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 extensors - the interpretation of which is complicated by previous described ontogenetic increase of moment arms for these muscles.

Fig. 1

Function space plot (fascicle length vs. PCSA, normalized by appropriate exponents of body mass) for muscles of the pectoral limb of Alligator mississippiensis. Schematic anatomy of left pectoral limb is shown in left-lateral view on left, cranial view on right.

Fig. 3

Scaling exponents of muscle properties vs. body mass for the pectoral limb of Alligator mississippiensis. Blue crosses indicate regression slopes for body mass, black diamonds indicate regression slopes for PCSA, pink circles indicate regression slopes for fascicle length. Error bars show upper and lower 95% confidence intervals, dashed lines indicates relevant value for isometry.

Fig. 4

Scaling exponents of muscle properties vs. body mass for the pelvic limb of Alligator mississippiensis. Blue crosses indicate regression slopes for body mass, black diamonds indicate regression slopes for PCSA, pink circles indicate regression slopes for fascicle length. Error bars show upper and lower 95% confidence intervals, dashed lines indicates relevant value for isometry.

Fig. 2

Function space plot (fascicle length vs. PCSA, normalized by appropriate exponents of body mass) for muscles of the pelvic limb of Alligator mississippiensis. Schematic anatomy of left pelvic limb is shown in left-lateral view on left, cranial view on right.