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Spontaneous prosocial choice by chimpanzees

Spontaneous prosocial choice by chimpanzees. Horner et al. Background - Altruism. Altruism commonly seen in humans Chimps closest biological relative Contradiction between observational and experimental studies Altruistic or empathetic behaviors seen observationally (sharing food)

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Spontaneous prosocial choice by chimpanzees

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  1. Spontaneous prosocial choice by chimpanzees Horner et al.

  2. Background - Altruism • Altruism commonly seen in humans • Chimps closest biological relative • Contradiction between observational and experimental studies • Altruistic or empathetic behaviors seen observationally (sharing food) • Lack of consistent evidence for prosocial behavior in controlled experimental conditions • Two primary altruistic experimental methods • Giving Assistance Tests (GAT) • Prosocial Choice Tests (PCT)

  3. Background - Giving Assistance Tests • Choice between giving help or doing nothing • Giving an out-of-reach object for a human or conspecific • Provided a conspecific with a needed tool • Bonobos in particular show prosocial tendencies in GAT • Communication greatly increases chance of receiving needed help • Children similarly generally grant help only to vocal partners

  4. Background - Prosocial Choice Tests • Subject chooses between two outcomes that are identical to self • One choice rewards only self, the other rewards self and a peer • Chimps have not shown consistent preference for the prosocial choice • However other monkeys (e.g. capuchin monkeys) have shown prosocial preference

  5. Background – Current Study • Confounding factors may be responsible for lack of significant results • Complex testing apparatus • Distracting rewards • Limited contact between subject and partners • Horner et al.’s goal: reduce these extra factors

  6. Methods - Overview • Subject chooses a token from the bucket • Tokens represent either selfish or prosocial choice • Food given to either just subject, or both subject and partner, based on token

  7. Methods - Participants • 7 Adult female chimpanzees (6 in data) • All members of a long-established 12 chimp living group • All experienced in a token exchange program • Each chimp was tested with three different partners • One with a significantly affiliative relationship • One with a significantly negative relationship • One with a neutral relationship • Each subject given a preference tests, to control for any color preference in the tokens

  8. Methods – Contingency Training • Two participants were chosen and randomly designated as either the actor or the partner • Actor given a set of 10 tokens, 5 selfish and 5 prosocial (distinguished by differing color) • Experimenter requested them back one-by-one • Actor rewarded when giving any token back, partner rewarded when prosocial token given back

  9. Methods - PCT • Subject given tub filled with 30 tokens, 15 of each • Hands a token to experimenter, who puts token on easily visible platform, and replaces token in tub • Experimenter gives banana treat, wrapped in a noisy paper wrapping, to either the subject or the subject and partner • Partner can be easily seen and heard

  10. Methods – Control Trials • Subjects were run through experiment again, with identical procedures until the PCT, including preference test and contingency training • A new color scheme selected for tokens • PCT the same, except for the lack of a partner • If a prosocial token chosen, experimenter still pretends to give a food reward to an imaginary partner

  11. Results • Prosocialv.s. selfish token • Bias towards prosocial choice • No partner control group • chimps choose randomly

  12. Results

  13. Results • Found no correlation for choices based off of previous behavior when roles were reversed • High ranking chimps found to be more prosocial than subordinate apes • No significant difference between related individuals vs. non related • No correlation between prosocial behavior and outside affiliation (grooming pairs)

  14. Results- Actor/partner interactions • Chimps interacted frequently • Behavior of partner categorized as: • Neutral • Attention-getting • Direct requests and pressure (DRP) • Partners engaged in the non-neutral behaviors more often following selfish choices by the actor

  15. Results- Actor/partner interactions

  16. Results- Actor/partner interactions • Actors acted prosocial towards neutral partners, and even more prosocial towards attention-getting partners • Actors did not act differently from chance level for DRP partners

  17. Results- Actor/partner interactions

  18. Discussion • Chimps favor the prosocial option • Factors such as kinship, affiliation, dominance rank or reciprocity have no effects • Matches what chimps do in the wild when sharing a common goal

  19. Discussion • However, this study does have different results than other studies done in a lab • They believe this was due to • Close proximity • Wrapping the treats in paper • Avoiding complex apparatuses • Results also differ from popular suggestion that prosocial choices happen more often between related monkeys

  20. Discussion • They do acknowledge that chimps could be acting on reciprocity for events happening outside of the experiment • How do you think this could be controlled for?

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