CROCODILES: CHARACTERISTICS, ANATOMY, DIFFERENCES WITH ALLIGATORS

CROCODILES

mugger crocodile

Crocodiles are the largest reptiles. They are also the largest of all predators that spend some time on land. The biggest crocodiles are bigger than the biggest tigers and lions and polar bears. In the water only Great White sharks and Killer Whales are bigger predators..

A big crocodile, according to National Geographic writer Rick Gore, "is cunning enough to stalk a human, strong enough to bring down and dismember a water buffalo, yet gentle enough to crack open its own eggs to release its young." Dr. Daphne Soares, a neuroscientist at the University of Maryland, told the New York Times, “I absolutely love these creatures. They're beautiful, elegant and goofy at the same time...”An alligator looks like nothing so much as a big, amphibious and grievously misunderstood kitten. Sure, it sports thick scales and bulging bony knobs called osteoderms rather than fur, and 80 teeth to the house cat's 30, and a tail.”

Describing why crocodile are not loved like some other animals, Wayne King of the New York Zoological Society said: "They're not cuddly. They don't have big soulful eyes like seals. Most of the animals the world is concerned with are beautiful, or they tug at your heartstrings. Crocodiles have a pretty toothy leer. They eat dogs in Florida — sometimes even people. Who could love them?" [Source: Rick Core, National Geographic, January 1978]

Explaining mankind's fascination with crocodiles and other dangerous animals the eminent biologist E.O. Wilson once wrote: "We're not just afraid of predators, we're transfixed by them, prone to weave stories and fables and chatter endlessly about the them, because fascination creates preparedness and preparedness, survival. In a deeply tribal sense, we love our monsters."

Websites and Resources on Animals: Reptile Database reptile-database.org ; Reptileweb reptilesweb.com ; Reptile Phylogeny whozoo.org/herps/herpphylogeny ; Animal Diversity Web animaldiversity.org ; BBC Earth bbcearth.com; A-Z-Animals.com a-z-animals.com; Live Science Animals livescience.com; Animal Info animalinfo.org ; World Wildlife Fund (WWF) worldwildlife.org the world’s largest independent conservation body; National Geographic National Geographic ; Endangered Animals (IUCN Red List of Threatened Species) iucnredlist.org

Crocodilians

crocodile head

There are 28 species of crocodilians, in three families — A) Crocodylidae ( "true" crocodiles.crocodiles, 18 species); B) Alligatoridae (alligators (two species) and caimans and close relatives (six species); and C) Gavialidae (gharials and false gharials (two species). Crocodiles are essentially Old World creatures living in Asia and Africa and alligators and caimans are New World ones, living in the Americas. Gharials and false gharials live in Asia and can be easily recognized because they have long, very slender snouts

Crocolilians live in 100 tropical and sub-tropical countries. All reside within 4000 kilometers of the equator and all have adapted to a semi-aquatic life, although as recently as 3000 years ago there may have been a terrestrial crocodilian species on New Caledonia. Some crocodiles may venture into larger bodies of fresh or salt water, but all must lay their eggs on dry land. The only crocodilians that live outside the tropics are American alligators and Chinese alligators and they still cannot tolerate climates colder than temperate climates. No crocodilians venture out of lowlands; it is speculated that none ever lived above 1000 meters (3260 feet) above sea level. [Source: Danny Goodisman, Animal Diversity Web (ADW), September 9, 2002]

Danny Goodisman wrote in Animal Diversity Web: All crocodilians have a similar body shape, with a head held horizontally in front of the body, four legs which project from the sides, heavy scales which function as armor, and a heavy muscular tail. Their front feet have five separate toes and their rear feet have four partially-webbed toes. Their eyes are on the top of their head, close together to allow for binocular vision (the field of vision of the two eyes intersects, to provide more accurate depth perception in front of the animal). The nostrils are crescent-shaped and valvular, and set at the end of the snout, which allows breathing even when the animal is almost entirely submerged. [Source: Danny Goodisman, Animal Diversity Web (ADW), September 9, 2002]

