Parietal x Frontal
Parietal x Frontal
A crocodilian skull. (Illustration by Brian Cressman)
recorded in several species. Changes in mood, such as those caused by stress, and environmental temperature can dull skin color. Long-term change can be effected by the environment, with individuals from shaded areas becoming darker as black pigment (melanin) accumulates in the skin.
The crocodilian head always draws attention. The skull, although massive and sturdy, is infiltrated with air spaces. These spaces reduce weight without compromising strength, and provide extensive areas for muscle attachment and expansion. Two pairs of openings on either side of the cranium classify the skull as diapsid. There is considerable variation in skull and jaw morphology across all 23 species, and this has an ecological significance: broad jaws are reinforced by bony ridges to resist strong bite forces for crushing prey, while slender jaws slice with little resistance through water to seize slippery prey.
The head houses all the major sense organs, vital for navigation, communication, and hunting. Senses are concentrated on the dorsal surface, so they remain exposed even when the head is partially submerged. Remarkably, a crocodile can hide its entire body below the water while maintaining maximal sensory input from its surroundings. As masters of stealth and ambush, crocodiles have no equal.
The eyes of the crocodile are placed high on each side of the head, turrets that provide 270° of widescreen coverage plus 25° of binocular overlap directly ahead to accurately judge distance. The pupil, round and dilated at night to permit maximum light entry, is compressed to a thin vertical slit during daylight to protect the sensitive retina. Inside the eye, cone cells on the retina provide color acuity by day, and high densities of rod cells give excellent low-light sensitivity at night. These rods can change shape to further alter sensitivity. A layer behind the retina, the tapetum lucidum, is im pregnated with guanine crystals to reflect light back across the visual cells. This effectively doubles visual sensitivity at night, and shining a beam of light directly into a crocodile's eye rewards the observer with a fiery red eyeshine. Visual cells are most concentrated in a horizontal band across the back of the retina, a fovea providing highest visual acuity where crocodiles need it most—along the same plane as water. To focus (accommodate), crocodilians change the shape of their lens using the ciliary body.
Three eyelids cover each eye. The upper lid contains bony ossification to protect the eye, large bony palpebrals in caimans lending "eyebrows" to their appearance. The lower lid lacks ossification and is responsible for closing the eye. The third eyelid, the nictitating membrane, sweeps laterally over the cornea to clean the eye and protect it from abrasion underwater. Although the nictitating membrane is transparent except for the ossified leading edge, crocodilians still see poorly through it. Lachrymal (tear) glands lubricate its passage via ducts connected to the nasal cavities. Fluids may even accumulate when the crocodile remains out of water—real "crocodile tears," yet an unlikely source for the popular myth.
The ears are located immediately behind the eyes, the eardrum protected by an elongated flap of skin. Hearing sensitivity can be altered by opening a slit in front of the flap, or lifting the flap upward. When submerged, the ears normally close, as hearing becomes secondary to the ability to feel vibrations through the water. Detectable frequencies range from below 10 Hz to over 10 kHz, and sound pressure levels below -60 dB can be detected within certain bandwidths. In other words, crocodilians have excellent hearing, on a par with birds and mammals. Peak sensitivities range from 100 Hz to 3 kHz depending on the species, which coincides with the bandwidth of calls produced by juveniles. Vocalization is well developed in crocodilians, with over 20 different call types from both juveniles and adults recognized.
Crocodilians can breathe when submerged by exposing the dorsal margin of their head and hence their raised nostrils. Inhaled air passes through sinuses separated from the mouth by a bony secondary palate, where any chemicals in the air are detected by sensory epithelial cells. The presence and direction to food is easily discerned, and smell plays an important role in chemical communication. In early crocodyliforms, the internal nostrils (choanae) opened in the front of the mouth, but over millions of years they moved back to the throat, a phenomenon termed post-nasal drift. The palatal valve, a fleshy extension of the tongue, completely seals the throat from the mouth, hence crocodilians breathe easily near the surface even if the mouth is flooded with water. The glottis, an opening to the trachea and lungs, is located directly beneath the choanae. By varying tension in muscles lining the opening, exhaled air is forced through a constriction capable of relatively complex vocal sounds. Amplification is provided by expanding the throat using the hyoid apparatus, a curved cartilage beneath the glottis. A curious bend in the trachea of several species may further amplify the sound, similar to the long, curved necks of cranes.
The tongue lies between each mandibular bone of the lower jaw, behind the mandibular symphesis (fusion). Hence in slender-snouted species with extended sympheses, the tongue is greatly reduced. Although relatively immobile, the tongue can be pushed against the roof of the mouth to manipulate objects or pulled down to create a pouch for hatch-lings. Typically bright yellow or orange, the tongue's color may provide a social or warning signal when the jaws gape. Pores cover the surface of the tongue, through which "salt glands" produce a saline fluid in brackish or sea water in Croc-odylidae and Gavialidae. Alligatorid pores play no role in salt secretion, supporting theories of a more recent marine dispersal phase for the Crocodylidae. Chemoreceptors lining the tongue detect chemicals in water, yet little is known of their sensitivity. Their importance is implied in their ability to detect food underwater, and in the role of pheromones secreted from chin and paracloacal musk glands.
