Asymmetries in amphibians: a review of morphology and behaviour.

Morphological and behavioural asymmetries in amphibians are reviewed. Among the characteristics considered are: (1) the asymmetry of the shoulder girdle (epicoracoid overlap); (2) the distribution of the left and right variants of its structure in amphibian populations; (3) asymmetry in the position of the spiracle(s); (4) asymmetric order of forelimb emergence from opercular chambers in tadpoles; and (5) preferential forelimb use in adult amphibians. I show that there are no direct cause-and-effect relationships between these characteristics, which would explain their development. Other asymmetries, such as asymmetry of the visceral organs, turning behaviour of tadpoles, asymmetries in the length and weights of the long bones, and some neuromorphological traits, also show few examples of relationships. However, the simultaneous absence of many asymmetries in some amphibians and their presence in others suggests a common cause, which affects all of these asymmetries indirectly, presumably very early in ontogenesis.