Interlimb coordination during climbing by primates: a test of the Vilensky hypothesis
Authors: Granatosky Michael C , Schmitt Daniel , Hanna Jandy B 
Published: November 2023

Primate horizontal locomotion is characterized by a trio of unusual features, including hindlimb‐biased weight support, greater diagonality, and high degrees of forelimb protraction. All of these features are thought to have been influenced by early primates' adoption of a thin‐branch niche. This interpretation has however been challenged based on non‐primate mammals that occupy a similar niche, but do not demonstrate these characteristics. Vilensky (1994) proposed that diagonality evolved in response to greater use of more vertically oriented supports. Nyakatura et al (2008) provided data on tamarins in support of this hypothesis. However, both of these studies examined limited primate species and only on inclined supports, rather than purely vertical supports. Here, we examine diagonality during climbing of vertical supports compared to walking across horizontal supports to the hypothesis first proposed by Vilensky.711 symmetric gait sequences were collected during horizontal walking and vertical climbing by 8 species of primates spanning over an order of magnitude of body weight. Trials were video recorded and footfall sequence were determined for each trial from these recording. From these data, speed, diagonality, interlimb coordination, forelimb and hindlimb duty factor, and handhold patterns were determined and compared between horizontal walking and vertical climbing.During horizontal quadrupedal locomotion, most primates tended to utilize diagonal‐sequence diagonal‐couplet gaits and trots. These gait types involve the contralateral forelimbs and hindlimbs moving together in near‐unison, and therefore limit the portion of the stride that the limbs are arranged as a unilateral bipod. In contrast, during climbing all species demonstrated decreased diagonality (ANCOVA, p<0.0001), thereby increasing the proportion of trots and lateral‐sequence lateral couplet gaits. The degree of this decreased diagonality appears to be species specific, with shorter limbed species demonstrating a greater decrease than longer limbed species. The amount the limbs are in contact with the support also increases during climbing, thereby increasing the number of limbs that are in contact with the support at any one time.Although most primates exhibit primarily diagonal sequence, diagonal couplet gaits during horizontal walking, it appears that climbing does not follow a similar pattern. Diagonality does not increase on vertical supports, as suggested by Vilensky and supported by Nyakatura, although diagonal couplets are retained. Our results are similar to those recently reported by Schoonaert et al (2016) for bonobos. Additionally, duty factor index appears to be increased during climbing. We suggest that these phenomena are due to the propulsive requirements of vertical climbing, which causes hindlimb duty factor to increase disproportionately relative to the forelimb in some primates.Support or Funding InformationThis project was supported by the National Science Foundation, Leakey Foundation, Force and Motion Foundation, National Science Foundation's Graduate Research Fellowship Program, Molly Glander award, Department of Veterinary Sciences at the Michale E. Keeling Center at UT MD Anderson Cancer Center, and Duke University Internal Funding.