Walking, Running, And Crawling

Crocodilians, individually and collectively, have a diversity of locomotor patterns. When moving unhurried on land, a crocodile proceeds with a stately high walk but, if frightened it will plunge down an embankment in an inelegant belly crawl. A few species can even gallop across a beach with hare-like bounds. No crocodilian however shows any skills as a climber and certainly none has developed skills as an aerialist.

The high walk of the crocodilians is unique among the living reptiles. Turtles and lizards walk with a sprawled posture; the limbs project outward from the body, rather than downward, so that the body is hardly raised above the ground. In contrast, the limbs of a bird or mammal extend directly beneath the body and elevate it high off the ground. The high walk of crocodilians is much more similar to the mammalian walking posture than to the typical sprawled reptile one. Crocodilians hold their limbs nearly, although not completely, vertical beneath the body, resulting in an elevated posture that is high enough to raise much of the tail off the ground.

The walking gait of the crocodilians shows the same sequence of limb movements as in all four- limbed animals – right fore, left hind, left fore, right hind, and so on in sequence. This diagonal sequence provides a well- balanced tripod of support as the animal moves forward. With an increase in speed, the diagonally opposite limbs begin to move forward almost simultaneously and faster in a trot-like gait. The resulting bipedal support is less stable but the quick back and forth shift between diagonal support pairs establishes a dynamic equilibrium.

However, if the limbs move too fast this equilibrium is lost and the crocodilian, especially if it is an adult, appears to stumble into a new mode of locomotion, crashing onto its chest and belly with the limbs splayed to the sides. With side-to-side twists of the body and rowing-like swings of the limbs, the crocodilian thrashes and slides on its belly into the water. This belly crawl is most effective on steep shorelines for fast escape but can also be used to slip quietly into the water.

In smaller crocodiles the running gait can change into a bouncing gallop – the hind limbs push the crocodile forward in a leap; the body straightens; the forelimbs extend and catch the body at the end of the leap; the hind limbs swing forward as the back bends; and then the hind limbs leap the animal forward as the back bends. This bouncing gallop produces speeds of 3- 17 kilometers (2-10 miles) an hour, not the 15-30 kilometers (9 – 19 miles) an hour of some mammals, but consistently faster than the 0.3- 4.5 kilometers (0.2 – 3 miles) an hour of the crocodilian high walk.



Posted in Structure and Function

Thermoregulation

Crocodilians are poikilothermic, that is, they rely on external sources of heat to raise their body temperatures. Unlike mammals, crocodilians lack insulation (such as hair or feathers) and have no internal means (such as shivering) to raise their body temperature above that of the environment. Consequently, crocodilians are today restricted to those parts of the world where the average temperature of the coldest month of the year does not drop below 10 – 15 degrees Celsius (50 – 59 degrees Fahrenheit).

Body temperature depends upon heat exchange between an animal and its environment, and is strongly influenced by solar radiation and conduction in water. Water can be used both as a source of heat during periods of low temperature and as a heat sink when the animal is too warm. Large crocodilians are influenced only slightly by ambient air temperatures. However, as body size decreases body temperature is more dependent on the convective environment.

Thermoregulation (the raising and lowering of body temperature) of crocodilians is determined primarily by behavior. Thermal selection (choosing temperature regimes to regulate temperatures) is a complex behavioral process that varies among different species of crocodilians, especially those with temperate and tropical ranges, and is also a function of a number of influences on individual animals. Size, sex, feeding activity, health, and social behavior all influence thermal behavior.

Specific heat-seeking and heat-avoiding behaviors on land and in water are the primary methods of regulating body temperatures in crocodilians. The significant differences in the thermal behavior of different species result in correspondingly different body temperatures.

Crocodiles surface in the water in the early morning for a brief period then submerge and remain submerged for most of the day. To aid in cooling they move onto land at night. In winter Indopacific crocodiles and Common Caimans spend more time basking during the day. Common Caimans and American Crocodiles will continue to bask in winter even when air temperatures are lower than water temperatures. A consequence of these different thermal behaviors is that tropical species have lower and more variable body temperatures than temperate species.

Contrary to many published reports, crocodilians do not survive cold winter temperatures by hibernating. Most species remain active during the warmer periods of winter. External and internal factors modify crocodilian thermoregulation. Circadian rhythms, climatic conditions, social interactions, and reproductive state all influence thermal behavior. Selected body temperatures are affected by nutritional status, age, health, social context, and the temperature of incubation for a particular animal. Crocodilians move between land and water as a response to light-dark cycles as well as to seasonal differences in temperature. Larger animals may exhibit social dominance over smaller animals and may keep them from areas with preferred temperature regions. A variety of behaviors associated with reproduction, especially courtship and nesting, may also take precedence over thermal behaviors.



