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Crocodilians as we know are one of the oldest species in the world. They are found in almost every continent of this world. Crocodilians are known for their gigantic body, strong jaw and their hunting skills. Crocodilians have survived over so many years and thus we can see a big evolution in them. Talking about the anatomy we can see that the anatomy of the crocodilians is very interesting, the skull of the crocodilians is so developed that while they are in water they can manage to keep just their nose and eyes out of the water, which helps them to hide themselves from their pray and keeping their senses active. They have got a four chambered heart which helps them to hold their breath for almost 5 hours when they are below water. Their tail plays a very important role when they are in water, it helps them create a hydrodynamic thrust and helps them to propel. We can observe different locomotion patterns in crocodilians which is another interesting area while studding the evolution of the crocodilians. As we know crocodilians are oviparous they have to come out and lay their eggs in a self-dug hole, we can find different digging mechanism in them. The other very interesting phenomenon which we can find in them is that it mechanics of twist feeding in water. Crocodilians usually spin under water to subdue their pray. Research on crocodilians is done from a long time, with various different research techniques like using force platform to study the locomotion pattern, use of transducers to study the bite force of the crocodilians.
Crocodilians belong to the Crocodylidae family. Crocodylidae family which also includes Alligators, Caimans and Gharials along with crocodiles are the closet relative of birds as these are the only two species which survived from Archosauria. The members of this family are basically large reptiles which can be found throughout the tropics of Africa, Asia, America and Australia. Crocodilians usually live in fresh water habitats like rivers, lakes and water lands though there are few types like the saltwater crocodile which are found in brackish water. The origin of crocodilians can be traced back by millions of years so as a result of which lot of interesting anatomical changes can be found in them which helped then to survive over the ages.
Existing crocodilians are the only know living tetrapod which have nearly used full range of “recognised quadrupedal terrestrial locomotion patterns” from highly abducted, laterally undulating ‘sprawling' gait to more erect walking, asymmetrical (bounding and galloping) gaits(1).Crocodilians show an interesting terrestrial walking styles and limb motions that are an important aspect in understanding there history and evolution. Crocodiles are the species often used as a resource to study evolution of locomotion of this diverse group by palaeobiologists.
The study based on the anatomy of crocodilians has one of the longest histories of any taxon (2). First recorded observation of Crocodilians dates back to as early as 440 B.C. by Herodotus (2). Crocodilians are well known for their predator skills. Their large body combined with strong jaws and teeth have helped then in the above aspect. The musculoskeletal anatomy of the Crocodilians is an important factor for its diverse gaits, but only qualitative studies of muscle tendon unit anatomy exists (1).Crocodile have a fairly long tail and short limbs. And unlike most mammals their limbs project sideways from their body. One of the most distinctive features of the crocodile body is its elongated snout. Irrespective of the width of the snout, the head is on an average one seventh of the total length of its body (3). The crocodile body has been evolved in such a way that when in water it can hide itself by exposing a very small portion of its head containing its eyes, cranial platform and nostrils. All the sensory organs are above water thus keeping it alert for any potential prey.
The jaw of the crocodile consists of a single row of teeth that do not meet when its jaws are closed. The teeth of the upper jaw encircle the teeth of the lower jaw which makes then visible when the jaws are closed. Unlike alligators or caimans, an enlarged fourth lower tooth fits into the notch in the upper jaw. The teeth of the crocodile are conical in shape. Crocodile's teeth regrow when they fall off. Crocodile teeth are designed to seize and hold prey and are not adapt at chewing it. Hence crocodile tend to swallow their food when they are small enough. Another interesting fact about the crocodilian jaw is that the set of muscles used to open their jaws are comparatively weak and this is a reason why when captured the crocodilian jaws are tied shut first.
Adult crocodile have blunt teeth which are robust and resistant to bending and shear stresses during impact when compared to their sharper and slender juvenile teeth of same volume (4). These modifications happen simultaneously with the change in the diet of the crocodile. From data's collected from research it was found that muscle cross sectional area determined the muscle force generated, and the bit force of the crocodile depended on the jaw muscle that contributed in the bite. The crocodilian bit force is as high as 9452N (4) which is much higher than that of spotted hyena which has the highest relative and absolute bit force for a carnivores. The relative and absolute bit force of a crocodilian could also be higher than some of the tetrapod dinosaurs (4).
Figure 2 shows the bite force vs. total length of the crocodile. In the figure (a) represents diet of insects and small fishes, (b) represents frogs, small reptiles, (c) represents medium sized fishes, birds, small mammals, (d) represents large fishes, snakes, large birds, (e) represents turtles, medium to large mammals. The slope of the graph increases steadily and there is no sudden changes to it due to change in dietary (4).
Crocodile has got a typical vertebrate eye, having a vertical pupil and a reflective layer behind the retina. The eyes of the Crocodile have evolved in such a way that their eyes have moveable eyelids along with a third transparent eye lid called a nictating membrane which protects their eyes by covering them when they are submerged in water. Eye sight of crocodile is very good when compared to other reptiles.
