Pelvic function in anuran jumping: Interspecific differences in the kinematics and motor control of the iliosacral articulation during take-off and landing.

Abstract

Although the anuran pelvis is thought to be adapted for jumping, the function of the iliosacral joint has seen little direct study. Previous work has contrasted the basal "lateral-bender" pelvis from the "rod-like" pelvis of crown taxa hypothesized to function as a sagittal hinge to align the trunk with take-off forces. We compared iliosacral movements and pelvic motor patterns during jumping in the two pelvic types. Pelvic muscle activity patterns, iliosacral anteroposterior (AP) movements and sagittal bending of the pelvis during the take-off and landing phases were quantified in lateral bender taxa Ascaphus (Leiopelmatidae) and Rhinella (Bufonidae) and the rod-like Lithobates (Ranidae). All three species exhibit sagittal extension during take-off, therefore, both pelvic types employ a sagittal hinge. However, trunk elevation occurs significantly earlier in the anuran rod-like pelvis. Motor patterns confirm that the piriformis muscles depress the urostyle while the longissimus dorsi muscles elevate the trunk during take-off. However, the coccygeoiliacus muscles also produce anterior translation of the sacrum on the ilia. A new model illustrates how AP translation facilitates trunk extension in the lateral-bender anurans that have long been thought to have limited sagittal bending. During landing, AP translation patterns are similar because impact forces slide the sacrum from its posterior to anterior limits. Sagittal flexion during landing differs among the three taxa depending on the way the species land. AP translation during landing may dampen impact forces especially in Rhinella in which pelvic function is tuned to forelimb-landing dynamics. The flexibility of the lateral-bender pelvis to function in sagittal bending and AP translation helps to explain the retention of this basal configuration in many anurans. The novel function of the rod-like pelvis may be to increase the rate of trunk elevation relative to faster rates of energy release from the hindlimbs enabling them to jump farther. J. Morphol. 277:1539-1558, 2016. © 2016 Wiley Periodicals, Inc.