Crocodilia Classifications

Crocodiles, alligators, and gharial belong to separate biological families. The order Crocodilia consists of 28 extant species belonging to nine genera grouped into three families based on modern molecular studies.

alligator head

A) Family Crocodylidae (True crocodiles)
Genus Crocodylus: fourteen species
Genus Mecistops: two species
Genus Osteolaemus: two species
[Source: Wikipedia]

B) Family Alligatoridae (Alligators and caimans)
Genus Alligator: two species
Genus Caiman: three species
Genus Melanosuchus: one species
Genus Paleosuchus: two species

C) Family Gavialidae (Gharial and false gharial)
Genus Gavialis: one species
Genus Tomistoma: one species

Differences Between Crocodiles and Alligators

How can you tell crocodiles and alligators apart? Crocodiles have: 1) relatively narrow snouts; 2) lower teeth that are visible when their mouths are closed; and 3) a special notch on either side of their upper jaw for the fourth tooth of the lower jaw. Alligators have a wider, more rounded snout, their lower teeth are not visible and rest inside their upper teeth and mouth and they lack the notch. In comparison to American crocodiles, alligators are dark brown to black in color whereas American crocodiles are more greenish-grey.

The most obvious external differences between crocodiles and alligators are visible in the head. Crocodiles have narrower and longer heads, with a more V-shaped shout while the snout of alligators and caimans is more U-shaped. Another obvious trait is that the upper and lower jaws of the crocodiles are the same width, and the teeth in the lower jaw fall along the edge or outside the upper jaw when the mouth is closed. This means all teeth are visible, In contrast alligators possess small depressions in the upper jaw into which the lower teeth fit. Also, when the crocodile's mouth is closed, the large fourth tooth in the lower jaw fits into a constriction in the upper jaw. For hard-to-distinguish specimens, the protruding tooth is the most reliable feature to define the species' family. Crocodiles have more webbing on the toes of the hind feet and can better tolerate saltwater than alligators and caiman due to specialized salt glands for filtering out salt, which are present, but non-functioning, in alligators. Another trait that separates crocodiles from other crocodilians is their much higher levels of aggression.[Source: Wikipedia]

In a study published in March 2018 in the journal Royal Society Open Science, scientists from Japan reported that they had identified another feature that differentiated crocodiles and alligators: Alligators tend to have shorter humerus bones in their forelimbs and shorter femurs in their hind limbs than crocodiles. “This information could help explain differences in their ecology and locomotion, including the strange fact that, while small crocodiles have been observed to bound and gallop, alligators have not,” Julia Molnar an evolutionary biologist from the New York Institute of Technology College of Osteopathic Medicine, told the New York Times. She said the limb differences can affect things like speed and leverage in the animals. [Source: Nicholas St. Fleur, New York Times, March 12, 2018]

According to the New York Times: “The differences are small, and not something you could easily spot in the wild. But the finding may provide insights into the ways in which the two reptiles move. Masaya Iijima, a vertebrate paleontologist from Hokkaido University in Japan and lead author on the study, measured more than 120 alligator and crocodile skeletons from nearly a dozen museums across the world. Then he analyzed the results using a statistical model. The specimens mostly belonged to extinct crocodilians, which is the supergroup that encompasses both alligators and crocodiles, as well as caimans and gharials.

Though they look remarkably similar, alligators and crocodiles diverged evolutionarily during the Late Cretaceous period some 80 million years ago. To put that into context, humans and chimpanzees split ways about seven million years ago. Both reptiles also survived the mass extinction event that wiped out the dinosaurs, and since then have remained relatively unchanged. That includes the differences seen in their limb proportions, according to Dr. Iijima.

“He found that over the course of millions of years, crocodiles tended to have longer humerus bones and femur bones than alligators. That suggests that the muscles attached to those bones extended farther on crocodiles than on alligators. The newly discovered difference is not nearly as apparent as the differences seen in the reptiles’ snouts. “I didn’t notice it in the museum, but when I put them into the statistical test then I found it,” Dr. Iijima said. “It’s a very tiny difference, but an important difference.” Though he found a statistical difference between the two superfamilies, Dr. Iijima noted that does not necessarily mean the differences have had a noticeable effect. “It is surprising that this has not been investigated before, but many aspects of crocodilian biology have been neglected,” said John Hutchinson, an evolutionary biomechanist at the Royal Veterinary College at the University of London.

