Functional specialization and ontogenetic scaling of limb anatomy in Alligator mississippiensis

Abstract

Crocodylians exhibit a fascinating diversity of terrestrial gaits and limb motions that remain poorly described and are of great importance to understanding their natural history and evolution. Their musculoskeletal anatomy is pivotal to this diversity and yet only qualitative studies of muscle-tendon unit anatomy exist. The relative masses and internal architecture (fascicle lengths and physiological cross-sectional areas) of muscles of the pectoral and pelvic limbs of American alligators (Alligator mississippiensis Daudin 1801) were recorded for an ontogenetic series of wild specimens (n = 15, body masses from 0.5 to 60 kg). The data were analysed by reduced major axis regression to determine scaling relationships with body mass. Physiological cross-sectional areas and therefore muscle force-generating capacity were found to be greater in the extensor (anti-gravity) muscles of the pelvic limb than in the pectoral limb, reflecting how crocodylians differ from mammals in having greater loading of the hindlimbs than the forelimbs. Muscle masses and architecture were generally found to scale isometrically with body mass, suggesting an ontogenetic decrease in terrestrial athleticism. This concurs with the findings of previous studies showing ontogenetic decreases in limb bone length and the general scaling principle of a decline of strength : weight ratios with increasing size in animals. Exceptions to isometric scaling found included positive allometry in fascicle length for extensor musculature of both limbs, suggesting an ontogenetic increase in working range interpreted as increasing postural variability -in particular the major hip extensorsthe interpretation of which is complicated by previous described ontogenetic increase of moment arms for these muscles.

Disparity and convergence in bipedal archosaur locomotion

“…Rather than ignore these crucial aspects of locomotor biology, we explore the disparity in hindlimb muscle parameters available in the literature for living archosaurs and lepidosaurs [19][20][21][22][23]. First, we focus on how muscle mass is functionally distributed within the limbs of a number of taxa from these groups by calculating the percentage of hindlimb muscle capable of inducing specific directional torque at each joint (e.g.…”

“…For example, muscles with both relatively large PCAs and FLs are expected to be designed for exerting comparatively high forces over long contraction distances. Relatively smaller PCAs are considered indicative of lower force capability and shorter FLs of reduced contraction working range or distance [21]. In a study of Alligator muscle function, Allen et al [21] conceptualized the distribution of locomotor muscles in this function space into a number of categories, which are subsequently followed here.…”

“…Relatively smaller PCAs are considered indicative of lower force capability and shorter FLs of reduced contraction working range or distance [21]. In a study of Alligator muscle function, Allen et al [21] conceptualized the distribution of locomotor muscles in this function space into a number of categories, which are subsequently followed here. Specifically, muscles with relatively large PCAs and FLs are considered 'high-power specialists', muscles with high PCAs but shorter fibres are considered 'force specialists' and muscles with low PCAs and relatively long fibres are considered 'displacement specialists'.…”

“…size and architecture) and overall locomotor style. Specifically, we compare living taxa with diverse locomotor strategies in terms of how muscle mass is functionally distributed within their hindlimbs [19][20][21][22][23], and use the concept of muscle 'function space' [21] to explore structure-function links in muscle design. Combining knowledge of these parameters in extant taxa with data from our modelling analysis addresses two fundamental questions in evolutionary biomechanics: how are animals adapted to the functions they perform and is this discernable from fossil evidence?…”

Digital dissection and three-dimensional interactive models of limb musculature in the Australian estuarine crocodile (Crocodylus porosus)

“…Many studies describe the limb anatomy of A . mississipiensis [11,14,20,23–25], with some comparing A . mississipiensis to a number of other crocodilian species [22], and individual species such as the South American Caiman latirostris [26], Southeast Asian Crocodylus siamensis , South American Crocodylus acutus , an unknown species of crocodile [27], and African Osteolaemus tetraspis [20].…”