Crocodilians spend much of their time swimming in water. They swim by folding their legs against their body and moving their tails from side to side. Because they spend so much time underwater, crocodilians do not have external ears. Instead they have internal ears that are protected by a flap of skin. These ears are located behind the eyes.
Crocodilians' eyes are well adapted to underwater life. They have two sets of eyelids. The outer eyelids, like human eyelids, close from top to bottom. The inner eyelids close from the sides toward the center of the eye, like window drapes. Crocodilians use these transparent inner eyelids to see underwater.
The early crocodilians that stayed in the rivers of South America and those that moved to the rivers of India and Africa developed long snouts with 108 sharp, needle-like teeth to help them catch fish. Today, the only living ancestors of these early crocodilians are the long- snouted gharials, named after the Hindi word for "pot" (because of the pot-shaped structure on the snout of the male). Gharials live only in the freshwater rivers of Eastern India and Nepal.
The early crocodilians that moved to the wetlands of Europe and Asia developed blunt snouts and larger but fewer teeth (66 teeth) so that they could catch larger prey. These crocodilians became known as "true" crocodiles. True crocodiles eventually moved to Africa and North America and developed into different types-the Nile crocodile, Indonesian crocodile (Tomistoma), estuarine crocodile, and American crocodile.
Scientists believe that the first alligators evolved from crocodiles about 70 million years ago in North America (near present-day Montana). These animals were stockier and had blunter snouts than their crocodile ancestors. They moved to Europe and South America across land bridges. The alligators that moved to Europe became extinct. But the alligators that moved to South America moved into freshwater rivers, developed thinner snouts than modern alligators, and grew more "scutes," or plates, on their skin for protection. They developed into the modern caiman, found in South and Central America.
The last group of crocodilians to evolve were the "true" alligators of China and the southeastern United States. Scientists believe that these alligators evolved from crocodiles in ancient Florida about 20 million years ago. Some of the alligators moved to Asia over the land bridge. The others remained in the Southeastern United States.
In Florida, American crocodiles bask along the banks of sheltered coastal mangrove areas and in dens, or tunnels, along the shores of creeks. They range from Lower Biscayne Bay to upper Key Largo and west along the mangrove mainland in Florida Bay to Cape Sable. The salinity of these areas varies from highly saline near Key Largo to slightly brackish in Lower Biscayne Bay. Within that range the largest nesting populations are found near Joe Bay and Madeira Bay in Northern Florida Bay, on upper Key Largo, and on the edges of the cooling canal system of Florida Power and Light Company's Turkey Point plant, located south of Miami (Biscayne Bay).
The American alligator can be found throughout low wetland areas from the extreme southeast corner of Virginia southward to the Lower Florida Keys, west to the Rio Grande in Texas, and northward up the Mississippi River to southeastern Oklahoma, eastern Arkansas, and northwest Mississippi. Most alligators live around shallow freshwater lakes or ponds, freshwater marshes, swamps, rivers, and creeks. Some can also be found in salt marshes and certain inshore marine habitats. However, alligators prefer freshwater and will nest only in freshwater areas. Alligators dig dens in mud banks or in the bottom of larger ponds. In Florida these dens are known as "gator holes."
Crocodiles tend to be lighter colored than alligators. Crocodiles are grayish-green to olive-brown across the back and have white or yellow undersides. Young crocodiles have dark stripes across the back of their tails. In contrast, alligators have black or dark bodies and their young have bright stripes and blotches of yellow on their bodies.
Crocodiles have longer, more tapered snouts than alligators. Within their long, tapered snouts, crocodiles have 66 teeth, and the fourth tooth on either side of the lower jaw is exposed when the mouth is closed. Alligators have a broad snout with 40 teeth and an internal socket in the upper jaw so that the fourth tooth on either side is not exposed when the mouth is closed.
Crocodiles tend to be longer than alligators. Male crocodiles can grow to 4.6 m (about 15 ft) and females can grow up to 3.9 m (about 12.8 ft). Alligators can grow to 3.5 m (about 11.2 ft) for males and 2.5 m (about 8.2 feet) for females.
