Postnatal allometry of the skeleton in Tupaia glis (Scandentia: Tupaiidae) and Galea musteloides (Rodentia: Caviidae)--a test of the three-segment limb hypothesis.
During the evolution of therian mammals, the two-segmented, sprawled tetrapod limbs were transformed into three-segmented limbs in parasagittal zig-zag configuration (three-segment limb hypothesis). As a consequence, the functional correspondence of limb segments has changed (now: scapula to thigh, upper arm to shank, fore arm plus hand to foot). Therefore, the scapula was taken into account in the current study of the postnatal growth of the postcranial skeleton in two small mammalian species (Tupaia glis, Galea musteloides). Comparisons were made between the functionally equivalent elements and not in the traditional way between serially homologous segments. This study presents a test of the three-segment limb hypothesis which predicts a greater ontogenetic congruence in the functionally equivalent elements in fore and hind limbs than in the serially homologous elements. A growth sequence, with decreasing regression coefficients from proximal to distal, was observed in both species under study. This proximo-distal growth sequence is assumed to be ancestral in the ontogeny of eutherian mammals. Different reproductive modes have evolved within eutherian mammals. To test the influence of different life histories on ontogenetic scaling during postnatal growth, one species with altricial juveniles (Tupaia glis) assumed to be the ancestral mode of development for eutherians and one species with derived, precocial young (Galea musteloides) were selected. The growth series covered postnatal development from the first successive steps with a lifted belly to the adult locomotory pattern; thus, functionally equivalent developmental stages were compared. The higher number of allometrically positive or isometrically growing segments in the altricial mammalian species was interpreted as a remnant of the fast growth period in the nest without great locomotor demands, and the clearly negative allometry in nearly all segments in the precocial young was interpreted as a response to the demand on early locomotor activity. Different life histories seem to have a strong influence on postnatal ontogenetic scaling; the effects of the developmental differences are still observable when comparing adults of the two species.
Schilling
Nadja
N
Institut für Spezielle Zoologie und Evolutionsbiologie mit Phyletischem Museum, Friedrich-Schiller-Universität, Erbertstr. 1, D-07743 Jena, Germany. nadja.schilling@uni-jena.de
Petrovitch
Alexander
A
D000375:Aging / Q000502:physiology; D000818:Animals; D001696:Biomechanical Phenomena; D005260:Female; D005552:Forelimb / Q000033:anatomy & histology / Q000502:physiology; D006614:Hindlimb / Q000033:anatomy & histology / Q000502:physiology; D008297:Male; D010802:Phylogeny; D012377:Rodentia / Q000033:anatomy & histology* / Q000502:physiology*; D013045:Species Specificity; D014417:Tupaia / Q000033:anatomy & histology* / Q000502:physiology*