Jeannette P. Ward

Handedness as a function of sex and age in a large population of Lemur

A population of 194 lemurs (Lemur spp.), 116 males and 78 females, from 1 to 30 years of age, was assessed for lateralized hand use in simple food reaching with a minimum of 100 reaches per animal. A hand preference was present in 80% of the population with a bias for use of the left hand that was most characteristic of male lemurs and young lemurs. The results confirm the presence of lateralization in prosimians, and we interpret the sex and age differences in relation to current theories of neural lateralization.

Posture and laterality in the bushbaby (Galago senegalensis).

A colony of adult bushbabies was assessed for hand preference in order to determine what the proportion of preference for the left and right might be for the population and whether hand preference for individuals was reliable. 25 animals were tested in an apparatus demanding a vertical stance. The population bias was expressed in a distribution skewed toward a left-hand bias. With 16 animals available for retesting, the test-retest reliability coefficient was 0.651, significant at the 0.01 level. When 12 of the original subjects were tested in an apparatus demanding a quadrupedal stance, hand preference shifted to a bimodal distribution. Test-retest reliability was 0.864, significant at the 0.01 level. Significant correlations were not found between two types of test nor between the two retests. Neither sex nor length of laboratory residence served to predict hand preference. These results were discussed with the view that the postural adjustments required by a bipedal stance may have shaped the development of handedness in humans.

Behavioral laterality in captive bonobos (Pan paniscus).

Behavioral laterality was studied in a captive group of 11 bonobos (Pan paniscus). In an observational method a significant left lateral bias in carrying and a right lateral bias in leading limb were found. Direction of lateral bias in carrying and leading limb was enhanced when subjects maintained a bipedal posture. Analysis of bimanual feeding behavior revealed a significant right-hand bias for eating when subjects were holding food with their left hand. In an experimental method a significant shift toward greater right-hand use was found when subjects were required to maintain a bipedal, rather than a quadrupedal, posture. There was no evidence of lateral bias for measures of face touching, self-touching, or gestures. The results are discussed in the context of previous reports of primate laterality.

Patterns of Lateralized Behavior in Prosimians

In the preceding chapter, Harris has thoroughly documented the enduring interest of the human species in the question of lateral asymmetry in the behavior of nonhuman primate species. The appearance of this book with reports of a variety of nonhuman primate laterality studies originating in many parts of the world testifies to the resurgence of interest in this topic. This chapter reports the incidence and patterning of laterality phenomena in species of prosimians, a major group of nonhuman primates much less studied in all research contexts than their anthropoid counterparts. The relative neglect of these most primitive of primates, which represent the stem from which the primate order originated, has been undergoing remediation and there now exist many sources of information about this group (see for example, Charles-Dominique, 1977; Charles-Dominiqueet al., 1980; Doyle,1974; Doyle & Martin, 1979; Jolly, 1966; Martin, 1990; Martin, Doyle, & Walker, 1974; Tattersall, 1982).

Posturally related variations in the hand preferences of the ruffed lemur (Varecia variegata variegata).

The hand preferences of 5 semi-free-ranging black-and-white ruffed lemurs were assessed by using three distinct testing procedures. Testing conditions varied in the extent to which they required animals to make a whole body postural adjustment prior to making a reach. Minimal bodily adjustment was necessary for free foraging, whereas discrete food presentations on land (DFP-land) and in a moat (DFP-moat) promoted a gross reorientation of the animals entire body. In the DFP-moat condition 4 animals exhibited exclusive use of the left hand, and only 1 of 515 reaches was made with the right hand. Similarly, all 5 animals showed a pronounced left hand preference in the DFP-land condition. The free-foraging condition revealed a hand preference for only 1 of the 5 subjects, and that preference was weak in comparison with those measured in the other two test conditions. These findings indicate that whole body postural adjustments critically influenced the expression of hand preference and should be taken into consideration in future studies of primate hand preferences.

Black lemur (Lemur macaco) hand preference in food reaching

The present study assessed the hand preferences exhibited by 33 black lemurs during routine feeding. Individual animals displayed hand preferences that were consistent across observations separated by as much as seven months. Within this population, 20 were left hand preferent, 12 right preferent, and 1 was ambidextrous. Correlational analysis of age and percentage left hand use indicated an inverse relationship in which younger animals tended towards the preferential use of the left hand and older animals the preferential use of the right hand. Similar analysis found no relationship between either sex and hand preference or familial relationship and hand preference. The skewed distribution of age in this sample renders tentative conclusions regarding age-related variations in hand preference. It is suggested that if the hand preferences of the black lemur are not age-related, then this species may be characterized as having a bias towards the preferential use of the left hand for food reaching.

Hand Preferences and Whole (Galago senegalensis)

The hand preferences in prey capture and whole-body turning biases after prey capture were assessed in 10 lesser bushbabies (Galago senegalensis) in 8 conditions designed to manipulate posture, visibility of prey and angle of reaching. Each subject received 60 trials in each test condition for a total of 480 trials. Seven subjects had a left-hand preference in food reaching, three right and none were ambipreferent. Eight subjects had a left whole-body turning bias, one right and one had no bias. No correlation was found between reach preference and turning bias. Bipedal posture facilitated the use of the dominant hand, whereas other manipulated conditions did not have a significant effect on hand use. A neuraxial arousal system is postulated as mediator of the bipedal effect on hand use.

Lateral bias in feeding and brachiation inHylobates

Lateralized hand use in gibbons was assessed for both food reaching and leading limb in brachiation. Sex and age effects were found in hand preference for food reaching. Adult females were all very strongly right hand preferent, whereas adult males had no across group consistent preference. Within the female group there was a strong correlation between age and strength of right handedness. When compared in terms of absolute strength of hand preference, females were found to be more strongly lateralized than males. Leading limb preference in brachiation was scored into vocal and non-vocal categories. Three subjects had a shift in preferred leading limb from the non-vocal brachiation condition to the vocal brachiation condition. This shift may be influenced by the arousal effects of species typical vocalization. The results of this study underline the importance of consideration of such factors as sex and age when interpreting behavioral lateralization data. The exploration of laterality in many different response measures is important to the achievement of a complete understanding of behavioral lateralization in primates.

Analyses of feeding lateralization in the small-eared bushbaby (Otolemur garnettii): A comparison with the ring-tailed lemur (Lemur catta).

Feeding related lateralization was examined in a population of 23 small-eared bushbabies (Otolemur garnettii). The three measures used to determine lateralization were food reaching, holding, and manipulation. Sex and age differences were found, with adult females showing a strong right bias and adult males a left bias. Juvenile males were weakly lateralized and less consistent across measures than adult animals. The use of standard scores to assess lateralization allowed species comparisons to be made. The results of this study were compared with results from a previous study on lateralization in the ring-tailed lemur (Lemur catta). Species comparisons found sex differences to be a stronger factor in lateralization than species differences.

Multiple measures of motor lateralization in human primates (Homo sapiens).

Laterality was assessed in 100 human (Homo sapiens) subjects through the use of five measures of motoric behavior in three categories: locomotor, manual, postural. Locomotor measures included leading limb for initiation of walking and whole-body turning. Performance and questionnaire measures were used to assess lateral hand bias. Postural bias was assessed as the weight distribution in quiet standing. Population-level biases on the manual and leading limb measures were to the right; on posture and turning, to the left. Locomotor measures were found to be altered by marching experience. Only the manual measures were correlated. We compare the results with those of nonhuman primate studies that have used similar measures and discuss some patterns of laterality common to human and nonhuman primates.