When walking, most mammalian quadrupeds use lateral-sequence gaits, where hindlimb footfall is followed by the ipsilateral (same-side) forelimb. By contrast, primates generally use what is known as a diagonal-sequence gait;after one hindlimb makes ground contact, the contralateral (opposite-side)forelimb is the next to touch down. But why is the gait of primates distinct from that of other mammals? What selective forces might have led them to develop this alternative gait? In a recent paper, Pierre Lemelin and colleagues from Duke University provide empirical support that diagonal-sequence gaits probably evolved in arboreal habitats, where fine-branched canopies select strongly on locomotor morphology and gait.

Interestingly, rather than working on primates, Lemelin and colleagues used didelphid marsupials (American opossums) to test for functional links between arboreality and diagonal-sequence gaits. Why opossums? Because they include species known to use diagonal-sequence gaits. Additionally, opossums can be found in a broad range of habitats, including forest canopies. Lemelin and colleagues reasoned that if there is some causal link between arboreality and diagonal-sequence gaits in primates, a similar link should be found in opossums.

They studied two species: Monodelphis domestica, the short-tailed opossum, which generally forages on the ground, and Caluromys philander, the woolly opossum, which spends much of its time on the thin branches of tree canopies. The team filmed 3–4 animals from each species walking on three surfaces: a flat runway (to simulate the ground), a 28 mm-diameter pole and a 7 mm-diameter pole (representing tree branches of differing size). The team then analysed the locomotor sequences and calculated the degree of diagonality in each gait as the percentage of the locomotor cycle in which ipsilateral forelimb footfalls followed hindlimb touchdowns. Values of 0.25–0.5 imply low diagonality (lateral-sequence walking)whereas values of 0.5–0.75 imply high diagonality (diagonal-sequence walking).

The more terrestrial species, Monodelphis, commonly exhibited diagonality values less than 0.5 and was nearly incapable of coordinated movement across the poles. By contrast, the arboreal species, Caluromys, generally used diagonal-sequence gaits on all surfaces. Moreover, it exhibited greater diagonality when moving on poles than on the flat surface and had no trouble negotiating the slender 7-mm pole. The authors suggest that the use of diagonal-sequence gaits in the woolly opossum and many primates is not coincidental. Rather, the need for moving about and foraging on narrow tree branches has led to the evolution of this gait.

What advantages might diagonal-sequence gaits provide on narrow unpredictable supports? Lemelin points out that earlier workers had proposed that diagonal-sequence gaits reduce yaw, which may be important for stability. Additionally, recent work by Cartmill's group suggests that diagonal-sequence gaits, in conjunction with grasping hindfeet, allow animals such as Caluromys and many primates to maintain a firm grip with a grasping hindfoot while the contralateral forelimb tests out potentially risky footing. If a branch were to give way under the forelimb, the animals could readily pull themselves back and save themselves from taking a potentially nasty tumble.

Lemelin, P., Schmitt, D. and Cartmill, M.(
). Footfall patterns and interlimb coordination in opossums(Family Didelphidae): evidence for the evolution of diagonal-sequence walking gaits in primates.
J. Zool. Lond.