Crocodiles

Crocodile Facts

Classification:

Kingdom: Animalia

Subkingdom: Bilateria

Phylum: Chordata

Subphylum: Craniata

Class: Reptilia

Subclass: Archosauria

Order: Crocodyla

Suborder: Eusuchia

Families: Alligatoridae (alligators and caimans, four genera, seven species), Crocodilidae (crocodiles, three genera, fourteen species), Gavialidae (garials, two genera, two species)

Geographical location: Every continent except Antarctica and Europe

Habitat: Variable depending on species; freshwater lakes, rivers, streams and swamps, estuaries, and oceans

Gestational period: Approximately three months

Life span: Up to seventy years

Special anatomy: Quadrupedal; biserial dermal armor consisting of two rows of osteoderms running on either side of the vertebral column; secondary palate with fleshy valve; ear flap; verticalized braincase; elongated carpals; pubic bone is excluded from the acetabulum; extensive air sinus systems extending from the Eustachian tubes to the middle-ear cavity and skull bones

Classically, the order Crocodilia is divided into six suborders: Protosuchia, Archaeosuchia, Mesosuchia, Thalattosuchia, Sebecosuchia, and Eusuchia. Crocodilia are familiar inhabitants of rivers, swamps, and lakes, distributed in the tropical and subtropical regions of North and South America, the Caribbean, Africa, Asia, and Australia. However, crocodilians did not begin their evolution as amphibious reptiles. The earliest recognizable crocodilians arose in the Upper Triassic and were terrestrial in nature. Protosuchians are small, averaging below two meters in length. They have terminal nostrils, a single row of dermal armor on either side of the vertebral column and elongated carpal bones and hindlimbs. They show other classic morphological features of crocodilians—the pubic bone does not contribute to the formation of the hip socket as in all other reptiles. Before giving rise to Mesosuchia, the first amphibious crocodilians, the protosuchids, gave rise to another side group— Notosuchia. Mesosuchia appear to be the ancestors of the living Eusuchia. The Thalattosuchia are Mesozoic marine crocodilians, most of which have longirostrine (having long, thin jaws) mandibles adapted for fish eating. Some Thalattosuchia, the geosaurs, were well adapted to the marine environment, having reevolved a tail fin for aquatic locomotion.

The living Crocodilia are all placed within the suborder Eusuchia. They are divided into three families, the Gavailidae, Alligatoridae, and Crocodylidae, the latter with its two subfamilies, the Crocodylinae, and the Tomistominae. The gharials or gavials of Nepal and India comprise the family Gavialidae. They are longirostrine crocodilians specialized for fish eating and grow to nearly nine meters. Their limbs are reduced more than in most crocodilians, and they are unable to climb over obstacles of more than a half meter high. The Crocodylidae are divided into alligatorines, which include the American and Chinese alligator and the caimans, and the Crocodylinae, which include members of the genus Crocodilus, Osteolaemis, and Tomistoma. The placement of the fourth mandibular tooth in crocodilians has often been cited as the difference between these families, in that in alligatorines the fourth mandibular tooth resides in a socket in the upper jaw when the mouth is closed. Within the crocodiles this tooth fits in a groove in the upper jaw and thus is visible when the jaw is closed. The longirostrine crocodilians, such as gharials, tomistomines, and Crocodilus johnstoni, have longirostrine jaws and have interdigitating teeth. Thus, the tooth character is equivocal in these taxa. Another method of dividing the living forms into the subfamilies has to do with salt tolerance. With one exception alligatorines are salt intolerant. Crocodiles are often found in brackish waters and some, such as Crocodilus porosus, have been found hundreds of miles off the coast of Australia. Unlike alligatorines, crocodiles have well-developed salt glands in the tongue. Gharials, although not marine, have salt glands in the tongue as well as an orbital salt gland. This trait may indicate an oceanic origin for gharials.

In 2008, archaeologist Richard Oslisly, while exploring remote caves in the African country of Gabon, discovered a number of African dwarf crocodiles that seemed to display behavior previously unobserved and atypical of the species. After further study, including return trips with crocodile experts that involved taking blood samples and sequencing DNA, the researchers hypothesized that the crocodiles were likely evolving to adapt to life in the harsh environment of the caves, surviving in the darkness by eating bats and crickets. As testing revealed that the cave crocodiles had a set of genes not found in the African dwarf crocodiles living outside of the caves, they theorized that the crocodiles may be mutating to form a new species. Further studies were being conducted as of 2018 to determine whether this is the case and what the implications may be.

Aquatic Adaptations

Eusuchians are better adapted to the aquatic environment than their mesosuchian ancestors. The secondary palate is well developed, extending the internal nostrils back to the pharynx. The external nostrils face dorsally so the crocodiles can stay hidden in the water. Here, a fleshy valve separates the mouth from the pharynx and the internal nostril openings. Eusuchians also have an earflap or Ohr Klappe that closes off the external ear from the water during dives. The Eusuchia have enhanced the dermal armor and have a biserial or double row of osteoderms running on either side of the vertebral column. This dermal armor was once thought to be evolved for protection, but this system is tied to back muscles that aid the crocodilians in bending the vertebral column for locomotion. Dermal armor was widespread in early archosaurs but is best developed in eusuchian crocodilians. All crocodilians have a strong bite. Although crocodilians have jaw adductor muscles in the temporal region of the skull, the largest jaw adductor originates on the palate. Except for fish-eating species that have sharp, curved, conical teeth, most crocodilians have more squat-shaped teeth in the rear of the jaw for crushing. Crocodilians will often position turtles in the posterior region of the jaw to crush the shells of those they cannot swallow whole.