In Joe Bay and Madeira Bay, females prefer beach nests. Therefore, they often must travel away from the protected coastline and out across open water, to the more saline waters of the Keys in Florida Bay. Because the water level is lower on these islands than on the inshore creek banks, island nests have a higher rate of success. However, the island location presents a problem for hatchlings, who have to travel from the island nest sites across open water back to the mangrove coastline.
In early May (around the 12th), each female crocodile lays 8 to 80 eggs during a single night. The female covers the eggs and leaves the nest. She does not return to the nest until late July or early August- 85 days later. When the mother returns, she places her head across the mound and listens for barking sounds from the nest. These sounds indicate that the offspring have broken through their eggs with their "egg teeth."
After the eggs have hatched, the mother leaves the hatchlings, which remain along the shore or bank for the next two weeks. They feed on the yolk that they ingested while breaking out of their eggs. After a few weeks, the crocodile hatchlings begin the journey back to the coastline and creeks.
Alligators use vegetation to build nest mounds on raised banks in freshwater wetlands during the months of June and early July. They construct their nests in just a few days and return to the same site to construct a new nest year after year. Females make a mound approximately 1 m (3 ft) high with twigs, branches, and other debris. Once the mound is complete, the female digs a depression about 30 cm (about 12 in) deep and fills the bottom with rushes, roots, and mud. She then deposits 20 to 50 eggs and covers them with mud.
Female alligators stay near the nest for the next 65 days. The eggs hatch in mid-August. When the female alligator hears high-pitched barking indicating that the hatchlings are breaking through the eggs with their egg teeth, she opens the nest. The mother alligator cares for her young for a period ranging from a few days to several months. Unlike crocodiles, which make very little noise, alligators make a bellowing sound to call their young together.
Dr. Frank Mazzotti and Laura Brandt have observed during their work in capturing and tagging crocodiles that they are not as aggressive as alligators. Unlike alligators, which put up quite a struggle when they are captured, crocodiles generally only roll a few times before submitting to their captors. Scientists also have observed that when a male American crocodile in captivity feels threatened, he arches his back and slaps his head on the surface of the water. Unlike the American alligator, the crocodile does not bellow or hiss.
In addition to this habitat loss, crocodiles are affected by re-routing of freshwater through artificial canals. Sometimes re-routing of water changes the volume (amount) of water in a site, causing flooding- which threatens the crocodiles' habitat, especially the nests. In other cases, re-routing of water changes the water's quality-the delicate balance of salinity required for healthy species. This can decrease the available freshwater, which the crocodiles (especially the hatchlings) depend on for drinking. Changes in the population size of the American crocodile can tell scientists quite a bit about a site's ability to support life. When one species is used by scientists as an indicator of certain conditions in an environment, that animal is known as an indicator species.
The current plans for re-routing freshwater flow in the Everglades will affect the entire habitat. The demise of the American crocodile may be just the first indication of this impact.
In order to determine the "health" of the crocodile population in the Everglades, Frank and Laura visit the sanctuary three times a year for a period of three days and two nights. They locate crocodile nests in late May. In August, they return to these nests to tag, measure, and record the hatchlings' success. In January, Frank and Laura move to the coastline and creek banks to find the hatchlings from August and determine their growth and movement. Adults are also measured for size and movement.
To identify and keep records about individual crocodiles, scientists remove scutes from the top of each crocodile's tail. For example, they may remove the third scute on the right side of the tail and the second scute on the left side of the tail. This crocodile is then identified as "Right 3, Left 2." Each time scientists recapture "Right 3, Left 2" they can add information to that crocodile's record to help them chart the crocodile's growth and movement. This process of tagging and recapture is also used for sharks and is discussed further in Investigation 13.
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Gene Carl Feldman
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Todd Carlo Viola, JASON Foundation for Education (todd@jason.org)
Revised: 17 Oct 1995