Abstract
BackgroundFactors determining patterns of laterality manifestation in mammals remain unclear. In primates, the upright posture favours the expression of manual laterality across species, but may have little influence within a species. Whether the bipedalism acts the same in non-primate mammals is unknown. Our recent findings in bipedal and quadrupedal marsupials suggested that differences in laterality pattern, as well as emergence of manual specialization in evolution might depend on species-specific body posture. Here, we evaluated the hypothesis that the postural characteristics are the key variable shaping the manual laterality expression across mammalian species.Methodology/Principal FindingsWe studied forelimb preferences in a most bipedal marsupial, brush-tailed bettong, Bettongia penicillata in four different types of unimanual behavior. The significant left-forelimb preference at the group level was found in all behaviours studied. In unimanual feeding on non-living food, catching live prey and nest-material collecting, all or most subjects were lateralized, and among lateralized bettongs a significant majority displayed left-forelimb bias. Only in unimanual supporting of the body in the tripedal stance the distribution of lateralized and non-lateralized individuals did not differ from chance. Individual preferences were consistent across all types of behaviour. The direction or the strength of forelimb preferences were not affected by the animals’ sex.Conclusions/Significance Our findings support the hypothesis that the expression of manual laterality depends on the species-typical postural habit. The interspecies comparison illustrates that in marsupials the increase of bipedality corresponds with the increase of the degree of group-level forelimb preference in a species. Thus, bipedalism can predict pronounced manual laterality at both intra- and interspecific levels in mammals. We also conclude that quadrupedal position in biped species can slightly hinder the expression of manual laterality, but the evoked biped position in quadrupedal species does not necessarily lead to the enhanced manifestation of manual laterality.
Highlights
Asymmetry in motor activity, especially in the use of the limbs, appeared to be much more widespread among vertebrates than previously thought [1,2,3]
Analysis revealed that 11 subjects (92%) displayed left forelimb preference and one (8%) – right forelimb preference, there was a significant prevalence of the left-forelimb preferent individuals
The present study showed that brush-tailed bettongs displayed significant left-forelimb preference at the group level in four different types of their usual behaviour where one forelimb is used
Summary
Especially in the use of the limbs, appeared to be much more widespread among vertebrates than previously thought [1,2,3]. In prosimian primates the strength of motor laterality increases in a row of six species from the strongly quadrupedal mouse lemurs, Microcebus murinus, with a horizontal orientation of the body’s long axis, to the more bipedal galagos, Galago senegalensis and G. moholi, which typically rest or feed vertically and move by vertical clinging and leaping [29,30,39]. Primate quadrupeds with the horizontal long body axis, such as mouse lemurs, exhibit no increase of manual preferences even when shifting from quadrupedal to vertical or bipedal positions [40], that is, the species-typical posture may have more influence on the laterality and, has stronger predictive power than within-subject effect of postural change. We evaluated the hypothesis that the postural characteristics are the key variable shaping the manual laterality expression across mammalian species
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