Crocodile Size and Age

Crocodilians range in size from Cuvier's dwarf caiman, which only grows to about 1.5 meters (five feet) long, to the saltwater crocodile, which grows to seven meters (23 feet) long. Male crocodilians are larger than females. /=\

Large adult crocodiles can weigh more than a ton. Large male crocodiles reach a certain length and then they start to broaden out so they look more like a tank than a crocodile. No one knows exactly how long a crocodile can live. Specimens in zoos have survived until their late 60s but some scientists speculate that some they may reach an age of 80 years old.

Saltwater crocodile caught in the the Johnstone River near Innisfail Australia in 1903

In July 2012, a huge crocodile known as Lolong was officially named the largest crocodile in captivity, the Guinness World Records. Lolong measures 20.24 feet (6.17 meters) and weighs more than a ton, Guinness spokeswoman Anne-Lise Rouse said in a statement. The Guinness website said: “Lolong’s weight was also measured at a nearby truck weigh-bridge and verified as approximately 1,075 kilograms. The reptile took the top spot from an Australian crocodile that measured more than 17 feet (5 meters) and weighed nearly a ton. See Saltwater Crocodiles

The world’s oldest known crocodile, named Henry, resides at the Crocworld Conservation Center in South Africa and turned 124 in December 2024 according to The Guardian. It's difficult to determine a crocodile's age. Sometimes is estimated based on size. The rings on the femur bone can be counted similarly to rings on a tree but that can generally only be done after a crocodile dies.[Source: Emily Wind, The Guardian, November 6, 2024].

Crocodile Characteristics

Crocodilians can adopt a more straight-limbed posture than other reptiles, have gizzards, a four-chambered heart and birdlike respiratory systems and are ectothermic (cold blooded). Crocodiles warm their bodies by lying in the sun, and they open their mouths to release some of heat if they become too hot. Their backs are protected with armored plates made of dermal bone. They can float just at the surface of the water, and can adjust their buoyancy by ingesting stones. [Source: lowryparkzoo.com]

Telling male and female crocodiles apart is very difficult. In the old days breeders often had to wait until female gave birth to say for sure she was a female. In some cases zoologist discovered that reason why a supposed “breeding” pair of crocodiles fought rather than mated was that they were both males. Today sex is determined by flipping a crocodile on its back and reaching up is rear end.

The crocodile’s immune system is more powerful than that of humans. It is capable of killing the HIV virus and fending off severe infections and diseases that would kill people. “A crocodile wallows in mudholes, lagoons and other microbial Club Meds, yet it can suffer the most harrowing sort of injury - a limb torn off, its belly ripped open, its lower jaw sheared away - without so much as shedding a crocodile tear. "Crocodiles have tremendous robustness against bacterial infection," Dr. Ross said. "The sort of wound that would leave any of us severely septicemic doesn't seem to touch them." That immunological ferocity has inspired researchers at the Johns Hopkins to begin screening crocodile blood in search of new antibiotics.”

All crocodilians possess integumentary sense organs located in the skin and covering much of the animal’s body including the body, tail, cloaca and inner surfaces of the legs, as well as on the head and jaws. These are likely used to detect changes in pressure caused by touch or the movement of prey in water; this sense is likely used for hunting in murky water. [Source: Katie Foster, Animal Diversity Web (ADW) /=]

Crocodile Skin

crocodile skin

The skin of crocodiles is covered with non-overlapping scales composed of the protein keratin and often studded with bony plates called scutes. Lizard scales are similar, but lack the bony plates. The scales are shed individually, so crocodilians do not molt (shed their skin all at once) like snakes do. This skin does not provide much insulation, preventing crocodilians from inhabiting cooler climates. [Source: Danny Goodisman, Animal Diversity Web (ADW), September 9, 2002]

