Abstract
Extant Crocodylia are exceptional because they employ almost the full range of quadrupedal footfall patterns (“gaits”) used by mammals; including asymmetrical gaits such as galloping and bounding. Perhaps this capacity evolved in stem Crocodylomorpha, during the Triassic when taxa were smaller, terrestrial, and long-legged. However, confusion about which Crocodylia use asymmetrical gaits and why persists, impeding reconstructions of locomotor evolution. Our experimental gait analysis of locomotor kinematics across 42 individuals from 15 species of Crocodylia obtained 184 data points for a wide velocity range (0.15–4.35 ms−1). Our results suggest either that asymmetrical gaits are ancestral for Crocodylia and lost in the alligator lineage, or that asymmetrical gaits evolved within Crocodylia at the base of the crocodile line. Regardless, we recorded usage of asymmetrical gaits in 7 species of Crocodyloidea (crocodiles); including novel documentation of these behaviours in 5 species (3 critically endangered). Larger Crocodylia use relatively less extreme gait kinematics consistent with steeply decreasing athletic ability with size. We found differences between asymmetrical and symmetrical gaits in Crocodylia: asymmetrical gaits involved greater size-normalized stride frequencies and smaller duty factors (relative ground contact times), consistent with increased mechanical demands. Remarkably, these gaits did not differ in maximal velocities obtained: whether in Alligatoroidea or Crocodyloidea, trotting or bounding achieved similar velocities, revealing that the alligator lineage is capable of hitherto unappreciated extreme locomotor performance despite a lack of asymmetrical gait usage. Hence asymmetrical gaits have benefits other than velocity capacity that explain their prevalence in Crocodyloidea and absence in Alligatoroidea—and their broader evolution.
Highlights
Extant Crocodylia have long been known to use almost all forms of walking and running locomotor modes present in quadrupedal mammals
We found that Crocodyloidea and Alligatoroidea used different gaits — the former adopted a wide range of asymmetrical gaits, whereas the latter only employed symmetrical gaits
We recorded the first documentation of asymmetrical gaits in the critically endangered Philippine crocodile Crocodylus mindorensis — all four of the juveniles studied were remarkably adept at the full range of crocodylian gaits (e.g., Supplementary Movie S6)
Summary
Extant Crocodylia have long been known to use almost all forms of walking and running locomotor modes (e.g., footfall patterns) present in quadrupedal mammals. Bounding (synchronized left-right forelimb and hindlimb motions separated by an aerial phase) and galloping (slightly asynchronous left-right motions; essentially a slower version of bounding) gaits have been described for some Crocodylus species, especially C. johnstoni[27,28], C. porosus[29] and C. niloticus[30,31]. The kinematics of asymmetrical gaits have been noted to be highly variable within and among Crocodylia, covering almost all possible footfall combinations[1,38] and disparate correlations of velocity and kinematic parameters such as stride length or frequency[27,28,29] Some of this variation appears size-related — Zug[29] noted that only young C. porosus
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