The End-State Comfort Effect in Vervet Monkeys (Chlorocebus pygerythrus)

Restricted (Penn State Only)
Anthony, Sarah
Area of Honors:
Bachelor of Science
Document Type:
Thesis Supervisors:
  • Daniel J Weiss, Thesis Supervisor
  • Kenneth Levy, Honors Advisor
  • Second order planning
  • End-state comfort
  • Vervet monkey
  • Comparative psychology
  • Development
  • Primates
  • Motor planning
  • Cognition
Adult humans anticipate the consequences of forthcoming actions. In second order motor planning, an individual adopts a grasp or action not just based on the immediate task demands, but in anticipation of what is going to be done next. The end-state comfort (ESC) effect is a subclass of second order motor planning effects that emphasizes terminal positions. Evidence for the ESC effect has previously been found in multiple species of nonhuman primates and there is a clear developmental trajectory to the use of ESC in humans. This study attempted to find evidence for ESC in a species on nonhuman primate not previously tested and to fill a critical gap in the literature and ask whether there is a developmental trajectory for nonhuman primate ESC akin to what is observed in human children. We tested two groups of juvenile vervet monkeys (Chlorocebus pygerthrus), young juveniles (aged 4-7 months) and older juveniles (15-30 months) and two mature adults. A cup-manipulation-task was used where a cup was placed in either an inverted or upright orientation and the monkeys had to pick it up in order to retrieve a marshmallow stuck inside. We expected that the monkeys who demonstrated ESC would deploy a thumb-down (inverted) grip on the stem in order to facilitate rotation of the cup and extraction of the marshmallow in a more stable and comfortable position. We found that overall, 12 out of 25 monkeys inverted their grip in at least one of the inverted cup trials thereby providing an existence proof for the ESC effect. When analyzed by age group, there seemed to be a developmental trend. The young juveniles had a lower proportion of individuals who used an inverted grip at least once relative to the group of older juveniles. The young juveniles also exhibited a higher frequency of other grips relative to the older juveniles indicating greater variability in the strategies employed to complete the task.