SUMMARY:The excavatory movements of the spade-snouted amphisbaenid Leposternon microcephalum (Reptilia, Squamata) was studied with the aid of videofluorscopy (X-ray) techniques. This allows the observation of skull and column movements along tunneling, and a more detailed motion observation, being so a novel approach for amphisbaenian excavatory researches. A single specimen of Leposternon microcephalum was kept in a glass terrarium filled with semoline, and filmed with a scopy (X-ray) machine. Fixed anatomical marks on the head of the specimen were put in drawings from the framed recordings. Selected sequences of the recordings were fragmented in isolated frames for motion observation. The analysis of the recordings revealed a repetitive pattern of excavatory cycles, with retreating and downward bending of the head before its upstroke to compact the substrate tunnel roof. Follows a dropping of the head, which lays over the substrate giving support for the next retreating and downward head bending. This is an essential step that was neglected in earlier cycle descriptions. The initial downward head bending was not previously properly described for spade-snouted amphisbaenians. The excavatory movements of spade-snouted amphisbaenians are usually treated as a two-stepped cycle, but the evidence that this excavatory cycle has three steps is given here.

Most studies on excavation behaviour of Amphisbaenia have been based on descriptive analysis through visual observation or external body motion records. Herein, we recorded the excavatory gaits of the shovel-headed amphisbaenid Leposternon microcephalum using videofluoroscopy. This technique films by X-ray emission, allowing a more detailed analysis of the amphisbaenid's underground locomotor behaviour and performance. Thus, we described, for the first time, its ascendant excavatory cycle and backward movement. Furthermore, we analysed its performance through the quantitative data (e.g. speed, travel distance, frequency, time) of each fossorial gait, including the three-step excavatory cycle previously described in the literature. When comparing the three-step and the four-step excavatory cycles, the first presented high average speed and short travel distances. Our original hypothesis that there was a relation between retreat/ downward movement of the head and the intensity of burrowing activity was not corroborated by the regression analyses. This movement seems to be just a part of the motion needed to perform the excavatory cycle, not a propulsion step influencing burrowing activity. The results presented in this work contribute to a better understanding of L. microcephalum fossorial behaviour. Further studies can be performed to better describe and compare excavation patterns and performance among different amphisbaenian skull morphotypes (round headed, keel headed, shovel headed and spade headed).

Amphisabenia is a group of squamates adapted for a fossorial lifestyle. The skull is the animal's the main digging tool, and can present one of four principal shapes. The shovel‐headed shape is considered to be the most specialized for digging. The South American genus Leposternon presents a shovel‐headed morphotype, and is widely distributed on this continent. The general shovel‐headed skull pattern may vary considerably, even within the same genus, and we hypothesized that this variation may be influenced primarily by body size and geographical factors. This study investigated the variation in skull size and shape among five Leposternon species, and examined the potential relationship between this variation and the size of the specimens and bioclimatic variables, through a geometric morphometric approach. Significant morphological variation was found among the species, and was also related systematically to body size and the geographical distribution of the specimens. As even subtle differences in the skull size or shape may represent significant modification in bite force and digging capacity and digging speed, the cranial variation found among the Leposternon species and specimens may have a direct influence on their diet and locomotor performance. Our results, together with direct observations of some of these species, suggest that shovel‐headed amphisbaenians may be able to penetrate different soil types under a range of climatic conditions, especially considering the ample, but often sympatric distribution of the species studied here.

BackgroundMorphological descriptions comparing Leposternon microcephalum and L. scutigerum have been made previously. However, these taxa lack a formal quantitative morphological characterization, and comparative studies suggest that morphology and burrowing performance are be related. The excavatory movements of L. microcephalum have been described in detail. However, there is a lack of studies comparing locomotor patterns and/or performance among different amphisbaenids sharing the same skull shape. This paper presents the first study of comparative morphometric variations between two closely related amphisbaenid species, L. microcephalum and L. scutigerum, with functional inferences on fossorial locomotion efficiency.MethodsInter-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 videofluoroscopy and then compared. The influence of morphological variation on the speed of digging was tested among Leposternon individuals.ResultsLeposternon 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 share the same excavatory movements. The animals analyzed reached relatively high speeds, but individuals with narrower skulls dug faster. A negative correlation between the speed and the width of skull was determined, but not with total length or diameter of the body.DiscussionThe 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 closely related amphisbaenids with the same skull type would exhibit the same excavatory pattern. The negative 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 suggest trade-offs between excavation speed and compression force during burrowing in this species.