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Session Schedule & Abstracts

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Sunday 3rd July, 2016

Symposium: Morphology & evolution of the Xenarthra 3

Room: Salon H   2:30 pm–3:15 pm

Moderator(s): S. Bargo & J. Nyakatura
XEN3-1  2:30 pm  Architecture of dorsovertebral muscles corresponds with derived function of the vertebral column during suspensory locomotion in two toed sloths (Mammalia: Xenarthra). Nyakatura J.A.*, Humboldt U. Berlin; Stark H., FSU Jena
Abstract: Extant sloths are highly specialized in upside-down quadrupedal locomotion and completely lost the ability for asymmetrical gaits otherwise typical of terrestrial mammals. In order to study the consequences of the evolution of this derived locomotor mode on the morphofunction of the vertebral column we first analyzed in vivo intervertebral movements during locomotor trials of the sloths using x-ray motion analysis. To achieve this, sloths were trained to move on a motorized "tread-pole" in front of two perpendicular x-ray image intensifiers. When employing upside-down quadrupedal locomotion, dorsoventral flexion/extension of the sloth's spine is insignificant when compared to asymmetrical gaits of terrestrial mammals. Lateral bending and torsion of the spine, however, is as large as or even larger than what has been observed in right-side up terrestrial mammals. Subsequently, we tested whether this aberrant function of the vertebral column is also reflected in the muscle architectural properties of the dorsovertebral muscles. We used layer-wise dissection of the transversospinal system, the longissimus, and the iliocostales to facilitate the 3D digitization of individual muscle fascicles. We determined fascicle length and orientation, the anatomical cross sectional area, muscle volume, and the moment arms to the intervertebral joints. These parameters were found to be in agreement with the previously observed kinematics: they favor extensive lateral bending and torsion, but are disadvantageous for powerful dorsal extension of the spine. The agreement of architectural properties with in vivo function documented in this study further characterizes the specific functional morphology of highly derived extant sloths.

XEN3-2  2:45 pm  Architectural specializations of the forelimb musculature of the three-toed sloth (Xenarthra: Bradypus variegatus). Olson R*, Ohio University; Cliffe RN, Swansea University; Glenn ZD, Ohio University; Thomas DR, Youngstown State University; Kennedy SJ; Butcher MT, Youngstown State University
Abstract: Sloths exhibit a range of arboreal locomotion that involves suspension by one or more limbs for extended periods of time. Despite these abilities that require great strength, their skeletal muscle mass is quite low; however, their overall limb form has been extremely modified for suspensory behaviors. The biomechanical functions of these modifications remain poorly understood due to the lack of available data on their limb morphology. To develop an initial understanding, we evaluated muscle architecture in the forelimb of (Bradypus variegatus) using measurements of moment arm (rm) belly mass (MM), belly length (ML), fascicle length (LF), pennation angle (θ), and physiological cross-sectional area (PCSA). From these properties, estimates of isometric force, joint torque, and power were quantified. Our findings show that the limb retractors account for the greatest percentage of total forelimb muscle mass. Significant mass is also distributed to the limb adductors, elbow flexors, and digital flexors. Notably, the humeral mediale, radial, and ulnar heads of m. flexor digitorum profundus each have an isometric force capability similar to that of the massive and highly pennate m. subscapularis. Moreover, all of the antebrachial flexors have high PCSA:MM ratios indicating the substantial force production needed for prolonged suspension. No forelimb muscle has the capacity for high power, while numerous muscles have an architectural arrangement for applying large joint torque, and this capability is greatest among the limb retractors. The retractor and flexors muscles expected to exert large joint torque have low values for FL:rm, and these are opposed by relatively high ratios for the limb protractors, and elbow and digital extensors. Collectively, these architectural specializations provide a means to compensate for the low muscle mass of sloths and suggest further modifications for sustained high force/joint torque.

XEN3-3  3:00 pm  Diagnostic imaging in Linnaeus's two-toed sloth (Choloepus didactylus)—pregnancy diagnosis and fetometry. Thielebein J*, Martin-Luther-University Halle-Wittenberg; Wujciak D, Radiological Community Practice ; Kiefer I, University of Leipzig
Abstract: The knowledge of the reproductive physiology and biology of the two-toed sloth is fragmentary at best. So the data of the gestation period in the literature vary from 150 to 322 days. This study is intended to demonstrate the pregnancy diagnosis and the fetometry in the two-toed sloth with ultrasonography. The ultrasound investigations to visualize the genital system in this species can be performed transcutaneous or transrectal. Only the transcutaneous examinations can be performed without immobilization, if the animals are in medical training. The transcutaneous pregnancy examination is executed with a transducer frequency from 6 to 10 MHz. In total 23 pregnancies (n=10 animals) were diagnosed with certainty. The ultrasound investigations were also used to detect the fetal development (n=8 animals). The first indication of an existing pregnancy is the visualization of the gestational sac at the end of the first month of pregnancy. With the beginning of the second month of the gestation period, the embryo is detectable. In the same gestation month the cardiac activity can also be recognized. The crown-rump length of the fetus is only measurable in the third month. For the following months the fetal development is represented on the biparietal diameter (BPD) and the abdominal diameter (AD). The total gestation period in a two-toed sloth lasts 330 to 350 days. In a range from 294 to 316 days, the fetal development could be visualized with ultrasound (n=7 animals). A shorter fetal development of 265 days was registered in one female.

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