Abstract
In quadrupeds the musculature of the hindlimbs is expected to be responsible for generating most of the propulsive locomotory forces, as well as contributing to body support by generating vertical forces. In supporting the body, postural changes from crouched to upright limbs are often associated with an increase of body mass in terrestrial tetrapods. However, felids do not change their crouched limb posture despite undergoing a 300-fold size increase between the smallest and largest extant species. Here, we test how changes in the muscle architecture (masses and lengths of components of the muscle-tendon units) of the hindlimbs and lumbosacral region are related to body mass, to assess whether there are muscular compensations for the maintenance of a crouched limb posture at larger body sizes. We use regression and principal component analyses to detect allometries in muscle architecture, with and without phylogenetic correction. Of the muscle lengths that scale allometrically, all scale with negative allometry (i.e. relative shortening with increasing body mass), whereas all tendon lengths scale isometrically. Only two muscles' belly masses and two tendons' masses scale with positive allometry (i.e. relatively more massive with increasing body mass). Of the muscles that scale allometrically for physiological cross-sectional area, all scale positively (i.e. relatively greater area with increasing body mass). These muscles are mostly linked to control of hip and thigh movements. When the architecture data are phylogenetically corrected, there are few significant results, and only the strongest signals remain. None of the vertebral muscles scaled significantly differently from isometry. Principal component analysis and manovas showed that neither body size nor locomotor mode separate the felid species in morphospace. Our results support the inference that, despite some positively allometric trends in muscle areas related to thigh movement, larger cats have relatively weaker hindlimb and lumbosacral muscles in general. This decrease in power may be reflected in relative decreases in running speeds and is consistent with prevailing evidence that behavioural changes may be the primary mode of compensation for a consistently crouched limb posture in larger cats.
Highlights
In terrestrial tetrapods, where there are evolutionary increases in body masses there tends to be changes in limb posture from crouched to upright to avoid potential increases in stresses within the supportive tissues, whose relative strengths tend not to vary (Biewener 1989, 1990, 2005)
The fascicle lengths for M. extensor digitorum lateralis and M. vastus intermedius showed significant allometry: the M. lateral digital extensor fascicles scaled with negative allometry, and M. vastus intermedius scaled with positive allometry (Table 4)
For the muscle belly masses two muscles initially showed significant allometry; the M. vastus intermedius scaled with negative allometry and the M. gluteus medius scaled with positive allometry (Table 5, Figure 1)
Summary
In terrestrial tetrapods, where there are evolutionary increases in body masses there tends to be changes in limb posture from crouched to upright to avoid potential increases in stresses within the supportive tissues, whose relative strengths tend not to vary (Biewener 1989, 1990, 2005). Extant felids are unusual in that they maintain the same crouched posture from the smallest species to the largest (Day and Jayne, 2007) throughout their ~1 - 300kg range of body masses (Cuff et al, 2015). Larger felids (above cheetah, Acinonyx jubatus, size) seem to suffer from reduced locomotor performance relative to their smaller relatives (e.g. range of speeds: Garland, 1983; Day and Jayne, 2007), which may be emphasised more strongly in Felidae than in some other mammals due to their conserved limb postures. Diameters and cross-sectional areas of those bones scale with positive allometry, meaning long bones become relatively more robust (and stiffer and stronger as a consequence) in larger felid species (Doube et al, 2009; Lewis and Lague, 2010; Meachen-Samuels and Van Valkenburgh, 2009; Meachen-Samuels and Van Valkenburgh, 2010)
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