High densities of dome pressure receptors (DPRs) cover scales on the head, particularly around the jaws. Disturbances of the water surface create pressure waves easily detected by DPRs, rapidly alerting the crocodile to potential prey near the head. Crocodilians also react rapidly to movement underwater (such as fishes) even when vision is unavailable. Similar pressure receptors, Integumentary Sense Organs (ISOs), are located on the caudal margin of body scales in Crocodyl-idae and Gavialidae, but not Alligatoridae. Their function is not fully understood, nor is the reason why alligatorids entirely lack them. However, evidence of DPRs exists in extinct crocodyliforms, suggesting their sensory role in water has long been part of their repertoire.
Betraying their terrestrial origins, crocodilians are surprisingly mobile predators on land. Although lacking the stamina for pursuit, their explosive force catches most prey unaware. Like all archosaurs, the hind limbs are significantly larger and stronger than the forelimbs, suited to the croco dilian propensity for launching the body forward at speed. Five toes are present on the front feet and four on the back, although residual bones from the fifth still exist. The inner three toes terminate with strong, blunt nails that provide traction; the outer one (back feet) or two (front feet) lack claws and bend backward during walking. There is extensive webbing between the toes on the back feet, but webbing is minimal or absent from the front feet.
The limbs are used to crawl, walk, and gallop. The crawl employs the limbs alternately to slide the body across mud, sand, or grass. When sufficiently motivated, the limbs can propel the body forward in a slithering manner at much greater speed, up to 6.2 mph (10 kph). In the uniquely crocodilian high walk, the feet rotate inward toward the body and support it from below. Lifting the head and belly clear of the ground enables the crocodile to traverse obstacles or rough terrain. In a few species, the front and hind limbs move in tandem to gallop. This springlike gait accelerates the crocodile up to 10.6 mph (17 kph) for several seconds until the safety of water can be reached. Cuban crocodiles (C. rhomb-ifer) remind us of the frightening aggression of their terrestrial ancestors when they gallop toward a threat.
Water is clearly the crocodilian's preferred domain, a home for prey that live in the water and a magnet for prey that live on land. Mobility is possible through the powerful tail, which makes up half the body's total length. Flattened dorsoven-trally to provide extensive surface area for propulsion, the tail is undulated laterally by powerful muscles. Limbs are swept back during rapid swimming, although when moving slowly they help the crocodile steer, brake, reverse, or walk across the bottom. So powerful is the tail that it can drive hundreds of pounds (or kilograms) of crocodile vertically out of the water to capture prey several feet (or meters) overhead.
Internally, the pleural cavity contains the lungs, and the visceral cavity houses major organs associated with digestion and reproduction. These cavities are separated by the bilobed liver and diaphragmaticus, a sheet of muscle analogous to the diaphragm in mammals. Inhalation is achieved by contracting the diaphragmaticus, which pulls the liver backward and expands the pleural cavity. Thoracic (intercostal) muscles also expand the chest, and reduced pressure in the lungs draws air in through open airways. To exhale, the diaphragmaticus and thoracic muscles relax, compressing the pleural cavity and forcing air out of the airways. Crocodilians control their buoyancy primarily through the volume of air in their lungs. By moving the liver, hind legs, and tail, subtle postural changes are also possible. When diving, air is forced out of the lungs and the crocodilian, which is considerably heavier than water, sinks rapidly. Swimming may facilitate this sinking, and by sweeping the hind legs forward the crocodilian can reverse and submerge simultaneously. Swimming or pushing off the bottom returns the crocodilian to the surface, and positive buoyancy is achieved by filling the lungs with air. Stones called gastroliths in the stomach typically comprise 1-5% of the crocodile's total weight, and their presence may provide additional ballast.
Situated between the lungs is the most complex heart in the animal kingdom, apparently the result of adaptation to
the demands of the crocodiles' semiaquatic lifestyle and their size. Unlike other reptiles, the crocodilian heart is fully divided into four chambers, as are the hearts of birds and mammals. Uniquely, valves under nervous and hormonal control can alter blood flow. These ensure that vital oxygenated blood circulates between essential areas during oxygen stress, such as while diving, while deoxygenated blood is sent to nonessential areas. During rest and normal exercise, blood in the right ventricle passes via a coglike valve to the pulmonary arteries and lungs to acquire oxygen. During diving, this valve constricts, and deoxygenated blood is diverted to the left aortic arch that leads to the nonessential visceral organs—a pulmonary-to-systemic shunt. Only a small volume is used to collect residual oxygen in the lungs. A second valve, the foramen of Panizza, connects the base of left and right aortic arches. The right aorta directs blood to the head, limbs and tail, and these vital areas require oxygenated blood during oxygen stress. The foramen of Panizza allows oxygenated blood to pass from right to left aortas (to visceral organs) only during rest and normal exercise, cutting them off when not needed.
Biochemical adaptations complement the action of the heart. Crocodilian blood contains complex hemoglobin molecules capable of carrying more oxygen molecules than those of any other vertebrate. Crocodilians also endure much higher levels of lactic acid (produced when oxygen is scarce) in their blood than any other vertebrate. Blood pH has been measured below 6.1 without serious consequences, a level that would kill any other vertebrate. The result? A submergence time of nearly two hours when quiescent, even longer under cool conditions. American alligators have remained trapped under ice for eight hours and survived. Heavy activity substantially reduces submergence time, but crocodilians need only to outlast their prey. The blood also houses complex an tibacterial proteins capable of fighting off infection. Living in bacteria-filled waters, where injuries from fights are common, a strong immune system is essential. Crocodile blood has even been shown to kill "superbugs" for which scientists have no known cure.
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