Biomechanical Determinants of Transverse and Rotary Gallop in Cursorial Mammals

1. What they did

Running mammals have two kinds of gallop. In the transverse gallop, the leading hindfoot and the leading forefoot are on the same side of the body, so the footfalls cross from one side to the other. In the rotary gallop, they are on opposite sides, and the footfalls seem to rotate around the body. Horses are known to use only the transverse gallop, cheetahs and racing dogs the rotary gallop. The authors asked which species use which gallop, and why.

• Classifying gallops. They screened 1650 films and video clips of free-ranging mammals and selected 351 clear sequences of straight galloping. From these they placed 89 species of hoofed mammals and carnivores into three groups: transverse at any speed, rotary at any speed, or speed dependent (transverse at lower speed, rotary at high speed). A list by R. McNeill Alexander and the literature were the starting point.
• Collecting body and ecology data. For each species they gathered 23 measures from the literature and museum specimens: maximum speed, body mass, body and limb lengths, limb bone lengths and thicknesses, spine vertebra measurements, and traits such as habitat, activity time, social life, feeding and whether the species is mainly predator or prey.
• Analysing gait. Using slow-motion footage, they timed the footfalls in 152 strides and measured how far the hindlimb swings at high speed.
• Comparing the groups with statistics, including analyses that take into account how closely related the species are.

2. What they found

Who gallops how

• 37 species (42%) used only the transverse gallop, 35 (39%) only the rotary gallop, and 17 (19%) switched from transverse to rotary at high speed.
• All horses, zebras, asses and the rhinoceros used the transverse gallop; all cats used the rotary gallop; most dogs and their relatives (wolf, jackals, African wild dog, coyote) were speed dependent.

Body build

• Transverse gallopers were slower and heavier, with proportionally thicker limb bones.
• Rotary gallopers had the longest feet relative to the upper limb (longer metacarpals and metatarsals compared with the humerus and femur) and were relatively faster.
• Speed-dependent species had thin, slender limb bones, light bodies and high relative speed, but in other ways (body shape, foot length) they were closer to the transverse gallopers.
• The cheetah, a rotary galloper, had the highest absolute speed (29.2 metres per second) and relative speed. The goitered gazelle was fastest in body lengths per second (26.3).
• Spine stiffness, measured from the vertebrae of 21 species, did not differ between the groups.

Ecology and behaviour

• The transverse gallop was more common in diurnal, gregarious species of open grasslands and plains, most of them prey.
• Rotary and speed-dependent gallopers were more common in mixed habitats and among crepuscular (dawn and dusk) species. Rotary gallopers were more common among solitary predators.

Footfall timing

• In rotary strides, the feet were on the ground for a shorter part of the stride (duty factor about 23% versus 29% in transverse strides), and the forefeet touched down for less time than the hindfeet.
• Rotary strides usually had two flight phases (one with legs gathered under the body, one with legs stretched out); transverse strides usually had one, with the legs gathered, as in the horse.
• The hindlimb swung through a wider angle in rotary strides (about 91 degrees) than in transverse strides (about 66 degrees). The angle increased with relative speed and decreased with body mass.

3. What they concluded

• A large number of mammal species use only one kind of gallop. The type of gallop depends on a mix of body build, mechanics and environment, not only on speed, slope and terrain.
• In the authors’ words, an animal with a lean body and long legs that lives as a solitary predator or prey, active at dusk in mixed habitats, is most probably a rotary galloper. Heavy, diurnal, gregarious prey of open country with shorter legs tend to be transverse gallopers.
• Rotary gallop suits nimble animals that need sudden sprints and sharp turns among obstacles; size and long limbs favour animals of wide-open spaces. Gregarious predators such as canids, which need endurance, use both gallops at different speeds.
• Shared gallop types among related species are more often due to similar bodies and ecology than to inheriting the gait itself. The red fox and Arctic fox, close relatives, use different gallops, which the authors link to their different environments.
• The most important determinants are body mass, the relative lengths of limb segments, body shape (height to length) and how far the limbs swing.
• The spinal nerve networks of running mammals could probably produce all running gaits; each species uses only some of them because of its body and environment. The results should also help infer the gaits of extinct four-legged animals.

6. Other technologies, skills or innovations

• Classifying gaits from documentary footage and online video, combined with the authors’ own high-speed filming (Casio Exlim EX-FH20 at 210 frames per second).
• Counting video frames to time each footfall and tracing frames to measure limb swing angles.
• Museum bone measurements with digital and analogue callipers.
• Analyses that correct for how closely related the species are (phylogenetically independent contrasts).