Date Published: July 28, 2011
Publisher: Hindawi Publishing Corporation
Author(s): Jandy B. Hanna, Daniel Schmitt.
Primate locomotor evolution, particularly the evolution of bipedalism, is often examined through morphological studies. Many of these studies have examined the uniqueness of the primate forelimb, and others have examined the primate hip and thigh. Few data exist, however, regarding the myology and function of the leg muscles, even though the ankle plantar flexors are highly important during human bipedalism. In this paper, we draw together data on the fiber type and muscle mass variation in the ankle plantar flexors of primates and make comparisons to other mammals. The data suggest that great apes, atelines, and lorisines exhibit similarity in the mass distribution of the triceps surae. We conclude that variation in triceps surae may be related to the shared locomotor mode exhibited by these groups and that triceps surae morphology, which approaches that of humans, may be related to frequent use of semiplantigrade locomotion and vertical climbing.
From Aristotle’s thoughts in De Motu Animalium , to Borelli’s  comprehensive review of biomechanics in the 1600s, to Muybridge’s  original documentation of gaits in horses around the turn of the last century, animal movement has been a vibrant and productive area of research providing insights into critical aspects of form-function relationships and selection pressures on limb and body design in many vertebrates, including primates. In addition to capturing photographic plates of the different gaits of the horse, Muybridge also took stop motion images of other mammals, including a nonhuman primate. His famous collection of plates and prose, published in 1887, was the first available to researchers interested in animal locomotion  and spawned a new generation of scientists interested in locomotion. Following Muybridge’s observation that the baboon “disregards the law governing the walk” (3 : 30), multiple researchers created hypotheses concerning primate locomotor evolution, which were based on observed differences between primate and nonprimate locomotion (e.g., [4, 5]). For example, building on Muybridge’s observations on footfall, in which he argued for a differential functional role of the forelimb and hindlimb in primates, forty years later, De la Croix  commented on the unique aspects of what he called “the pithecoid gait” (6 : 53 and Figure 3 therein), which he argued was “the gait used by the early ancestors of man.” (6 : 53, referring to ).
This paper is a literature review. Data on the functional morphology of the triceps surae (TS) were compiled from the literature and compared across a variety of primates and a few, nonprimate species. Much of the literature concerning TS is qualitative; thus, the section depicting the initial results presents those descriptions. Subsequent to that, quantitative data on fiber type distribution and relative mass of the muscles composing TS are presented. Finally, possible patterns of TS morphology are discussed with reference to literature regarding the biomechanics of movement. The literature compiled and presented includes data from both wild and captive animals, dry and wet weights of the muscles, and animals of known and unknown ages. To attempt to account for some of the differences likely caused by the lack of controls, the fiber type and mass data are presented as percentages, relative to the total percent of the individual muscle (in the case of fiber type distribution) or the total mass of TS (in the case of percent mass). Despite these attempts for better control, data should be interpreted with caution.
This sample is, at best, limited and there are some questions that remain unanswered. First is how does phylogeny affect TS variation? Although Langdon  suggested that there is a “typical TS morphology” within primate phylogenetic groups, the sample does not clearly illustrate this suggestion. Additionally, in other mammalian groups, similar variation within phylogenetic groups is present. For example, within Carnivora, the black bear is described as having a well-developed soleus and three heads of gastrocnemius, but no plantaris , while the polar bear is reported to have only a plantaris and two heads of gastrocnemius . Finally, Ray  reports the presence of all four muscles in the Malay bear, with plantaris being approximately the same size as the lateral gastrocnemius. In another group of carnivores, the cat has a large plantaris and a small soleus, while canids have no soleus at all [85, 90]. Similar diversity in TS morphology is exhibited in other species, such that African elephants exhibit a robust soleus and limited plantaris , while hippopotami lack a soleus [91, 140], but exhibit a “fleshy” plantaris .
Much of the convergent TS morphology of great apes, atelines, and lorisines appears to be related to a shared locomotor mode, and biomechanical data may support this hypothesis. Clearly, more data on the dynamics of vertical climbing need to be collected before definite conclusions can be drawn about the relationship between TS morphology and vertical climbing. If such a relationship does exist, however, the implications for primate locomotor evolution would be twofold.