Posted in Structure and Function

The Senses

Crocodilians have a small but complex brain. The outer covering or cortex of the cerebrum contains nerve endings for intelligence or learning. The cerebrum also controls the senses and voluntary actions. The cerebellum controls the involuntary actions of muscular balance and coordination. The medulla controls the activity of internal organs and glands. The olfactory lobe is for the sense of smell and the optic lobe for the sense of sight.

Unlike other senses, which respond to physical stimuli, taste and smell are responses to chemical stimuli. The sense of smell in crocodilians is well developed. Olfactory nerve endings lie in nasal cavities that open into paired nostrils placed on top near the front tip of the upper jaw. A chemosensory organ near the roof of the mouth present in other reptiles is much reduced in crocodilians. In laboratory experiments, juveniles responded to airborne odors from the cloacal gland secretions of adult males.

Crocodilians have a fine sense of hearing and quite a variety of acoustic behaviors. They have external ear openings covered with a moveable flap to reduce water intrusion during diving. The inner ear is well developed, and hearing is accomplished by a very thin rod (stapes) that extends from the inner ear to the eardrum (tympanum). Crocodilians vocalize (or respond to vocalizations) in a variety of contexts. Adults of some species bellow during the courtship season, larger animals hiss or snarl warnings at intruders, and smaller ones bark “distress calls” to adults. Tape recordings of hatching crocodiles still in eggs have been used to elicit nest- opening behavior by adults attending nests. Adult crocodilians will also charge humans holding a vocalizing hatchling.

Crocodilians have typical vertebrae eyes with a rigid, spherical eyeball enclosing the light responsive retina. The eyes are set laterally on the top of the head and are more prominent in alligators and caimans than in crocodiles. The eyes have a vertical pupil that can open wider at night to allow more light to enter than would be possible in a rounder pupil. An additional adaptation is the reflective layer (tapetum lucidum) that lies behind the retina. The cells of the tapetum contain guanine crystals, which form a mirror like layer reflecting most of the incoming light back through the light- receptor cells of the eye (producing the characteristic yellow-orange- red eyeshine familiar to nighttime crocodile observers). Light receptors include both cones and the more numerous rods. So presumably crocodiles can see colors.

Crocodilians have well developed movable eyelids and a transparent third eyelid called a nictitating membrane. Although covering the eye while the crocodile is submerged, the membrane does not allow the eye to focus underwater. Crocodilians apparently use vision to capture prey above the surface of the water but must rely on other sensory cues underwater.



Posted in Structure and Function

Croc Structure and Function

Crocodilians are the most advanced of all reptiles. Most of these “advancements”, such as the four chambered heart, diaphragm, and cerebral cortex are internal, while their primitive external morphology reflects their primarily aquatic habitats. Crocodilians are elongated, armored, and lizard-like, with a muscular, laterally shaped tail used in swimming. The snout is also elongated, with the nostrils set at the end to allow breathing while most of the body remains submerged in water.

Their skin is formed by a thick dermal layer covered with non overlapping epidermal scales or scutes. The surface layer of each scute sloughs off individually rather than shedding in large patches. Bony buttons or plates called osteoderms are embedded in the skin, forming a dorsal armor. The teeth are thecodont, that is, they are set in sockets in the jaw rather than fused to the top (acrodont) or side (pleurodont) of the jaw as in other reptiles. The crocodilian skull is diapsid with two openings in the temporal area that allow expansion of the jaw muscles.

The snouts of crocodilians range from broad (alligators) to very narrow (gharials) and, to some extent, are related to the major components of their diets-narrower snouts usually indicating fish eating species. Crocodilians have two pairs of short legs with five toes on the front feet and four toes on the hind feet; the toes on all feet are partially webbed. The success of this body design is evidenced by the relatively few changes that have occurred since crocodilians first appeared in the late Triassic, about 200 million years ago. This is merely a summery of the physical characteristics of the crocodilian; more detail is expressed in further pages of this website.



