This experiment tested how posture affects the hand preference in tufted capuchin monkeys (Cebus apella).
The research indicated that in fact there may exist a preference for use of the right-hand when in an upright posture standing bipedally. This preference, according to the study is exhibited in the capuchins when reaching bipedally. However the monkeys did not necessarily exhibit a preference when using tools to probe for a treat. In addition the results showed that there was little or no bias in hand preference neither for reaching nor for tool use when standing quadrupedally. The authors link this finding with the human retreat from quadrupedalism and our subsequent right-hand preference that has been selected for almost exclusively, equally exemplified in all human cultures, through the use of complex tools.
The experiment consisted of 11 male and 5 female capuchins at various developmental levels all of which contributed 50 trials each, over 8 months. In order to conduct the quadrupedal reaching analysis the researchers placed a piece of fruit on the base of the subject’s cage. To evoke the bipedal reaching response the fruit was instead place 40-50 cm above the floor on the cage mesh. The probing response was evoked using an apparatus which was filled with a sweet sugar syrup the apparatus was mounted to the cage both at the bottom of the cage (quadrupedal response) and at shoulder heighth (bipedal response).
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In all cases a researcher took note of the hand used for the retrieval or tool use actions.
In order to more accurately and systematically measure each response the researchers developed a formula that yields a handedness index (HI), a score that indicates hand preference. The formula is as follows: [(R-L)/(R+L)] , R = the number of right-handed reponses and L = the number of left-handed responses. Next this HI score
was used as an absolute value so that regardless of right or left hand preference a measure is reached that shows overall preference strength. This scale places a positive value to right-handed actions and a negative value to left-handed actions. According to this scale there was a significant rise in frequency of right-handedness in bipedal reaching and a greater frequency overall in bipedal action. Contrarily there was little interaction in the bipedal tool use.
The authors offer several mechanisms through which bipedalism may affect a particular hand preference. One view offered is that the quadrupeds, not habitually standing bipedally, are influenced by the ‘greater specialization’ required in order to perform such manual actions. The authors keenly take note that this would not explain this experiment’s findings pertaining to the use of complex tools and the apparent lack of handedness associated with it. It is perplexing as to why no hand preference would be associated with reaching and not tool use.
The authors recognize bipedalism as a trait that truly distinguishes man from other primates. They note several schools of thought concerning why handedness may have occurred evolutionarily. One such school of thought asserts that bipedalism is an evolutionary effect of tool use. That is primate ancestry may have realized the advantage of tool use and selection passed traits of handedness that in turn allow greater manual dexterity while standing bipedally. The cultural significance of this dexterity increase is evident in all aspects of human tool use. On the other hand one very much contradicting school of thought suggests that selection pressures in ancient primates may have been biased toward handedness due to feeding and postural support benefits.
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Regardless of untied evolutionary ends this experiment provides us with several useful conclusions: 1)posture significantly affects hand preference in tufted capuchin monkeys. 2)Tufted Capuchins exhibit a right-hand bias in the case of reaching but not in tool use. 3)The monkeys do exhibit a greater overall hand-preference (not accounting for direction) for the use of tools than for reaching. 4)Bipedal reaching aids a slight right-hand bias in manipulative primates.
All of these conclusions can be helpful in the search for man’s cultural roots as we attempt to tie our ancestral components together. This journal article does a very good job of explaining the terms and methodology used in such a way that a lay anthropologist might easily comprehend the meaning to which this research alludes. For anyone who is interested in this subject or any concerning primate correlations to our own cultures, this article is quite interesting and opens up a wealth of intellectual queries.
Westergaard, Gregory Charles, Heather E. Kuhn, and Stephen J. Suomi (1998) Effects of Upright Posture on Hand Preference for Reaching vs. the Use of Probing Tools by Tufted Capuchins (Cebus apella).
American Journal of Primatology, 44:147-153.
Call No.: QL 737.P9 A566 1998