On the other hand: Current issues in and meta-analysis of the behavioral laterality of hand function in nonhuman primates

Abstract

The last decade has seen a resurgence of interest in laterality of function in primates, especially in hand use as it links to handedness and language in Homo sapiens. Manual lateralization of behavior in humans reflects asymmetry in cerebral structure, which must have evolved from nonhuman progenitors. To what extent is hand function lateralized in our nearest living relations? First, we address current issues of theory and methodology: statistics, measurement, variables, setting, sensory modality, and sample size. Specific topics include preference vs. performance, posture, bimanuality, inheritance, and arm asymmetry. We categorize the published literature in a descriptive, classificatory framework of five levels that range from Level 1, ambilaterality, to Level 5, human-like handedness. In a meta-analysis we put 241 published data-sets to a methodological test of seven criteria and code the 48 survivors onto the levels framework, by taxonomic grouping (prosimian, New World monkey, Old World monkey, ape, chimpanzee). Primates at Level 1 are mostly wild or naturalistic populations performing spontaneous species-typical behavior patterns. Most primates are at Levels 2 and 3, that is, individually lateralized to either side, especially on complex, demanding or practiced tasks, usually as devised in captive settings. Only chimpanzees show signs of population-level bias (Levels 4 and 5) to the right, but only in captivity and only incompletely. We conclude that nonhuman primate hand function has not been shown to be lateralized at the species level-it is not the norm for any species, task, or setting, and so offers no easy model for the evolution of human handedness.

Simple Reaching Is Not So Simple: Association Between Hand Use and Grip Preferences in Captive Chimpanzees

“…Note that subjects were required to locomote ≥2 steps between each trial to ensure that the experimenter could not influence hand use by the position of the food and that another piece of food was not thrown into the cage until the prior piece had been consumed to ensure that each response was discrete. Accordingly, we secured independence of data points in the assessment of hand preference (Lehman, 1993;Hopkins, 1999;McGrew and Marchant, 1997).Grip type was either thumb-index, middle-index, or single digit ( Figure 1). Thumb-index gripping occurred when a subject abducted the thumb to the lateral side or tip of the index finger to secure the food item.…”

“…Some researchers have suggested that population-level handedness is restricted to captive populations (McGrew and Marchant, 1997;Palmer, 2002) and therefore is an artifact of being born and raised by humans. Others have argued that differences in the types of measures and how handedness is assessed is a more likely explanation for discrepancies between wild and captive ape populations (Hopkins, 1999; Hopkins and Cantalupo, in press).…”

“…When the chimpanzee acquired the raisin, the experimenter recorded both the hand and the grip type used to pick it up. We recorded only one reaching response each trial to assure independence of data points (McGrew and Marchant, 1997, cf. Hopkins, 1999.…”

“…Moreover, preferential use of the right hand should be greater when subjects employ a thumb-index grip compared to other types of grip preferences. Combining the Yerkes and Bastrop data generated a reasonable sample size to evaluate the effect of certain subject variables-rearing history, sex, and offspring parity-on handedness: Rearing history is of particular theoretical interest because McGrew and Marchant (1997) suggested that population-level handedness is restricted to captive-born, human-raised chimpanzees, whereas Hopkins & Cantalupo (in press) have questioned their interpretation. Parity is also of theoretical interest because studies in human subjects have indicated that there are higher incidences of left-handedness in first-born and latter born individuals.…”

“…Chimpanzee; hand preference; grip preference There has been considerable historical (Ettlinger, 1988;Fagot and Vauclair, 1991;Marchant and McGrew, 1991;Warren, 1980) and contemporary (Corballis, 1992, in press; Hopkins and Cantalupo, in press;McGrew and Marchant, 1997;Palmer, 2002) debate regarding the presence or absence of population-level hand preferences in nonhuman primates and the factors that influence the expression of handedness in nonhumans. The question of whether nonhuman primates express population-level asymmetries is of considerable theoretical interest because of the alleged role of hemispheric specialization in the evolution of complex human behavior including tool-use, bipedalism and language (Bradshaw and Rogers, 1993).…”