A crocodile’s bony, leathery skin is so tough that many bullets can not penetrate it. Natalie Angier wrote in the New York Times, “As scientists are just beginning to appreciate, that body plan is panzer, a tropical tank from the skin in. Beneath its scaly sheath and craggy osteoderms is another layer of armor, built of rows of bony overlapping shingles, or osteoscutes, that are both strong and flexible. And beneath that formidable barrier is an immune system that merits the modifier: it is virtually immune to defeat. [Source: Natalie Angier, New York Times, October 26, 2004]

Crocodile Anatomy

Crocodiles are considered to be more advanced than other reptiles. Their hearts have four chambers like mammals and birds, but there is a pore between the left and right ventricles which allows some mixing. Four-chambered crocodile hearts are almost as advanced as those of mammals and birds, allowing crocodiles to stay submerged underwater for as long as an hour by slowly pumping blood through their bodies. Crocodile and bird hearts have elaborate plumbing in their hearts with valves that keep oxygenated and unoxygenated blood separate. The reptile heart by contrast mixes the two kinds of blood. In cold weather alligators can stay underwater for up to 24 hours without breathing.

Crocodiles have a large chunk of skin in their throats called a palatal valve that prevents water from flowing into their lungs and drowning it when it opens its mouth. They also have muscles that pulls the liver backward, inflating the lungs. They serve the same purpose of a diaphragm in humans.

According to Animal Diversity Web: Their thoracic and abdominal cavities are separated by a muscular diaphragm, used in breathing. Their nostrils close when the animal dives; the nostrils are separated from the mouth by a bony palate (like mammals have, although other, reptiles, don't) and a valve in the back of the mouth. This allows crocodilians to breathe when their entire body except their nostrils are submerged, and also when holding prey. [Source: Danny Goodisman, Animal Diversity Web (ADW), September 9, 2002]

They have two openings in their skull behind their eye sockets; it has been suggested that these openings help the attachment or functioning of crocodilians' powerful jaw muscles. Their stomachs are the most acidic recorded for any vertebrate, allowing them to digest even the bones and shells of prey animals. Their digestion is also aided by a muscular gizzard containing stones to help break down food

Crocodile Head and Teeth

crocodile tooth

A crocodile’s ears, eyes and nostrils are located on the top of it head. This allows it to breath, hear, smell and see above water with just a small part of its body exposed. If you are brave enough to go swimming in croc infested waters keep your eyes pealed for beady crocodile eyes, peeping above the surface.

Crocodiles have no lips so water leaks in and out of their mouth. When they eat they have to lift their head out of the water so water doesn’t go down their throat and lift their head up so gravity forces the food down their throat. Even if a crocodile didn’t have any teeth, it could easily kill a man by crushing his skull with its powerful it jaws.

A crocodile's teeth are constantly growing. Their teeth are always falling out and new ones grow in. Danny Goodisman wrote in Animal Diversity Web: Crocodilians' teeth are mostly identical, thick-walled cones embedded in the outside of the jaw. Rear teeth are more cylindrical and blunter than front teeth. The teeth are not cemented into the jaw but simply attached to the outside, and can be broken off. Crocodilians can replace their teeth an indefinite number of times (as opposed to mammals, which only replace individual teeth once). [Source: Danny Goodisman, Animal Diversity Web (ADW), September 9, 2002]

Unusual Scales on a Crocodile’s Head

Crocodiles have lumpy, three-dimensional skin patterns on their scaly heads which differ from the sleek and smooth scales and patterns found on snake and lizard heads. How this unique skin formed on crocodiles has long puzzled scientists. In a study published on December 11, 2024 in the journal Nature, a group of researchers said the pattern of scales on the face and jaws of crocodiles is formed by a mechanical process of skin folding and not genetics. [Source: Laura Baisas, Popular Science, December 12, 2024]

Laura Baisas wrote in Popular Science: Typically, animal skin appendages including hair, feathers, and scales are controlled by specific genes when an embryo is developing. There are some exceptions to this rule–including in crocodile heads. According to Michel Milinkovitch, a study co-author and physical biologist who helps lead the Laboratory of Artificial & Natural Evolution at the University of Geneva in Switzerland, the fact that their body scales and head scales develop so differently also sets them apart as animals. Crocodile body scales develop from what scientists call a polka dot pattern of gene expression during the embryo’s development. This is when particular genes turn on in specific and localized regions of tissue or an organ. “So, at each spot of high gene expression, cells become fated to form a skin appendage–a hair, a feather or a scale, depending on the species,” explains Milinkovitch.

