Animals can be classified by their mode of locomotion. The word digitigrade is one such classification, used to classify animals (especially mammals). Digitigrades walk on their toes instead of walking while touching the ground with their heels. Cats, dogs, hyenas, and walking birds are the extant digitigrades. The digitigrade locomotion causes the distinctive hooked shape of doglegs.
Characteristics of Digitigrades
Digitigrade animals have anatomical differences from other animals. Long carpals and tarsals characterize these animals. They also walk on their distal and intermediate phalanges. A bone found in digitigrades that would correspond to the ankle found in humans is located higher in these mammals. Their legs also include metatarsals or metacarpals which are similar to the bones that form the arch of the foot and the palm in humans.
Digitigrades move more quickly and quietly compared to other animals. The foot posture is positioned in a way that the heel plays a part in supporting the body weight when walking, running or standing. Most of the digitigrades have two sets of legs, one optimized for running and another for jumping.
Digitigrades are different from plantigrades which usually walk with the soles of their feet on the ground and the unguligrade which walk only on the distal-most tips of their digits. Plantigrade animals include humans while unguligrades consist of horses, donkeys, and cattle.
Semi-digitigrade animals have an immediate character between the plantigrade and digitigrade foot. Their heel does not rest on the ground in walking, but they are not raised so high as those in the digitigrade foot. Examples of semi-digitigrade include elephants, capybara, pigs, and hippos in that they walk, stand, or run with an elevated heel that does not come into contact with the substrate. The cat tribe has the intermediate form because their three lower tiers of the feet rest on the ground as they run, stand, or jump. On the other hand, dinosaurs are both digitigrade and semi-digitigrade.
Advantages and Disadvantages of Digitigrade Locomotion
Digitigrades are faster in running and jumping as the digitigrade legs get extra leverage from their ankle which gives them a spring in their step. The shape of their legs increases the speed of running by increasing the length of the leg and by improving the storage and recovery of energy in the lower limbs of the animal or bird. This digitigrade locomotion also tends to generate less sound which gives predators advantage over their prey. Large ground birds such as ostriches can use their kick to lethal effect for primary defense.
Bipedal Digitigrades, however, are not very stable, and it is easier to knock them over compared to plantigrades and unguligrades. This is due to the smaller surface area in touch with the ground and they have a lower load bearing capacity for this reason.