Body and skull morphometric variations between two shovel-headed species of Amphisbaenia (Reptilia: Squamata) with morphofunctional inferences on burrowing

Abstract

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

Skull variation in a shovel‐headed amphisbaenian genus, inferred from the geometric morphometric analysis of five South American Leposternon species

“…The longissimus dorsi , for example, is an extensive dorsal muscle that originates at the base of the head (proximal portion) and runs backward along the dorsal region of the animal, forming the distal portion (Navas et al, ). In the shovel‐headed amphisbaenians, specifically L. microcephalum , even small variations in head width may have a major impact on burrowing compression force (Navas et al, ) and speed (Hohl et al, ). In this case, the analyses of the morphological configuration of the skull presented here indicate that the larger Leposternon specimens or species are able to exert a higher compression force, but dig more slowly, reflecting a trade‐off between compression forces and burrowing speeds, as suggested by Hohl et al ().…”

“…The excavatory cycle of Leposternon microcephalum and Leposternon scutigerum has been described as: (a) an initial static position with the gular and anterior body regions lying over the tunnel floor; (b) a retreating and downward bending of the head, with the tip of snout touching the floor substrate; and (c) a continuous upward and forward movement of the head, which compacts the granules of the substrate against the tunnel roof, while the pectoral region compresses the tunnel floor. This cycle ends with the animal dropping its head to return to its initial static position (Barros‐Filho, Hohl, & Rocha‐Barbosa, ; Hohl et al, ; Hohl, Loguercio, Sicuro, Barros‐Filho, & Rocha‐Barbosa, ). The shovel‐headed morphotype is considered to be the most specialized for digging, which allows these amphisbaenians to penetrate heavily compacted soils (Gans, ; Gans, ; Hohl et al, ; Kearney, ).…”

“…The characteristics of the amphisbaenian head are also useful for phylogenetic studies (Kearney, ), including those that discuss about homoplasy and adaptive convergence related to fossoriality (Gauthier, Kearney, Maisano, Rieppel, & Behlke, ; Mott & Vieites, ), and provide valuable insights into the ecology of these animals through studies on their fossorial locomotion (e.g., Barros‐Filho, Hohl, & Rocha‐Barbosa, ; Gans, , ; Hohl et al, ; Hohl et al, ; Kaiser, ; Navas et al, ) and masticatory biomechanics (e.g., Baeckens et al, ). For example, small variations in the width of the posterior skull segment may have a relatively large impact on burrowing compression force and speed (Hohl et al, ; Navas et al, ), as well as bite force (Baeckens et al, ).…”

A farewell to arms and legs: a review of limb reduction in squamates

“…Functional studies of limb-reduced and limbless reptiles (e.g. Gans, , 1962Gans, , 1986Gans, , 1994Gasc, 1974;Leonard, 1979;Dial, Gatten & Kamel, 1987;Walton, Jayne & Bennet, 1990;Gans, Morgan & Allen, 1992;Gans & Fusari, 1994;Renous, Hofling & Gasc, 1998;Benesch & Withers, 2002;Bergmann & Irschick, 2010;Hohl et al, 2017;Morinaga & Bergmann, 2020) have often based their observations on locomotory assays. These are used as a tool to analyse and compare how animals move relative to the substrate, dissecting this movement into several analytical components such as speed, and in the case of limbreduced lizards, amplitude and wavelength relative to body undulation [for mathematical models of body undulation and lateral bending in squamates, see Raveshky (1960), Daan & Belterman (1968) and Navas et al (2004)].…”