Crocodiles as Social Organisms

Crocodilians have complex mating rituals with both male and female mating displays. Both sexes engage in bellowing behavior, although male bellowing is of a deeper tone, as well as head splashing to announce their presence to potential mates. Moreover, males will create subaudible vibrations in the water above their backs, vibrating the water so that it “dances” above the animal’s back, as a sexual display. Males may use bellowing to define territories during the mating season. Males will fight over territory and may not tolerate the presence of other sexually mature males in their territory during the breeding season. Females may approach males and, by a series of rubbing moves, indicate their intention to mate. Musk glands located under the lower jaws are used as a sexual perfume to arouse the male. The male also has musk glands that are used in response to the female scent. Mating occurs in the water, with the male on top of the female. Males have a copulatory organ, a grooved penis that is inserted into the cloaca of the female.

Many crocodilians, such as the American alligator and saltwater crocodile (Crocodylus porosus), build aboveground nests using foliage, while others, such as gharials, may dig nests in the sand. Temperature-dependent sex determination occurs in crocodilians. Low nest temperatures, below 30 degrees Celsius, produce females, whereas higher temperatures, above 33 degrees Celsius, produce males. Intermediate temperatures may produce males or females depending upon species. Thus, both sexes are found in a nest since the temperature at the top of the nest may differ from the temperature at the bottom. While egg numbers vary, many crocodilians lay forty eggs or more. In populations studied, only 2 percent of the hatchlings survived to maturity. When hatching occurs, crocodilians use an egg tooth to break the eggshell. Hatchlings inside the nest will bark or grunt to signal to the mother, who may defend the nest and may be nearby to come and help dig out the hatchlings. Female crocodiles may defend the hatchlings from predators that include other crocodiles. In some species, she will carry the hatchlings to the river in her mouth.

Diet and Locomotion

Diet is dependent upon the stage of life and the adult size. When crocodiles are juveniles they may eat insects, arachnids, small frogs, and snakes. Adults may eat snails, fish, turtles, small mammals, and birds. The larger species, such as Crocodylus niloticus, may take large prey, such as wildebeest. Crocodiles may hunt cooperatively in bringing down large prey that a single individual cannot handle alone. Other species, such as caimans, may chase fish into the paths of waiting members of the population, or may herd fish to the shore for easy capture. Crocodilians are quite adept at stealthy behavior. They remain submerged and will slowly approach prey until they can leap or lunge to grab the prey in their viselike jaws. It is not the teeth that kill a prey but rather the crushing power of the jaws. If the prey is too large to kill outright, crocodiles will usually drown the prey animal and then tear it apart by rolling movements of the body. In this way two or more crocodiles can cooperate by holding on to the same prey animal and tearing it apart by combined rolling actions.

Aquatic locomotion is accomplished by undulations of the tail. There is a double row of elongated scutes running down the length of tail that may aid in swimming. The limbs are normally held against the body during swimming. Crocodiles show versatility in their terrestrial locomotion. Crocodiles are mainly quadrupedal in stance and gait; the limbs can be held at about 70 degrees under the body. Some of the smaller species, such as Crocodylus johnstoni, can gallop, while the saltwater crocodile, C. porosus, has been observed to run bipedally. All crocodilians are capable of leaping to capture prey. They appear to be fond of belly sliding as a means of locomotion over short distances, especially in entering the water.

Physiology and Size

Crocodilians are cold-blooded animals and must thermoregulate by means of evaporative cooling, using the mouth, and basking and shade usage behaviors. Crocodilians, like most reptiles, lack sweat glands. Even though crocodilians essentially have a four-chambered heart, there remains a small opening between the ventricles persisting as the foramen of Panizza at the base of the systemic and pulmonary trunks. Nonetheless, the crocodile heart is too small for the body to maintain a high metabolic rate. The stroke volume of the heart and the heart versus body mass ratio determines whether the animal may maintain high activity levels. Crocodiles have a metabolic rate that is about 4 percent that of an adult human. However, small crocodilians have higher metabolic rates than do larger species.

Crocodilians come in all sizes. Some of the caimans are small species, such as Paleosuchus, which may reach little more than five feet in length. However, other caimans, such as Melanosuchus, the black caiman, may reach up to twenty feet. Crocodiles also vary in length. The dwarf crocodiles of Africa, Osteolaemis tetraspis, may reach over six feet in length. The largest crocodilians are crocodiles. Crocodylus niloticus may reach over twenty-one feet in length, and specimens of Crocodylus porosus have been known to reach lengths of nearly thirty feet. Despite their size, some species of crocodilians, such as the gharials that are over twenty feet in length, are relatively harmless to humans because they are specialized for fish catching and cannot open their mouths very wide.

Principal Terms

Biserial Dermal Armor: Bony plates running alongside of the vertebral column

Eusuchia: The living Crocodilia

Meosuchia: The first aquatic crocodilians of the Mesozoic

Protosuchia: The first crocodilians of the Triassic

Secondary Palate: The palate forming a shelf in the mouth that places the internal nares far back in the throat

Bibliography

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