Background. Morphological descriptions comparing Leposternon microcephalum and L. scutigerum are known. However, these taxa lack of a formal quantitative morphological characterization and comparison between their morphometric patterns. Studies suggest that morphology and burrowing performance seem to be related. For example, the robustness of the head in L. microcephalum is positively associated to the digging force. The excavatory movements of this species were described in detail. However, there is a lack of studies comparing locomotor patterns and/or performance among different amphisbaenids sharing the same skull pattern. Likewise, morphofunctional interpretations of the morphological variations are scarce. This paper presents the first study of comparative morphometric variations between two adaptively close amphisbaenid species,L. microcephalum and L. scutigerum, with functional inferences of fossorial locomotion efficiency.
Methods. Inter-specific morphometric variations were verified through statistical analyses of body and cranial measures of L. microcephalum and L. scutigerum specimens. Their burrowing activity was assessed through X-ray videoflouroscopy and then compared. The influence of morphological variations on the speed of the digging performance was tested among Leposternon individuals.
Results. Leposternon microcephalum and L. scutigerum are morphometrically distinct species. The first is shorter and robust with a wider head while the other is more elongated and slim with a narrower head. They seem to share the same excavatory movements. However, L. scutigerum presented higher averages for speed, travel distance and frequency of excavatory cycles, and lower values for cycle duration in relation to that found for L. microcephalum. The animals analyzed reached relatively high speeds, but individuals with narrower skulls dig faster. A negative correlation between the speed and the width of skull was determined, but not with total length or diameter of the body.
Discussion. The morphometric differences between L. microcephalum and L. scutigerum are in accord with morphological variations previously described. Since these species performed the same excavation pattern, we may infer that adaptively close amphisbaenids with the same skull type would exhibit the same excavatory pattern. The locomotor performance values suggested that L. scutigerum is a faster digger than L. microcephalum. Such differences are supported by morphometric and morphological features. The negatively correlation between head width and excavation speed is also observed in others fossorial squamates. The robustness of the skull is also related to compression force in L. microcephalum. Individuals with wider heads are stronger. Thus, we suggested trade-offs between excavation speed and compression force during burrowing activity for this species.