Field Guide to Animal Tracks and Scat of California. Lawrence Mark Elbroch

      To better visualize and understand tracks on the ground, we must study whole animals, starting with their feet. Feet make footprints, and footprints are the building blocks of track patterns. Here we begin with the morphology of mammal feet, then move on to study the morphology of mammal tracks, then look at how mammals move, and finally delve into the science and interpretation of mammalian track patterns on the ground.

       Foot Morphology

      Bones

      Thousands of years ago, the first mammals had five toes on each foot (Hildebrand and Goslow 2001). Over great lengths of time, through evolution and specialization, feet and legs evolved and became more varied. The foot structure of extant shrews is believed to reflect that of the first mammals.

      The skeletal structure of the forefeet consists of the carpal bones, metacarpal bones, and phalanges, while that of the hind feet consists of the tarsal bones, metatarsal bones, and phalanges. The original mammals were plantigrade, meaning that they were supported by all the bones of their feet while they were moving, whereas today's mammals have diversified in structure and function. In general, modern plantigrade animals have relatively short limbs and prefer to walk, because the construction of their feet is not well adapted for jumping or running for long distances. In contrast, long-distance runners have long limbs, and the area of their feet that is in contact with the ground is as small as possible. To obtain a firm grip on the ground, the foot must exert the greatest possible pressure to dig into it. Because pressure is equal to force per unit area, the contact area must be as small as possible to ensure the greatest possible pressure. For example, in deer the second and fifth toes are reduced to form dewclaws, and their weight is supported on the third and fourth toes; thus, they have long legs and small feet relative to their size.

      Bears and humans are examples of extant plantigrade species. Digitigrade species, such as felids (cat family) and canids (dog family), support themselves on the distal ends, or heads, of the metacarpal bones and the phalanges of the forefeet, and the distal ends of the metatarsal and phalanges of the hind feet. Even-toed unguligrade species such as deer and bison, and odd-toed unguligrade mammals such as horses, walk on the distal phalanges of the third and fourth toes, the equivalent of our toenails.

      In animals with five well-developed toes (or digits), they are numbered from 1 to 5 beginning with the innermost toe, that which corresponds with the human thumb (see the accompanying illustrations of woodrat and river otter feet on pages 33 and 34). The third toe in most mammals is the longest and largest, followed in order of size by the fourth, second, fifth, and first. In some mammals the first toe is tiny and only makes a weak impression, or none at all. If all five toes are showing and the shortest toe is on the left side of the footprint, the track was made by a right foot. If only four toes are showing and the shortest toe is on the left side of the track, then it was made by a left foot.

      Big-eared woodrat feet.

      With evolution, however, some species developed structures and gaits that required different foot structures. Bang and Dahlstrom (1972) note that many species, such as deer, began to evolve longer bone structures to aid in running, while at the same time losing toe 1 altogether to become more streamlined.

      Pads

      Tough horny layers of skin cover elastic masses of connective tissue that protect bones and other foot structures from rough ground. In most mammals these pads are naked (rabbits are an exception), and the spaces between these pads are filled with fur (some species have completely naked feet, like Striped Skunks). The thick toe and metacarpal or metatarsal pads that compose the “palms” of each track are covered in sweat glands and deposit scent with each step (see the accompanying illustrations of river otter feet on page 34).

      River otter feet.

      The pads at the tips of the toes are called the digital pads. The pads that form the “palms” of footprints are called the metacarpal pads on the front feet and the metatarsal pads on the hind feet, and they correspond to the respective bones that they protect. In many animals, such as Bobcats and Coyotes, the metacarpal pads of the front feet and metatarsal pads on the hinds are fused to form one large pad. In addition to these, some animals have one or two additional pads to the posterior. The larger pad on the front feet is the carpal pad and covers the carpal bone. When there is a second, it is another metacarpal pad linked with a reduced and sometimes redundant inner digit (see the figure of the woodrat on page 33). In mammals, carpal pads are found only on the forefeet.

      Feet

      If the front end of an animal is larger and heavier than its hind end, the forefeet will be larger and broader than the hind feet. The forefeet support the head, chest cavity, and forequarters of the body, which are often heavier than the hindquarters. Some mammals, such as bears, rodents, and otters, have larger hind feet and more massive hindquarters than forequarters.

      The forefeet are generally rounder in shape than the narrower hind feet, because the forelegs are almost perpendicular to the ground while the hind legs are positioned such that the heel is held at an angle to the ground. A cylinder that is perpendicular to a plane has a circular cross section, while a cylinder that meets a plane at an angle has a larger, elliptical cross section.

      Evolutionary adaptations of feet reflect specific types of locomotion, as well as their use in some animals as tools or weapons. Predators have soft pads for stealth, and some have sharp claws to hold down their prey, or short, blunt claws that aid in traction. Some animals have claws that are tools for digging, and others that are adapted for grooming. Feet adapted for soft, muddy ground require a large contact area for support. Sharp, pointed hooves can dig into soft, sandy terrain to obtain a firm grip, while hooves must be rounder to better grip firm ground. Aquatic mammals often have webbed toes or a stiff fringe of hair to increase the surface area of their feet and therefore the resistance with which they pull themselves through the water. Animals adapted to arboreal environments have sharp claws to dig into the bark of trees, or opposable joints or toes to grasp branches.

      Male Bighorn Sheep, approximately seven years of age. Note the huge disparity in size between the front and hind feet.

      The more massive ungulates, such as American Bison, have broad, round hooves, while lighter deer have slender, narrow hooves. Very sharp, pointed hooves are an adaptation for speed, especially in soft, sandy

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