Abstract
Crocodiles and their kin (Crocodylidae) use asymmetrical (bounding and galloping) gaits when moving rapidly. Despite being morphologically and ecologically similar, it seems alligators and their kin (Alligatoridae) do not. To investigate a possible anatomical basis for this apparent major difference in locomotor capabilities, we measured relative masses and internal architecture (fascicle lengths and physiological cross-sectional areas) of muscles of the pectoral and pelvic limbs of 40 individuals from six representative species of Crocodylidae and Alligatoridae. We found that, relative to body mass, Crocodylidae have significantly longer muscle fascicles (increased working range), particularly in the pectoral limb, and generally smaller muscle physiological cross-sectional areas (decreased force-exerting capability) than Alligatoridae. We therefore hypothesise that the ability of some crocodylians to use asymmetrical gaits may be limited more by the ability to make large, rapid limb motions (especially in the pectoral limb) than the ability to exert large limb forces. Furthermore, analysis of scaling patterns in muscle properties shows that limb anatomy in the two clades becomes more divergent during ontogeny. Limb muscle masses, fascicle lengths and physiological cross-sectional areas scale with significantly larger coefficients in Crocodylidae than Alligatoridae. This combination of factors suggests that inter-clade disparity in maximal limb power is highest in adult animals. Therefore, despite their apparent morphological similarities, both mean values and scaling patterns suggest that considerable diversity exists in the locomotor apparatus of extant Crocodylia.
Highlights
Smaller individuals of some species of extant Crocodylia use asymmetrical bounding and galloping gaits for rapid terrestrial locomotion
We took the bulk of anatomical data from two species, American alligators (A. mississippiensis, n = 15, body mass ~0.5–57.7 kg), and Nile crocodiles (Crocodylus niloticus, n = 16, body mass ~0.1–278 kg)
We found no qualitative differences between these results and those obtained using body mass as a normaliser
Summary
Smaller individuals of some species of extant Crocodylia use asymmetrical bounding and galloping gaits for rapid terrestrial locomotion. Gatesy, 1991; Blob & Biewener, 1999; Blob, 2001; Willey et al 2004), such asymmetrical gaits have yet to be observed in Alligatoridae [ Reilly & Elias (1998) describe one individual American alligator (Alligator mississippiensis) as ‘attempt(ing) to gallop’ for part of a stride] This suggests that Alligatoridae, despite being morphologically more similar to Crocodylidae than either is to Gavialoidea Anecdotal evidence (e.g. Cott 1960; Singh & Bustard, 1976) supports the inference that both Crocodylidae and Gavialoidea may lose the use of asymmetrical gaits past a certain size boundary (~2 m total length) Such ontogenetic gait loss is rare among tetrapods, so analysis of crocodylian asymmetrical gaits is a source of comparative data on quadrupedal gaits in general (crocodylians represent the only non-mammalian vertebrates known to use asymmetrical quadrupedal gaits), it is a useful case study in vertebrate locomotor ontogeny
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