However, crocodile head scales are a bit different from their body scales. While taking a blood sample from a Nile crocodile, Milinkovitch was struck by this unusual pattern of scales on its jaws and face, where some of the polygons had unconnected edges. This unusual pattern couldn’t really be explained by the primary genetic understanding of how sales form.

Milinkovitch suspected that a mechanical process was at play–and not genetics. Determining the precise mechanism has eluded scientists, as crocodile embryos are pretty difficult to come by. It took over 10 years for Milinkovitch and his colleagues to gather enough embryos to conduct this new study. Once the team had several embryos to work with, they then combined experiments with the embryos and computer simulations to generate a 3D mechanical growth model that details the patterning of head scales on a crocodile.

They discovered that the scales are self-organizing through some familiar mechanical processes including compressive folding. This folding begins when the skin is growing faster than the underlying bone and when the skin itself becomes either more elastic or stiff. This folding and physical change then produces the irregular geometric patterns in the head scales as the crocodile grows in a distinctly different way.

Crocodile Senses

Crocodilians have keen senses of smell, sight, and hearing. They have elongated open-ear canals. Their ears are covered by flaps which close to prevent water from entering them. Crocodilian eyes are immobile spheres covered by three eyelids. They have vertical, cat-like pupils which dilate to allow them to see well in the dark. A layer of tapetum at the back of their eyes also greatly increases their ability to see at night and makes their eyes glow in the dark. Crocodilians do not see well underwater. [Source: Danny Goodisman, Animal Diversity Web (ADW), September 9, 2002]

Crocodile eyes have a special transparent third eyelids — nictitating membranes —that can be closed when the animal submerges, allowing them to see under water. Crocodile pupils "tilt a little as the angle of the head changes, so that prey will always be in focus.” Wrestlers who flip a crocodile over and rub its stomach and “mesmerize” the crocodile or “put it to sleep” are actually disorienting the creature. Upside down the crocodile’s pupil's can't adjust and the world becomes nauseating conglomeration of images.

Some crocodilians have dark bumps all over their upper and lower jaws or all over their body. These bumps have nerves that are connected directly to the brain. For a long time it wasn’t known what these bumps did. In the early 2000s, it was discovered that they enable crocodilians to detect movement in the water. Alligators that were deprived of sight and hearing were able to swim directly to a drop of water using the bumps.

In the early 2000s, Dr. Daphne Soares, a neuroscientist at the University of Maryland, discovered a kind of sixth sense unique to crocodilians. Natalie Angier wrote in the New York Times, “She has determined that the mysterious little bumps found around the jaws of some crocodile species and across the entire bodies of others, which naturalists had long observed but never before understood, are sensory organs exquisitely suited to the demands of a semisubmerged ambush predator. The pigmented nodules encase bundles of nerve fibers that respond to the slightest disturbance in surface water and thus allow a crocodile to detect the signature of a potential meal - an approaching fish, a bathing heron, a luckless fawn enjoying its last lick of water.[Source: Natalie Angier, New York Times, October 26, 2004]

Image Sources: Wikimedia Commons

Text Sources: Animal Diversity Web animaldiversity.org ; National Geographic, Live Science, Natural History magazine, David Attenborough books, New York Times, Washington Post, Los Angeles Times, Smithsonian magazine, Discover magazine, The New Yorker, Time, Reuters, Associated Press, AFP, Lonely Planet Guides, CNN, BBC, Wikipedia, The Guardian, Top Secret Animal Attack Files website and various books and other publications.

Last updated February 2025