The Belly Crawl
The High Walk
Crocodylian locomotory patterns can be broken down into two different categories: Aquatic and Terrestrial. Since crocodylians are semi-aquatic animals, they have adapted to both life on land and life in the water. Even with this in mind though, there are some forms of locomotion one would not expect from a crocodylian.
Even though they have been described as slow lumbering animals when on land they really should not be viewed that way as crocodylians have a variety of different ways to move when on terra firma.
The belly crawl is the one that most people notice, when viewing crocodylians. In the belly crawl the legs are splayed out to the sides and move about in an oar like fashion. This is normally done slowly and for a short distance (it's mainly used to get in the water), but crocodylians are also able to move fast in a belly crawl. In this process the animal both moves it's body sinuously and slightly up and down. The legs still move sideways, but in a more diagonal progression so that the body rises and falls as the animal moves. The faster the crocodylian goes, the more sinuous the movements. A fast moving crocodylian in a belly crawl has it's entire body thrashing from side to side, which might look ungainly at first, but when one sees the distance covered by it, this "ungainly" movement can become quite fluid.
Now the common analogy used for the belly crawl is that of the way in which a lizard moves. It is usually stated that when a lizard moves with its splayed limbs, it doesn't raise its body that far off the ground and is usually stuck dragging its stomach and tail behind, but this is only a half truth. Some of the shorter legged lizards such as anguids, have bodies that are very long, and legs that are very short. The effect of this bodyplan is, of course, that the lizards drag most of their bodies on the ground when walking. On the other hand, we have more common longer legged lizards such as the varanids and iguanians, which are able to not only carry their bodies off the ground, but also around two thirds of their tails too. Turtles, with the exception of the fully aquatic variety, are all able to carry their bodies off the ground.
Then of course there are snakes, but they sort of speak for themselves in that department.
The belly crawl is the most commonly seen form of locomotion in crocodylians, not so much for it's frequency of use, as it is the times at which they usually viewed (e.g. resting and sliding into the water).
One of the more popular characteristics of being a reptile is that of having a sprawling stance, but not all reptiles are the same, and here one witnessess one of the transitional elements of crocodylians. For along with the more typically seen sprawling gait, crocodylians can also walk in a near to fully erect stance (depending on species). They accomplish this by using a suite of characteristics which one can learn about here.
Crocodylians use the "high walk", as it is termed, to traverse longer distances than would be comfortably possible with the belly crawl. By raising their bodies this much higher above the ground, they are able to see farther and avoid obstacles such as rocks and sticks. By having the legs directly underneath the body, these animals can more easily support their great bulk.
High walks generally don't go very far or very fast. They tend to be used just to get from one basking spot to another or to the water. Still there are instances where crocodylians (e.g. Mugger & Nile crocs) have traversed many miles overland. Their "high walks" make this mode possible.
Yes, you heard right, crocodylians can gallop. This is the most amazing form of crocodylian locomotion, for unlike the usual ways of walking employed by most vertebrates, where the legs move - right fore - left hind - left fore - right hind - and so on producing a stable tripod stance, in the gallop, the legs move completely different.
In this gait the back limbs shove off pushing the animal up and forward. Then the back straightens and the arms outstretch to catch the body as it reaches the ground. Now the back bends and the hind limbs swing forward to grip the ground and shoot the animal forward again
So now instead of the stable tripod, the animal is in the more unstable bipodal position with only two legs touching the ground at a time
Of course seeing pictures of crocodylians galloping, while cool, is nothing compared to actually seeing them in action. So for those who are interested, Adam Britton of crocodilian.com has taken digital movies of a Crocodylus johnstoni in a gallop along with other movies of croc behaviour.
To view the movies head on over to crocodilian.com's crocs on film site.
The gallop of a crocodylian is different from that of a horse and looks more like that of a cat. It is still quite a sight and produces speeds of up to 3-17 kilometers (2-10 miles) per hour.
The gallop is never (or at least has never been observed) used in hunting and is employed only by a crocodile that is frightened and attempting to flee. This form of locomotion is seen in the smaller species of crocodylian and can probably be used in the young of all species, but as the animals get bigger the chances of them doing this falls shorter. To date, there has been no documented case of a galloping adult.
Another uncommonly known thing that crocodylians can do is jump. This is a behaviour that has been observed best in the Colombian crocodile (Crocodylus rhombifer) although there is no reason to doubt that it happens in other crocodylians too (as the picture above demonstrates)
To accomplish this the croc firmly places its hindlegs on the ground and lifts first up with its forelegs and then its hind. This is the same way as a dog or cat would do it, just on a MUCH larger scale. This is probably used to get things hanging in branches and such, maybe even a low flying bird. Both juvenile and adults Colombian crocs have been seen doing this, although it gets increasingly rarer the larger the adult.
