ABSTRACT
Observations on five species of rodents, vole (Microtus socialis), gerbil (Gerbillus dasyurus), jird (Meriones tristrami), dormouse (Eliomys melanurus) and jerboa (Jaculus orientalis), revealed that, during the period when their neonates share a matching morphology, they also share the same forms of quadruped locomotion (gaits). The order in which the different gaits develop is similar in all species, beginning with the basic gaits of lateral walk and trot. Gaits and body morphology do not undergo further changes in voles, whereas the other species incorporate more specialized gaits later in ontogeny, when the adult body morphology has been attained. Gerbils and jirds incorporate a bounding gait, dormice incorporate galloping and jerboas incorporate bipedal running. Species with more specialized locomotion thus undergo more developmental stages than those with less specialized locomotion. Except for the jerboa, the nesting period was roughly the same for all species, but those with more specialized locomotion exhibited earlier onset of the basic gaits as if condensing their development in order to reach the adult gait within the same nesting period. Consequently, the adult gait emerges approximately 10 days before the end of nesting, regardless of nesting duration. Since growth rate does not seem to account for the differences in morphology and onset of gaits, the heterochrony in the observed species probably stems from differences in the duration of growth, which seems to be the key factor in the diversion from the basic common morphology. The present results reconfirm the traditional generalities of functional morphology derived from cross-species comparisons. In addition, they provide another perspective by comparing form and function within the same individuals in the course of ontogeny.