Posted in Structure and Function

Respiration and Circulation

Crocodilians are adapted to breathing in an aquatic environment. The nostrils are located on the tip of an elongated snout and close reflexively against incoming water during dives. Like mammals, crocodilians have a well- developed secondary plate made up of maxillary, palatine, and pterygoid bones meeting in the roof of the mouth. The nasal passages extend above the secondary palate, opening directly into the throat behind a valve formed by a fleshy fold at the back of the palate meeting with a similar fold on the tongue. This separates water from inspired air, allowing the crocodile to breathe while submerged (except for the nares) or while holding prey.

Crocodilian lungs have numerous large chambers, each filled with many smaller individual chambers, giving these advanced reptilian lungs a spongy appearance due to pockets of trapped air. The lungs are filled by suction as the trunk muscles raise the ribs, expanding the body cavity. As the air pressure in the cavity around the lung decreases, atmospheric pressure inflates the lungs.

Crocodilians have a counterpart to the mammalian diaphragm – a muscular septum separating the lung cavity from the periodontal cavity. This system provides for more effective lung ventilation. Unlike other living reptiles, the crocodilian has four chambers in the heart, as in mammals. A septum completely divides the ventricle, separating the flow of oxygenated blood from deoxygenated blood within the heart. Some mixing of the blood occurs when the arteries carrying oxygenated blood from the left ventricle “communicate” with arteries from the right ventricle via the foramen of Panizza.

Crocodilians can adjust their peripheral circulation in response to temperature, movement, diving, or fear. Obviously an increase in peripheral blood flow would be an advantage to an animal heating up for it would maximize heat transfer from the exterior to the body core; a decrease in peripheral blood flow would reduce heat loss in cool periods. Crocodiles may decrease peripheral circulation during diving, which would reduce flow to muscles while maintaining an oxygenated blood supply for the heat and brain.

Crocodilians have two main physiological responses for adjusting peripheral blood flow. They can speed up (tachycardia) or slow down (bradycardia) their heart rate, and/or they can widen (vasodilate) or narrow (vasoconstrict) their blood vessels. By using these responses in combination crocodilians can effectively control the flow of blood and, consequently, oxygen and heat to different portions of the body.



Posted in Structure and Function

Locomotion how the crocs move

How crocodilians move is very much a consequence of their body form. Although crocodilians spend a great deal of time on land, they are usually basking there with little movement. However, crocodilians do move on land to satisfy thermal or reproductive requirements, to move from one body of water to another, to escape from disturbance, or to feed.

Although not equal to their grace and ease of movement in water, terrestrial locomotion can be surprisingly coordinated and rapid. There are three distinct styles of movement on land – the high walk, the gallop, and the scuttling belly run.

In contrast to their limited ability to move on land, crocodilians are excellent swimmers. Swimming (both at the surface and underwater) is effected by the lateral S-shaped undulations of the tail. The limbs are held closely against the body, streamlining the profile and reducing drag. Crocodilians generally cruise slowly with a gentle sweeping motion of the tail. However, when being chased or chasing prey a crocodile can move quite rapidly, even leaping out of the water in a ” tail walk” reminiscent of dolphins. When stationary the limbs are held away from the body, apparently to aid in maintaining or adjusting position.

Despite their excellent swimming abilities Crocodilians, for the most part, avoid areas with strong wind and wave action. In calm water a crocodile needs to keep only its nasal disc above the surface of the water to breathe. In rough water the snout has to be raised at a steep angle above the surface during inspiration. This apparently makes swimming more difficult. Crocodilians therefore seek protected water and frequently come ashore when calm water is not available.



Posted in Structure and Function

Sex Determination

The sex of a bird or mammal is determined at conception by particular sex chromosomes. Not all vertebrates however, have sex chromosomes; in some vertebrates, sex is determined at a later stage of development or growth – certain fish, for example, change sex even as adults, depending on the social situation or the environment. In crocodilians, many turtles, and some lizards, the temperature experienced by the embryo in its egg is a major determinant of hatchling sex, a type of environmental sex determination referred to as temperature dependent sex determination (TSD).

By simply altering or controlling egg temperature during development, it is possible to produce in a predictable manner the desired sex or a predetermined sex ratio. Among crocodilians TSD was first reported in 1982 for American Alligators, based on artificial incubation of eggs at constant temperatures in a laboratory. High temperatures of 32 – 34 degrees Celsius (90 – 93 degrees Fahrenheit) result in male American Alligators; low temperatures of 28 to 30 degrees Celsius (82 – 86) yield females. At intermediate temperatures over a narrow range, both male and female American Alligators are produced in varying proportions.

