Functional adaptations in the forelimb muscles of non‐human great apes

Abstract

The maximum capability of a muscle can be estimated from simple measurements of muscle architecture such as muscle belly mass, fascicle length and physiological cross-sectional area. While the hindlimb anatomy of the non-human apes has been studied in some detail, a comparative study of the forelimb architecture across a number of species has never been undertaken. Here we present data from chimpanzees, bonobos, gorillas and an orangutan to ascertain if, and where, there are functional differences relating to their different locomotor repertoires and habitat usage. We employed a combination of analyses including allometric scaling and ANCOVAs to explore the data, as the sample size was relatively small and heterogeneous (specimens of different sizes, ages and sex). Overall, subject to possible unidentified, confounding factors such as age effects, it appears that the non-human great apes in this sample (the largest assembled to date) do not vary greatly across different muscle architecture parameters, even though they perform different locomotor behaviours at different frequencies. Therefore, it currently appears that the time spent performing a particular behaviour does not necessarily impose a dominating selective influence on the soft-tissue portion of the musculoskeletal system; rather, the overall consistency of muscle architectural properties both between and within the Asian and African apes strengthens the case for the hypothesis of a possible ancient shared evolutionary origin for orthogrady under compressive and ⁄ or suspensory loading in the great apes.

The hominins: a very conservative tribe? Last common ancestors, plasticity and ecomorphology in Hominidae. Or, What's in a name?

“…Lower stride frequencies are consistent with the finding of Myatt et al. (, ) that no consistent quantitative differences exist in either hindlimb or forelimb muscle architecture between the orang‐utans and the nonhuman African apes, which would imply that longer stride lengths in vertical climbing must be offset by lower stride frequencies. Myatt et al.…”

“…Although Myatt et al. () found no differences in hindlimb muscle architecture in a larger sample of species and individuals, available evidence still suggests that muscle moment arms of orang‐utans and gorillas differ (Payne et al. ) and, kinematically, gorillas appear to have a smaller abduction of the hip, but greater flexion of the swing‐leg, whereas orang‐utans keep an extended posture of the knee and have higher hip extension (see e.g.…”

Getting a grip on tetrapod grasping: form, function, and evolution

“…Arboreal mammals tend to have relatively more muscle mass devoted to the forearms and shanks, and less to the epaxial musculature (Grand, 1983). Myatt et al (2012) reported that the digital flexors of chimpanzees, bonobos, gorillas, and orangutans had the largest PCSAs of all of the distal forelimb muscles. For those that tend to cling and climb, however, the flexors might play a relatively more important role for counteracting the force of gravity.…”

“…Schieber (1995) noted that, among mammals, the ability to individuate finger movement increases from ancestral to derived taxa, and is reflected to some extent in species-level differences in muscle structure. For instance, in macaques digits I, II, and V are controlled by relatively fewer multi-tendoned muscles, whereas in humans (and gibbons; Myatt et al, 2012;Diogo, Richmond & Wood, 2012) they tend to be operated by separate mono-tendoned muscles (partly a result of a separate m. flexor pollicis longus belly), resulting in a greater degree of digital independence (Schieber, 1995). In certain frog genera, the forearm muscles are highly differentiated and appear to control each digit individually (e.g.…”

Mechanical similarity across ontogeny of digging muscles in an Australian marsupial (Isoodon fusciventer)

“…Physiological cross‐sectional area (PCSA) is calculated from muscle mass ( m m ) and muscle architecture data; fiber length (FL) and pennation angle (the angle between the line of action of the muscle and the fibers), and is considered a better representation of muscle force production than muscle mass alone (Moore et al, ). As PCSA is a representation of maximum isometric force production for individual muscles, the PCSA values of muscles in a limb can be used to interpret biological functions in correlation with skeletal morphology (Myatt et al, ; Thorpe, Crompton, GÜnther, Ker, & McNeill, ). Scratch‐digging mammals have relatively large PCSA values for muscles used as main movers in the power stroke, such as the triceps brachii long head, which reflect a greater number of fibers per unit volume of muscles; this allows them to produce a greater force for scratch‐digging (Moore et al, ; Olson et al, ; Rose et al, ; Rupert et al, ).…”