More injuries in left-footed individual lizards and Sphenodon

Abstract

In lizards and Sphenodon, often the fourth toes of individuals with intact tails have more subdigital lamellae on the right than on the left side, and the opposite situation frequently occurs in individuals with injured tails. The difference between intact and injured individuals in morphological directional asymmetry was statistically significant (P<0.05) in 11.4% among 193 species from various lizard families. Lizard families varied in extent and direction of association, but no phylogenetic constraints were detected within genera. Statistical significance was greater in samples from homogenous geographic origin than from heterogeneous ones. Among gekkonid species, the difference was stronger in those with cursorial (terrestrial) habits, than in those with scansorial (rupestral or arboreal) habits. In Scincidae, loss seems more often lethal in left-footed than in right-footed individuals. Statistically significant associations between morphological left-side dominance and tail injury exist also in three independent lineages with reduced limbs (Anguidae, Scincidae and Teidae). Hence such association is probably not a result of limb function. Rather, left-side dominance seems to be the symptom of an unknown, perhaps organism-wide, detrimental trait. Polymorphism in morphological dominance existed in all species, suggesting advantages and disadvantages in different situations to both phenotypes. We propose the hypothesis that an inversion of side dominance may occur in a single trait without systematic inversion of side dominance in all traits of the body. Inversion in a single trait causes incompatibility in multiple-trait functions. Such a mechanism, rather than cultural conventions, could increase accident proneness also in left-handed Homo sapiens, and could explain increased proneness to accident and warfare mortality in left-handed men, beyond the possible involvement of cultural factors.