meltzoff)

• 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 months

Enculturation

• “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)

Grub

Kenya

Noelle

Christopher

Pongo

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

Scoring

• 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): 12.0 sec• Kenya (5 behaviors): 35.0 sec• Noelle (2 behaviors): 17.5 sec

• Group median = 17.5 sec• % displayed with in 60 sec: 92%• % displayed within 30 sec: 67%

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: 50%• % displayed within 30 sec: 33%

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: 24 years, 2 month– Katie: 15 years, 6 months– Anja: 24 years, 10 months– Dona: 14 years, 7 months

Phase 3– Rita: 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: Chimpanzees

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 intelligence.

• 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.