The nasal disc consists of two nostrils which are on the snout of the crocodile. Each nostril has a protective flap at their opening. The crocodiles have canals that pass through the bone of the snout, connecting the nostrils to the back of the throat. The canals are the chambers through which the crocodile's sense smell (3).
Crocodiles have a heart with four chambers which enables them to stay under water for long period of time without breathing. The four chambered heart prevents the mixing of oxygenated blood from the lungs with the deoxygenated blood from the body and keeps their blood oxygen rich. Crocodile valves have cog teeth which mesh together to control the flow of blood to the body. These cog teeth are controlled by the amount of adrenalin in the blood. According to the current research the cog teeth close when the crocodile is relaxes due to the absence of adrenaline.
The high level of flexibility of the crocodile's feet enables it to support the crocodile in various positions. The front feet of the crocodile are made up of five toes and the back feet has four. The feet's are equipped with webbing which aid the crocodile during swimming. But they are not used by crocodile to propel themself in water and are use more for changing direction and making fast turns in water.
The structure of crocodilian forelimb is similar to that of most other tetrapod, where in extrinsic forelimb musculature attaches the pectoral girdle to the axial skeleton, long with a bony attachment which is present between the coracoid and the interclavicle (2). Extensor compartment and a flexor compartment divide the brachium into two parts. But the nerve structure to each part is not well defined when compared to other tetrapod. The antebrachium is also divided into flexor and extensor compartment, which is similar to that of birds (2).
Tail plays a very important role in tetrapod locomotion on both land and as well as in aquatic habitats. The role of the tail in tetrapod when they are walking on land is that it acts to counter balance an elevated trunk of a biped and the other use is to regulate the slide frequency(5). The orientation of tail also results in improving the limb muscle functions in crocodilians.
Semi-aquatic animals such as crocodilian use its tail for creating hydrodynamic thrust. Tail usually is not as handy to crocodilians on land has it is in water. In land the tail lowers the acceleration of the crocodilians as they are not elevated from the ground and they have to be dragged across the land. In water the tail provides force to propel itself (5). It is only because of their large tail that the crocodilians have become successful in hunting in the water.
The skin of the crocodile is covered with scales of different shapes and sizes. The scales on the belly tend to be square and flat, on the upper back and tail they are raised like ridges and on the side the scales are round with raised centre. The raised section on the back of the crocodile is because of “osteoderms” which are isolated and discrete blocks of bones. The scales on the tail unlike the once on the back do not contain osteoderms, and are only hardened scales called scutes. These scutes on the tail increase the surface area and also aid the crocodile during swimming (3).
Crocodilians have different gaits during locomotion, which include the ones used for sprawling posture, belly walk, the high walk (in juvenile crocodiles) and true gallop. Study of tarsi, pelvis and hind limbs of earliest crocodile conducted showed that these animals had adaptations for erect posture (6).
The high walk is used during most terrestrial activity and is a symmetrical walking gait. It was observed that at least two feet of the crocodilians remained in contact with the ground during high walk. It was found that during high walk peak vertical forces were
significantly higher for hind-limbs than for fore-limbs. Vertical force applied by the tail was also quite high, about 18.5% and 14.4% of hind and fore-limb value (5). Table 1 shows the various values of forces and time parameter in a crocodilian high gait. It was also found by the researches that both hind and fore-limb mediolateral impulse reduced notably with speed (5). During belly walk the crocodile remains on its belly and slides down a slope like the banks of a river with the posterolateral thrusts of it legs. This is used an escape method by sliding down a bank into the water for a hasty retreat (7).
It has been found that in a walking trot the diagonal fore limbs and hind limbs supported an estimated 36.8% and 51.3% of the body weight respectively. And the remaining 11.8% present of the body weight was supported by the crocodilian tail (5).
The crocodilian gallop is an asymmetrical gait and it is not used by the crocodilians often. In this gallop the fore and hind limbs work in serial pairs (7). The crocodilian gallop is of the form of a rotary gallop as there is no support after the hind limb lift off. And it is during this period that the extension of the body takes place. The galloping has mostly been seen in infant crocodilians which are of size less than 2 meters (7). According to researchers size could be a reason for larger crocodilians not galloping (7).
The centre of gravity for a crocodilian is approximately located at 70% of the gleno-acetabular distance. The centre of gravity of a crocodilian is closer to the hip joint than the shoulder joint (5). The ground reaction force for the crocodilians varied about its body weight. Figure 6 shows the ground reaction force during a foot fall. Fx represents the mediolateral force, Fy for crainocaudal force, Fz for vertical forces. The negative and positive values of crainocaudal forces represent the braking and propulsive effort.
The main organ used by the crocodile for swimming is its tail. When swimming the crocodile's hold their legs close to their body and they flex their body in a wave formation from head to tail which provides them the forward thrust. When diving they lift their front limbs up to almost a vertical position helping it to direct its head downwards. The hind limbs function as stabiliser during swimming in water.
