Morphological and kinematic specializations of walking frogs

Abstract

The anuran body plan is defined by morphological features associated with saltatory locomotion, but these specializations may have functional consequences for other modes of locomotion. Several frog species use a quadrupedal walking gait as their primary mode of locomotion, characterized by limbs that move in diagonal pairs. Here, we examine how walking species may deviate from the ancestral body plan and how the kinematics of a quadrupedal gait are modified to accommodate the anuran body plan. We use a comparative analysis of limb lengths to test the hypothesis that quadrupedal anurans shift away from the standard anuran condition defined by short forelimbs and long hindlimbs. We also use three-dimensional high-speed videography in four anuran species (Kassina senegalensis, Melanophryniscus stelzneri, Phrynomantis bifasciatus, and Phyllomedusa hypochondrialis) to characterize footfall patterns and body posture during quadrupedal locomotion, measuring the angle and timing of joint excursions in the fore- and hindlimb during walking to compare kinematics between limbs of disparate lengths. Our results show frogs specialized for walking tend to have less disparity in the lengths of their fore- and hindlimbs compared with other anurans. We find quadrupedal walking species use a vertically retracted hindlimb posture to accommodate their relatively longer hindlimbs and minimize body pitch angle during a stride. Overall, this novel quadrupedal gait can be accommodated by changes in limb posture during locomotion and changes in the relative limb lengths of walking specialists.

The impact of pelvic lateral rotation on hindlimb kinematics and stride length in the red-legged running frog,Kassina maculata

“…Despite a sixfold increase in speed, little variation in footfall pattern was observed (figure 2; [5, fig. 5]).…”

“…Our current knowledge of frog walking derives from recent observational studies [4,5]. Among several species tested, frogs walk by maintaining each limb in ground contact for at least 50% of the stride duration (i.e.…”

“…Among several species tested, frogs walk by maintaining each limb in ground contact for at least 50% of the stride duration (i.e. duty factor ≥50% [4]) and show remarkably similar footfall and limb kinematics across speed and species [5]. Additionally, frogs employ a quadrupedal limb contact cycle where the forelimb (on the leading side) makes contact first, followed by the leading hindlimb then the trailing hindlimb and, finally, the trailing forelimb [5].…”

“…Furthermore, the use of these models, based on experimental kinematic data, provides a novel and non-invasive method of studying the influence of pelvic lateral rotation on the hindlimb. Shedding light on the musculoskeletal function of the pelvic apparatus during walking, along with recent findings [4,5], may bring us closer to understanding the ways in which modern frogs overcome their jumping ‘specializations’ and provide insight to the driving forces behind the evolution of the unique anuran pelvic apparatus.…”

Peer Review #1 of "Digital dissection of the pelvis and hindlimb of the red-legged running frog, Phlyctimantis maculatus, using Diffusible Iodine Contrast Enhanced computed microtomography (DICE μCT) (v0.1)"

“…As such, frogs have been used as models for understanding the biomechanics of jumping (e.g. Calow & Alexander, 1973;Roberts & Marsh, 2003;Astley & Roberts, 2014;, swimming (Gillis & Biewener, 2000;Nauwelaerts & Aerts, 2002;Richards & Biewener, 2007;Clemente & Richards, 2013) and walking (Ahn et al, 2004;Reynaga et al, 2018). One particular region of interest in anurans is the morphological variation in the sacrum and pelvis, thought to play a large role in the locomotor versatility observed across anuran taxa (Emerson, 1979).…”