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 substrate, and act like spikes to prevent slipping. Pronghorns, which prefer open terrain, have to rely on speed to escape being captured, and have sharp, pointed hooves. While habitat specialization results in foot structures with certain advantages, it also places the same animal at a disadvantage in alternative habitats. For example, the large, webbed feet of beavers are ideal tools for aquatic living but cumbersome obstacles while negotiating land at speed.
Track Morphology
Most mammal tracks are composed of claws, digital pads, metacarpal or metatarsal pads (the palms), carpal pads and other heel structures, and the negative space in between these structures (see the figure on page 36). To start, study beautiful footprints in damp, slightly muddy earth, wet sand, a thin layer of loose dust on firm substrate, or a thin layer of fresh snow. In these conditions you will be able to see clear, complete tracks and better understand them. An individual animal's footprint will vary according to its age, mass, sex, condition, and the substrate in which it steps, and these nuances can only be recognized once you have enough experience with a species to know what a “normal” track might look like.
North American River Otter tracks.
The specific details of track morphology for different species are covered in depth in the species accounts at the end of the book. However, as an introduction to the topic, consider these eight key questions to species identification and some examples of why they might be useful:
1. How asymmetrical is the track? Track symmetry is the comparison between the right and left sides of the track. In a perfectly symmetrical track, the left and right sides align and match perfectly. However, nearly every mammal track is asymmetrical to some degree, and it is the degree of asymmetry that will be useful.
For example, the front tracks of Bobcats are often much more asymmetrical than the hind tracks. The opposite is true for river otters, where the front track often appears more symmetrical than the hind track.
2. How many toes can you see in the track, and are they all on the same plane? Shrew tracks have five toes on both front and rear feet, and they all tend to register in footprints. Hares have five toes on their front feet (although the innermost toe is tiny and easily overlooked in the track or does not register at all) and four toes on their hind feet. Canid and felid tracks tend to register four distinct toes, even though they have five toes on their front feet; digit 1 on each front foot sits on a higher plane on the inside on the leg and registers when they run.
The loping tracks of a river otter. From right to left, right front, left front (below)–right hind combo, left hind. Compare the relative sizes and symmetries between front tracks and hind tracks. Study the shapes of the digital pads and claw marks.
3. What is the shape of the digital pads? Teardrop-shaped toes are characteristic of felids, especially females, as well as the digits in mink and smaller weasel tracks. In Fisher tracks the bulbous toe pads are separate from the metacarpal or metatarsal pads, while in raccoon tracks the cigar-shaped toes tend to be connected to the metacarpal or metatarsal pads.
4. Can you see nails? Look closely, because sometimes nails register as tiny pricks in the ground, as in Gray Foxes; in other instances they are massive and difficult to miss, as in American Beavers. Are they sharp or blunt? Are they coming straight out from the toes, or do they curve down from a location on top of the toe? In deep snow, Bobcat tracks often show claws, which register in the wall of snow in front of the track, on a plane higher than the digits. Domestic Dog tracks tend to have large, blunt nails in comparison with the sharp, thin nails of Coyotes. The outer nails of Coyotes also often register so close to the inner toes that they are overlooked.
5. What is the overall shape of the metacarpal or metatarsal pads (palms)? In some animals, including canids and felids, the metacarpal pads have fused together to form one palm pad. Characteristics of felid tracks include an anterior (leading) edge with two lobes, termed bilobate, and a posterior edge with three. The palm pads of felid tracks also tend to fill the overall track. This means that, in terms of area, they compose a larger proportion of the track than do the palm pads in tracks of canids. In both canids and felids, the palm pads are much larger on the front feet than the hind.
A comparison of the negative space in the front feet of three carnivores.
6. Can you make out a heel in the track? All mammals in the order Rodentia, from mice to beavers, excepting the porcupine, register two round pads behind their metacarpal pads in their front tracks. One is a carpal pad on the heel and the other a modified metacarpal pad. In the front tracks of weasels, Ringtails, and bears, a carpal pad can be found, and in some mammals the heels of the hind feet register only as impressions.
7. Can you see hair in the track, and if so, where? Rabbits lack naked pads and have completely furred feet. Red Foxes have very small toe pads surrounded by lots of hair, and Striped Skunks have no hair between their toes at all.
8. What is the shape in between the toes and palm pads, here called the negative space?