Crocodilians have a multi-cameral lung. The lung is characterised by rows of tubular chambers, which are broad and saclike in cranial and ventral lung. There are cubicles (ediwlae) in the inner surface of the chamber of which capillary-bearing walls are often perforated. There is no frequent extra bronchial communication among chambers. The trabecular epithelium is same as that of different reptiles and of other mammals. There is consistent distribution of (64%) non-vascular smooth muscles in trabecular and 36% in interedicular walls, is consistent with the theory that these two antagonistically oriented muscle group come together to enhance lung patency (8).
DIGGING MECHANISM IN CROCODILIANS:
As mentioned above crocodilians are semi aquatic animals. They spend most of their time in water and spend comparatively very less time on lands, mainly for thermoregulatory basking (9). Usually digging activities are not common among crocodilians. As we know crocodilians are oviparous, they have to come out and lay their eggs in a self-dug hole. Crocodilians even dig holes sometimes to burry themselves in extreme cold or draught conditions. It is even sometimes seen that crocodiles dig hole to burry surplus food when the food is abundantly available.
A wide array of digging behaviour can be observed in the process of nest creation by crocodiles. Crocodilians often use hind limbs, fore limbs and snouts for digging while digging the hole for their nests, it is even seen that in the grassy terrain crocodiles even use their jaw to dig hole. Crocodilians, while digging holes in order to make chambers for their eggs uses “alternating scooping movement” (9) of the board; and in the process of excavating the eggs “alternating strokes of forefeet” are used (9). Varity of behaviours such as biting, vigorous thrashing of the body and tail and digging with the snouts and hind limbs (9) can be found at the time of nest opening.
We can see scratching strokes of fore limbs and ramming thrust of the head by few species of crocodilian family in the process of den formation, in adverse time. In some other species of crocodilian it can be seen that along with using the forefeet and hind feet they use their tail for excavation of soil in the process of den formation. By using all the above techniques a crocodile can create a den of about 10m long and 3m deep (9). It is found that due to the short limbs and well developed appendicular muscle help crocodilians to perform their digging operations.
MECHANICS OF TWIST FEEDING IN WATER:
Crocodilians do a spinning manoeuvre when in water to subdue their prey. This manoeuvre is also known as death roll. During this manoeuvre it performs a rapid rotation along the longitudinal axis of its body. The crocodilians get there hind limbs and fore limbs close to their body and its head and tail will be canted with reference to its bodies longitudinal axis. The angle of bend of its head and tail with respect to its body was averaged to be 49.20 and 103.30 respectively (10).
The figure 7 show the rotation pattern in crocodilians during death roll. This manoeuvre not only aids the crocodilians in subduing their prey but also helps them to reduce the pray to small pieces. The crocodilian teeth are good for grasping their pray, but are not very adept at chewing hence when spinning it helps them tear off the pieces and swallow. Spinning motion can help in tearing apart large pray as their tissues are weak in torsion (10).
Figure 8 shows the variation of shear force generated during death roll based on the length of the crocodilian at different rotational speed.
RESEARCH TECHNIQUES USED FOR STUDYING CROCODILIANS:
There are quite a few techniques that are developed and used in the study of crocodilians, and are used depending of the situation. The most important factor that has to be taken into account while selecting the research techniques is the type of data that is required to conduct the analysis
In studying the gait of the crocodilians, Alligators of a specified weight range were kept understudy and were made to walk over a period of time. The alligators were made walk across Kistler force platform (5) which was placed on the trail. Figure 9 shows the different patterns used to record the ground force exerted by different parts of the body. To record the trail the researchers used Motionscope 500 camera which recorded at 250 frames per second (5). The recording was used to calculate mean forward speed, stride duration and footfall pattern. Hildebrand gait graph is used to determine kinematic gait by plotting limb phase against duty factor (5).
When studying the bite pattern of the crocodilians the researchers needed access to crocodilians of larger age group as the bite pattern and tooth structure differed a lot in juvenile and adult crocodilians. Different types of transducers are used as to fit within the mouth of the crocodilian. Transducers are basically fitted with strain gauges or piezoelectric load washers are used. Table 2 show some of the transducer types that are used in determining bite force.
In studying the death roll in crocodilians, juvenile are often used as it is difficult to record data from fully grown crocodilians. The data obtain from the juvenile are than scaled for larger crocodilians using certain mathematical functions. High speed video cameras are used to record the movements (10).
Most of the cases the studies are based on crocodilians bred in captivity. This is because it is highly difficult to capture and conduct studies on crocodilians in the wild. The breeding habitats are kept in a highly controlled manner when conducting research. The feeding pattern and diets are regulated during research periods. Three dimensional analysis packages in mechanics like Ansys are often used as the initial step in understanding the structure and stress concentration in a biological structure (11). This provides information as to from where to gather the required data.