Implications of Primate Functional Asymmetries for the Evolution of Cerebral Hemispheric Specializations. In: Ward JP, Hopkins WD, editors. Primate Laterality. New York: Springer; 1993. pp. 319-41. [DOI: 10.1007/978-1-4612-4370-0_15]

Forelimb preferences in quadrupedal marsupials and their implications for laterality evolution in mammals

BACKGROUND

Acquisition of upright posture in evolution has been argued to facilitate manual laterality in primates. Owing to the high variety of postural habits marsupials can serve as a suitable model to test whether the species-typical body posture shapes forelimb preferences in non-primates or this phenomenon emerged only in the course of primate evolution. In the present study we aimed to explore manual laterality in marsupial quadrupeds and compare them with the results in the previously studied bipedal species. Forelimb preferences were assessed in captive grey short-tailed opossum (Monodelphis domestica) and sugar glider (Petaurus breviceps) in four different types of unimanual behaviour per species, which was not artificially evoked. We examined the possible effects of sex, age and task, because these factors have been reported to affect motor laterality in placental mammals.

RESULTS

In both species the direction of forelimb preferences was strongly sex-related. Male grey short-tailed opossums showed right-forelimb preference in most of the observed unimanual behaviours, while male sugar gliders displayed only a slight, not significant rightward tendency. In contrast, females in both species exhibited consistent group-level preference of the left forelimb. We failed to reveal significant differences in manual preferences between tasks of potentially differing complexity: reaching a stable food item and catching live insects, as well as between the body support and food manipulation. No influence of subjects' age on limb preferences was found.

CONCLUSIONS

The direction of sex-related differences in the manual preferences found in quadrupedal marsupials seems to be not typical for placental mammals. We suggest that the alternative way of interhemispheric connection in absence of corpus callosum may result in a fundamentally distinct mechanism of sex effect on limb preferences in marsupials compared to placentals. Our data confirm the idea that non-primate mammals differ from primates in sensitivity to task complexity. Comparison of marsupial species studied to date indicate that the vertical body orientation and the bipedalism favor the expression of individual- and population-level forelimb preferences in marsupials much like it does in primates. Our findings give the first evidence for the effect of species-typical posture on the manual laterality in non-primate mammals.

Do left-handed competitors have an innate superiority in sports?

This study assessed handedness distributions among (a) sporting competitors (n = 1,112) and nonsporting university students (n = 1,112), (b) sporting competitors engaging in interactive (n = 576) and in noninteractive sports (n = 536), and (c) sporting competitors engaging in direct interactive (n = 219) and indirect interactive (n = 357) sports. Chi-squared showed that there were statistically significant differences in proportions of left-handed persons in (a) sporting competitors and nonsporting university students, (b) sporting competitors engaging in interactive and noninteractive sports, (c) sporting competitors engaging in interactive sports and nonsporting university students, and (d) sporting competitors engaging in direct interactive and indirect interactive sports. It appears that left-handers are more common among those who engage in competitive manual activities. This superiority of the left-handers may be fully explained by a consideration of tactical or strategic factors associated with handedness during sporting interactions. The results with important implications for the measurement and evaluation of handedness are discussed in the light of the current findings on laterality.

Frequency-dependent maintenance of left handedness in humans

The percentage (10-13%) of left handedness in human has apparently not changed since the Neolithic. Left handedness is heritable and appears to be repeatedly associated with some reduced fitness components; the persistence of left handedness implies that left handers have a fitness advantage in some situations. We propose that left handers have a frequency-dependent advantage in fights and for that reason a fitness advantage. To test this hypothesis, left handedness frequencies in the general population and in sporting individuals (both students and the sporting elite) have been compared, as sporting performance is likely to be a good indicator of fighting abilities. The higher proportion of left-handed individuals in interactive sports (reflecting some fighting elements), reaching 50% in some sports categories, but not in noninteractive sports, is consistent with the fighting hypothesis. The greater frequency of left handedness in males than in females is also consistent with this hypothesis, as male-male fights are universally more frequent than other combinations. The frequency-dependent advantage in fights of left handers might explain the stability of left handedness.

A speculation about the parallel ear asymmetries and sex differences in hearing sensitivity and otoacoustic emissions

Hearing sensitivity and the prevalence of spontaneous otoacoustic emissions show parallel ear and sex differences in humans. Here it is proposed that these lateral asymmetries and sex differences may all result from differences in the 'strength' of the efferent inhibition delivered to individual cochleas. Specifically, it is proposed that the amount of efferent inhibition is relatively less in right ears and in females than in left ears and males. While it is unclear why or how ear asymmetries or sex differences would develop in the efferent system, by assuming that they do, one can begin to explain a number of basic facts of hearing. It is even possible that the lateral asymmetry in the efferent system may be related to the well-known cortical asymmetries that are believed to underlie speech perception, speech production, and other human abilities.