In the wild, males compromised 60 – 80 percent of the young alligators sampled, whereas among farm – reared animals, only 10 – 25 percent were male. In this instance, artificial incubation of the eggs had resulted in “cooler” temperatures and, consequently, a preponderance of females.
This is different for Crocodiles, because with Crocodiles, females are produced at both Higher and lower temperatures, whereas males are produced at intermediate temperatures. There are differences as such among each species of Crocodilians, and conservation programs must adapt to these differences.

The nest is also an important factor in sex determination. A nesting female will often make trial nests and appears to be very selective in choosing a nesting site. Thermal cues probably play a major role in nest selection and construction. It is not surprising that, in many crocodilian nests, all of the siblings are of the same sex. Sometimes incubation temperatures vary enough within a nest to produce males in the top layer of eggs and females at the bottom or vice versa. The temperature at incubation is dependent on the weather, droughts and low water levels result in higher temperatures, rain and high water levels result in low temperatures.

In the wild, sex ratios are strongly skewed in favor of females, but also fluctuate from year to year. As such, a changing climate could lead to the overproduction of one sex and the eventual extinction of the species.



Posted in Reproduction and Hatchlings

Nest Mortality

Unlike most species of birds that lay relatively small clutches of eggs but may produce two or three broods each breeding season (thus increasing the chances of survival of at least one brood) crocodilians normally lay a large number of eggs but do so only once a year. Mean clutch size varies both within and between species but analysis of clutch size for most species indicates that between 20 and 80 eggs are laid.

Incubation periods for crocodilian eggs tend to be lengthy in comparison with those of birds, and vary from 35 days for small species to 90 – 100 days for larger species. Most female crocodilians guard their nests from predators and may fast for the entire incubation period in order to remain close to the nest. Birds incubate their nests and therefore have some control over the results. They can rapidly rebuild a nest to lay another clutch of eggs in the event of a natural disaster or loss through predation.

Crocodilians however, are not so lucky. The eggs once laid – either in a hole in the ground or within a mound of debris – are subject to the elements and crocodilians have no control over nest conditions, such as temperature of the eggs, humidity in the egg chamber, flooding, or a multitude of other environmental parameters.

Heavy rainfall or even a prolonged cloudy period can drop incubation temperatures below the level required for the embryos to survive. The developing embryos, which depend on oxygen diffusion through porous eggshells, may drown during prolonged periods of heavy rain. Conversely lack of rainfall causing lowered moisture and humidity levels in the nest during extended hot, drought periods will lead to overheating or desiccation of the eggs and a large percentage, or even whole clutches, may perish.

Nests are often built on river banks, marsh impoundment dikes, floating mats of vegetation, lake shores, and other sites subject to flooding and devastation by tropical storms, hurricanes, or typhoons. Although the female may have laid the eggs at a time when substrate and air temperatures were ideal and when water levels were suitable, and although the eggs may have been laid above the normal high-water flood levels, an entire seasons crop of eggs can be destroyed by flooding or lakeside wave action.

Climate, however, is not the sole cause of egg and embryo mortality. Excessive dampness in the nest can encourage fatal fungal growths on the eggs. Noxious gases emanating from putrid eggs in tightly packed and mud covered nests can cause mortality of healthy eggs in the same nest.

Mortality, particularly at colonial nesting sites, can also be attributed to females inadvertently digging up and destroying clutches previously laid by other females. Occasionally a large percentage of eggs in a clutch are pierced by the sharp claws of the females hind feet, and several eggs in a clutch may be squashed or dented by the females careless nesting habits. Turtles utilize the nests of some crocodilians to lay their own eggs and may damage some of the crocodilian eggs.

Thin-shelled eggs that crack or dent easily and even unshelled eggs are occasionally found; these have usually been laid either by very young or very old females. There are also instances of complete hatching failure because the female has not returned to release her offspring, imprisoned beneath a hard- baked layer of clay from which they cannot dig out unaided.



Posted in Reproduction and Hatchlings

Courtship

In all species, the mating system is polygamous; that is each male mates with a number of different females. Large males set up mating territories or establish dominance hierarchies. In other species, breeding takes place within territories occupied throughout the year. The number of reproductive males relative to the number of productive females is known as the operational sex ratio. In crocodilians this ratio varies with social structure and group size.

Establishment of territory and dominance proceeds over courtship and mating. A large male advertises his dominance by swimming conspicuously within his territory with much of his body exposed on the surface, periodically engaging in species-specific assertion displays. Headslapping (explained in the communication page) is often accompanied by vocalizations, exhalations, and/ or subaudible vibrations.

