Legs
Bipedal Hopping
Both hopping and skipping are saltorial motions in which the front limbs remain elevated off the ground, and the resulting track patterns include paired hind footprints. Few mammals use bipedal motion, and in California the most likely culprit will be a kangaroo rat or kangaroo mouse. Birds, too, hop and skip. In hopping trails, paired hind tracks appear right next to each other, or nearly so, and the gait is the typical kangaroo-style hop. This pattern is possible because both hind feet hit the ground simultaneously. Technically, feet would only truly hit simultaneously if the feet were placed exactly next to each other, or the animal were coming straight down. However, for our purposes we'll use the word “simultaneously” to mean at the same time, or nearly so.
Bipedal Skipping
In skipping trails, tracks are also paired, but each hind foot lands completely independently of the other. Looking at the trail pattern, a hop becomes a skip when one hind foot registers completely in front of the other hind track. When kangaroo rats skip, they stay very close to the ground and take very long strides. Their hind feet rotate forward, one striking down before the other, and as the body moves forward over this foot, the second foot touches down. Their momentum continues, propelling the body forward over the second foot. As the body continues forward, the first foot to have touched down lifts up behind the animal. Continuing forward, the second foot joins the first behind the animal, then the animal lifts off, and together the hind feet rotate forward to begin another cycle. Momentum is more horizontal than vertical, and very little energy is wasted in rise.
Interpretation of Track Patterns
Until now we have discussed how an animal is moving in the technical sense of which limbs are moving when and in what order limbs make contact with the ground. Next, we leap into speculative interpretation based upon our initial understanding of how an animal is moving, and ask ourselves why an animal is moving in a particular way. Because we can never know what an animal was thinking or exactly what an animal was doing at a particular moment (unless we actually witnessed the event), we speculate on its behavior and make our best guess. As in all aspects of tracking, we build a working hypothesis and test it as we continue to follow the animal. As we gather more information as we follow the animal, we either toss out our original hypothesis and create a new one, or continue to refine and support it.
Natural Rhythms and Energy Efficiency
Gaits and their associated track patterns are reflective of energy conservation, substrate, and behavior. Energy efficiency is a tremendously important variable in interpreting trails, as well as in predicting how a given animal will move in a given depth of substrate. It is certainly true that the slower an animal moves, the less energy it expends; however, it's more complicated than this. Every species possesses an anatomy, locomotion, and set of behaviors that improve its fitness. Fitness is quite simply the quality of success experienced by an animal, and in ecology, fitness is measured by how well an animal reproduces and perpetuates its genetic lineage. An animal has high fitness when it produces numerous offspring that themselves produce many offspring. Think of the measure of success as the number of grandchildren an animal has, because having grandchildren indicates successful production of offspring that were, in turn, able to survive and reproduce.
To simplify things further, let us focus upon only energy input and output. Each species balances its energy intake, meaning food collection, and energy output, which includes the ground covered to locate and/or pursue said food. Weasels have high metabolisms, insatiable appetites to meet their energy requirements, and gait preferences that propel them quickly over long distances. A weasel that walks too often may save energy in the short term but is denying that which makes it a weasel, and will die of starvation in the long term because it does not gather enough food to meet its energetic requirements. The balance point between energy intake and output influences the ways animals move, and the common gait that an animal has adapted to use for traveling is called its natural rhythm (Elbroch 2003). Let's compare Bobcats with Coyotes.
Each species has a body structure and biology that balances its individual energy intake and energy expenditure. Bobcats walk. Cats move through the forest slowly and stealthily, sitting and pausing frequently to study their environment, hoping to see or sense potential prey before they themselves are noticed. They may even lie in wait for prey and do not generally cover long distances while hunting. When a potential prey is selected, they stalk in and, when close enough, explode with enough speed to catch their intended victim before it is aware of them, or before it can escape. They grip their prey with their curved claws and deliver a killing bite to the head or neck.
On the left is the trail of a trotting Coyote and the animal's resulting drag lines, and on the right, the clean walking trail of a Bobcat. Study the distance between footprints and the width of the trails in both animals.
Coyotes trot through the bush, hoping to catch smell, sound, or sight of potential prey, or to startle something to flee before them. Of course Coyotes do occasionally stalk, but in general, Coyotes cruise longer distances than medium-sized cats, allowing scents and sounds to betray the presence of prey species. When opportunities present themselves, they run down their intended prey, gripping and subduing it with their teeth. Their claws, which are never sheathed, project straight out from their toes and aid in traction while running.
Can Coyotes walk? Of course. Can Bobcats trot? Yes. Every animal is capable of a variety of gaits and speeds; however, each animal will have one or two gaits that are the most energy efficient for them, and they use these most of the time. Their common gaits are their natural rhythm.
Understanding the natural rhythms of animals is important, because it gives us a place to start. Think of an animal's natural rhythm as its normal speed, or middle ground. Each time an animal shifts from its normal speed, there is a reason and thus an opportunity for us to speculate why. Consider yourself as a starting point to better grasp how one might approach trail interpretation. You have your typical walking gait, speed, and subsequent track pattern. Should you be late for work or very focused upon a specific destination, your pace and track pattern will change. If you are hungry and stand within sight of five great restaurants, you will probably wander a bit and then move with determination once you have decided where to dine. You jump if you are scared and run when your life is threatened. The list of possibilities goes on and on. Why wouldn't all these sorts of changes be apparent in wild animal trails? The answer is that they are.
Certain gaits are associated with specific environmental conditions (substrates), while others betray behaviors or intentions. From a series of footprints you can determine whether an animal feels exposed and potentially uncomfortable, or whether it is hunting. And if you really become familiar with an area and its inhabitants, the way an animal behaves and moves may betray that it is out of its usual territory, or allow you to identify a transient or trespasser.
Interpretation of track patterns is advanced tracking and only comes with experience and lots of mistakes. The more natural history information you have about a species and the more time you spend trailing a particular animal, the better armed you are for this process. Regardless of experience, there will always be trails that will perplex you. Be reconciled with the fact that tracking is not a perfect science but a lifetime of learning.
Beginning Interpretation
With experience, interpretation can be highly detailed, and we discuss advanced aspects of interpretation later in the book. Here we encourage you to begin by interpreting the way an animal is moving as one of three categories: slow, normal (natural rhythm), or fast.
1. Slow—a speed slower than its normal gait (natural rhythm), that might indicate foraging, stalking away from danger, hunting, scent marking, exploring another animal's scent post, or numerous other potential behaviors.
2. Normal—its natural rhythm, the common gait in which an animal moves. The animal is exhibiting little or no undue