Though lacking any arboreal (or even scansorial) adaptations, there have been occasional cases where one has found crocodylians up bushes and trees. These are mostly seen in alligatoroids (probably due to the more coverage given to them) and in youngsters at that. The animals don't usually climb too high (which is good since they lack any grasping appendages) and it is probably done to get at a better basking spot or in search of food and avoid predators. Crocodylians have been seen climbing everything from ferns and other low hanging brush, to potted plants and chain linked fence.
Now for the second portion of crocodylian locomotion:
This form of locomotion, no doubt, deserves the most coverage, since this is what extant crocodylians are most adapted for.
When crocodylians are swimming around lazily enjoying the day they can be seen on the surface of the water with their arms and legs outstretched to keep them from rolling over.
The tail is either not moving (if in calm water) or is in a constant state of flux (if in moving water) as the crocodylian continually makes minute changes to compensate for the moving water. The entire upper portion of the body is usually out of the water and the animal can just basically observe everything thats going on. By making slight kicks to the side with it's legs and by swishing the tail slightly the animal is capable of turning itself around.
All of this is for relaxation purposes only and is never used in hunting or hiding (since the main goal of both is to stay hidden). The other position crocodylians stay in above water is the one used in hunting. The body is held near vertical underwater while the upper portion of the head (containing the five basic senses) on top. The limbs are either held away or underneath the body for stabilzation purposes and the tail doesn't move (or moves slightly). In this position the crocodylian can survey it's potential prey without getting caught. The vertical position allows better control for diving which just so happens to be next.
Crocodylians underwater move in two different ways. The first way is by means of the limbs. In this form the tail becomes secondary. The crocodylian basically walks along the bottom of the river or lake bed, taking the normal step and kicking off almost like it were walking on land. Sometimes crocodylians will only use their longer hindlegs for this and leave the forelegs tucked near the body. This form of locomotion is for relaxation purposes mostly and can only be done in areas without any strong currents. For strong currents crocodylians do something different.
When moving fast or against a current, crocodylians tuck their limbs to their body and use the tail as the main propulsionary unit. This is decreases drag from the limbs and increases underwater speed. Hunting crocodylians employ this mode when moving into a new position quickly and dealing with fast moving water. For the most part crocodylians avoid areas with fast moving or choppy water. This probably has to do with taking in air. Whereas a crocodylian in need of a breath can just stick the tip of it's noise out in calm water, it needs to raise the entire head up in choppy water. So crocodylians probably aim for the calm water just for the ease of breathing. A crocodylian that is moving very fast underwater will be undulating it's entire body. The speeds reached are pretty impressive, around 50 kilometers (30 mph's) per hour.
Leaping, lunging and tail walking
Seeing how closely related these last three are, they will all get covered under this section.
This occurs when crocodylians have built up enough speed underwater that they are able to launch themselves up and out at any object in their path. Commonly used to attack oncoming rivals during the mating season, the animals can close the gap faster this way than underwater (less friction). It is also used for fast attacks on shore animals. This usually happens when the animal has detected the crocodylian's presence and is attempting to flee. A leap can carry a crocodylian several times it's length onto shore.
This is a lot like leaping except it's used to grab prey near the water. The animal digs it's hindlegs in the embankment and curls the tail. Then at the right moment it surges forward by rotating the legs back and unfurling the tail (much like a spring). The main difference between this and leaping is that it is done from a stationary position. A crocodylian doing this can cover many feet in mere seconds. Once beaching itself the crocodylian may continue it's attempt to capture it's prey by once again swinging the rear legs and launching itself forward some more. It may even anchor it's head to the ground in the process of swinging the legs forward as it continues it's attempt. Eventually this might be turned into a run. By doing this crocodylians have been seen running near vertical up small cliffs reaching in upwards of 1.5 meters (5 feet). Mind you this is all going on in seconds of time. One minute a the animal is in the water and the next it could be 5 or 6 meters from the water. An impressive, if not downright shocking, move.
This form of movement is the one that the tourist industry loves. It is used by crocodylians attempting to snatch low flying birds and animals in trees. Oh yes and of course lunchmeat held out by tour operators on tour boats in Australia. In order to do this the croc usually will station itself vertically underwater with the tail curved back against the ground and the legs tensed up like springs. When the animal spots it's prey above the water it jumps with it's hind legs and pushes off with it's tail. This shoots the animal straight up and at it's prey. These tail walks can take a crocodylian many feet into the air. Young animals are able to shoot their entire bodies out of the air plus several feet more. Adults are usually capable of getting all the way up to their hind legs. The difference in height achieved by both young and old is probably nill. A crocodylian doing this is pretty good at judging the distance needed to reach it's prefered prey. Tail walking can even be done when moving although the angle needed to be reached is so steep that it could only be done in deep water. By doing this with the momentum achieved by swimming the animals can reach even loftier heights. The tail walk, much like the gallop, is an amazing, but rare, thing to see, unless of course your on a tour boat:)