Large males dominate group breeding by patrolling territorial boundaries and by approaching other animals. Subdominant males are actively challenged and excluded by vigorous chases and open-mouthed lunges. Combat between males contesting dominance involves head – to – head physical contact; for example, sparring with the jaws or head ramming and posturing with raised inflated bodies. Head ramming is a particularly brutal display of strength characteristics of Indopacific crocodiles. Two similar- sized males align themselves with their bodies parallel and facing in the same direction. The two first swing their heads together with loud “thud”. These contests apparently do not do much damage and often go on for an hour or more. American crocodiles contesting dominance fight with open jaws directed at the rivals jaw. “Bites” during these encounters don’t do too much damage, although they do serve to hold the rival down while he is overpowered.

Females are tolerated within the territories and indicate submission upon approach by snout lifting, vocalizing with specific sex calls, and/ or by submerging. In some species, females tolerate other females during courtship but sometimes a group of females form their own dominance hierarchies within male territories. Still, in other species the females will defend their individual territories within the male’s territory.

Receptive females typically initiate courtship and tolerate other females nearby as they court and mate. Dominate males often interfere with courtship attempts by subdominant males. After such an interruption, the female will typically resume courtship with the dominate male once he has displaced the subordinate. In large aggregations, females move with impunity between the territories of different males, and may court and mate with several males in succession.

Although female – female aggression is more subdued during courtship than during nesting, social hierarchies form among females and may affect when and how often a female mates with the dominant male.

Courtship and mating consist of a sequence of attraction or advertisement behaviors followed by pair formation, precopulatory behaviors, and finally copulation. Dominate males approach females to initiate courtship or females approach a dominant male, often after a male- male aggressive interaction or an advertisement display. During pair formation and precopulatory activity, males and females engage in a variety of female specific behaviors, which include snout contact, snout lifting, head and body rubbing and riding, conspicuous male displays, vocalizations, exhalations, narial and guttural bubbling, circling, and periodic submergence and reemergence.

Mating occurs when the male mounts the female by moving onto her dorsum, and positioning his tail underneath the female’s tail, and then copulation begins. It is difficult to observe because it takes place underwater, but successful copulation takes several minutes and may last 10 – 15 minutes or longer. In captive groups, a pair may copulate on repeated occasion for several days.



Posted in Reproduction and Hatchlings

The Crocodile God of Ancient Egypt

The ancient Egyptians reverence the crocodiles of the Nile and held them to be sacred. Since crocodiles appeared in great numbers during the annual September flooding of the Nile, the Egyptians associated them with the eternal sustenance of their lands. Sobek (or Souchos), the crocodile-god, was one of 438 ancient Egyptian deities and was though to be the son of Neith, the oldest of the goddesses. Inn the ancient pyramid texts both Sobek and Neith were depicted as gods who would “endure forever”, while the affairs of state and the fortunes of other great gods could fluctuate. Whether the crocodile was first made sacred out of terror or because the god of such fearful beasts was in need of placation is impossible to tell. But certainly by 2400 B.C. Sobek had emerged as one of the great gods.

Sobek was normally depicted with a human body and a crocodiles head. He carried a staff in his left hand and a symbol of eternity (the ankh) in his right. Incorporated in his headdress was the sun disc, since by the time of the New Kingdom (1400 B.C.) Sobek was being worshiped as a manifestation of Ra, the sun god, and was known as Sobek-Ra (Ra was one of the supreme Egyptian gods in whose chariot the deceased god-king pharaohs rode across the sky during the day).

According to Herodotus, the ancient Greek historian, Egyptians in the Nile Delta “treated crocodiles with every kindness”, and young crocodiles were often adopted as pets and carefully mummified at death. Elsewhere however, crocodiles were looked upon as enemies and were hunted for their meat. Hunters at Elephantine on the upper Nile would beat pigs to make them squeal and so lure crocodiles to a bait and hook.

The ancient story of the founding of Crocodilopolis, the major center for the worship of Sobek, may be the source of a later European folk tale – “The Gingerbread Man”. King Menes, the first of the pharaohs, was set upon by his own dogs while out hunting. In his flight he came to Lake Moeris, where a large crocodile was basking in the sun. Apprising the situation quickly, the beast offered the desperate king a ferry service to the site of the city Menes then founded and which the Greeks named Crocodilopolis. The story symbolizes Egypt’s flight from evil (the dogs) and chaos (the water). Crocodiliopolis became a favorite city for the New Kingdom Pharaohs and, under the Greek rulers of Egypt, it became a thriving Metropolis.



Posted in Mythology
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