Not so fast: Speed effects on forelimb kinematics in cercopithecine monkeys and implications for digitigrade postures in primates

Abstract

Terrestrial mammals are characterized by their digitigrade limb postures, which are proposed to increase effective limb length (ELL) to achieve preferred or higher locomotor speeds more efficiently. Accordingly, digitigrade postures are associated with cursorial locomotion. Unlike most medium- to large-sized terrestrial mammals, terrestrial cercopithecine monkeys lack most cursorial adaptations, but still adopt digitigrade hand postures. This study investigates when and why terrestrial cercopithecine monkeys adopt digitigrade hand postures during quadrupedal locomotion. Three cercopithecine species (Papio anubis, Macaca mulatta, Erythrocebus patas) were videotaped moving unrestrained along a horizontal runway at a range of speeds (0.4-3.4 m/s). Three-dimensional forelimb kinematic data were recorded during forelimb support. Hand posture was measured as the angle between the metacarpal segments and the ground (MGA). As predicted, a larger MGA was correlated with a longer ELL. At slower speeds, subjects used digitigrade postures (larger MGA), however, contrary to expectations, all subjects used more palmigrade hand postures (smaller MGA) at faster speeds. Digitigrade postures at slower speeds may lower cost of transport by increasing ELL and step lengths. At higher speeds, palmigrade postures may be better suited to spread out high ground reaction forces across a larger portion of the hand thereby potentially decreasing stresses in hand bones. It is concluded that a digitigrade forelimb posture in primates is not an adaptation for high speed locomotion. Accordingly, digitigrady may have evolved for different reasons in primates compared to other mammalian lineages.

Electromyography of wrist and finger flexor muscles in olive baboons (Papio anubis)

“…Specifically, faster trials were those with stance durations of <20 video fields, whereas slower trials had stance phases lasting >30 video fields (video recorded at 60Hz; see below). As seen in prior studies (Patel, 2009;Patel, 2010a), this dichotomy in speed selection corresponded well with the hand posture used by both animals: digitigrade postures were used when moving slowly and palmigrade-like postures were used when moving more quickly. One important point that must be noted is that the behavioral preferences of the subjects coupled with variable success in EMG recording resulted in unequal samples of step cycles analyzed for each muscle and for each behavior (Table1).…”

“…baboons) adopt digitigrade hand postures only during slow walking, and use less digitigrade or more 'palmigrade'-like hand postures (i.e. the forepaw equivalent to plantigrade hindfoot postures) when running and using asymmetrical gaits (Patel, 2009;Patel, 2010a;Patel and Polk, 2010;Patel and Wunderlich, 2010). These primates transition to palmigrady at faster speeds because of the inability of their mobile distal forelimb joints to passively resist extension moments at the wrist joint while experiencing higher GRFs -effectively forced palmigrady (Patel, 2009;Patel, 2010a).…”

“…the forepaw equivalent to plantigrade hindfoot postures) when running and using asymmetrical gaits (Patel, 2009;Patel, 2010a;Patel and Polk, 2010;Patel and Wunderlich, 2010). These primates transition to palmigrady at faster speeds because of the inability of their mobile distal forelimb joints to passively resist extension moments at the wrist joint while experiencing higher GRFs -effectively forced palmigrady (Patel, 2009;Patel, 2010a). Whether forced or not, palmigrady at faster speeds when GRFs are higher is beneficial in distributing those higher forces over a larger contact area and thereby dynamically moderating the stresses acting on relatively fragile hand bones and accompanying soft tissues (Patel, 2010b;Patel and Wunderlich, 2010).…”

“…Janis and Wilhelm, 1993;Isbell et al, 1998;Reilly et al, 2007). Digitigrady may improve walking efficiency in nonhuman primates by increasing functional forelimb length, allowing them to take fewer but absolutely longer strides to travel a given distance (Patel, 2009). In addition, as the generation of muscular force is a large contributor to active metabolism (e.g.…”

Exploring Third Metacarpal Capitate Facet Shape in Early Hominins

“…We chose five locomotor categories to examine, including palmigrade/digitigrade quadrupedalism (QUAD), knuckle-walking (KW), brachiation (BRAC), vertical climbing (VC), and overall substrate usage (SUB), and assigned taxa values based on published reports of the proportion of time species spend performing different behaviors. Palmigrade and digitigrade hand postures used during quadrupedalism were grouped into the same category since recent studies have shown variation in use of these hand postures within a species, and metacarpal traits do not always clearly distinguish between ''palmigrade'' and ''digitigrade'' quadrupeds (Patel, 2009(Patel, , 2010Patel and Polk, 2010). Extant taxa were not assigned to a single locomotor category.…”